Where Structural Steel and Concrete Meet

5/27/2018 Where Structural Steel and Concrete Meet

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Where structural steel and concrete meet

Prof. J.W.B. StarkDelft University of Technology / Stark Partners

Delft, The [email protected]

ABSTRACT

Traditionally "steel structures" and "concrete structures" formed more or less two differentworlds in structural engineering. Fortunately this situation is changing rapidly. It is nowrecognised that each of the two materials have advantages and disadvantages and that oftenan optimal solution is found by combining both materials. This may be a combination of steeland concrete in an element as is the case in "Composite steel-concrete construction" or thecombined use of concrete elements and steel elements in "Mixed construction". For the designof composite steel-concrete elements specific design standards have been developed. Howeverfor Mixed construction a combined use of steel design standards and concrete designstandards is necessary. It is important that the design rules for the two materials are consistent,especially for those components connecting both materials. However, in the past the design

standards and recommendations for concrete and steel have been developed separately. Soevidently at this moment there are considerable differences in design assumptions andtreatment of various aspects. In the paper design methods for connections between structuralsteel and concrete will be discussed. The methods will be illustrated for column bases, beingthe most frequently used type of connection between steel and concrete, though the informationcan also be used for related types of connections.

INTRODUCTION

In the past for the design of a building the choice was normally between a concrete structureand a steel structure. Looking at recent practice there is an evident tendency that designersalso consider the combined use of concrete and steel in the form of composite or mixed

structures as a serious alternative. Use of composite elements in the form of compositebeams, composite columns and composite slabs is already common practice in many countries.

Applications are supported by accepted Standards or Recommendations as for example theEuropean Standard: EN 1994 - Eurocode 4. However, this supporting material is not availablefor mixed constructions where (reinforced or prestressed) concrete elements and structuralsteel elements are used in combination. The elements itself are covered by the respectivedesign standards for concrete and steel. But in many cases the joints where the elements meetform a black spot as far as Design Standards and information is concerned. So the designerhas to develop design models based on a creative interpretation of methods and rules in use forconcrete and steel. It is of course a complication when these design methods for the differentmaterials are not consistent. In the past the Design Standards and Recommendations for


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concrete and steel have been developed separately. So evidently at this moment there are stillconsiderable differences in design assumptions and treatment of various aspects. Someexamples of these differences will be illustrated in this paper.

TYPOLOGY

Many different details exist depending on the type of members to be connected, the actions tobe transferred and the performance requirements. An exhaustive treatment of all possibledetails is not possible in the context of this paper. Just to give an idea two categories arediscussed.

Column bases

This is one of the most commonly used details. The steel column is connected to a base plate,which is attached to the concrete foundation by some form of so-called holding downassembly. A typical detail is shown in Figure 1. The system of column, base plate and holdingdown assembly is known as a column base. The holding down assembly comprises two, butmore commonly four (or more) holding down bolts (anchors). These may be cast-in-place, orpost-installed to the completed foundation. Cast-in-place bolts sometimes have some form oftubular or conical sleeve, so that the top of the bolts are free to move laterally, to allow the baseplate to be accurately located.

Fig. 1 - Typical detail of a column base

Base plates for cast-in assemblies are usually provided with oversized holes and thick washerplates to permit translation of the column base. Anchor plates or similar embeddedarrangements can be attached to the embedded end of the anchor assembly to resist pull-out.Post-installed anchors may be used, being positioned accurately in the hardened concrete.Post-installed assemblies include, for instance, torque-controlled expansion anchors, under-cut

anchors and bonded anchors.

Connections of steel beams to concrete walls or columns

A stiff concrete core often provides the stability of a multi-storey steel frame. The steel beams ofthe floors are connected to the wall of the concrete core (see Figure 2a). To provide sufficientfire resistance sometimes (prefabricated) concrete columns are used instead of steel columnswith fire protection. In Figure 2b a connection is shown as used in a refurbishment project wherenew steel floor beams are connected to existing concrete columns by means of an extendedend plate connection.

