Specially Funded R&D Program

PCISFRAD Projects 1 arid 4

Summary Paper

Moment ResistantConnections and

Simple Connections

I'Charles W. DolanVice PresidentABAM Engineers Inc.Federal Way. Washington(on leave at Cornell University)

John F. StantonAssociate ProfessorUniversity of WashingtonSeattle. Washington

Richard G. AndersonManaging DirectorConcrete Technology AssociatesTacoma, Washington

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CONTENTS1. Research Objectives and Background ............... 64

2. Moment Resisting Connections—Descriptionand Findings .......................... 65

3. Simple Connections—Descriptionand Findings .......................... 69

4. Frame Test ...................................... 72

5. Conclusions and Recommendations ................ 72— Moment Resisting Connections— Simple Connections— General

6. Closure ......................................... 74

References.........................................74

Note: This paper is a condensation of PCI-SFRAD Project No. 1/4, Moment ResistantConnections and Simple Connections. The fullreport is available from PCI Headquarters at$20.00 to firms supporting the sponsored re-search. $30.00 to PCf Members (non-support-ing firms), and $60.00 to non-PCI Members.

The summary paper, and the full report, arebased on a research project supported by thePCt Specially Funded Research and Develop-ment (PCISFRAD) Program. The conduct of the

research and the preparation of the final reportsfor each of the PCISFRAD projects were per-formed under the general guidance and direc-tion of selected industry Steering Committees,However, it should be recognized that the re-search conclusions and recommendations arethose of the researchers. The results of the re-search are made available to producers, en-gineers and others to use with appropriate en-gineering judgment similar to that applied to anynew technical information.

PCI JOURNAL/March-April 1987 63

The successful structural performanceof precast concrete systems depends

upon the connection behavior, Theconfiguration of the connections affectsthe constructibility, stability, strength,flexibility and residual forces in thestructure. Furthermore, connectionsplay an important role in the dissipationof energy and redistribution of loads asthe structure is loaded.

The PCI Specially Funded Researchand Development Programs 1 and 4(PCI 1/4) focused on the actual behaviorof commonly used connections. The twoprograms were combined in order to de-

vote maximum effort to the physicaltesting of connections in common use.PCI 1/4 consisted of individual tests ofeight simple connections, eight momentresisting connections and one momentresisting frame test.

This condensed report summarizesthe test program, describes the testspecimens, and presents the basic find-ings and conclusions reached during theinvestigation. The Research Report'contains a complete description ofthe research program, as well as de-tailed descriptions of the individualtests.

1. RESEARCH OBJECTIVES AND BACKGROUND

Simple connections are designed toresist or transmit shear and axial loads.The Project Steering Committee se-lected the simple connections for thisprogram. The study team reviewed theprevailing design approaches for eachconnection, formulated a design capac-ity based on the nominal specified mate-rial strengths, and predicted capacitiesbased on the actual measured materialproperties. One test was performed oneach connection. The results of the testswere compared to the predictions andconclusions were drawn.

Moment resisting connections are es-sential to develop frame action in pre-cast buildings. The connections mustdevelop sufficient strength to resist theapplied loads and must have sufficientstiffness to limit the sidesway of thestructure. The Project Steering Com-mittee selected the moment resistingconnections for investigation based onthe experience ofthe members.

In this study, connections wereexamined for structural performance, asmeasured by load and deflection be-havior, and for cost effectiveness andconstructibility. Emphasis was placedon the behavior of the connection sub-jected to gravity loading and lateral

loading due to wind or equivalent seis-mic loading. Although rigorous seismicanalyses were not performed, several ofthe connections were subjected to mod-erate cyclic loadings in order to observetheir behavior.

A two-bay by two-story moment re-sisting frame was constructed. Theframe included several moment resist-ing connections similar to those testedindividually. The frame was tested as ageneral confirmation of the findings ofthe individual tests and offered an op-portunity to refine some of the detailsand recommendations derived from theindividual tests.

The moment resisting frame designmethodology used to define the testloadings and some of the connectiondetailing included a review of the rec-ommendations of the 1985 UniformBuilding Code (UBC). 2 This design ap-proach was influenced by the consid-eration that the implementation of mo-ment resisting connections may be sub-jected to a building department reviewand that UBC is a widely recognized ac-ceptance standard.

Many structures subjected to lateralloadings from wind or seismic forces aredesigned to resist equivalent static

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loads. Analyses are performed usinglinear elastic methods and the resultingeffects are factored to obtain strengthdesign values. The equivalent staticloads are specified in codes, such asUBC, and are predicated on the as-sumption that the structure will possessan assumed level of ductility roughlyequivalent to that of a well detailedmonolithic structure.

