POST-TENSIONED IN BUILDINGS · POST-TENSIONED INBUILDINGS Preface The development of reliable prestressing techniques has certainly been the most important innovation in the field

4.1VSL REPORT SERIES

POST-TENSIONEDIN BUILDINGS

General Objectives in Building Design

Applications of Post-Tensioning

in Building Structures

The VSL Hardware for Use in Buildings

Details and Layouts Improving the

Constructability

Preliminary Sizing

of Post-Tensioned Floors

Examples

PUBLISHED BYVSL INTERNATIONAL LTD.

P OST-T E N S I O N E D I N B U I L D I N G S

PrefaceThe development of reliable prestressing techniques has certainly been the most importantinnovation in the field of structural concrete. It enabled concrete construction to competesuccessfully within areas that had previously been dominated by steel construction, including long-span bridges, high-rise buildings, pressure vessels and offshore structures.Today, prestressing and, in particular, post-tensioning is a mature technology, providingefficient, economic and elegant structural solutions for a wide range of applications.

Surveys indicate vast differences in the use of post-tensioning among different countries.While the wide spread can largely be explained by differences in local needs, standards,education and habits it appears that the potential offered by post-tensioning is far frombeing exploited, especially in building structures. Too many building structures, for whichpost-tensioning would provide a clearly superior solution, are conceived, designed and builtas non -prestressed. For too long, non-prestressed and prestressed concrete have beentreated as completely seperate entities and hence, prestressing is not yet regarded as afamiliar and desirable construction option by many developers, architects, engineers andcontractors.

Post-tensioning in buildings is not limited to floor slabs. Post-tensioning of foundations,transfer beams and plates, post-tensioned masonry and the combination of precast elements with cast-in-place concrete by means of post-tensioning offer other interestingopportunities. Developers, architects, engineers, contractors, educators and students willfind the present report to be most informative in this regard. It describes the application ofpost-tensioning within the overall context of building construction and it yields a sufficientbasis for corresponding preliminary designs; special information required for the finaldimensioning and detailing will be given in a companion report.

VSL should be commended for continuing their tradition to disseminate state-of-the art information on post-tensioning and it is hoped that through this and related efforts anincreasing number of companies and individuals will benefit from the use of posttensioningin buildings.

Zurich, 30th April 1992

Prof. Dr. Peter MartiETH Zurich


Contents1. Introduction 2

2. General Objectives in the Design of Building Structures 4

3. Applications of Post-Tensioning in Buildings 103.1 Floor Systems . 103.2 Moment-Resisting Frames . 163.3 Transfer Beams and Transfer Plates 183.4 Wall Panels and Service Cores 193.5 Post-Tensioned Foundations and Ground Anchors 213.6 Post-Tensioned Masonry Walls 223.7 Other Applications 22

4. The VSL hardware for Use in Building Structures 234.1 The System 234.2 To Grout or not to Grout? 23

5. Preliminary Sizing of Post-Tensioned Floors 25

6. Details Improving the Constructability of Post-Tensioned Floors 27

7. Examples . 30

8. References . 37

Authors:

Franz A. Zahn, PhD, Dipl. Ing. Hans R. Ganz, Dr. sc. techn., Civil Engineer ETH

Copyright 1992 by VSL INTERNATIONAL LTD., Switzerland -All rights reserved - Printed in Hong Kong

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1. IntroductionIt is no secret that the key to the successfulconstruction of new buildings is successfulplanning. Successful planning starts from thevery beginning with good communication andclose cooperation between all parties involvedin the project, in particular the owner, thearchitect and the engineer. As soon as acontractor has been nominated he too shouldbe included in the planning team. In this wayone of the key aspects of successful planning,the constructability of the building, can beaddressed properly as part of theevaluation process of various concepts. This isof paramount importance for the success ofthe project since constructability mostmarkedly affects the time to completion of aturn-key project and thus the final cost to theowner. Because the major part of the total costof large developments is financing cost ratherthan actual construction cost, the completiontime is often a more important considerationthan material consumption. With this in mind itfollows that successful planning means toalways maintain an overall perspective of theproject, that is to consider the building as awhole rather than looking at individual parts inisolation. Since the various parts of a buildingstrongly influence oneanother, in particular inthe way they are constructed, optimization ofone part may well be detrimental to another.

another, in particular in the way they areconstructed, optimization of one part may wellbe detrimental to another.

Even when considering only the constructioncosts (Fig. 1.1), it is evident that optimization ofstructural material consumption alone will resultin relatively modest overall savings since onone hand the structural cost makes only about30 to 50% of the total construction cost and onthe other hand more than half of the structuralcost is labour cost, related mainly to formwork.

Any significant saving in construction cost cantherefore only be achieved by means that alsoaffect the labour cost and the non-structuralcost for cladding, electrical and mechanicalservices, lifts, fit-out, etc.

The most cost significant structural element of abuilding is the floor framing. Fig. 1.2demonstrates the relative contribution of thefloor framing to the total structural cost per unitfloor area. While for low-rise buildings thiscontribution is almost 100 %, the cost forcolumns and walls including their foundations,and for the lateral load resisting systembecomes increasingly significant for tallerbuildings. The floor framing system affects thecost in two ways:

First it has a direct influence on the rest of thestructure in that its weight determines the sizeof columns, walls and foundations, and itsstructural depth determines the total buildingheight and thereby the quantity of cladding

and vertical trunk lines. In seismic areas thefloor weight also determines the member sizesof the lateral load resisting system. Fig. 1.3shows the split-up of the total structural weightof a 49-storey building. While the floor framingaccounts for just over 50 of the total, any reduction of floor weight would cause acorresponding weight reduction also for the peripheral frames and the service core andwould thus affect almost the entire structuralweight.

The second way the floor framing systemaffects the cost of the building relates to thetotal construction time: Both the time required toconstruct one floor and the time lagbetween the structural completion of the floorand the commencement of fit-out work such aselectrical and mechanical services,suspended ceilings and decorating, are majorfactors influencing the time to completion of thebuilding. These considerations demonstrate thatthe optimization of the floor framing with regardto weight, structural depth and constructabilitygoes a long way towards successful planning.However, one should not make the mistake ofcomparing the cost of one floor system againstthe cost of another without considering the arryover effects on other parts of the structure,including the non-structural parts, and onfinancing cost.

In some countries, including the U.S.,Australia,South Africa and Thailand, a great number oflarge buildings have been successfullyconstructed using posttensioned floors. One ofthe main reasons

Fig. 1.2: Contribution of Floor Framing to Total Structural Cost [1]

Fig. 1.1: Split-up of Total Cost for Buildings

Labour Labour Labour Labour

MaterialMaterial

Material

Material

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for this success is the improved constructabilityof post-tensioned slabs: less material to bohandled and placed, simpler and lesscongested reinforcement, earlier stripping offormwork and often simpler formwork. Apartfrom shorter overall construction time andsavings in material and labour cost, post-tensioning allows more architectural freedom:Larger columnfree spaces providing moreflexibility in the subdivision of commercial andoffice floors, wide-spanning or boldlycantilevering floors that leave generous spacefor lobbies or public areas, slender elegantroofs for show rooms or exhibition halls, toname a few examples.

Fig. 1.3: Split-up of Total Structural Weight for a 49 Storey Office Building of the "Tube-in-Tube" Type (adopted from [2]

In addition, the reduction of structural heightand weight as outlined above, and theimproved deflection and cracking behaviourcontribute to the success of post-tensionedfloors.

Fig. 1.4 shows the total post-tensioningconsumption and the percentage used inbuildings in various countries in the year 1990.It is evident that there are huge differences.While in the U.S. and Australia more than 75 %of the total posttensioning was built into buildingstructures, this market made less than 10 % inmost European countries. Theobjective of this report is to encourage the useof post-tensioning in buildings, particularly incountries where this idea is not yet

widely accepted, by demonstrating itsadvantages and benefits. The report isintended to provide useful backgroundinformation to owners, architects, engineersand contractors, and to relate to them the positive experience made in areas where theuse of post-tensioning in buildings iscommonplace.

Specifically, Chapter 2 summarises the majordesign objectives, including suggestions howthesb objectives can be met. This should helpthe reader to rationally select an efficientoverall structural concept for a building. Then,in Chapter 3 a wide range of post-tensioningapplications are illustrated, includingfoundations, structural walls and service cores,moment-resisting frames, transfer beams andplates, and masonry walls. In recognition oftheir key role in building structures, floor framing systems are discussed in greaterdepth. These illustrations demonstrate thatpost-tensioning can make a significantcontribution to the success of building designs.After a brief review of the VSL post-tensioninghardware in Chapter 4, Chapter 5 presentssome background information to enable thereader to determine preliminary sizes of floorframing members, and to estimate approximate reinforcing and prestressing steel quantities.The content is not intended to serve as a designaid to engineers. Technical design issues will bethe subject of the second volume of this report.Tendon arrangements, connection andanchorage details for post-tensioned floors arediscussed in Chapter 6. Finally,

Fig. 1.4: Total Annual Post-Tensioning Consumption and Percentage Used in Building Structures inVarious Countries (1990 Figures)

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Chapter 7 presents two examples that reiteratethe contents of Chapters 3, 5 and 6. While post-tensioning is a very attractive repair andstrengthening method, this report is limited toapplications in new construction.

Note: Saving weight of floor framing resultsin significant savings in core and outer tube,particulary so in seismic area


2. General Objectives in the Design of Building StructuresBuildings can be classified in many differentways. They can be distinguished by their use oroccupancy, by the construction materials, bytheir owners (public / private), or by their height(low-rise / high-rise). Here two representativebuilding types, distinguished primarily by

trunk lines, the storey height and therefore thestructural height of the floor framing must beminimised. In order to maximise rentable spaceand flexibility of occupancy the floor framing isusually required to have relatively long spans,which is in conflict with the objective to

and deflection limitations or avoidance ofexpansion joints.

Tables 2.1 and 2.2 also include suggestions asto how each design objective can be met.These suggestions include

Fig. 2.1: Typical High-Rise Building underConstruction

Fig. 2.2: Typical Low-Rise Building under construction

the predominant direction in which theconstruction progresses, are used todemonstrate some general objectives to beconsidered in the conceptual design.

For typical medium to high-rise office ormulti-purpose buildings similar to the exampleshown in Fig. 2.1, the construction progressesvertically, floor by floor. The high repetition rateof identical floors and the floor-by-floorconstruction sequence imply a number ofdesign objectives typical for this type ofbuilding. Table 2.1 summarises some of theseobjectives and how the project benefits if theobjectives are met. For instance, in order tominimise the overall construction time, one ofthe prime design objectives must be to achievea fast floor cycle, that is to minimise the timerequired to complete a floor. In order tominimise the size of vertical members andfoundations the floor weight must be kept aslow as possible. In order to save on cladding,vertical structural members and vertical service

minimise structural floor height and weight.

