Antiquated Structural Systems Series – Part 7

7/21/2019 Antiquated Structural Systems Series – Part 7

http://slidepdf.com/reader/full/antiquated-structural-systems-series-part-7 1/3STRUCTURE magazine December 2008

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Antiquated Structural Systems SeriesPart 7 By D. Matthew Stuart, P.E., S.E., F. ASCE, SECB

For this series of articles, “antiquated” has been defined as meaning outmoded ordiscarded for reasons of age. In reality, however, most if not all of the systems thathave been and will be discussed are no longer in use simply because they have beenreplaced by more innovative or more economical methods of construction.However, this article deals with a method of construction that is still very much

in use today. Therefore, this installment will provide a history of how the post-tensioning industry developed in the United States.The historic, original construction practices described in this article may still be

encountered in existing structures. Therefore, the primary purpose of this seriesof articles will be fulfilled, which is to compile and disseminate a resource ofinformation to enable structural engineers to share their knowledge of existingstructural systems that may no longer be in use but are capable of being adapted orreanalyzed for safe reuse in the marketplace of today and the future.

the amount of total reinforcing steel inthe concrete section because the tendons

were made of higher strength materialthan conventional reinforcement.

Button-Headed TendonsThe button-headed system involved the

use of parallel, ¼-inch-diameter, 240-ksi, cold-drawn wires, each with an ap-proximate tensile capacity of 7 kips. The

wires were combined together, generallyin groups of seven, to form tendons. Toanchor the tendons at each end, the wires

were threaded through holes in a steel bear-ing plate and then an externally threaded,circular stressing washer. A “button” wasthen cold-formed on the end of each wirevia impact of a hammer on the end of the

tendon. The buttons prevented the wiresfrom passing back through the holes inthe circular washer and, as a consequence,allowed them to become anchored duringand after the field tensioning operation.The tendon assemblies were then coated

with mastic for corrosion protection and wrapped in heavy wax paper to preventthe tendon from bonding to the concrete.The above process was completed in apre-assembly shop prior to shipment tothe construction site.

Once the tendons were at the job site,the assemblies were placed in the forms,the concrete was cast, and the wires werestressed once the concrete achieved aminimum strength. The stressing process

was performed with a hydraulic jack, which was attached to the threaded wash-er. A steel shim was inserted between the

bearing plate and the washer to preventthe circular anchorage from retractingback against the bearing plate. As theshim plate was prefabricated, the lengthof the plate parallel to the tendon had tomatch exactly the calculated elongation.This system of anchoring the tendon wasinherently flawed because if the actualelongation did not match the length ofthe shim plate, field corrections had tobe made. A second problem associated with thebutton-headed system was the fact that

the stressing washer and shim plate hadto be external to the concrete in orderto facilitate the stressing operation. Thisresulted in placing this portion of theanchor assembly either outboard of theconcrete or within a formed stressingpocket. In either case, it was necessaryto cast a continuous pour strip or fill inthe pockets after the stressing operation

was completed. Another problem presented by thebutton-headed system was the need touse bulky couplers when intermediate

stressing of the tendons was necessary.Intermediate stressing procedures wererequired any time a continuous framedslab was longer than approximately 160feet. This was because the frictional re-straint of the wires during the tensioningoperation limited the practical stressinglengths of the tendons to approximately80 feet. By stressing the tendons at eachend anchorage, a maximum of 160 feetcould be tensioned without having to in-troduce an intermediate stressing pointcoupler. The couplers usually consisted

Post-TensionedConcrete Construction

In the 1950s, post-tensioned concrete was introduced into the United States

from Europe, primarily as a part of thelift-slab construction industry. However,the first post-tensioned structure con-structed in the U.S. was the Walnut LaneBridge in Philadelphia, between 1949and 1950. This structure consisted ofprecast girders that were post-tensionedusing the Magnel system developed inBelgium; i.e., the tendons were stressedafter the concrete was cast, as opposed topretensioning in which the tendons arestressed between abutments or withinself-stressing forms before the concrete is

cast. From these beginnings, this systemof “prestressing” both cast-in-place andprecast concrete structures and memberscontinues to this day.

Lift-Slab Construction

Originally, the lift-slab industry usedconventional reinforcing in the framedslabs that were first stack-cast on theground and then lifted into position viahydraulic jacks located at the tops of thebuilding columns. Typical slab spans anddepths ranged from 25 to 30 feet and from

10 to 12 inches, respectively. However,because of these span lengths and therelated excessive dead load deflections,as well as the stresses associated with thestripping process, cracking of the slabs

was very common.In order to correct these inherent de-

ficiencies, the industry turned to theavailable European BBRV (button-head-ed) post-tensioning system (Figure 1).This system of post-tensioning allowedthe reduction of slab thicknesses to as lit-tle as 8 inches, along with a reduction in

Figure 1. Reprinted with the permission ofthe Post-Tensioning Institute from Ken Bondy,“Post-Tensioned Concrete in Buildings, Pastand Future - An Insider’s View,” PTI Journal,December 2006, pp. 91-100.