Test specimens used in this program

were not detailed for the specific con-finement criteria of UBC Seismic Zone3 as it was felt that many of the connec-tions would be used for construction inZones 1 and 2. Thus, these test resultswould be applicable for these locations.Consequently, some specimens failed atloads slightly lower than those whichmight have been reached if details suchas closer stirrup spacing and 135 degreehooks were used.

2. MOMENT RESISTING CONNECTIONS-DESCRIPTION AND FINDINGS

Eight basic moment resisting connec-tions were studied. They are defined inthe following paragraphs and summar-ized in Fig. 1. The test results are sum-marized in Table 1. Tabular results aregiven for Specimens BC28 and BC29even though they represent the samebasic connection type.

Connection BC-15 is a classicalwelded plate connection. In this con-nection, a steel plate is welded to theprincipal longitudinal reinforcement. Acompanion plate is cast into the adjacentprecast member, in this case a columnelement. A closure plate is field weldedbetween the two embedded plates tocomplete the connection.

Under negative moment, the connec-tion failed when one of the bars frac-tured at the end of the weld and whenthe field welds failed. Failure was a re-sult of direct tension and the prying ac-tion of the beam end rotating about theouter edge of the column corbel (seeFig. 2). Detailed designs which reducethe eccentricity of the load path throughthe connection and which minimize theprying due to the corbel should furtherimprove the connection performance.

The positive moment capacity of theconnection reached a value comparableto the negative moment design value.The connection displayed limitedenergy dissipation under cyclic loading.

Connection BC16A obtains its mo-ment capacity by using continuous re-inforcement cast into a composite top-ping. The moment resistance of thisconnection was considerably greaterthan predicted. In addition, the connec-tion displayed considerable ductility.While the connection displayedstrength and ductility, the energy dissi-pation during cyclic loading was low.The strength of the connection was ad-versely affected by the prying action ofthe beam rotating about the corbel.

Connections BC25 and CC1 arebeam-to-column and column-to-columnconnections. Each connection is madeby bolts between end plates cast into thecolumn elements. Two different con-nection configurations were tested.Specimen BC25 was loaded with a 300kip (1330 x 10 1 N) axial load and highstrength bolts provided moment resis-tance. Specimen CC1 was loaded with a40 kip (178 x 10 3 N) axial load and usedASTM A-307 anchor bolts.

Both specimens were loaded cyclicly.Specimen CC1 did not fail after threefull cycles. Specimen BC25 failed on thethird cycle by bursting of the ties andbuckling of the reinforcement. Bothconnections exceeded their predictedcapacity and, in the case of SpecimenBC25, by a factor of nearly two. Neitherconnection dissipated much energy

PCI JOURNAL/March-April 1987 65

CONNECTION DESCRIPTION

s^BC15 BEAM TO COLUMN CONNECTIONS

USING WELDED PLATES FOR THEPOSITIVE AND THE NEGATIVE CON-NECTIONS. r------

BC16A BEAM TO COLUMN CONNECTIONUSING CONTINUOUS REINFORCINGTHROUGH THE COLUMN ANDCAST-IN-PLACE TOPPING. POSI-TIVE MOMENT CONNECTION USESWELDED PLATES._

BC25 & CC1 A COLUMN BASE DETAIL MADECONTINUOUS BY BOLTING. BC25IS FOR EXTENDING COLUMNSTHROUGH LARGE BEAMS. CC1 ISA COLUMN-TO-COLUMN EXTEN-StON. t—

- sS

BC26 A PRECAST BEAM CONSTRUCTEDINTO A CAST-IN-PLACE COLUMN.

BC27 A PRECAST BEAM POST-TEN-SIONED TO A COLUMN OR OTHERFRAME MEMBER.

Fig. 1. Moment connections.

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A PRECAST BEAM MADE CONTINGUS USING DYWIDAG THREADBARS SCREWED INTO COUPICAST IN THE COLUMN

BC99

CONNECTION DESCRIPTION

BC28 & BC29 A PRECAST BEAM INSTALLED ONA GROUTED OR PARTIALLYGROUTED DOWEL.

Fig. 1 (cant.). Moment connections.

during cyclic loading.The bolt capacity limited the strength

of the connection and bolt yielding ac-counted for almost all of the energy dis-sipation. Specimen BC25 could developthe full bolt capacity if additional tieconfinement was used. The suddenbuckling failure of the bars and the pos-sible implications for frame collapsesuggest that this connection may not bereliable as the principal moment resist-ing element. A connection locatedcloser to the point of inflection, ratherthan the point of maximum moment, maybe more appropriate and 135 degreehooks should be used on ties near theconnection plates.