For typical large area, low to medium-risebuildings similar to the one shown in Fig. 2.2,the predominant direction of constructionprogression is horizontal, with somesimultaneous vertical progression. Thecompletion of anentire floor is therefore not on the critical path tothe same extent as for high-rise buildings. Also,the repetition rate of identical floors, and thetotal number of floors is usually relatively smallso that the structural height and the weight ofthe floor framing do not normally play assignificant a role as they do in the design ofhigh-rise buildings. Table 2.2 summarises someof the design objectives for this type of buildingand the corresponding benefits for the project.While the design objectives related to theconstructability are similar to those listed forhigh-rise buildings, the different usage oflow-rise buildings, e.g. industrial, retail, parking,often implies some specific requirements suchas strict cracking

the use of simple and efficient formwork, post-tensioning, pre-fabrication of reinforcingassemblages, complete or partial pre-fabrication of entire concrete elements, thechoice of a suitable floor framing system, simpledetails, high degree of standardization, and theuse of high early strength concrete.

Post-tensioning helps to meet each single oneof the design objectives. The reasons for thisare different in each case and are listed as footnotes under the tables. The most prominentones are that post-tensioning allows the floorframing to be more slender, solving the problemof the conflicting needs for long spans and smallstructural depth, and that it replaces asignificant amount of reinforcement, thusreducing steel quantities and allowingstandardization and simplification of thereinforcement. Further reasons why post-tensioning helps to improve the design are thatusually the concrete quantities are reduced andthat the

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Benefits for the project

• post-tensionning 1)

• simple, standardised details for reinforcement• simple, standardised details for formwork• post-tensionning 3)

• use of self-supporting falsework that only needs to be supported near vertical elements

• high early strength concrete• post-tensionning 6)

• high early strength concrete• simple reinforcing and formwork in large

pre-assembled units• simple details with high repeatability• pre-fabrication of critical path elements

(columns, beams, slab soffits, or walls)• post-tensionning 3) 4) 5)

Benefits for the project

• saving on vertical structuralmembers, cladding, mechanical risers, lift stairs, air conditioning(volurne to be heated or cooled)

• flexibility of occupancy, • maximum rentable space

• saving on vertical structural members and foundation and, in seismic areas, on lateral load resisting system

• improvement of constrructability and thus saving of time

• saving of time • avoidance of

clashes between different trades• reduction of required number of formwork

sets

• direct saving cf time, indirect saving of time by atiowing building fit-out to start earlier (early access)

Overall Objective

smallest-possible floor-to floor height

largest-possible columnfree space, i. e. long spans

high repeatability from floor to floor

no back-propping wherever possible


Table 2.1: Objectives Of Concrete Floor Design

Medium to High-Rise Officeand Multi-Purpose,Building

characteristic construction progression: vertical

lowest-possible weight of floor • light-weight concrete• ribbed or waffle slab• slab with voids• post-tensionning 2)

quickest-possible floor cycle

1) post-tensioning allows greater span/depth ratio2) for a given span post-tensioned floors require less

concrete3) if a significant part of the load is resisted by post-tensioning

the non-prestressed reinforcement can be simplified andstandardised to a large degree. Furthermore, materialhandling is reduced since the total tonnage of steel(non-prestressed + prestressed) and concrete is less than for aR. C. floor

4) post-tensioning allows earlier stripping of formwork

5) assembling of precast elements by post-tensioning avoids complicated reinforcing bar connections with insitu closure pours, or welded steel connectors, and thus can significantly reduce erection time

6) usually the permanent floor load is largely balanced by draped post-tensioning tendons so that only the weight of the wet concrete of the floor above induces flexural stresses. These are often of the same order as the design live load stresses. Hence back-propping of one floor below is usually sufficient

Why does post-tensioning help to meet the design objectives?

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• post-tensioning 1)

• same as above

• lay-out of columns and walls to avoid restraint of shrinkage and temperature shortening

• temporary or permanent separation of floor from restraining vertical elements

• careful concrete mix design • careful curing of concrete • well distributed non-prestressed steel • post-tensioning 6)

• high early strength concrete • simple reinforcing and formwork in large

pre-assembled units • simple details with high repeatability • pre-fabrication of critical path elements

(columns, beams, slab soffits, walls) • post-tensioning 2)3)4)

•simple, standardised details for reinforcement•simple, standardised details for formwork•post-tensioning 2)

How the objectives can be met

•post-tensioning

• pre-cambered formwork • sufficient stiffness of multi-span floors

post-tensioning• high early strength concrete • post-tensioning 5)

• use of self-supporting falsework that only needs to be supported near vertical elements

• high early strength concrete • post-tensioning 5)

Benefits for the Project

• flexibility of occupancy, • maximum rentable space

• improvement of constructability and thus saving of time

• saving of time• reduction of required number of

formwork sets

~• direct saving of time, indirect saving of time

by allowing building fit-out to start earlier (early access)

• (requirement)

• (requirement)

• improved riding surface, for fork lifts, easy-to-clean surfaces

• more rentable space for given total building height.

• shorter ramps

for some commercial and industrial developments: strict deflection limitations

for some commercial and industrial developments: strictlimitation of crack widths


Table 2.2: Objectives Of Concrete Floor Design

Low to Medium-Rise, Large AreaMulti-Purpose Commercial Buildings

characteristic construction progression: predominantlyhorizontal, in one or two directions

Overall Objectivelarge column-free spaces, i.e. large spans

high repeatability from stage to stage and from floor to floor

quickest-possible turnaround of formwork

no back-propping wherever possible

for some warehouse orindustrial buildings :floor free of expansion joint

for parkings (typically):lowest-possible floor-to-floorheight

1) post-tensioning allows greater,span/depth ratio, thus moreeconomical for large spans

2) if a significant part of the load is resisted by post-tensioningthe non-prestressed reinforcement can be simplified and standardised to a large degree. Furthermore, material hand-ling is reduced since the total tonnage of steel (non-prestressed + prestressed) and concreteis less than for a R.C. floor

3) post-tensioning allows earlier stripping of formwork4) assembling of precast elements by post-tensioning avoids complicated

reinforcing bar connections with insitu closure pours, or welded steel connectors, and thus can significantly reduce erection time

5) usually the permanent floor load is largely balanced by draped post-tensioning tendons so that only the weight of the wet concrete of the floor above induces flexural stresses. These are often of the same order as the design live load stresses. Hence back-propping of one floor below is usually sufficient

6) post-tensioning usually balances most of the permanent loads thus significantly reducing deflections and tensile stresses

7) the P/A stress provided by post-tensioning may prevent tensile stresses causing the floor to crack.

Why does post-tensioning help to meet the design objectives?

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formwork can be stripped earlier than fornon-prestressed floors. Also, the oftenrequired strict limitation of deflections and crackwidths can be effectively achieved by post-tensioning. Since the draped prestressingtendons typically balance a significant part oft h epermanent floor loading, deflections andcracking are substantially reduced compared toa reinforced floor. In addition, the in-planecompression forces from the prestressedtendons neutralize tensile stresses in theconcrete to a degree, delaying the formation ofcracks.

Another important factor that helps to speed upthe overall construction time is theformwork. Large highly mechanized formsystems for the floor framing, such as fly formsor table forms (Figs. 2.3 and 2.4) and climbingforms for wall systems such asservice cores (Fig. 2.5) are typical examples of

efficient formwork.

Pre-fabrication of large reinforcing assemblages,often including post-tensioning tendons andanchorages, is another good example of howthe construction time can be reduced (Fig. 2.6).Pre-assembling of reinforcement isfacilitated by the use of post-tensioning sincesteel quantities are reduced and reinforcingdetails can be simplified and standardized to ahigh degree.

Yet a higher level of rationalization is thepartial or even complete pre-fabrication ofentire concrete elements such as columns,beams, wall panels or slab soffits. In this waythe setting up and stripping of formwork and theplacing of steel and concrete is mainly carriedout off the critical path, resulting in time savingand a smaller workforce required on the jobitself. Some typical examples of precastelements are shown in Figs. 2.7 to 2.9: Precastcolumns, continuous over two storeys in Fig.2.7, precast wall panels in Fig. 2.8 and precastcolumns with precast drop panels mounted ontop in Fig. 2.9. This example demonstrates howa flat slab with drop panels can be constructedwithout a major increase of formworkcomplexity. Fig 2.10 shows an example of aone-way slab and band beam floor built entirelywith precast concrete soffits and shell beamswhich will act compositely with the insituconcrete. Fig. 2.11 (a) shows three differenttypes of shell beams. The possiblearrangement

of post-tensioning tendons in the band beamsand in the slab is also indicated in the diagrams.The precast elements normally contain most, ifnot all of the bottom reinforcement, includingbeam stirrups where needed, so that only thepost-tensioning tendons and the top

reinforcement have to be placed on site.Theshell beams can be combined with differenttypes of slabs, ranging from insitu slabs cast on conventional formwork to proprietory precastfloor panels such as

Fig. 2.4: "Flying Formwork" Panel Viewed from Inside of Floor under Construction

Fig. 2.3: Large "Flying Formwork" Panels

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hollow core or double T slabs, as shown in Fig.2.11 (b).

Finally, a note on the concrete properties forpost-tensioned parts of buildings. In generalthere is no need to specify higher

concrete strengths than for structural parts ofnormal reinforced concrete. However, in somecases the choice of high early strengthconcrete may lead to significant time savingssince it allows earlier stripping of formwork.Because the stresses in a post-tensioned

slab or beam without super-imposed loadingare typically very small, except for the localzones around the anchorages of theprestressing tendons, the prestress can beapplied at a time when the concrete is still

still quite "green", provided that the anchoragezones have sufficient strength. The use oflarger bearing plates or of precast anchorageblocks containing standard anchorages, are twopossible ways to allow early stressing and thusearly formwork stripping even when normalstrength concrete is used. In order to minimiseshrinkage and creep related cracking of theyoung concrete it pays to use carefullyformulated mixes and to properly cure theconcrete for a sufficiently long period of time.

Fig. 2.5: VSL Climbing Formwork is an Efficient Means toConstruct Complex Service Cores in High-Rise Buildings

Fig. 2.6: Site Pre-Fabricated Reinforcement, Including Strand andAnchrorages, Speed up Construction

Fig. 2.7: Another Example of Efficient Construction: 2-Storey Pre-Fabricated ConcreteColumns

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Fig. 2.8: ,Pecast Wall Panels

Fig. 2.8: Precast Wall Panels

Fig. 2.10: Floor Framing Constructed with Integrated Formwork

Fig. 2.11: Floor Framing with Integrated Formwork: Sections

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3. Applications of Post-Tensioning in BuildingsApart from floor systems there are many otherpossible applications of posttensioning inbuilding structures that can result insignificant savings. The list includes moment-resisting frames, shear walls,service cores, transfer beams and plates,foundations, masonry walls, hangers and ties.In this chapter each of theseapplications, as well as post-tensioned floorsystems, are discussed in some detail. Theadvantages offered by posttensioning arereviewed and some typical tendonarrangements are shown for the differentapplications. Since floor systems have by farthe greatest impact on the cost of buildingstructures they are treated in more depth thanthe other applications presented.

