Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

The PCI headed stud anchorage research program: Scope andhighlights of findings

Neal S. AndersonConcrete Reinforcing Steel Institute, Schaumburg, Illinois, USA

Donald F. MeinheitWiss, Janney, Elstner Associates, Inc., Chicago, Illinois, USA

ABSTRACT: The construction industry has relied on the headed stud anchor to provide connection betweenconcrete and multiple other structural elements for many years. In the past, the design engineer followed numerousaccepted rational methods of designing for allowable strength and serviceability in accordance with acceptedstandard procedures.

Recent changes in the requirements in ACI 318, as well as new developments from testing completed by PCI,have started to impact the design engineer’s practice in designing anchorages to concrete. Failure to stay currentwith new design procedures can result in liberal design solutions.

The historical overview discusses the chronology of design methods based on various boundary conditionsand load applications. The standard for designing headed stud anchor connections has been impacted by newtesting results compiled by PCI. This article is an overview of the current state-of-the-art approach used byprecast structural designers for headed stud anchor design.

1 BACKGROUND

For decades, the precast concrete industry has reliedheavily on welded headed studs on anchorage plates toconnect precast concrete elements to other structuralelements. Formal headed-stud design concepts haveexisted in the Precast/Prestressed Concrete Institute’s(PCI) PCI Design Handbook since the early 1970s(PCI 1971). These early design concepts accounted formultiple anchors with variable spacings or those foundclose to free edges. New design concepts in theAmeri-can Concrete Institute’s (ACI) Building Code Require-ments for Structural Concrete Appendix D (ACI 3182002) raised questions about the PCI design approachbecause the new design model produced designs moreconservative than PCI’s design approach, even thoughthe PCI approach worked successfully since 1971.

In 2002, ACI Committee 318, in Appendix DAnchoring to Concrete, included new provisions fordesigning all types of anchors, including headed studs,embedded bolts, and various mechanical expansionanchors. These new design provisions presented asimple physical model, which easily accommodatesanchor spacing in two directions and the effects of freeedges. The origins of the ACI design model date to themid-1960s. However, the design model was more fullydeveloped in the European Kappa method of the late

1980s and put into a comprehensive/transparent formwith the development of the Concrete Capacity Design(CCD) model (Fuchs, Eligenhausen & Breen 1995).The motivation to create a chapter in the ACI Codewas to rationally design both cast-in-place anchoragesand post-installed mechanical anchorages.

The formal codification of the design requirementsfinally provided a design engineer with the method-ologies to consider many of the geometry and memberinfluences that can affect the capacity and the overalldesign of an anchorage group. Prior to the ACI cod-ification of design provisions, post-installed anchorsgenerally had to be designed using manufacturer’s cat-alogs/procedures and a standard factor of safety, usu-ally 4. Hence, the codification of procedures greatlyfacilitates the general design of concrete anchors.

However, when thoroughly examining the researchliterature on headed studs and embedded bolts, theauthors found that the database for tests on headedstuds and embedded bolts loaded in shear was domi-nated by post-installed anchors. The database on ten-sion is more substantially populated by headed studsand embedded bolts.Although there are no known sys-temic design problems with headed anchors designedusing the pre-ACI 2002 procedures, it is fair to ques-tion the earlier design procedures when the new andold procedures give different solutions.

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2 PCI’S RESEARCH INITIATIVE

The ACI Appendix D design method was calibratedusing an extensive database, dominated mostly bypost-installed anchor tests. This may have skewed thedesign equations toward post-installed anchors. Con-sequently, some current design provisions for headed-stud connections and other cast-in anchorages may notbe totally appropriate.

In the mid 1990s, the Prestressed/Precast Con-crete Institute (PCI) initiated a headed stud researchprogram to create a database for headed-stud groupconnections. Originally, the PCI research program wasconceived in response to the proposed ACI AppendixD provisions where certain provisions for the cast-in-place anchorage were found more liberal than the CCDapproach. Because of the limited amount of researchdata on shear, PCI initiated an industry-sponsoredresearch project to satisfy these objectives:

• Provide justification for the design procedures usedin the past, which through Code implementation andadoption are now considered unconservative.