Courtesy of Larry Krauser, GeneralTechnologies Inc.



of large, high-strength, externally threadedsteel studs that were screwed into an internallythreaded hole in the stressing washer.

Formed-in-Place Construction

At the same time that lift-slab constructionwas being advanced, separate companies spe-cializing in post-tensioned, formed-in-placeconstruction began competing for the sameprojects. However, it was not until allianceswere made between the post-tensioning com-panies and the emerging flying form industrythat formed-in-place, post-tensioned con-struction really began to become competitivewith lift-slab construction. By the 1960s,formed-in-place, post-tensioned constructionhad eclipsed lift-slab construction as thepredominant method of constructing multi-story, cast-in-place concrete buildings.

Post-Tensioned Strand

The use of spirally woven, seven-wire strandas post-tensioned tendons was introduced

into cast-in-place, post-tensioned construc-tion from the precast/prestressed concreteindustry in the early 1960s. By the end of thesame decade, the post-tensioning strand sys-tem had replaced the button-headed tendonsystem. Up until the early 1970s, unbonded,mono-strand tendons were installed greasedand spirally wrapped with two layers of heavykraft paper. By 1975, plastic sheathing was ex-clusively used for greased, unbonded strand.The use of strand was identified because

of the ability of the tendons to be securedwith wedge anchors, which eliminated theprimary disadvantage of the button-headedsystem – the lack of flexibility of the steelanchor shims. The first wedge anchor for usein the post-tensioned industry was patentedby Edward K. Rice of Atlas PrestressingCorporation (Figure 2 ). The wedge anchorwas constructed with a tapered, high-strengthwire coil with no bearing plate. The plateshown in Figure 2 was only provided to securethe anchor to the inside face of edge form.This method of installation also eliminatedthe other disadvantage of the button-headedsystem – the need for a continuous pour stripor infilled stressing pockets.The wedge anchor assembly was installed

with a small, two-piece, round rubber grom-met that was positioned between the anchorageand the inside face of the edge form. Thegrommet allowed the anchorage to be re-cessed a few inches in from the edge form,which created a round hole in which the noseof the tensioning jack could be inserted. Thegrommet also filled in the space between thecoil and the strand, preventing cement pastefrom entering the assembly during concreteplacement. After tensioning was completed

and any excess strand was removed beyondthe outboard end of the anchorage, the re-maining hole was filled with grout for a flushfinish with the edge of the slab.The seven-wire strand system was also

more conducive to intermediate stressinglocations, eliminating the final disadvantageof the button-headed system. With a strand,the cable could be placed as a continuoustendon with a wedge anchor assembly simplyslid down the length of the strand to the in-termediate stressing point.The anchorage force associated with the

coil wedge assembly was transferred to theconcrete by direct tension resistance to the

lateral forces generated by the wedges on the

inside of the coil. As a result, localized failureof the concrete, particularly with lightweightmixes, was common. This inherent deficiencyof the coil anchor resulted in the developmentof a ductile iron anchorage plate in the mid-

1960s. The ductile iron assembly consisted ofa flat bearing plate combined with a center,open, tapered barrel ring where the wedges

were secured. The current end anchorageplates in use today are very similar to theseoriginal bearing assemblies.

Methods of Analysis

Because of the indeterminate nature of acontinuous, cast-in-place concrete beam orslab and the secondary effects induced bydraped post-tension tendon uplift forces at

the structural supports, the analysis of posttensioned structures was originally difficulThe analysis of cast-in-place, post-tensionestructures became much easier, however, aftea simplified method of analysis called “loabalancing” was presented by T.Y. Lin in 1963 ACI Journal paper.The load balancing method of analysis basi

cally involved idealizing the vertical forceapplied to a structure from the draped tendons as a series of upward uniform loads on

the member. The load balancing methoallowed structures to be accurately analyzeusing standard structural engineering methodsuch as moment distribution. This greatlsimplified the process of post-tensioned designmaking it as easy as designing a conventionallreinforced structure, thereby helping to increase the popularity of post-tensioned construction from the late 1960s into the 1970s