3.1 Floor Systems

The multitude of different floor systems adesigner can choose from are reviewed withrespect to the selection criteria and thecompatibility with post-tensioning. Floorsystems can be classified in different ways, forinstance insitu versus precast floors,single span versus multi span floors, slab onbeams versus flat slab, one-way versustwo-way systems, etc. Table 3.1 presents a classification into two main categories, namelyone-way systems and two-waysystems. Each of these is further sub-dividedinto different groups, depending on whetherbeams are used, and if so, whether the beamsare of the wide shallow type often referred to as"band beams", or conventional narrow beams(including supporting walls as a limit case interms of support stiffness for the slab). Each ofthese groups can then be further sub-divided byslab type (flat solid slab, flat voided slab, bothwith or without drop panels, ribbed slab, waffleslab), beam type (solid or voided beams, bothwith or without drop panels), and constructionmethod (different combinations of insitu andpartly precast construction, including the use ofsteel trough decking as composite slabformwork).

The selection of the floor framing system depends on a number of factors , the main onesbeing:

- typical span rangethe suitability and economy of the different floor framing systems depend on the span length

- ratio of span in x-direction to span in y-directionfor nearly square column grids two-way systems are more suitable than one-way systems

- super-imposed loading (light/ heavy)for heavy loading floor systems with beams are more suitable than flat slabs

- overall structural height of the floor framing determines the total building height and thus the cost for cladding and vertical services (particularly important for high-rise buildings)

- constructabilitydetermines the overall construction time and thus the final cost to the owner economy of the floor system material consumption versus labour cost, relative cost of concrete, steel and formwork, local availability of proprietary systems such as double T, hollow core or similar

- flexibility for the lay-out of under-ceiling mechanical/electrical services free routing underneath the soffit versus penetrations through beams

- structural weight per unit area (average)determines the size of vertical supporting members and foundations and, in seismic areas, of the lateral load resisting system. The use of ribbed slabs. waffle slabs or voided slabs helps to minimise the weight

- requirements for in-service behaviour (deflections, cracking) and for strength depending on these requirements stiffness, moment capacity, or both are important criteria

- is the floor framing part of the lateral load resisting system?floor systems with beams are preferable if frame action is required

- exposed soffits/ suspended ceilingsflat plates or waffle slabs are more aesthetic where the soffit is exposed

"Constructability" incorporates a whole range of factors, including- whether or not large fly form panels or table forms can be lifted out through the

sides of the completed floorif not the use of composite (lost) formwork such as precast concrete soffits and shell beams, or steel trough decking should be considered.

- crane access and capacitythis determines to what extent precast elements and large formwork units can be used

- formwork cost and number of re-uses of identical forms in the buildingfor many re-uses a higher level of form complexity can be justified economically

- complexity of connection detailsconnections between floor framing and previously cast parts, such as precast edge beams or wall panels, climb-formed service cores etc. deserve careful consideration

- punching shearbeams, drop panels or column capitals versus flat plate with punching shear reinforcement

- are the floor edges straight or irregular with re-entrant corners?irregular and indented edges are simpler to form for flat slabs than for slab/beam systems

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Table 3.2 summarises the main features, theadvantages and disadvantages, and somepossible post-tensioning arrangements for thefloor systems of Table 3.1. The post-tensioningtendons are marked in blue. Depending on thesystem, it may be more economical to post-tension only the beams, or the slab, or both. Forthe floor systems with beams, only the oneswith band beams are shown in Table 3.2. Thearrangement of slab tendons in equivalentsystems with narrow beams or supporting wallsis similar. Naturally, narrow concrete beams,regardless of whether they are precast orcast-in-place, can also be post-tensioned. Forsystems 1 and 2 (Table 3.2) the tendons caneither be concentrated in the "column strips", i.e. narrow bands along the grid lines, or they canbe partly concentrated in the "column strips"and partly distributed in one or both directions.While column strip tendons are very easy toplace, the arrangement shown as option (c)requires careful planning of the placingsequence since the tendons inter-weave. Onthe other hand, option (c) provides betterin-service performance because the balancingloads from the draped tendons are moreuniformly distributed. Option (b) thus appears tobe a good compromise of the two.

Referring to Table 3.2 it is evident that forincreasing span lengths and superimposedloads the use of auxiliary stiffening elementssuch as ribs, beams or drop panels increasesthe complexity of the floor framing, and thus thecost for formwork. However, even for lightloading as for office floors the choice of a complex system may prove to be economical,particularly so for high-rise buildings. Comparedto flat plates, floor systems with beams, ribs,drop panels or voids possess a better structuralefficiency, that is flexural stiffness and strengthfor a given weight per unit floor area, hence thefloor weight can be reduced significantly,resulting in savings for vertical members andfoundations. On the other hand, the formwork isre-used many times in highrise buildings so thatthe complexity is relatively insignificant in termsof formwork cost per unit floor area. It shouldalso be remembered that the combination ofprecast soffits, drop panels or shell beams withinsitu concrete allows the construction ofcomplex floor systems with rather simple, if anyformwork, as shown in Figs. 2.9 to 2.11.

Are there any disadvantages? The short answeris no. One argument frequently used againstpost-tensioning by owners and contractors isthe lack of flexibility to accommodate floorpenetrations, either planned or as part of futurechanges to meet specific tenant requirements.For planned or future small penetrations thecontrary is the case: Because of the reducedreinforcement content the rebars are usuallyspaced further apart, leaving more flexibility forsmall penetrations. There is no doubt thatdrilling or coring small holes into or throughpost-tensioned floors requires a certain amountof discipline in order to avoid cutting anyposttensioning strands. However, it is verysimple to locate the tendons in a floor to makesure that small penetrations or fixing holes missthem. In most cases the construction drawingswill give a good estimate where it is safe to drill.If the drawings show tendons close to the

proposed penetration then the exact tendon location can readily be determined on site withthe aid of a metal detector. Drilling of smallholes for fixing dowels is generally safeanywhere provided the drill has got anautomatic switch that is triggered by contactwith steel. Since the tendons usually have aminimum concrete cover of 30 to 50 mm smallholes less than 20 to 30 mm deep can be drilledsafely anywhere in the slab.Large penetrations for new stairs, lifts or airconditioning ducts require a careful designcheck by an engineer, regardless of whether thefloor is post-tensioned or not. If the post-tensioningtendons are arranged in column strips or beamsonly, there are relatively large slabpanels between these strips where there are notendons at all so that penetrations can be readily accommodated with very little, if anyadditional strengthening.

In Chapter 2 the general objectives in the design of building structures and the different ways inwhich post-tensioning can help to meet these objectives, werereviewed. The specific advatagesof post-tensioning the floor framing are once more summarised below:

- reduction of structural depththis in turn results in a reduced building height and corresponding savings in cladding and vertical services, or allows to fit additional storeys into the given maximum building height

- increase of span lengthallows larger column-free areas and thus more flexibility in the floor use

- reduction of floor weight and of material consumption the size of columns,walls and foundations is reduced and less material is used for the floor framing itself

- flexibility in layout of serviceswith post-tensioning it is often possible to choose a floor system with a flat soffit while a corresponding reinforced concrete floor would need beams or ribs

- improved constructabilityfaster construction because less material is to be handled and placed. Simpler details, higher degree of standardization, less congestion of reinforcement, often minimum reinforcement that

can be pre-assembled in large units- improved cracking and deflection control

Due to the load-balancing effect of the draped prestressing tendons a typical post-tensioned floor is more or less free of flexure under its self weight , i.e. virtually no tensile stresses and deformations exist. Cracking and deflections are almost exclusively caused by super-imposedand live loads (apart from volume change effects due to drying shrinkage and temperature changes) and are normally reversible when the loads are removed. Long-term (creep) deflections are thus mainly due to permanent super-imposed loads which ar typically applied no sooner than 6 months after the floor was cast. Because the creep coefficient for long-term loads applied at that concrete age is much smaller than for loads acting from the start, the creep deflections of post-tensioned floors are further reduced compared to those expected for non-nrestressed floors.

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Table 3.1 Classification of floor systems

band beam + slabsolid "beam" strips with voided

1-way-slabnarrow beams or walls + slab

solid slab

ribbed slab

voided slab

solid slab

ribbed slab

voided slab

solid beams solid beams

voided beams

beams with drop panels

slab insitu on formwork

slab insitu on precast soffit

slab composite (steel troughs)

any combination of any combination ofany combination of

slab precast with insitu topping slab insitu on formwork


slab composite (steel troughs)

slab precast with insitu topping

beam insitu on formwork

beam insitu in precast shells

beams insitu on formwork

beams insitu in precast shells

beam partly precast

steel beams

and

and

allinsitu

1-way-systems

12

beams precast


narrow beams or walls in 2 directions + slabFlat slabs

band beamsin 2 directions + slabwaffle slabs

solid slab

voided slab

waffle slab

solid beams

solid slab

voided slab

waffle slab

solid slab

voided slab

beams with drop panels

with solid bands

with solid «capitals»

solid slab

voided slab

without drop panels

with drop panels

all insitu any combination of



slab insitu on formwork


slab on formwork




beams partly precast

beams precast

any combination of

steel beams

insituon precast soffit

allinsitu

and

and

Table 3.1 Continued

2-way-systems

13


Table 3.2

* light: 2-5 kN/m2; medium: 5-10 kN/m2; heavy: greater than 10 kN/m2

14

disadvantages

low punching shear capacity, excess concretefor longer spans and unequal spans in x andy, greater deflections than other types• tendons in type (c) less easy to place• type (a) less effective in load balancing

expensive formwork• for tendon arrangements see under (1)• sensitive to pattern loading

handling, placing and material cost of void formers• tendon arrangement (b)

less easy to place

more expensive formwork than (1), even moreso for ribbed slab, less flexible services layoutthan (1) / (2), particularly so with ribbed slaband conventional beams i. e. types (c), (d)

fire-rating of steel trough decking, less flexibleservices layout than (1) / (2), particularly sowith conventional R. C. or steel beams (b), (c)

formwork still more expensive than (2) / (4),beams interfere with services, particularly sowith conventional beams i. e. types (c),(d)• for (a): less effective for load-balancing• for (b): tendons are less easy to place

greater structural depht without paths forservices (around drop panels or betweenbeams) results in greater storey height orreduced ceiling space, expensive forms• type (a) less effective for load balancing

1

2

3

4

5

6

7

Remarks

drop panels are typicallyone third of the spanlength with a total thickness of 1.5 to 2 timesthe slab thickness

attractive in combinationwith precast soffits

type a and b: beams andribs should have samethickness. narrow conventional beams maybe precast and can eitherbe post-tensioned orconventionally reinforced

tendons are placed between the folds of thesteel troughs

narrow conventionalbeams may be precastand can either be post-tensioned or conventionally reinforced

note: ideally the "beams"and "drops" have thesamethickness as the waffleribs

advantages

lowest cost formwork, flexibility in columnarrangement, flat ceiling, greatest flexibilityfor under-ceiling services layout• tendons in types (a) and (b) easy to

place• types (b) and (c) give best

load-balancing(i. e. better deflection control)

compared to (1): better punching shear capacity, less concrete consumption for longer spans or heavier loading, less congested top reinforcement over columns• tendon arrangements see under (1)

has got all the advantages of (1) but lessweight (important in seismic zones), or, forsame weight longer spans possible. for same weight, loading and spans punchingshear capacity and deflections are bettercompared to (1)• tendon arrangement (a) easy to place

same as (2), longer spans possible in onedirection, lighter floor or longer slab span possible with ribbed slab• tendons are easy to place and very

effective for load-balancing

economical formwork, no form stripping required, otherwise same as (4)

allows long spans in both directions andheavy loading, deflections can be kept small, can carry concentrated loads• for (a): tendons are easy to place• for (b): very effective for load-balancing

types (a) and (b) have less weight than (6) and (2) respectively for same spans and loading. attractive exposed ceilings great flexibility in services layout•tendons are easy to place,even for arrangement (b)


15

Table 3.2 (continued)


If large penetrations coincide with prestressingtendons some strengthening along the edges isrequired and must be installed prior to cutting, inthe same way as for reinforced concrete floors.