The bolts in Specimen CC1 deformedplastically during the first loading. Sub-sequent load cycles displayed littlemoment resistance until the anchorplates regained contact with the elon-gated bolts.

Connection BC26 simulates a cast-in-place connection. It is awkward to

construct since it requires shoring, yetits similarity to cast-in-place construc-tion makes it the logical benchmark forstrength and ductility comparisons.

The specimen was tested monotoni-cally. Its strength and ductility ex-ceeded predictions. The specimenfailed when shear ties pulled out_ In thiscase, stirrup confinement using 135 de-gree anchorage provisions could haveadded capacity and ductility to the con-nection. The fact that the flexuralstrength at failure exceeded the pre-dicted strength is due to strain harden-ing of the reinforcement. When strainhardening effects are included, the pre-dicted strength agrees well with the ul-timate strength.

Connection BC27 is a fully post-ten-sioned connection. The specimen waspost-tensioned with strand in eachcorner. The specimen displayed goodductility and the strength slightly ex-ceeded the predictions. Failure oc-curred by fracture of one of the strands.

PCI JOURNAL/March-April 1987 67

Table 1, Results of moment resisting connection tests.

PositiveNegative moment moment

Maximum rotation

Design Predicted Measured Measuredmoment capacity capacity capacity Negative Positive

Connection (kip-in.) (kip-in,) (kip-in.) (kip-in.) (percent) (percent)

BC15 1428 1904 2I85 1450 3.7 1.20BC I6A 1428 1904 3500 1218 10.0 4.00BC25 1587 2488 4535 4228 4.0 4.00CC1 1020 1632 1523 1575 4.0 3.65BC26 1428 1904 3100 No test 12.5 No testBC27 2086 2575 2388 No test 8.3 No testBC28* 291 399 576 No test 4.1 No testBC29* 291 399 540 318 3.9 3.75BC99 1428 1904 2186 768 4.0 2,90

" The self weight of the beam is 164 kip-in. oar 51 percent of the reported capacity.Metric (SI) conversion factor. 1 kip-in. = 113 N•in.

Post-tensioning provided good initialstiffness, equivalent to that of uncrackedconcrete with a modulus of elasticity of7400 ksi (51,000 MPa). This stiffnessremained effective until the initialpost-tensioning force was overcome.

Connections BC28 and BC29 are

Fig. 2. Connection BC15.

dowel connections which are designedto eliminate field welding. The totalmoment capacity of these connectionswas comparable to the design value.The low magnitude of the achievablemoment and the large deformations re-quired to develop this moment makesthem unlikely choices as principalsources of moment resistance for frameaction. They do provide some resistanceand may he useful in some situations.

Connection BC99 is designed to ob-tain full moment resisting capacity byusing mechanical couplers in lieu offield welding. A cast-in-place closure isrequired to complete the connection.Field reports and experience buildingthe test specimen indicated that it canhe constructed quite easily.

The test specimen failed by suddenrupture of the connectors at approxi-mately the predicted capacity (see Fig.3). Investigation of the specimen deter-mined that the couplers were manufac-tured to develop only the specified yieldstrength of the bars. Thus, the couplershad no reserve capacity nor did theydisplay any ductility. The coupler man-ufacturer makes a coupler which can

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Fig. 3. Specimen BC99 —top couplers at failure.

develop the ultimate capacity of the bar.The high strength coupler was used inthe frame test of this connection and theconnection behaved well.

The availability of two similar cou-

piers indicates a potential quality con-trol problem. The sudden loss of ductil-ity with the yield strength couplersmakes them undesirable for use in mo-ment resisting frames.

3. SIMPLE CONNECTIONS - DESCRIPTION AND FINDINGS

The eight simple connections are de-scribed in the following paragraphs andare shown in Fig. 4. A summary of thedesign, predicted and measured capac-ity, is given in Table 2. Where appropri-ate, the recommendations of the PCIDesign Handbook' s were used to designand detail the connections. Thus, someverification of the Design Handbookwas obtained.

Connections SB1 and SB2 are de-signed as the top and bottom connec-tions to resist end torsion in a spandrelbeam. The estimated force in these con-nections is 5 to 15 kips (22 x 103 to 67 x103 N) based on a range of assumptionsregarding the length of the spandrelbeam and the eccentricity of the beambearings. The connections consist of

plates or angles east into the precastelements and a closure plate or anglewhich is field welded to the embeddedplates.