• Create a database of tests to (a) justify acceptingand conforming to the provisions of ACI 318-02Appendix D, (b) modify the ACI procedures, (c)refine the design procedures as currently publishedin the PCI Design Handbook (PCI 1999), or (d)create a database of tests to write a new design pro-cedure independent of ACI 318, which is permittedin the Code.

3 THE PCI RESEARCH PROGRAM

PCI conducted this research work in two phases. Phase1 of this study concentrated on examining headed-studanchorages loaded only in shear because of the lack ofheaded stud test data in the anchor database. Phase 2 ofthis study reviewed the tension-only information froman existing, moderately-sized database. However, themore important part of Phase 2 included experimentalstudies evaluating the affects of combined tension andshear on multi-stud connections. Working for the con-sulting firm of Wiss, Janney, Elstner Associates, Inc.(WJE), the authors conducted both of these projects inthe Northbrook, Illinois structural laboratory. In total,412 headed stud tests were conducted by WJE.

The shear research program, and ensuing revisionsto the PCI Handbook (the precast designers referencemanual) considered the geometric effects of the front-edge distance, corners, side-edge distance, back-edgedistance, and in-the-field type connections.

Welded, headed studs are designed to resist directtension, shear, or in the usual case, the combinationof the two. The design equations given in the Hand-book are applicable to studs, which are welded to steelplates or other structural members, and embedded

in unconfined, cracked or uncracked concrete(Anderson & Meinheit 2007a).

It is assumed that the steel plates are of sufficientthickness to prevent significant plate deformation andadequately transfer the applied load to the studs.In addition, the in–place connector design strengthshould be taken as the smaller value based on comput-ing both the concrete and steel characteristic capacityfor the anchorage configuration. Cracking is assumedto cause a reduction in capacity, similar to that inACI Appendix D. No cracked concrete tests wereconducted.

4 HIGHLIGHTS FROM THE RESEARCHRESULTS

4.1 Steel stud capacity

As presented in their paper (Anderson & Meinheit2001), the authors clearly showed that steel failure intensile and shear is only governed by the area of thestud shank and ultimate tensile strength of the studsteel (Fut). This is attributed to the welding metallurgyaffecting the headed stud mechanical properties in theheat-affected zone (HAZ) and a slightly greater studarea in the weld flash region.

Therefore, the steel capacity for a headed stud intension and shear is identical. This realization mayappear new; however, the compositely designed steel-concrete members have essentially used the samecapacity in tension and shear since the early 1970s.The shear failure shown in Figure 1 occurs at a loaddictated by the tensile ultimate strength.

4.2 Tension

As stated earlier, there is a substantial database of ten-sion tests on headed studs and embedded bolts. WJE’sreview of headed stud data in Phase 2 of the WJE/PCIresearch shows that the ACI provisions for calculating

Shear (V)

Figure 1. Steel failure of a headed-stud anchorage loadedin shear.

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the single anchor concrete breakout capacity are repre-sentative of headed stud behavior. Provisions for edgesand spacing using the CCD model are reasonable andgenerally conservative, except for the case where thereis an anchor spacing influence in two directions. In thiscase, the CCD design model is slightly unconservative(no more than 10 percent) for anchor spacings that areless than about 2hef .

4.3 Shear

The WJE/PCI research program developed new designshear strength provisions for headed stud anchoragesgoverned by a concrete breakout failure. The PCIdesign provisions are calibrated based on WJE exper-imental testing of headed-stud anchorages, which ispermitted by the ACI Code. Figure 2 shows the differ-ent edge conditions for shear, discussed further in thefollowing sections.

4.3.1 Front edgeThe front edge condition represents a majority ofshear-loaded connections found in engineering prac-tice. Figure 3 shows a photograph of a typical frontedge concrete breakout. The analyses of test data leadto a simpler design equation for the single stud break-out capacity. Unlike the ACI CCD model, the PCIequation is not a function of the anchor diameter or theeffective embedment depth; their influence on headedstud capacity is not as significant as the edge distance.