There are no problems in cutting bondedtendons. The cut ends will act as bondanchorages so that the tendon is still effectiveover the remaining length. For unbondedtendons, VSL has developed a special jackwhich is used to grip the strands from the twoends either side of the cut so that the energy isreleased in a controlled way when the strandsare cut. Before this jack can be applied thetendon must be exposed over a length of 1.0 to1.5 m by carefully cutting away the surroundingconcrete. When the strand has been cut thepressure on the jack is released, de-tensioningthe tendon completely. After the penetration hasbeen completed, new stressing anchorages areprovided at the edges, the tendons cut to lengthand restressed with a normal stressing jack.

In summary, small floor penetrations and fixingholes are no problem but a certain amount ofdiscipline is required by the trades. A well documented maintenance manual will certainly minimise the danger of unintentional cutting ofprestressing tendons. Large floor penetrationsare no problem either but require carefulplanning and design by an engineer, regardlessof whether the floor is post-tensioned or not.

3.2 Moment-Resisting Frames

In this context frames are understood to consistof columns and beams rigidly connected toresist moments and shears from lateral and gravity loads. Figs. 3.1

Fig. 3.1: Frames in a Low-Rise Building

Fig. 3.2: Frames in a High-Rise Building

and 3.2 show examples of frames in a lowriseand a high-rise building, respectively. While forlow-rise buildings the floor framing itself and itssupporting columns may have sufficientstiffness and strength to resist wind loads, formedium and high-rise buildings it is usuallynecessary to provide shear walls, peripheral orinternal frames, or both in order to brace thebuilding against side sway, particularly so inseismic areas.

The construction of moment-resisting framescan be quite time consuming due to the oftencomplex reinforcement, particularly in thebeam-column joints and in end columns of frames that predominantly resist lateral loads.End columns typically carry relatively smallgravity loads but large tension forces caused bylateral loads, which usually results in highreinforcement percentages. The beams in suchframes often have considerable top and bottomreinforcement in the end portions to resistpositive and negative moments resulting fromthe dominance of lateral loads. Replacing thedense reinforcement with a few high strengthprestressing tendons can lead to substantialsavings in construction time, since theremaining reinforcement is simple and caneasily be pre-assembled in beam and columncages or even as entire cruciform cages. Post-tensioning of frames has the added advantagethat the stiffness is increased or, conversely, themember sizes can be reduced. Both the beamsand the columns can be post-tensioned.However, often only the end columns willwarrant this. The beam tendons can becontinuous from end to end of the frame,

either with parabolic drapes to balance gravityloads, or straight in the top and bottom of thebeams, or as a combination of straight and draped tendons, depending on whether thedesign is dominated by gravity or lateral loads.The columns can either be post-tensioned bystrand tendons continuous with couplers atevery so many floors, or by stress bars coupledat every floor.

In order to reduce the construction time ofmoment-resisting frames, many contractorsprefer to partly or completely precast thecolumns and beams which then only need to beerected and connected on site. In these casespost-tensioning offers the additional advantagethat the prestress across the joints betweenprecast elements provides sufficient clampingforce to transfer shear in friction, avoidingreinforcing bar splices or couplers with the corresponding insitu concrete, or weldedstructural steel connectors. Because post-tensioned precast frames usually require onlysimple mortar joints they can be constructedquite expediently.

Fig. 3.3 shows three different arrangements ofprecast frames connected by post-tensioningtendons. The portion of the frame shown ineach case is from mid-span to mid-span of adjacent bays and from mid-height to mid-height of two successive floors, as highlightedin Fig. 3.2. The beam tendons are continuousfrom one end of the frame to the other and areinserted into cast-in corrugated steel ducts andstressed after the mortar in the joints has hardened. The columns could be post-tensioned with strand tendons continuous overseveral storeys, or with stress bars coupled atevery floor. It is evident that all elements mustbe temporarily propped and braced until all thetendons have been stressed.

Fig. 3.3(a) shows an arrangement suitablemainly for frames that predominantly resistgravity loads which explains why there are onlydraped tendons that pass through the columnsnear the top fibre of the beam. Fig. 3.3(b) and(c) show beams continuous through the columnand a cruciform arrangement , respectivelywhere the beams are joined at mid-span. Thesearrangements are more suitable than type (a) ifthe frame predominantly resists lateral loads.Then the moments at the beam

16


Fig. 3.4: Partly Precast Frame Fig. 3.5: Partly Precast Frame: Section through Beam

Fig. 3.3: Arrangements of Post-Tensionned Precast Frames (adopted from [3])

ends can be negative or positive and theirmagnitude is usually greater than at midspanwhich is why there are also some straight tendons in the bottom of the beam. The beam-column joint is within monolithic concrete sothat some nonprestressed beam steel can passthrough the column which significantly improves

the energy absorption capacity of seismic loadresisting frames designed to form plastic hingesin the beams [3]. A possible detail of the joint oftwo full-span beams and a column is shown inFig. 3.3(d).Temporary steel brackets stressed tothe column are used to support the precastbeams until the tendons have been stressed.

Another way to construct peripheral frameswithout the need for formwork is shown in Fig.3.4. The beams are partly precast as shellscontaining the bottom steel and the stirrups, asshown in the cross section Fig. 3.5. The outsidepart of the shell is precast to full height so thatno edge form is required. The inside part is

17


precast to the level of the slab soffit and cansupport precast soffit panels or steel troughdecking. The columns could be precast one ormore storeys tall, either as fully precast units oras precast shells that are later filled with insituconcrete. In either case the joint is left free ofconcrete so that the top steel and thepost-tensioning tendons can be readily placed.In this way the beams are monolithicallyconnected with the slab and the columns. Thebeamcolumn joints behave similarly tocast-inplace joints, hence frames constructed inthis way are adequate also in seismic regions[3].

depths and large reinforcement quantities.Post-tensioning is a very effective way toreduce both the depth and the reinforcementcontent. Fig. 3.6 shows the principle of apost-tensioned transfer beam. The prestressingforce enables an arch system to form within thebeam, transferring the column forces from theupper floors to the

Fig. 3.6: Principle of Post-Tensioned Transfer Beam

(a)Section Showing Location of Transfer plate

Note: Tendon profile typically governedby minimum raduis requierements

Fig. 3.7: Transfer Plate of Pacific Place, Hong Kong

3.3 Transfer Beams and Transfer Plates

In many high-rise hotel and office buildingslarge column-free lobbies are required atground level, often extending over severalfloors, while the hotel or office floors abovehave columns and walls at much closerspacing. The transition from the small supportgrid to the large column spacing in the lobby iseither by means of transfer beams or a transferplate. In order to transfer the high concentratedforces from the columns and walls of the upperlevels to the lower supports these beams andplates usually require considerable

In summary, the advantages of post-tensioningmoment-resisting frames are:

- increase of frame stiffness and/or reductionof member sizes

- reduction of reinforcement percentages, thus simpler details and consequentlyfaster construction cycle.

There are added advantages for frames constructed of precast elements:

- connections possess high strength and large stiffness

- simple mortar joints, no reinforcing bar laps or couplers and hardlyany in-situ concrete requiring formwork

- no welded steel connectors with the corresponding high-level quality assurance procedures

18

P OST-TE N S I O N E D IN B U I L D I N G S

supports. Part of the loads, including the selfweight of the beam, is balanced by the upwardacting deviation forces from the parabolic tendons. The deflection is thus reducedconsiderably. The in-plane compression stressprovided by the posttensioning tendonsimproves the cracking behaviour of the beam.As a guide line the post-tensioning shouldprovide a minimum average in-planecompression stress of about 0.5 to 2.0 N/mm2.The same principle applies to transfer plates.The draped tendons usually have to bestressed in stages as the construction of theupper storeys progresses. Otherwise thedeviation forces from the fully prestressedtendons could cause a failure of the beambecause they are not yet balanced by the fullcolumn loads from the upper storeys.

In some applications it will not be possible todrape the tendons because the correspondingradii would be smaller than the minimum recommended radius of curvature for thetendons. This is because the minimumthickness of transfer beams or plates is usuallygoverned by

so that usually only a minimum crackdistributing reinforcement is required in theextreme faces. Hence the reinforcement issimplified significantly which means that theconstruction time is reduced. This is particularlyimportant because the timely completion oftransfer beams or plates is crucial to theconstruction of the upper floors. Of course thecracking and deflection behaviour is also improved by straight post-tensioning tendonssince they too provide an inplane compressionstress.

An example of a large transfer plate is the oneused in the Pacific Place building in Hong Kong[5].This 4.5 m thick solid concrete slab transfersthe loads from the closely spaced supports ofthe apartment / hotel complex to the widely spaced supports of the commercial / parkingcomplex below as shown in Fig. 3.7 (a) and (b).The original design as a reinforced concreteslab required almost 500 kg/m3 of reinforcingsteel. Owing to the very high shear forces, areduction of the plate thickness was not practical, but a post

(right half). The reduced and simplifiedreinforcement permitted the contractor tocomplete the slab in a much shorter period oftime. The layout of the posttensioning tendonsis shown in Fig. 3.7 (c).

A variation of transfer beams and plates are stiffcaps on top of high-rise buildings. They areeither used to suspend floors constructed fromtop to bottom, using hangers (Fig. 3.8a), or toengage the peripheral columns to take part inresisting lateral loads in tension / compressionin order to increase the lateral stiffness of thebuilding (Fig. 3.8b).