Both connections performed in excessof the design requirements. The use ofan angle for the closure plate proved tobe resilient and appeared to be less sen-sitive to construction tolerances.

Connection CB2 consists of a series oftests on coil rod inserts. The pullout re-sistance of the five tests varied consid-erably and the pullout strengths weregenerally less than the capacities pre-dicted using design methods outlined inthe third edition of the PCI DesignHandbook. (Note that the second edi-tion of the PCI Design Handbook tendsto overestimate the strength of these

PCI JOURNAL March-April 1987 69

CONNECTION DESCRIPTION

SS2/WW6 SHEAR CONNECTOR FOR WALLSOR FLOOR SLABS.

W W3 SHEAR CONNECTOR FOR WALL PANELS,.

WW7 BOLTED SHEAR CONNECTOR

Fig. 4. Simple connections.

connections.) The lower strengths mayhave been partly due to bending in thetest panel, however, these inserts areoften used in such panels. The testssupported the manufacturers' recom-mendations that factors of safety of fouragainst pullout (ultimate strength) areentirely appropriate.

Connection CB3 is an embeddedshear connector designed to resist grav-ity or other in-plane shearing loads. Theconnector consists of reinforcement

welded to an angle which is cast into athin panel. The connector exhibitedgood ductility and strength. Failure oc-curred by fracture of one of the weldedbars and was preceded by extensivecracking of the panel.

Connections SS2, WW6, WW7 arein-plane shear connectors. The testspecimen consisted of connector platesfield welded or bolted to inserts castinto the precast panels. All of thewelded connectors exceeded the

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CONNECTION DESCRIPTION

SB1 WELDED PLATE CONNECTION TORESIST TWISTING OF A SPANDRELBEAM.

SB2 WELDED ANGLE CONNECTION TORESIST TWISTING OF A SPANDRELBEAM.

SB1 PLAN SECTION S62

CB2 COIL ROD INSERT

J=L J^-L

CB3 THIN WALL PANEL GRAVITY LOADCONNECTOR

Fig. 4 (cont.). Simple connections.

strength requirements for the connec-tion. The bolted connections were un-able to develop design strengths.

Several plate configurations wereexamined to determine if the ductility ofthe connector could he improved. It isdesirable to have the failure occur in theconnector plate material rather than inthe weld. At the same time, it is some-times desirable to reduce the stiffness ofthe connection perpendicular to theshear plane in order to limit the forces

due to shrinkage and thermal effects. Ofthe various alternatives, a flat bar in-clined to the shear plane exhibited sub-stantial promise.

Connection WW3 is an angle assem-bly used to erect precast panels and toprovide shear resistance across a hori-zontal joint between the panel and itsfoundation. The connection proved tohe quite ductile, tough and predictable.Even after the failure of some of thestuds it was able to retain a significant

PCI JOURNAL/March-April 1987 71

Table 2. Results of simple connection tests.

Connection

Designcapacity

(kips)

Predictedcapacity

(kips)

Measuredcapacity

(kips) DuctileSB1 5.0-15.0* 20.6 NoSB2 5.0-15.0* 58.8 Yes, did not tailCB2t 5.0 10,5 9.5+ NoCB2c 6.0 17.5 12.7- NoCB3 9.0 24.0 30.8 YesSS2 10.0-20.0* 22.0-27.0 31.9t No, solid platesWW6 10.0-20.0* 22.0-27.0 31.9t Yes, stress relievedWW3 10.0-20.0* 48.0 50.1 YesWW7b 7.6-12.5 3.0 Yes, slipped on boltsWFV7w 23.8 28.0 44.0 Yes, when welded

Nominal design range, no specific design value was developed.+ Average of test results reported.Metric (SI) conversion factor: 1 kip = 4448 N.

residual load. Because the studs lie-outside the panel reinforcement, the

use on long studs is vital to the connec-tion integrity.

4. FRAME TEST

A two-story, two-bay test frame wasconstructed using several different mo-ment resisting connections. The nega-tive moment connections all performedwell and the frame was able to sustainthree cycles of lateral drift of ±4 per-

cent. The frame was loaded to a lateraldeformation of 8 percent of the totalstory height before experiencing a sig-nificant loss of lateral resistance. Severalpositive moment connections failed asthe drift exceeded 4 percent,

5. CONCLUSIONS AND RECOMMENDATIONS

Conclusions and recommendationsare given for moment resisting connec-tions and simple connections.