For a multiple-stud connection with more than oneanchor row perpendicular to the shear force direction,the breakout surface and hence capacity was found tobe defined by the position of the rear or back studrow. The concept of Back Edge Distance (BED) fordesign was introduced in the PCI design procedure.The photograph in Figure 4 shows the rear stud clearlycontrols the location of the breakout crack formation;no parallel-to-stud-line splitting cracks occurred prior

Figure 2. Edge conditions for an anchorage loaded in shear.

to failure. The ACI design provisions are conservativerelative to the database developed from the WJE/PCIresearch testing.

4.3.2 CornersThe corner is considered to be a special case of ananchorage loaded toward the front-edge, but locatedsufficiently close to two intersecting edges such thata different concrete breakout path occurs, as seen inFigure 5. Corner tests showed the cattycorner stud,or stud diagonally opposite the geometric corner (seeFigure 6) defined the concrete breakout capacity. Theresearch found that the headed-stud corner influenceis greater than the ACI model implies. Moreover, widespacing between individual studs in the anchoragecan initiate a “zipper-type” failure behavior, directing

Shearload

Figure 3. Front edge concrete breakout representing acommon load condition for shear.

Shearload

Figure 4. Concrete breakout through the rear stud for a frontedge loading condition.

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Shearload

Figure 5. Corner breakout conditions for an anchorageloaded in shear.

Figure 6. Critical stud location for the corner breakoutcondition for an anchorage loaded in shear.

the crack toward the side edge, and causing a greatercorner influence.

4.3.3 Side edgeSide-edge conditions exist when the shear load isparallel to the free edge but the anchor(s) are closeenough to the free edge to cause a breakout of the freeedge parallel to the direction of the shear. The pho-tograph in Figure 7 shows a side edge breakout fora two-stud connection. A new single-stud, side-edgebreakout capacity equation was developed.

The ACI design procedure for side edge breakoutwas found to be very conservative for small side-edgedistances when compared to the WJE/PCI developeddatabase. The ACI design approach and equations forside edge breakout was based on limited testing ofsmall diameter, single anchors; no group influenceswere considered because of the lack of test data atthe time.

Shearload

Figure 7. Side edge concrete breakout for shear appliedparallel to the free edge.

V

Figure 8. Example of a pryout failure in shear occurring inthe field when short, headed studs are used in the connection.

4.3.4 Back edgeThe authors, to their knowledge, have not found anydiscussion of anchors near a free edge and loaded suchthat shear force is directed away from the free edge.For this condition of pure shear, testing found the back-edge distance has no influence on the headed-stud con-nection capacity, provided the anchor is not susceptibleto pryout.At very small edge distances, as small as 4da,the failure was consistently a steel shearing failure; thisedge distance is well within the minimum concretecover requirements for reinforcement protection.

4.3.5 In-the-field and pryoutWhen a headed-stud anchorage is sufficiently awayfrom all edges, termed in-the-field of the member,the anchorage capacity is consistently governed by thesteel stud(s), provided certain conditions exist with thestud geometry. If the stud is short and stocky, a con-crete breakout failure known as concrete pryout canoccur, as illustrated in Figure 8.

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Sufficient test data exists in the literature to proposea new concrete pryout capacity equation for headedstud anchorages (Anderson & Meinheit 2005, 2007b).The geometric characteristic of the headed stud thatdominates the pryout capacity is its stiffness. Thediameter is representative of the stiffness of the anchor,which was experimentally found as controlling thepryout capacity.

4.4 Combined tension and shear action

Phase 2 of the WJE/PCI research explored combinedtension and shear loading on a stud group.A test repre-senting the attachment of a structural steel member toan embedded steel plate with headed studs was tested.Present design provisions for capacity analysis use atri-linear interaction equation or diagram to describethe behavior, with truncation limits for minimum ten-sion and shear. Because there are many different failuremodes in tension and shear, one single interactiondiagram representing all the different failure modeswill make some combined loading designs liberal andothers conservative.