In summary, post-tensioning oftransfer beams and plates offersthe following advantages:

- significant reduction of reinforcement, thus steel fixing is simplified, reducing constructiontime

- in many cases a reduction of the beam depth or plate thickness (i.e. total building height and weight is reduced and material saved)

- greater stiffness and hence better cracking and deflection behaviour

shear force considerations. In those casesstraight tendons are arranged in the top and inthe bottom of the beam or plate, acting aschords. In that way arches similar to the systemshown in Fig. 3.6 can develop. The bendingmoments are mainly resisted by thepost-tensioning tendons

tensioned alternative design by VSLInternational Ltd. permitted reducing the

reinforcement content to 180 kg/m3 whileintroducing only 27 kg/m3 of posttensioningstrand. Fig. 3.7 (d) illustrates the typicalreinforcement of the original (left half) and of thealternative design

Fig. 3.8: Post-Tensioned Transfer Caps

3.4 Wall Panels and Service Cores

Reinforced concrete structural walls are used tobrace framed building structures against sidesway. Because these walls usually carry onlyrelatively small gravity loads they require ratherlarge quantities of vertical reinforcement in theirextremities. These vertical bars, acting astension chords, must be lap-spliced or coupledat every construction joint, or otherwise severalmeters long starter bars project from the pourlevel, hampering the concreting operations andposing a possible source of injuries. Thereplacement of most of the verticalreinforcement by high strength post-tensioningtendons therefore simplifies the steel fixing andthus results in overall construction time savings.Also, post-tensioning improves the crackingbehaviour of concrete walls.

Similar to post-tensioned columns, theprestress can either be provided by strand tendons continuous over several storeys,

19


Vertical joint with overlapping hair pins anda vertical reinforcing bar,filled with grout or insituconcrete

(b) Vertical Joint

Fig. 3.10: Details of Wall Panel

Fig. 3.9: Outside Shear Wall Constructed ofPrecast Panels Vertically PostTensioned

or by stress bars that are coupled at every storey. Strand tendons are inserted into cast-incorrugated ducts and coupled to dead-endanchorages provided at the wall base. Afterstressing from above the

Fig. 3.11: Service Core Constructed of PrecastBox Segments Vertically PostTensioned

ducts are then filled with cement grout for corrosion protection and to provide bond withthe surrounding concrete. Provided the wall istemporarily sufficiently stable (braced by floorslabs and supple

concrete closing the gaps left for bar splices orsteel connectors. Fig. 3.9 shows a typical outside shear wall of a mediumrise buildingunder construction. There are two precastpanels per floor. The principle of the horizontalconnection detail is shown in Fig. 3.10(a).Tendons are located in corrugated steel ducts.A seating pocket is left for the slab. Shear keysand/or location dowels can be provided tocentre the panels. Note that slab starter barsand shear keys or location dowels are notshown for clarity. The vertical joints do not needany formwork either (Fig. 3.10b). Overlappinghair pins provide the continuity of the horizontal reinforcement. The gap is filled with insituconcrete.

Provided that there is sufficient crane capacity,service cores can be constructed of precast boxsegments connected in the same way as shownfor wall panels in Fig. 3.10. In this way it is possible to get the service core "out of theground" in a rather short period of time, which isan important consideration with respect to thestart time for floor construction. Since the boxsegments are stable under their

mentary bracing), the tendons need only becoupled at every so many floors.

For walls constructed of precast panels,post-tensioning offers the added advantage thatthe prestress provides an active clamping forceto transfer shear across the interface betweentwo panels in friction [6].Hence the connectionsare simplified since neither welded steelconnectors are required, nor do reinforcing barsneed to be coupled. Consequently, there is noneed for formwork for insitu

self weight they do not need temporary bracing.For the vertical tendons to act as chordreinforcement they are best located near thecorners of the box cells. Fig. 3.11 shows aservice core constructed of precast segments.

Another type of precast wall panels is shown inFig. 3.12: Cruciform "Swiss Cross" panels canbe used to build perforated concrete wallsenclosing high-rise buildings of the "tube" type(Fig. 3.13).

Fig. 3.12: Cruciform Wall Panels Post-Tensioned Together to Form a Perforated Wall

20


Here too, only simple mortar joints are requiredif the panels are connected by post-tensioning.The horizontal strand tendons are continuousfrom end to end of the wall and are pushed orpulled into cast-in corrugated ducts. The verticalprestress is provided by stress bars coupled atmid-height of each floor. The panels must betemporarily braced for stabilization until all thetendons and bars have been stressed. The panels themselves contain only nominalshrinkage reinforcement. No reinforcing barscross the joints. The straight prestressingtendons in both faces of the 'beams` and"columns" provide the required capacity toresist negative and positive bending momentsas well as shear forces resulting from lateralloading.

but with the parabolic drape inverted: low pointsunder columns and walls, high points in thespans. Fig. 3.15 clearly shows the invertedtendon profiles in a raft foundation underconstruction. In many cases the foundation canbe designed to act as a stiff box (Fig. 3.16). Thewalls can be posttensioned to reduce the wallthickness and the reinforcement content; and toimprove the cracking behaviour.

For foundations with draped tendons it may benecessary to stress the tendons in stages asthe building rises since otherwise the deviationforces arising from the tendon curvature underthe columns would push off the columns whichinitially only carry a small portion of the finaldesign load. A more comprehensive summaryof the aspects to be considered in the design ofpost-tensioned foundations is given in [7].

A special form of post-tensioned foundationsare ground anchors and tension piles (Fig.3.17), which play an important role in resistinglarge overturning moments due to wind orearthquake loads, or in providing sufficientsafety against buoyancy uplift. Finally it is worthmentioning under-ground tension members thatprovide the horizontal tie at the base of arch orshell structures or inclined columns (Fig. 3.18).

Fig. 3.14: Different Types of Raft Foundations

Fig. 3.13: Typical "Tube" Type Building ThePart Shown in Fig. 3.12 is Highlighted

3.5 Post-Tensioned Foundations and Ground Anchors

The principle of a raft foundation is very similarto that of a floor slab turned upsidedown. Thedistributed soil pressure acts at the bottom surface and is held in equilibrium by thedownward-acting concentrated forces fromcolumns and walls. Similarly, a strip foundationacts like a beam turned upside-down. Fig. 3.14illustrates three different types of foundationmats that are the equivalents of a flat plate, aflat slab with drop panels and a one-way slab /band beam system. Posttensioning offoundation mats or beams offers similaradvantages as for floor systems: primarilyreduction of the thickness and reinforcementquantity and the corresponding reduction of theconstruction time, and improvement of thecracking and deflection behaviour, which in turnresults in an increase of the stiffness. The reduced raft thickness means less excavationand smaller concrete volume. The smallerconcrete volume, in turn, permits faster placingand is less critical in terms of the developmentof hydration heat. Reduced steel content meansless material to be placed and handled, andsimpler reinforcing details mean faster steelfixing. The tendons can be arranged similar asin the corresponding slab types,

In summary, the advantages of post-tensioning structural walls and service cores are

- reduction of reinforcement percentages, thus simpler details and consequentlyfaster construction cycle

- vertical tendons provide continuous chord reinforcement, no potential weak sections at multiple reinforcing bar laps

In addition, there are the followingadvantages for walls and wail systemsconstructed of precast panels:

- connections possess high strength and largestiffness

- simple mortar joints, no reinforcing bar laps or couplers and hardly any insitu concrete requiring formwork

- no welded steel connectors with the corresponding high-level quality assurance procedures

- increase of stiffness and/or reduction of member sizes of perforated outside walls of high-rise buildings of the "tube" type, particularly when assembled from precast cruciform panels

21


Fig. 3.15: A Post-Tensioned Raft Foundation under Construction Fig. 3.17 Ground Anchors and Tension Piles

(b) at base of inclined colums(a) at base of shell structure

Fig. 3.16: Stiff Basement Box Fig. 3.18: Under-Ground Tension Ties

3.6 Post-Tensioned Masonry Walls

Masonry walls usually carry only small, if any,super-imposed gravity loads. Unless they arereinforced they therefore possess relativelysmall flexural strength both in-plane and out ofplane. Unreinforced masonry walls have theadditional disadvantage of failing in a brittlemode once the tensile capacity of the joints isexceeded. Vertical post-tensioning tendonsplaced in the cores significantly increase thestrength and ductility of masonry walls. TheVSL post-tensioning system for masonry usesunbonded greased and plastic-sheathedstrands (`monostrands') that are inserted intosteel pipes placed in sections while the blocksor bricks are laid (Fig. 3.19). This is a far lesstime consuming operation than placing bondednon-prestressed reinforcement. Selfactivatinganchorages are placed at the base of the wallwhich grip the strands

when inserted. The tendons are stressed fromthe top of the wall. More detailed information onpost-tensioned masonry is contained in [8].

3.7 Other Applications

In addition to floor systems, momentresistingframes, transfer beams and plates, shear wallsand service cores, foundations, ground anchorsand masonry walls there are some specialapplications. These include stay cables tosupport large roof structures, or used as backtie: for large cantilever supports of grand standsor hangar roofs. They also include cable roofs,tension ties at the base of inclined columns orarches, and tension columns suspending floorsfrom a cap at the top of a high-rise building("hanging structures"). These applications arenot discussed further.

Fig. 3.19: Steel Ducts for VerticalTendons Placed in Cores of Masonry

22


4. The VSL hardware for Use in Building Structures4.1 The System

There are basically three VSL posttensioningsystems that are used in building structures:The VSL"monostrand" system, i.e. unbondedgreased and plasticsheathed single strands, theVSL bonded slab post-tensioning system withflat duct for up to 5 strands, and the bondedmultistrand system. Each of these systemscomprises the following components: Thetendon, a passive "dead-end" anchorage, astressing anchorage and a coupler. Thesecomponents are illustrated for the threesystems in Table 4.1. The stressing anchoragescan also be used as fixed ("dead-end")anchorages, which may be preferred in somesituations for practical or economical reasons.The monostrand tendons are delivered to siteas coiled pre-fabricated units, i.e. cut to lengthand with the dead-end anchorage alreadyinstalled. The stressing anchorages aresupplied separately and are fixed to the edgeformwork. The tendons are then rolled out, fixedto the support chairs that are typically spaced atabout 1 m centres, and coupled to thepre-placed stressing anchorages (or couplers atconstruction joints). Flat duct tendons can eitherbe pre-fabricated, i.e. up to five strands cut tolength and placed in the flat duct, the "onions"of the dead-end H-anchorage already formed,or the empty ducts are placed first. In that casethe strand is delivered on a dispenser and pushed-in hydraulically and cut to length. Theonions then have to be formed in place. In bothcases the stressing anchorages are deliveredseparately. The strands of the flat duct systemare stressed individually using a monostrandjack. After stressing, cement grout is injectedinto the ducts, providing bond to the concreteand corrosion protection of the strands andanchorages. This step is not required for themonostrand system since the strands arecorrosion-protected ex-works by grease and apolyethylene sheath. Round duct multistrandtendons can either be prefabricated or thestrands pushed into the empty ducts, as for flatduct tendons. The main difference, apart fromthe duct shape, is that all strands of amultistrand tendon are stressed simultaneously.

The monostrand system and the Flat ductbonded system are used mainly in slabs where,due to the small thickness an economical

tendon eccentricity can only be achieved by theuse of very small diameter tendon units. Wherethe tendon eccentricity is not so sensitive to thetendon diameter, e.g. in transfer beams andplates, foundations, frames, and in applicationswhere the tendons are placed centrically, e.g.walls and service cores, it is possible to use themultistrand system, allowing the use of fewerbut bigger tendon units.