FSteel column

Base plate

Grout

Concrete

Anchor


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a b

Fig. 2 - Connection composite beam-concrete core (a) ; steel beam-concrete column (b).

A great number of different forms of connection details are possible for these types ofconnection.

A treatment of all possible details is not possible in the context of this paper. Therefore thetreatment is restricted to column bases. This is one of the most commonly used types ofconnection. Another reason to focus on column bases is that this type of connection is explicitlycovered by the recently completed Eurocodes.

STANDARDS AND RECOMMENDATIONS

As the author is most familiar with the situation in Europe the treatment in this paper will focuson the design methods as covered by European standards and in particular by the Eurocodes.The column base connection is a typical detail where steel and concrete meet. But in addition to

steel and concrete there is in effect a third element and that is the connecting element in theform of anchors or fasteners. Each of these three composing elements is covered byEurocodes. But unfortunately the development of these Eurocodes was not fully coordinated sothat inconsistencies still exist in the various design approaches as will be illustrated in thispaper. The situation is as follows:

Steel

In EN1993-Eurocode 3 all the design rules for joints have been collected in a separate part ofEurocode 3: EN1993-1-8 [3]. In this part the design of column bases is not treated separatelybut is integrated in the so-called component-approach. The advantage is that the rules arefully consistent with the design approach for steel-steel connections. However, this way ofpresentation makes the rules not easy accessible for users. The rules are based on the results

of a project carried out within the framework of the European Project COST C1 (Semi-rigidbehavior of civil engineering structural connections) and the Technical Committee 10 of ECCS(European convention for constructional steelwork). For background information refer to arecent special issue of Heron [13].

Concrete

For concrete aspects reference is made to EN1992-1-1 [3] but this code does not give specificrules in all cases as will be demonstrated later. Furthermore the rules are only applicable if theanchorage has sufficient deformation capacity. This is often not the case for short anchors.


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Anchors

Recently CEN issued a series of drafts for an addition to EN1992-Eurocode 2 covering designrules for connections with short anchors [5] [9]. The content is based on an existing CEB-Design Guide [10] and EOTA (European Organisation for Technical Approvals) Guideline [11].

COMPARISON OF DESIGN APPROACHES

The design approaches in Eurocode 3 (steel world) and CEB-Guide/Eurocode2-PT4 (fastenerworld) are essentially different. This is illustrated in Figure 3 where the load distribution isshown for a column base subjected to compression and bending.

EN1994-1-8: Eurocode 3

Ultimate limit s tate

Plastic design Only equilibrium conditions Component method Base plate assumed to be flexible Compression concentrated under footprint

Serviceability limit state Rules for stiffness Classification for stiffness

CEB / EN1992-4

Ultimate limit s tate Elastic design Compatibility conditions Consideration of complete connection Base plate assumed to be rigid Linear distribution of strains (forces?)

Serviceability limit state Not covered

Fig. 3 Design approaches steel world versus fastener world

The following comments apply for the CEB method: The stiffness of the base plate required for the assumption of a rigid plate is expressed in a

maximum stress criterion and not a deformation criterion. Tests and inspection ofdemolished structures has shown that the compression always concentrates under stiffparts of the column section (flanges and web).

From the assumption that the plane at the interface of concrete and steel remains planefollows that the distribution of the displacements is linear. This does not necessarily implythat also the strain distribution is linear as is assumed in the design method.

F

M

F FC

F

M

t

c


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The calculation is more complicated than for the Eurocode 3 method. Unnecessary thick base plates or even stiffeners are required. Serviceability limit state is not covered.