Two-Way Construction Methods

Originally post-tensioned, two-way slab framing systems were constructed with columand middle strips, similar to those used iconventionally reinforced two-way slabs. However, because the continuous two-way tendonhad to be placed in a draped parabolic profil– near the top of the slab at the column linesand near the bottom of the slab at midspan the cables had to be placed in a basket weavpattern. This required that the tendons bnumbered and installed in a specific sequenceThis was difficult, particularly for structure

with irregularly spaced column grids.To avoid the complexities of a basket weav

tendon installation, an alternate method o

construction was conceived by T.Y. Lin & Associates and Atlas Prestressing Corporatiofor the infamous Watergate building locatein Washington, D.C. in the late 1960sThe system involved banding the tendon– grouping the strands together within narrow strip – in one direction along thcolumn line grid, while the tendons in thother direction were spaced uniformly abovthe banded tendons. The banded method oconstruction resulted in considerable labosavings over the basket weave system, and habecome the predominant method for placin

post-tensioning tendons in two-way slabever since.The structural adequacy and performanc

of the banded tendon layout was confirmethrough a number of laboratory tests at thUniversity of Texas, Austin in the early 1970sIn addition, the performance of the bandemethod of construction has also been provethrough the continued serviceability of thmany buildings that have been erected to dat

with this method of construction.

“...when ACI 318-77 was published,banded tendon distribution intwo-way slab construction was

recognized by the Code.”

Figure 2. Reprinted with the permission of the Post-Tensioning Institute from Ken Bondy, “Post-TensionedConcrete in Buildings, Past and Future – An Insider’sView,” PTI Journal, December 2006, pp. 91-100.



Building Codes andIndustry Organization

Up through the ACI 318-71 Code, cast-in-place, post-tensioning design was essentiallyignored, even though precast/prestressedmembers were addressed. However, when

ACI 318-77 was published, banded tendondistribution in two-way slab construction wasrecognized by the Code. This improvementcame about primarily through the formationand involvement of the Post Tensioning Insti-tute (PTI), which was established in 1976.

Common ProblemsThe two biggest problems faced by the post-

tensioning industry have been restraint crackingand corrosion, with the latter being the mostpervasive. Solutions to restraint cracking werediscovered fairly soon as the industry learnedhow to develop construction details to avoidthe problem. Addressing corrosion did not startto occur until the PTI developed improved

specifications in the mid 1970s.Concrete volume change occurs in both non-

prestressed and post-tensioned structures. Axial post-tensioning compression forces tendto close the restraint cracking associated withconcrete volume change within the span ofthe member. However, the compression forcesalso result in significant axial shortening ofthe concrete member at the ends. Therefore,any restraint that is provided at the ends ofthe member – e.g., at walls or columns – canand does result in significant cracking in thepost-tensioned member or the restraining

structural support. Solutions to this probleminclude the development of slip joints andpour strips.The early unbonded tendon coatings (grease)

and sheathings were very inadequate forprotecting the cables from corrosion, par-ticularly in harmful environments such asparking garages, where the use of deicingmaterials created caustic conditions for boththe concrete and the internal reinforcement.The actual extent of the problem did not re-veal itself until about 10 or 15 years afterthe earliest post-tensioned concrete structures

were constructed. The problem was cor-rected after the PTI developed specificationsthat became enforceable through certificationprograms created by the organization. Theimprovements established through the PTIspecifications included sheathings, coatingsand encapsulation systems for use in aggres-sively corrosive environments.

ConclusionThe post-tensioning industry is likely to con-

tinue to thrive worldwide for both buildingand bridge construction. Further advance-ments within the industry should continueto enhance the design and construction ofpost-tensioned structures. In fact, the authoranticipates that, sometime in the future, anadvanced chemical sheathing will be devel-oped for mono-strand construction that will

allow for conventional unbonded installa-tion and tensioning, yet subsequently createa long-term bonded condition in the absenceof internal ducts or grouting.▪

D. Matthew Stuart, P.E., S.E., F. ASCE,SECB is licensed in 20 states. Mattcurrently works as a Senior Project Managat the main office of CMX located in New

Jersey, and also serves as Adjunct Professor for the Masters of Structural EngineeringProgram at Lehigh University, Bethlehem,PA. Mr. Stuart may be contacted [email protected].

ReferencesDurability of Post-Tensioning Tendons: Technical Report; November 2001, The Internation

Federation for Structural Concrete, Status of the Durability of Post-Tensioning Tendons the United States, Cliff L. Freyermuth

Post-Tensioned Concrete: Five Decades of American Building Construction, Kenneth Bondy, Concrete Construction, December 2001

Post-Tensioned Concrete in Buildings, Past and Future, an Insiders View, Kenneth BBondy, PTI Journal Vol. 4 No.2, December 2006

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Antiquated Structural Systems Series – Part 7