In a slightly modified form the monostrandsystem is also used to post-tension masonrywalls. The principal difference is the self-activating anchorage used as the dead-endanchorage. The wedges are held by a spring sothat the strand can be pushed-in from above.The spring then pre-sets the wedges so thatthey grip properly as soon as the tendon isstressed. A reinforcing bar stud is provided to fixthe anchorage in place when concreting thewall base strip.

Fig. 4.1 shows a VSL stress bar, a high tensilealloy bar with a coarse thread that allowsanchoring the bar with end nuts. Thus there isno draw-in, which is a great advantage overstrand tendons for very short tendon lengths(say less than 5 to 10 m). The bars are self-supporting, easy to handle and can be readily coupled which makes them ideally suited toconnect precast elements, or in situations thatrequire vertical elements such as columns orwall panels to be stressed at every floor level.

Other VSL systems available for specialapplications are VSL ground anchors, VSL staycables and VSL external tendons. These aredescribed in the brochure "PostTensioningSystems"[9].

4.2 To Grout or not to Grout?

A question frequently asked by designers iswhether bonded or unbonded posttensioningshould be specified. It is not possible to give adefinitive answer to this question. The fact thatin some countries floors are post-tensionedalmost exclusively by unbonded monostrands(the U.S., Thailand, South Africa), while othersonly permit bonded post-tensioning (Australia)shows that opinions on this matter cover theentire spectrum from absolutely in favour toabsolutely against one or the other. Thereasons for this are related to local availability,design codes, availability of skilled labour,relative cost for manufacturing, handling andplacing, relative cost of prestressing strand andreinforcing steel, the cost for grouting, etc. Thebest approach to decide what is better is to takea look at the specific advantages of bonded andunbonded post-tensioning, and then to judgewhich aspects are more important in eachparticular case, keeping in mind that the overallaim should be to achieve a short constructiontime without compromising the quality.

Fig. 4.1: VSL Coarse Thread Stress Bar with Anchorage

The unbonded monostrand systemoffers the following advantages:

- thin, light and flexible tendons that allow maximum tendon eccentricity in relatively thin members and are easy to handle andplace

- small friction losses during stressing- corrosion protection of prestressing steel

ex-works- no grouting necessary

On the other hand, the advantages ofthe bonded systems include:

- full exploitation of the yield strength of the prestressing steel

- improved cracking behaviour by activationof bond forces, therefore less additional non-prestressed steel required for crack control than with unbonded system

- for the flat duct system: thin tendons allowing maximum tendon eccentricity in relatively thin members

- for the multistrand system: ability to transfer large forces using large tendon units

23


Table 4.1 The VSL hardware for use in building

24


Other decision criteria relate to the type of loading (small or large variable gravity loads)and whether or not the element is expected todevelop plastic hinges during large intensityseismic response. Generally the bondedsystem is to be preferred when the variable gravity loads are high in relation to thepermanent loads since only a small portion ofthe total load can be balanced by drapedtendons. The amount of additionalreinforcement required to resist the bendingmoments produced by full loading or patternloading would be substantially greater whenunbonded tendons were used. Bondedposttensioning is also to be preferred for beamsor columns of seismic load resisting framesrequired to dissipate energy in plastic hinges.

In chapters 2 and 3.1 it was stated thatpost-tensioned floors can be thinner for a givenloading and deflection limitation than reinforcedconcrete floors. This is primarily because of theload-balancing effect of the draped tendons, asillustrated in Fig. 5.1. In the span the deviationforces caused by the curved tendons act on theconcrete to oppose gravity. Where the tendoncurvature is inverted, i.e. over the grid lines between the columns, the deviation forces actdownward, inserting concentrated loads on the"column strip" tendons, i.e. the tendons runningalong the grid lines. These concentrated forcesare balanced by the upward acting deviationforces from the column strip tendons which inturn insert a downward acting force on thecolumns. Thus the system shown in Fig. 5.1 canbe compared to a net strung between thecolumns. When this net is stretched from all fouredges it inserts the load-balancing forces on theconcrete. The amount of prestressing steel canbe determined by the condition that the drapedtendons provide sufficient distributed deviationforce to load-balance a certain percentage ofthe floor self weight. This percentage dependson the ratio of total load to permanent load andis typically between 70 and 130 %. For typicaloffice or residential floors with live loads of 3 to4 kN/m2 and 1 kN/m2 additional permanentload one would normally balance 70 to 90 % ofthe self weight while for floors with higher liveloads more than 100 % of the self weight wouldbe load-balanced.The other effect responsible

for the improved deflection and crackingbehaviour of post-tensioned floors is the in-plane compression stress field in the concrete stemming from the anchorages of theprestressing tendons. Provided that there areno significant restraints, these compressionstresses neutralise a part of the flexural tensilestresses caused by the portion of the loadingnot balanced by deviation forces from thetendon drape. Typically the post-tensioning infloors provides an average in-planecompression stress of 1.0 to 2.5 N/mm2.

Now let us look at typical span-to-depth ratios ofpost-tensioned floors. For light loading, say upto about 3.5 kN/m2 and provided that punchingshear is not critical, a post-tensioned flat platecan be designed with a thickness of about 1/40of the larger span dimension (for interiorpanels), compared to about 1/30 for a flat platein reinforced concrete. If drop panels areprovided over the columns the span-depth ratiocan be increased to about 45 and 35 for interiorpanels of post-tensioned and reinforcedconcrete slabs, respectively. For highersuperimposed loading the span/depth ratiodecreases, particularly if the super-imposedload is predominantly variable in place andtime. Then the amount of posttensioning cannotsimply be increased to load-balance thesuper-imposed load so that in order to meet thedeflection limitations a greater floor thickness isrequired. This is illustrated in Figs. 5.2 and 5.3,

5. Preliminary Sizing of Post-Tensioned Floors

25


Fig. 5.2: Span/Depth Ratios for Flat Plates Fig. 5.3: Span/Depth Ratios for Flat Slabs with Drop Panels

where the span/depth ratios of a number ofpost-tensioned flat plates and flat slabs withdrop panels, respectively, built in various partsof the world over the last 10 years, are plottedagainst the total load normalized by the slab selfweight. The data derive from internal VSLrecords and from [1], [10]. In spite of the evidentlarge scatter both diagrams clearly show thetrend that the span/depth ratio decreasesmarkedly with increasing super-imposed load.The large scatter can be explained by four mainfactors: (1) there were certainly many differentrequirements for deflection and cracking, (2)there is always a trade-off between slabthickness and steel quantity, (3) punching shearcan be treated by either increasing the slabthickness or by providing shear heads or shearreinforcement, and (4) the total load-toselfweight ratio depends on the span length whichtherefore also affects the span/depth ratio. Thefact that general practice varies from country tocountry and even from engineer to engineer hasalso a lot to do with the wide range ofspan/depth ratios observed for any given load.However, ignoring a few extreme cases in thegraphs, it is evident that the data fall within acurved band with a width of about 10 to 15times I/d. To use these graphs for a rough-orderestimate of the slab thickness one has to firstassume a reasonable thickness and calculatethe total load-to-self weight ratio and then checkwhether the resulting I/d ratio falls within theband in the corresponding graph. For shortspans (say 5 m) the I/d ratio may be closer tothe upper edge of the band, while for very longspans (say 12 m) I/d ratios closer to the loweredge are more appropriate.

This reflects the dependence of the I/d ratio onthe absolute span length. It should beremembered, however, that the graphs are onlymeant to give some guidance in selecting a firstestimate of the slab thickness. Other steps inthe design such as calculating the punchingshear capacity or the expected deflection maydictate selecting a different thickness.

For total load-to-self weight ratios greater thanabout 2.5 and spans in excess of about 10 m,flat plates and flat slabs with drop panels willnormally no longer be economical. Other floorsystems with greater structural efficiency, that isstiffness and flexural strength for a givenaverage weight per unit floor area, should thenbe considered. Band beams in one or bothdirections, ribbed slabs with band beams,waffle slabs or voided slabs are all lighter thanan equivalent flat slab. For these systems it ismuch more difficult to give guide lines for thespan/depth ratio to be assumed. This isbecause there are additional variables, forinstance the spacing of band beams, the beamwidth, the slab thickness between the beams,etc. Therefore it will usually be necessary tostudy a number of variants before deciding onthe dimensions. Experience has shown thatband beam widths of 1/4 to 1/5 of the beamcentre line spacing and beam depths of 2.0 to2.5 times the slab thickness result ineconomical designs for floors with light tomoderate loading. When deciding on beamwidth, spacing and thickness the sizes ofreadily available plywood sheets should betaken into consideration so as to minimizecutting time and waste of formwork material.

As a very crude guide line for posttensioned one-way slab and band beam systems designedto support superimposed loads of 3 to 4 kN/m2,the slab thickness may be assumed to be 1/35 to1/45 of the clear span between beams, unlessgoverned by minimum thickness requirements.The span/depth ratio of the beams will most oftenbe between 20 and 30. With 3 to 4 kN/m2 floorloading a value of 28 is a reasonable estimate for2.4 m wide post-tensioned band beams spacedat about 8.4 m [10].

When sizing post-tensioned floors, edge andcorner panels should be distinguished from interior panels because of their differentboundary conditions. About 20% moreprestressing and reinforcing steel should beprovided, compared to the quantities derived forinterior panels. Where this is not practical thethickness should be increased, or the spanlength decreased by about 20 %. For simplespans a 20ù greater thickness should beselected anyway.

To estimate the total steel quantity in a floor it isuseful to know that the combined weight ofreinforcing steel and prestressing steelweighted by the yield strength ratio, i.e."converted" to normal reinforcement, is typicallybetween 80 and 130 kg/m3 of concrete. This isan average over the entire floor, includinginterior, edge and corner panels and taking intoaccount edge reinforcement. While for bondedpost-tensioning it may be assumed that thetendons will yield in the ultimate condition,unbonded tendons should be weighted only withthe effective prestress divided by the yield stressof the reinforcement.

26


6. Details Improving the Constructability of Post-Tensioned FloorsOne of the most effective ways to safeguardagainst construction delays and to avoidunacceptable crack widths is to specifyappropriate connection details and prestressingarrangements. In this chapter a number ofdetails are suggested for use in differentsituations.