COMPONENT METHOD

The component method consists of the following issues: identification, characterization,assembly, classification and modeling. The identification is the process of decomposing a jointin different components. Figure 4 shows the components of a column base. In thecharacterization of each component, the relevant mechanical properties are determined: theresistance, the stiffness and the deformation capacity. In the assembly, the mechanicalproperties of the components are combined in order to determine the resistance, the stiffnessand the rotational capacity of the joint. The joints may be classified in terms of the resistance,the deformation capacity or the stiffness. The purpose of the classification is simplification ofthe joint behavior for the frame analysis, for instance by classifying for stiffness as rigid.Modeling is required to determine how the (non-linear) mechanical properties of the joint aretaken into account in the frame analysis.

base plate in bending column web and flangebase plate and concrete anchor bolt

in shear and compressionand anchor bolts in tensionblock in copression in shear

Fig. 4 The major components of a column base

BASE PLATE AND CONCRETE BLOCK IN COMPRESSION

The resistance is determined by an equivalent rigid plate concept. In Figure 5 is shown how anequivalent rigid plate is defined to replace a flexible plate in case the base plate connection isloaded by axial force only. This rigid plate follows the footprint of the column.

Aeq

Ap A

c cc

c

c

c

Aeq

Ap A

Aeq

Ap A

Fig. 5 - Flexible base plate modeled as a rigid plate of equivalent area.

The resistance is now determined by two parameters: the bearing strength of the concrete andthe dimensions of the equivalent rigid plate.


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Dimensions of the equivalent rigid plate

The flexible base plate, with areaAp, is replaced by an equivalent rigid plate with area Aeq, see

Figure 5. This rigid plate area Aeqis composed of one T-stub under the column web and two T-stubs under the column flanges.

c

column

base plate

F

t

c tw

Sd FRd

L

Fig. 6 - T stub in compression

The additional bearing width c of the T-stub, see Figure 6, is determined on the basis of thefollowing assumptions: No plastic deformations occur in the flange of the T-stub, so that the flange remains

relatively flat. Therefore, the resistance per unit length of the T-stub flange is taken as theelastic resistance.

Mbp= t2fy/6 (1)

It is assumed that the T-stub is loaded by a uniform stress distribution. The bendingmoment per unit length on the base plate acting as a cantilever of span cis:

Mbp= fjdc2/2 (2)

The equivalent width c can be resolved by combining equations (1) and (2):

c = t [ fy / (3fjd) ]0,5 (3)

Bearing s trength of the concrete

The bearing strength of the concrete under the plate is dependent on the size of the concreteblock. EN1992-1-8 refers to EN1992-1-1;6.7 Partially loaded concrete as follows:

The design bearing strength of the joint fjdshould be determined from:fjd= j FRdu/ (beffleff) (4)

where:j is the foundation joint material coefficient, based on observations that the grout layer in

practice often shows imperfections and/or air bubbles. The value in EN1993-1-8 is 2/3provided that the characteristic strength of the grout is not less than 0,2 times thecharacteristic strength of the concrete foundation and the thickness of the grout is notgreater than 0,2 times the smallest width of the steel base plate. In cases where thethickness of the grout is more than 50 mm, the characteristic strength of the grout shouldbe at least the same as that of the concrete foundation.

FRduis the concentrated design resistance force given in EN 1992, where Ac0is to be takenas (beffleff) (see Figure 7).

FRdu= Ac0 fcd(Ac1/ Ac0)0,53,0 fcdAc0 (5)


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Fig.8 Comparison of tension regions in a column base and a beam-to column connection

The use of hooked anchors is restricted to material with yield strength 300 N/mm2.When the anchor bolts are provided with a washer plate or other load distributing member, noaccount should be taken of the contribution of bond.

As an alternative to the cast in place long anchors according to the reinforced concretetechnique also post installed short anchors according to the fastener technique are used.These anchors show other governing failure modes as illustrated in Figure 9.