This becomes a concern when steel failure maygovern one capacity, while concrete breakout governsthe other capacity. Instead of examining these fail-ure modes as mutually exclusive failure behaviors, theauthors have explored the combined action as a sepa-rate failure mode subject to modification by the levelof tension or shear on the anchorage.

The authors present analyses of combined test dataindicated that the failure in combined loading if notnear an edge would be reasonable predicted by the ten-sion breakout capacity. When the anchorage is closerto an edge, the failure again appears as a tension typeconcrete breakout.

Use of the tri-linear or an elliptical interactiondesign equation works for most cases of the data inves-tigated. The case where the tri-linear equation is lessconservative is when there is the possibility of a frontedge concrete shear breakout failure in addition to aconcrete tension breakout. Further refinements are stillneeded for this case.

5 SUMMARY

The WJE/PCI headed stud research initiative has pro-duced an alternate shear design procedure, whichbetter represents the behavior of headed-stud anchors.This design procedure also conforms to the ACI 318Building Code, Appendix D requirements. The PCIDesign Handbook – 6th Edition (PCI 2004) recog-nizes different types of failure modes associated with

a headed-stud anchorage depending upon the type ofedge condition, connection geometry, and the edgedistances in relation to the connection.

The front edge breakout mode was contained inprevious editions of the PCI Design Handbook, yethas been refined through this WJE/PCI research. Inaddition, the corner concrete breakout mode has beenfound to have a greater influence and revised con-nection capacity equations are presented. In the 6thEdition of the PCI Design Handbook, the concept of aside edge breakout has been introduced, as it was partof the WJE/PCI test program. Capacity equations fora connection adjacent to a side edge are presented forthe first time in this PCI Design Handbook.

Tension design of headed stud anchorages followsthe ACI Appendix D approach for the present. Thedesign equations were found by WJE to be goodrepresentations of headed stud behavior in tension.

REFERENCES

ACI Committee 318 –ACI 318. 2002. Building Code Require-ments for Structural Concrete (ACI 318-02) and Com-mentary (ACI 318R-02), American Concrete Institute,Farmington Hills, Michigan.

Anderson, N.S. & Meinheit, D.F. 2000. Design Criteria forHeaded Stud Groups in Shear: Part 1 – Steel Capacityand Back Edge Effects, PCI Journal, Vol. 45, No. 5,September/October, pp. 46–75.

Anderson, N.S. & Meinheit, D.F. 2005. Pryout Capacity ofCast-In Headed StudAnchors, PCI Journal,Vol. 50, No. 2,March/April, pp. 90–112.

Anderson, N.S. & Meinheit, D.F. 2007a. A Review ofthe Chapter 6 Headed Stud Design Criteria in the 6thEdition PCI Handbook, PCI Journal, Vol. 52, No. 1,January/February, pp. 84–112.

Anderson, N.S. & Meinheit, D.F. 2007b. Re-Evaluation ofPryout Breakout Capacity Design Provisions for HeadedStuds and Post-Installed Anchors Loaded in Shear, Pro-ceedings, 2nd International Symposium on Connectionsbetween Steel and Concrete, Universität Stuttgart, Ger-many (4–7 September 2007), Ed. by R. Eligehausen, et al.,ibidem-Verlag, Stuttgart, Germany, pp. 529–538.

Fuchs, W., Eligenhausen, R., & Breen, J.E. 1995. Con-crete Capacity Design (CCD) Approach for Fasteningto Concrete, ACI Structural Journal, Vol. 92, No. 1,January–February, pp. 73–94.

Prestressed Concrete Institute – PCI. 1971. PCI DesignHandbook, 1st Edition, Prestressed Concrete Institute,Chicago, Illinois.

Precast/Prestressed Concrete Institute – PCI. 1999. PCIDesign Handbook, 5th Edition, (PCI MNL 120-99),Precast/Prestressed Concrete Institute, Chicago, Illinois.

Precast/Prestressed Concrete Institute – PCI. 2004. PCIDesign Handbook, 6th Edition, (PCI MNL 120-04),Precast/Prestressed Concrete Institute, Chicago, Illinois.

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