Anotherway to reduce the restraint effects incases where walls cannot be arranged as inFig. 6.1 (a) is to temporarily or permanentlyseparate the floor from the stiff supportelements in such a way that relative horizontalmovements in one or both directions betweenthe floor and the walls

slab and wall. Fig. 6.2 (b) shows a detail for theconnection between a non-load bearing walland a slab edge, that allows free verticaldeflection of the slab. If elongated (flat) tubesare used this connection will also allow freemovement of the slab parallel to the wall. Inboth

Fig. 6.2: Restraint-Free Slab-to-WallConnections

Fig. 6.3: Expansion Joint Details

One of the first decisions in the conceptualdesign of a building is whether and whereexpansion joints are to be provided in the floorsystem. If there is no significant restraint by stiffvertical members, very large post-tensionedfloors can be constructed without anyexpansion joints since the compression stresscompensates shrinkage and temperatureinduced tensile stresses to a large degree. Fig.6.1 illustrates how the arrangement ofsupporting walls, stiff columns and servicecores positively or negatively influences thedesign in this respect. Wherever possible,arrangements according to Fig. 6.1 (a) shouldbe used. Expansion joints or other often costlymeasures to provide for good in-servicebehaviour of the floor are then avoided. Incases where for architectural reasons, wallsand service cores have to be arranged as inFig. 6.1 (b), prestressing does not guarantee acrackfree floor. This is because the prestressingforce is partly or completely lost due to theshear and flexural stiffness of the walls. Anyshortening due to shrinkage or cooling will thencause tensile stresses that are relieved as soonas a sufficient number of cracks have formed toprovide compatibility between the floor and theflexing supports. To avoid the possibility thatcompatibility is achieved by the opening of afew very wide cracks, continuous crackdistribution reinforcement must be provided.

can take place with little restraint while verticalsupport reactions are transferred. Forpermanent release joints proprietary connectingsystems are available that are usually based onthe principle of sliding dowels to transfer thevertical shear force from the slab to thesupporting wall. The basic principle of atemporary release joint is shown in Fig. 6.2 (a).Sliding material placed between the wall andthe slab allows relatively low-friction movementas long as the corrugated ducts placed aroundthe dowels cast into the wall are left open.Normally a significant part of the shrinkageshortening takes place over the first 2 to 6months after casting. After that period of timecement grout is poured into the ducts so thatafter the grout has hardened shear forces aretransferred between

cases the wall edge is laterally supported forout-of-plane loading due to wind or earthquake.

When expansion joints are required thedesigner has two principal options: the steppeddetail as shown in Fig. 6.3 (a) or the dowelconnection shown in Fig. 6.3 (b). A variety ofproprietary dowel systems are marketed, someof which can also be used for permanentrelease joints between supporting walls and theslab. The dowels allow relative movement inone or both horizontal directions while verticalshear force is transferred.

The connection of post-tensioned floors toprecast elements such as edge beams, orpreviously cast walls such as climb

Fig. 6.1: Influence of Wall Configuration on SlabRestraint

27


formed service cores is a very commonproblem, particularly in high-rise buildingconstruction. Fig. 6.4 shows a selection ofdetails both with dead-end and stressinganchorages. Details (a) to (d) do not requireaccess from the opposite face of the wall oredge beam. Stressing anchorages arranged asshown in details (c) and (d) of Fig. 6.4 are alsoused in situations where there is no scaffoldingalong the floor perimeter so that stressingcannot be carried out from the edge of the slab.Detail (d) is commonly referred to as a "pourstrip" and has the disadvantage that

a strip of formwork and a row of props must beleft in place until the slab has been completed. Ifthe pour strip is left open for a few weeks thisdetail also serves as a temporary release joint. Adisadvantage common to details (a), (c) and (d)is that a significant amount of reinforcementmust be provided for the positive moments andto tie the slab to the supporting wall or beam.Starter bars are also required to provide aclamping force to transfer shear across theconstruction joint. It is to be noted that the H-anchorage shown in detail (a) works primarily as

a bond anchorage and therefore the tendonforce is only fully developed at a similar distancefrom the edge as for the anchorage detailsshown in Fig. 6.4 (c) and (d). There are otherdead-end anchorages which develop the fulltendon force closer to the edge. It should benoted that stressing anchorages used as dead-ends require access to place the wedges. Thecorresponding detail is therefore similar to Fig.6.4 (c) but with the pocket closer to the wallsince no clearance is required for the stressingjack. Shear friction reinforcement is still requiredin that case since the tendon does not cross theconstruction joint.

In contrast, details (b) and (e) require only anominal amount of starter bars since theprestressing tendons cross the constructionjoint, providing most of the required shearfriction clamping force and positive momentreinforcement. The selfactivating dead-endanchorage for monostrand tendons is the sameas shown in Chapter 4 for post-tensionedmasonry. It can be cast into the wall or edgebeam with a piece of duct. The tendon is thenlocked by the spring-loaded wedges wheninserted into the anchorage.

Fig. 6.5 illustrates how stressing anchoragesand dead-end anchorages can be arranged inan alternating pattern to avoid stressing fromoutside the floor area and yet maintain asufficient amount of prestress close to thesupports. The edge beam in Fig. 6.5 is assumedto be cast-inplace. If a precast edge beam wereused, self-activating dead-end anchoragescould be cast-in. This would also be analternative for the fixed anchorage at the servicecore wall.

Fig. 6.6 shows a band beam arrangement whichallows air conditioning ducts to run along theperimeter and around the central service core ofa typical office floor. A thickened slab strip isprovided along these edges in order to reducethe effect of the step in the beam soffit on thestiffness and strength of the floor. All the tendonsare stressed from the faces of the step in thebeam soffit so that stressing pockets or pourstrips are avoided. Again, the dead-endanchorages could be replaced by self-activatinganchorages cast into the core wall and spandrelbeam, thus simplifying the reinforcementcrossing the construction joints.

Fig. 6.4: Details of Connections between Post-Tensioned Slabs and Previously Cast Walls or Edge Beams

Fig. 6.5: Alternating Tendon Arrangement with Stressing Pocket

28


sheet pile wall. Here centre-stressinganchorages can be placed at alternating sides ofthe centre support, near the points ofcontraflexure, allowing the stressing of alltendons from within the floor area.

In Chapter 2 a few possible measures tominimise the waiting time between concretingand stressing of the post-tensioning tendonswere mentioned. These measures include theuse of high early strength concrete, the use ofspecial (larger) anchor plates and the initialstressing to only 25 to 50 % of the finalprestressing force. For the monostrand systemthis implies stressing in two stages while with theflat duct system it is easier to fully stress 1 to 3strands of each anchorage. This will not locallyoverstress the concrete behind the anchoragesince in that case the anchorage is only partlyloaded. For the monostrand system, on theother hand, fully stressing every second orfourth strand would locally over-stress the youngconcrete since every strand is individuallyanchored. An elegant way to enable fullstressing at a very early stage is to integrate allstressing anchorages into precast concreteblocks set in the formwork. Fig. 6.8 shows anexample. The precast blocks also contain starterbars and bursting reinforcement. The stressapplied to the insitu concrete during stressing isthen much smaller than in the immediate vicinityof the anchorage so that the tendons can be fullystressed after a day or two. The stresses at thedead-end anchorage must be checked and, iffound necessary, they too must be placed inprecast blocks. This would be possible, forinstance, using selfactivating dead-endanchorages or normal (stressing) anchorageswith access to place wedges.

The details shown here were selected todemonstrate that connections and separations,tendon layout and anchorage details should becarefully designed to provide for efficientconstruction and maximum possible flexibility ofthe mechanical services layout. Details solelyderived from structural considerations do notnecessarily comply with this importantrequirement. Where-ever possible, the floorsystem and its details should therefore beselected in cooperation with the contractor andthe mechanical services engineer.

provide a centre-stressing anchorage. Fig. 6.7shows a situation where basement walls arecast directly against an insulation layer fixed to a

If all tendons must be anchored in thesupporting walls in spite of the lack of anyaccess for stressing from outside, VSL can

Fig. 6.6: Notched Band Beam to Provide Space for A. C. Ducts

Fig. 6.7:Centre-Stressing Anchorages

Fig. 6.8: Precast Anchorage Blocks

29


7. ExamplesTwo hypothetical examples are used to re-iterate the contents of Chapters 3, 5 and 6. First,a typical floor of a high-rise office building isdiscussed. The second example is a threestorey commercial building. In each example thefloor framing system is first selected from fourdifferent options. The selected floor system isthen further discussed, in particular the layout ofthe prestressing tendons and some connectiondetails, and the degree of load-balancing, theaverage compression stress, and the estimatedprestressing and reinforcing steel quantities aregiven.

7.1 High-Rise Office Building

Fig. 7.1 shows a part elevation of a highriseoffice building. The super-imposed floor loadingis 4 kN/m2. The typical floor plan is illustrated inFig. 7.2. The floor plan is divided into fourquadrants which have been pulled apart forclarity. Each quadrant shows a different floorframing system. The main elements of thestructure are the central service core whichcould be constructed by climbing formwork,peripheral frames constructed of precastcolumns and precast shell beams with insitujoints, and the floor framing system.

All four options shown in Fig. 7.2 involvecomparatively complex formwork, which iseconomically viable for high-rise buildings withmany re-uses of the forms. The bottom left handquadrant shows an arrangement of 500 mmdeep band beams running in one direction witha 270 mm voided one-way slab spanning in theother direction. The beams could be composedof precast shells and the slab could be cast onprecast soffits, so that only a small amount ofconventional formwork would be required.Mechanical services can run along the buildingperimeter where the beam depth steps back to350 mm. A variation of this system is shown inthe top right hand quadrant, where the voidedslab has been replaced by a 120 mm slab with350 mm ribs. The solution shown in the bottomright hand quadrant has band beams running inboth directions, with one-way ribbed slabs and350 mm waffle slabs as corner panels. Theformwork is more complex than for the other twooptions but the maximum depth is reduced by50 mm.

and the service core walls are formedconventionally.

Fig. 7.4 shows schematic details of the bandbeam section and some of the connections. Thefixed anchorages cast into the climb-formedwalls require access from inside the core so thatthe wedges can be placed. If the monostrandsystem were used this detail could be simplifiedby providing self-activating anchorages herewhich do not require an access pocket.

90% of the self weight is load-balanced bydraped prestressing tendons. The slab tendonsproduce an average inplane compressionstress of approximately 1.5 MPa while thecompression in the other direction amounts toabout 2.0 MPa. The prestressing steel quantityis 4600 kg, or 23 kg/m3 of concrete. Theaverage floor thickness is about 210 mm. Toestimate the reinforcing steel quantity, it isassumed that the equivalent non-prestressedtotal would be 120 kg/m3, i.e. at the upper endof the range given in Chapter 5. This is becausethe band beams are simply supported andtherefore require more reinforcement. Takingthe equivalent reinforcing steel quantity as 1590MPa / 500 MPa x 23 kg/m3 = 73 kg/m3, 37kg/m3 remain to be provided as additionalreinforcement. Had the unbonded system beenchosen this quantity would be greater.

Finally, the top left hand quadrant illustrates asolution with band beams running in bothdirections, spaced at 5.4 m, and 120 mm slabpanels. The major advantage of this solution isthat mechanical services can also run along theservice core walls since the beam soffits stepback at both ends. This is the reason why thissystem was selected as the preferred solution.