Fig. 9 Possible failure modes of short anchorsKey to figure 9:

a. Steel failureb. Pull-out failurec. Concrete cone failured. Splitting failuree. Concrete blow out failure

Rules for the design resistance based on the CEB-Design Guide [10] are given inprCEN/TS1992-4 [5]-[9]. In this document also rules are given for the load distribution and thedesign of the base plate, which are not consistent with EN1993-1-8 (see Figure 3).

Open questions

Is it necessary to consider prying forces for the design of the anchors? Is it possible to extend the application of hooked anchors for yield strength > 300 N/mm2? Threaded rods are often used as anchor. The bond strength of these anchors is not

covered. EN1992-1-1 does not give explicit rules for the resistance of load distributing devices as

washer plates. More specific rules are needed for the required reinforcement to avoid splitting and blow out

if long anchors are placed near edges of the concrete foundation. Harmonisation of the rules in EN1993-1-8 and EN1992-4 is required.

FC

M

end platein bending

F

M

F

base platein bending

a b c c

e

d d


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SHEAR RESISTANCE

According the CEB Design Guide for base plates with a grout layer thicker than 3 mm plastic

design is not allowed, friction forces underneath the base plate should be neglected and theshear capacity has to be calculated for the mechanism shear load with lever arm. For columnbases usually a grout layer with a thickness greater than 3 mm is used. Though it is realisedthat there may be uncertainties about the strength and quality of the grout layer, the CEBmethod will be very conservative in many practical cases. This was confirmed by COST/WG2[13] that compared design values with test results for column bases loaded in shear and with avarying thickness of the grout layer. In particular in case of low strength bolts and a thick groutlayer (60 mm) the experimentally obtained maximum shear load was many times (between 10and 25 !!) greater than the calculated characteristic shear strength of the connection.

According EN1993-1-8;6.2.8.1 one of the following methods may be used to resist shear force: Friction

The design friction resistance Ff,Rdis to be derived as follows:Ff,Rd = Cf,dNc,Ed (6)

where:Cf,d is the coefficient of friction. The following values may be used:

for sand-cement mortar Cf,d= 0,20 for other types of grout the coefficient of friction Cf,dshould be determined by testing

Nc,Ed is the design value of the normal compressive force in the column. Shear resistance of the anchor boltsThe design shear resistance of an anchor bolt Fvb,Rdis the smaller of F1,vb,Rd and F2,vb,Rd where:

F1,vb,Rd is the bearing resistance of the anchor bolt calculated as for a normal bolt

F2,vb,Rd = bfubAs/Mb (7)where:b= 0,44 - 0,0003 fybfybis the yield strength of the anchor bolt, where 235 N/mm2 fyb640 N/mm

2 Shear resistance of special elements such as block or bar shear connectors

For the design bearing resistance of a block or bar shear connector EN1993-1-8 refers toEN 1992. But this is not explicitly covered in that document.

Shear resistance of the surrounding part of the foundation

The design method in EN1993-1-8 is basedon the results of a research project, carriedout at TU-Delft [12].Due to the horizontal displacement, not onlyshear and bending in the bolts will occur, butalso the tensile force in the bolts will beincreased due to second order effects. Thehorizontal component of the increasing tensileforce gives an extra contribution to the shearresistance. The increasing vertical componentgives an extra contribution to the transfer ofload by friction.

Fig. 10 - Test specimen loaded by shear force and tensile force (Stevin, 1989).

Open questions

In EN1993-1-8 is given that the design shear resistance may be based on the summation:Fv,Rd = Ff,Rd + n Fvb,Rd (8)


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However summation seems to be in contradiction with 6.2.2(5) and 6.2.8.1(5) The shear resistance by friction is based on the normal compressive force in the column

Nc,Ed(see formula 6) . However it is expected that the normal force caused by bending will

also contribute to the friction resistance. The rules in EN1993-1-8 are based on tests on column bases with cast-in-place long

anchors. The applicability to short anchors should be investigated. The implications of use of oversized or slotted holes should be defined. More specific rules are needed for the required reinforcement in case of anchors placed

near edges of the concrete foundation.