Moment Resisting Connections

Connections BC15, 13C16A, BC25,CC1, BC26, BC27 and BC99 all deveI-oped strengths at least equal to the pre-dicted values based on their nominalstrengths and could be consideredstrong enough for their intended use.The available ductility varied consider-

ably among specimens. Only Connec-tion BC-26 exhibited as much energydissipation as would have been ex-pected from a well detailed monolithicjoint. Several connections indicated thatredesign would improve the ductilityperformance.

All connections which rely on corbelsas part of their erection or load carryingsystems will be subjected to prying ac-tions during Iarge deformations. De-signs which minimize the leverage cre-ated by the corbel should behave better.

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The tension load path through anumber of connections (such as Con-nection BC15 were intricate, leadingto important deformations which are notnormally considered in the design pro-cess. Connections which minimize theIoad path eccentricities should providebetter and more predictable behavior.The number of times that the load istransferred within the connectionshould also be minimized, since eachtransfer introduces additional possibil-ities for a failure.

Some of the connections containedless positive than negative moment re-inforcement, leading to lower flexuralstrength in the positive direction. Whilethis may be appropriate for gravityloadings, it may not be satisfactory fordeveloping frame action in the presenceof reversible cyclic loading. This isespecially true for connections whichare not precompressed by the additionof the dead loads.

Simple ConnectionsThe intended purposes of the simple

connections were quite diverse; hence,evaluation of their effectiveness must beon a case by case basis. Furthermore,the combinations of loads for which thecombinations are designed vary withapplication. Within these constraintsall the simple connections, with theexceptions of Connections SS2 andSB2, provided a reasonable level ofstrength.

Some of the specimens, such as WW3,deformed in ways which were not an-ticipated, emphasizing the need forthorough testing of standardized con-nections.

Connections such as WW6 and WW7may need strength in one direction andflexibility to accommodate shrinkage inthe other direction. To achieve thiscombination of behavior requires verycareful design, especially if the place-ment of the elements of the connectionalso has to allow for fabrication and

erection tolerances. Suggestions to meetthese goals are provided and some sug-gested details were tested.

It is generally not feasible to dissipateenough energy in the connections forthem to be considered the sole source ofdamping under reversible cyclic load-ing. However, connections which forcea significant inelastic deformation intothe member, such as CB3, or whichprovide substantial shear friction re-sistance, such as WW3, provide asignificant potential for energy dis-sipation.

The coil rod inserts of Specimen CB2failed at less than the predicted loads.However, further study is needed on theeffects of panel proportions and the ef-fects of other stress conditions in thepanels.

GeneralThe test results clearly showed the

need for imaginative design and detail-ing, i.e., not sensitive to minor changesin fabrication and erection, and the needfor good quality control. Some specificexamples are cited below.

The use of weldable reinforcement(ASTM A706) and the appropriate weldmaterials are essential for ductility.Even when weldable reinforcement isused, load eccentricities should beeliminated where possible. For exam-ple, bars in Specimen BC15 fractured inthe heat affected zone at the end of theweld with almost no necking. This loca-tion was subjected to severe kinkingcaused by eccentric load paths.

The original BC99 tests failed in abrittle manner because the highstrength couplers were not installed, Itis recommended that only full strengthcouplers be used for this connection.

Some weld plate designs may be pre-dicted on the plate yielding before thewelds fail. Overstrength plate steelcould then result in welds failing first,sometimes in a brittle fashion, when aductile failure was anticipated.

PCI JOURNAL/March-April 1987 73

6. CLOSURE

Since only one connection of eachtype was tested, it is imprudent to drawfurther conclusions at this summarylevel.

The detailed Research Report'contains considerably more test data and

interpretation of the behavior of theconnections. The authors feel that thedata is valuable to the design profession,the precast fabricator and to the buildingofficial reviewing precast concretestructures.

REFERENCES1. Stanton, J. F., Anderson, R. G„ Dolan,

C. W., and McCleary, D. E., "Moment Re-sistant Connections and Simple Connec-tions," PCI Specially Funded Researchand Development Program—ResearchProject No. 1/4. Prestressed Concrete In-stitute, Chicago, Illinois, 1886, 436 pp.

2. Uniform Building Code (1985 Edition),International Conference of Building Of-ficials (ICBO), Whittier, California

3. PCI Design Handbook - Precast and Pre-stressed Concrete, Third Edition, Pre-stressed Concrete Institute, Chicago, Il-linois, 1985,

NOTE: Discussion of this report is invited. Please submit yourcomments to PCI Headquarters by December 1, 1987.

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