Fig. 7.3 shows the adopted floor system,including the layout of the prestressing tendons.Each line represents a bonded tendon with 4strands 0.6" in a flat duct. It is to be noted thatthe unbonded monostrand system could havebeen selected as well but this would haveresulted in a greater amount of additional non-prestressed steel. All stressing can be donefrom within the floor area, either from stressingpockets or from underneath the beams. Accessfrom outside is not required. Where a fixedanchorage would fall within a column, an SOanchorage (see Table 4.1) is placed in a smallaccess pocket (necessary to place thewedges). This was preferred since an Hanchorage placed within the beamcolumn jointwas considered undesirable because of thereinforcement congestion there.The bandbeams are constructed of precast shell beamswith stressing anchorages already cast in. Theslab is cast on steel trough decking spanningbetween the shell beams while the 1.5 m widethickened slab strips along the perimeter

Fig. 7.1: Part Elevation of High-Rise Building Showing Arrangement of Precast Columns and Shell Beams with Insitu Joints

30


Fia. 7.2: Floor Plan Divided into Four Quadrants each Illustrating a Different Floor Framing System, and Corresponding Sections

31


Fig. 7.3: Floor Plan of Selected Floor Framing System Showing Arrangement of Prestressing Tendons, and Typical Sections

32


Fig. 7.4 Details

7.2 Three StoreyCommercial Building

Fig 7.5 shows two elevations of the building.The peripheral walls and frames areconstructed of precast wall panels connectedby vertical post-tensioning tendons, full-heightprecast columns and precast beams. Thebeams are connected to the columns by post-tensioning tendons. The interior columnsare precast too, but only to the slab soffit.

Fig. 7.6 illustrates four different floor framingoptions in four quadrants of the floor plan. Thefloor loading is 5 kN/m2 live load + 2 kN/m2 fornon-structural finishing concrete. Theunderlying soil is marginal for a spread footingfoundation so that in order to keep the cost forfoundations within reason the floor weight is tobe optimised. This objective almost rules outthe flat plate which would have a thickness ofabout 260 mm (i.e. I/35 for a load ratio of 2.0,which falls well within the band in Fig. 5.2). Theflat slab with drop panels would be a little lighterbut still about 230 mm thick with 2.8 x 2.8 m x400 mm drop panels (i.e. span/depth ratio of 40for a load ratio of 2.22 which falls well within

the band in Fig. 5.3). The band beam solutionwould be considerably lighter. The beams couldbe 9.0 / 4 = 2.25, say 2.4 m wide and about 8.0/ 26 = 0.31 m deep with a 160mm slab (= one40-th of the clear span). However, a flat soffitwas preferred for architectural reasons.

Therefore, the voided slab solution with solidbands in one direction was selected. It couldalso be 260 mm thick although the total load-to-self weight ratio is 2.5 which brings the selectedspan/depth ratio of 35 very close to the upperedge of the band in Fig. 5.2. This is not

Fig. 7.5: Elevations of Three Storey Building Showing Arrangement of Prestressing TendonsProvided to Connect Precast Elements.

33


Fig. 7.6: Floor Plan Divided into Four Quadrants each Illustrating a Different Floor Framing System, and Corresponding Sections

the prestressing steel will be taken as theequivalent of 1200 MPa / 500 MPa x 23 kg/m3

= 55 kg/m3 reinforcing steel. Since the edgeand corner panels are all about 20% shorterthan the interior panels the total reinforcementcontent is similar to that of interior panels andtherefore an average of the range given inChapter 5 may be assumed, i.e. approximately100 kg/m3. This leaves an additional quantity ofreinforcing steel of 45 kg/m3 to be placed.

Fig. 7.8 shows schematically some of thedetails at the connections between the slab andthe supporting columns, walls and edge beams.Fig. 7.8 (a) illustrates how the precast soffitpanels are arranged: The 1.8 m wide panels ofthe solid "band beam" strips run in N-S directionwith the "slab" panels running at right anglesand butting up to the "beam" panels. The "slab"soffit panels could be delivered to site with thepolystyrene void formers already attached. Onlya small patch of formwork is required at theinterior columns. All "beam" tendons arestressed from the outside edge (Fig. 7.8 b). Thisdetail also shows that no starter bars or weldedsteel connectors are provided at the horizontaljoints between the wall panels. The verticalbonded tendons provide the required shear andflexural capacity. Fig. 7.8 (c) and (d) show thatall stressing of the distributed tendons is donefrom stressing pockets, i.e. from within the floor.Finally, Fig. 7.8 (e) and (f) demonstrateschematically how the precast beams areconnected to the full height precast columns bybundles of unbonded monostrands. It is to benoted that the number of strands shown is onlyindicative. The top beam bars are continuousthrough the columns and are placed insitu, thusproviding bonded reinforcement across thejoints. A block-out in the column allows theplacing of these bars and provides a seat for theprecast slab soffit panels.

considered to be a problem since the stiffness isonly reduced insignificantly by introducingvoids. The slab is constructed on precast soffitswith polystyrene block attached.

Fig. 7.7 shows the scheme. Unbondednonostrands (bundles of 4) are concentrated inthe solid bands which act as )and beams, whilethe voided slab acts is a one-way slab.Uniformly distributed tendons (bundles of 2strands) are proiided in the slab direction,running in the ibs between voids. In view of the

relatively high permanent super-imposed loadof 2 kN/m2, 120% of the self weight are load-balanced by the draped tendons so that thedeflections are kept low. The selected amountof prestress results in 1.5 MPa averagecompression stress in the slab direction and 2.0MPa in the "band beam" direction.

The prestressing quantity is 5040 kg, or 23kg/m3 of concrete. The average floor weight is4.65 kN/m2. Since unbonded tendons do notnormally develop the full yield strength, eprestressing steel will

34


Fig. 7.3: Floor Plan of Selected Floor Framing System Showing Arrangement of Prestressing Tendons, and Typical Sections

35


Fig. 7.8 Details


8. References

(1) Fintel, M. and Ghosh, S.K., "Economics of Long-Span Concrete Slab Systems for Office Buildings-A Survey", Portland Cement Association, 1982

(2) Guggisberg, R., "Anwendung von Vorspannung am Beispiel eines Architektenwettbewerbs fur ein 160 m hohes Hochhaus", Heft 91, Institut fur Baustoffe, Massivbau and Brandschutz, TU Braunschweig, 1991 ISBN 3-89288-065-4

(3) Park, R., "Seismic Design Considerations for Precast Concrete in New Zealand",Vol.1, pp 1-38, Seminar on Precast Concrete Construction in Seismic Zones,Tokyo, October 1986

(4) Englekirk, R.E., "Seismic Design Considerations for Precast Concrete Multistory Buildings", PCI Journal, Vol. 35, No.3, pp 40-51 , May/June 1990

(5) VSL News Letter No.1 / 1989, VSL International Ltd., Berne, Switzerland, January 1989

(6) Annamalai, G. and Brown, R.C., "Shear Strength of Post-Tensioned Grouted Keyed Connections", PCI Journal Vol. 35, No.3, pp 64-73, May/June 1990

(7) "POST-TENSIONED FOUNDATIONS", VSL International Ltd., Berne, Switzerland, June 1988

8) "POST-TENSIONED MASONRY STRUCTURES", VSL Report Series No. 2, VSL International Ltd. Berne, Switzerland, 1990

(9) "POST-TENSIONING SYSTEMS", 4.90/1, VSL Imternational Ltd., Berne, Switzerland

(10) "DESIGN GUIDE FOR LONG-SPAN CONCRETE FLOORS", Cement and Concrete Association of Australia in collaboration with Steel Reinforcement Promotion Group, June 1988, ISBN 0 947 132 06 6

37

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BRUNEI DARUSSALAMVSL Systems (B) Sdn. Bhd.BANDAR SERI BEGAWANTel 673 - 2 - 22 91 53,

-221827Fax 673 - 2 - 22 19 54

CHILESistemas Especiales deConstruccion SA,SANTIAGOTel 56 - 2 - 233 10 57Fax 56 - 2 - 231 12 05

EASTERN EUROPEMIDDLE EASTAFRICA** (English speaking dart)

VSL (Switzerland) Ltd.Bernstrasse 9 3421LYSSACH Switzerland Tel 41-34-479911 Fax 41 - 34 - 45 43 22

PERUPretensado VSL del PeruSA,LIMATel 51 -14-760423,

- 76 04 26Fax 51 - 14 - 76 04 77

PORTUGALVSL Prequipe SA, LISBOATel 351 - 1 - 793 85 30Fax 351 - 1 - 793 09 01

SINGAPOREVSL Singapore Pte. Ltd.,SINGAPORETel 65 - 235 70 77/9Fax 65 - 733 86 42

SOUTH AFRICASteeledale Systems (Pty.)Ltd.JOHANNESBURGTel 27-11 -6137741/9Fax 27 - 11 - 613 74 04

SPAINVSL Iberica S.A., MADRIDTel 34 - 1 - 556 18 18Fax 34 - 1 - 597 27 01

SWEDENInternordisk SpannarmeringAB, DANDERYDTel 46-8-7530250Fax 46 - 8 - 753 49 73

SWITZERLANDVSL (Switzerland) Ltd.LYSSACHTel 41 - 34 - 47 99 11Fax 41 - 34 - 45 43 22

THAILANDVSL (Thailand) Co., Ltd.,BANGKOKTel 66-2-237 32 88/89/90Fax 66-2-238 24 48

WESTERN EUPOPESOUTH AMERICA,AFRICA* * (French speaking)

VSL France S.A.110, Avenue Verdun91520 EGLYFranceTel 33 - 1 - 69 26 14 00Fax 33 - 1 - 60 83 89 95

USAVSL CorporationATLANTA, GATel 1 - 404 - 446 - 3000Fax 1 - 404 - 242 - 7493

VSL Corporation DALLAS, TXTel 1 - 214 - 647 - 0200Fax 1 - 214 - 641 - 1192

VSL Corporation DENVER,COTel 1 - 303 - 239 - 6655Fax 1 - 303 - 239 - 6623

VSL CorporationHONOLULU, HITel 1 - 808 - 682 - 2811Fax 1 - 808 - 682 - 2814

VSL Corporation MIAMI, FLTel 1 - 305 - 592 - 5075Fax 1 - 305 - 592 - 5629

VSL CorporationMINNEAPOLIS, MNTel 1 - 612 - 456 - 0985Fax 1 - 612 - 456 - 9281

VSL CorporationPHILADELPHIA, PATel 1 - 215 - 750 - 6609Fax 1 - 215 - 757 - 0381

VSL CorporationSAN JOSE, CATel 1 - 408 - 866 - 5000Fax 1 - 408 - 374 - 4113

VSL CorporationWASHINGTON, DCTel 1 - 703 - 451 - 4300Fax 1 - 703 - 451 - 0862

VIETNAMVSL North East AsiaHONG KONGTel 852 - 590 22 22Fax 852 - 590 95 93

VSL COMPANIES AND LICENSEES

Documents

POST-TENSIONED IN BUILDINGS · POST-TENSIONED INBUILDINGS Preface The development of reliable prestressing techniques has certainly been the most important innovation in the field