STIFFNESS OF COLUMN BASES

Fitting within the design concept of EN1993-1-8 also design rules are provided to determine therotational stiffness of column bases. This is covered in EN1993-1-8;6.3.4 and the stiffnesscoefficients are included in table 6.11. The restricted space in this paper does not allowdiscussing this in detail.

In EN1993;5.2.2.5(2) also the classification boundary for rigid column bases is given.

REFERENCES

1. Stark, J., Hordijk, D., Where structural steel and concrete meet, Proc. Int. Symp.Connections between Steel and Concrete, Stuttgart, 2001, Rilem Pro21

2. Stark, J., Design of connections between steel and concrete to Eurocodes, Proc. Int. Symp.Connections between Steel and Concrete, Stuttgart, 2007, Rilem Pro21

3. EN 1993-1-8:2002, Eurocode 3 : Design of steel structures; Part 1.8: Design of joints.4. EN 1992-1-1, Eurocode 2 : Design of concrete structures - Part 1-1: General rules and rules

for buildings.5. prCEN/TS 1992-4-1:XXX, Design of Fastenings for Use in Concrete - Final Draft - Part 1:

General. CEN/TC 250/SC 2 N 677, 2008 (Document for formal vote).

6. prCEN/TS 1992-4-2:XXX, Design of Fastenings for Use in Concrete - Final Draft - Part 2:Headed Fasteners. CEN/TC 250/SC 2 N 678, 2008 (Document for formal vote).

7. prCEN/TS 1992-4-3:XXX, Design of Fastenings for Use in Concrete - Final Draft - Part 3:Anchor channels. CEN/TC 250/SC 2 N 679, 2008 (Document for formal vote).

8. prCEN/TS 1992-4-4:XXX, Design of Fastenings for Use in Concrete - Final Draft - Part 4:Post-installed fasteners - mechanical systems. CEN/TC 250/SC 2 N 680, 2008 (Documentfor formal vote).

9. prCEN/TS 1992-4-5:XXX, Design of Fastenings for Use in Concrete - Final Draft - Part 5:Post-installed fasteners chemical systems. CEN/TC 250/SC 2 N 680, 2008 (Document forformal vote).

10. Comit Euro-International du Bton: Design of fastenings in concrete. Design Guide. Parts 1to 3. Thomas Telford, CEB-Bulletin 233, januari 1997.

11. European Organisation for Technical Approvals: Guideline for European Technical Approvalof Metal Anchors for Use in Concrete. Part 1: Anchors in general; Part 2: Torque-controlledExpansion anchors; Part 3: Undercut anchors, Issue-1997. (Part 5: Bonded Anchors,Issue-March2002).

12. Bouwman, L.P., Gresnigt A.M., Romeijn, Research into the connection of steel base platesto concrete foundations (in Dutch), Stevin Laboratory report 25.6.89.05/c6, Delft.

13. Heron, Special issue: Steel column bases, Volume 53 (2008), issue 1/2, Delft.


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WORKED EXAMPLE FOR COLUMN BASE IN COMPRESSION, BENDING AND SHEAR

Column section IPE 240

Steelgrade S235

Baseplate: bp= 190 mmhp= 380 mmtp= 35 mm

Concrete grade C20/25

Concrete foundationB = 450 mm

H = 800 mm

Anchors M24; 8.8rolled thread

Spacing anchors: ha= 410 mmba= 120 mm

Edge distance: e = 35 mm

Loads:FSd;V= 85,5 kNFSd;H= 37,5 kNMS;d= 75 kNm

Detail calculation Clause

Distribution of internal forcesz.FT,l,E + (ha tf).NEd MEd= 0270.FT,l,E + (240 9,8). 85,5.10

3 75. 106= 0FT,l,E = 241,3 kNFC,r,E= FT,l,E + NEd= 241,3 + 85,5 = 326,8 kN

EN1993-1-86.2.8.1Fig. 6.18d

Design bearing strength

C20/25 fcd= ccfck/C= 1,0. 20/1,5 = 13,33 N/mm2

FRdu = Ac0.fcd.(Ac1/Ac0) 3,0.fcdAc0Assume c = 70 mmb1= beff= tf + 2c=9,8 + 2.70= 149,8 ; d1= bp= 190b2= 149,8 + 2.35 = 219,8 ; d2= 3.190 = 570

Ac0= 149,8.190 = 28462 ; Ac1= 219,8.570 = 125286

(Ac1/Ac0) = (125286/28462) = 2,10FRdu = 28462.13,33. 2,10 .10

-3 = 797 kN(6.6): fjd= jFRdu/ Ac0= (2/3)( 797000/28462)

fjd= 18,7 N/mm2

EN1992-1-1

3.1.6EN1992-1-16.7(Fig. 6.29)

EN1993-1-86.2.5

380

450

IPE 240

FSd;V

FSd;H

Ms;d

31070 70

190

I-I

II-II


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Detail calcu lation Cont. Clause

Addi tional bearing width(6.5): c = t [fy/ (3 fjd M0)]0,5

c = 35 [235 / (3 18,7.1,0)]0,5= 71,6 mm > Overstek

EN1993-1-86.2.5

Design compression resistance of T-stub(6.4): FC,Rd= fjdbeffleffbeff= 70 + 9,8 + 71,6 = 151,4; leff= bp= 190FC,pl,Rd = 18,7. 151,4. 190 = 538 kN > FC,r,E= 326,8 kN

EN1993-1-86.2.6.9Fig.6.4a

Verification plate cross-section II-II(1) Reference to 6.2.6.5(2) Prying force to be neglected.Table 6.2: FT,l,Rd= Mpl,i,Rd/ m (half T-stub)

Mpl,i,Rd= 0,25 leff t

2

fy/M0 = 0,25.190. 35

2

. 235 / 1,0Mpl,i,Rd= 13,674. 106

m = ( ha h )/2 = (310 240)/2 = 35 mmFT,l,Rd= 13,674.10

6. 10-3/ 35 = 390 kN > FT,l,E = 241,3 kN

Note: shear stresses in base plate neglected in this example

EN1993-1-86.2.6.116.2.6.5

6.2.4

Anchors

FT,l,E = 241,3 kN per anchor Ft;E= 241,3/2 = 120,7 kNTable 3.4: Ft,Rd= k2 fubAs/M2

Ft,Rd= 0,9. 800. 353. 10-3/1,25

Ft,Rd= 203,4 kN > Ft;E= 120,7 kN6.2.6.12(5): Use of hook not permitted for grade 8.8

Use washer plate, see fig. 6.14b6.2.6.12(6): When washer plates are used no account may be taken ofthe contribution of bond.

EN1993-1-86.2.6.123.6

ShearFrictional design resistance:(6.1): Design friction resistance Ff,Rd= Cf.d Nc,Ed

Fors and-cement mortar : Cf.d= 0,20Ff,Rd= 0,20. 326,8 = 65 kN > FSd;H= 37,5 kN

Design shear resistance of the anchor bolts:By the 2 anchors in compression region:

Fvb,Rd = b fubAs/Mb

b= 0,44 0,0003 fyb= 0,44 0,0003. 640 = 0,248Mbis not referred. Assume Mb= M2= 1,25Fvb,Rd = 0,248.800. 353. 10

-3/1,25 = 70 kN2 anchors: Fv,Rd = 140 kN > FSd;H= 37,5 kNBoth resistances by friction and shear resistance of anchors areseparately sufficient. Summation allowed by 6.2.2(8) form (6.3) is notrequired.

EN1993-1-86.2.2

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Where Structural Steel and Concrete Meet