AISC DESIGN GUIDE 8-1996 Partially Restrained Composite Connections.pdf

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1、Steel Design Guide SeriesPartially RestrainedComposite ConnectionsSteel Design Guide SeriesPartially RestrainedCompositeConnectionsA Design GuideRoberto T. LeonGeorgia Institute of TechnologyAtlanta, GeorgiaJerod J. HoffmanMeyer, Borgman and Johnson, Inc.Minneapolis, MinnesotaTony Staeger, RE.Hammel

2、 Green Larry Kloiber of LeJeune Steel provided input particularly in the practicalfabrication aspects of the connection; Dave Galey, Zina Dvoskin, and Johanna Harris of HGAs StructuralEngineering Department who helped developed the first draft of this guide and provided invaluable input andassistanc

3、e throughout the project; Bob Lorenz, Director of Education and Training, and Nestor Iwankiw, VicePresident of Technology and Research for AISC, whose patience and support made this document possible.The information presented in this publication has been prepared in accordance with recognized engine

4、eringprinciples and is for general information only. While it is believed to be accurate, this information should not beused or relied upon for any specific application without competent professional examination and verification ofits accuracy, suitability, and applicability by a licensed profession

5、al engineer, designer, or architect. Thepublication of the material contained herein is not intended as a representation or warranty on the pan of theAmerican Institute of Steel Construction, Inc. or the American Iron and Steel Institute, or of any other personnamed herein, that this information is

6、suitable for any general or particular use or of freedom infringement of anypatent or patents. Anyone making use of this information assumes all liability arising from such use. 2003 by American Institute of Steel Construction, Inc. All rights reserved.This publication or any part thereof must not b

7、e reproduced in any form without permission of the publisher.Part IBACKGROUND1. INTRODUCTIONPartially restrained connections, referred to as PR connec-tions in the LRFD provisions1and Type 3 connections in theASD provisions,2have been permitted by the AISC Specifi-cations since 1949. With some notab

8、le exceptions, however,this type of connection has not received widespread applica-tion in practice due both to (a) the perceived complexity ofanalysis required, and (b) the lack of reliable information onthe moment-rotation characteristics of the connections asrequired by design specifications. The

9、 notable exceptionsinvolve specific types of connections that have been demon-strated, through experience in the field and extensive analyti-cal work,3,4to provide equivalent response under designconditions to that of rigid connections. The Type 2 or “wind“connections allowed under the ASD provision

10、s are a goodexample of this approach. In these cases the specificationessentially prequalifies a simple connection under gravityloads as a rigid connection under lateral loads. In reality, ofcourse, these connections are neither fully rigid (FR) norsimple but partially restrained (PR). The code uses

11、 this arti-fice to simplify the analysis and design, but requires a guar-anteed rotational and strength capacity from these connec-tions.After 10 years of research and development a new type ofsemi-rigid connection, labelled the Partially Restrained Com-posite Connection or PR-CC,* can be added to t

12、his list.5-12Theword “composite“ is used to indicate that this connectionengages the reinforcing steel in the concrete slab to form thetop portion of the moment resisting mechanism under bothlive loads and additional dead loads applied after the end ofconstruction (Figure 1). The bottom portion is t

13、ypically pro-vided by a steel seat angle with web angles providing theshear resistance. This connection may be used to economizebeam sizes for gravity loading or to resist lateral loads inunbraced frames. The design of this type of system is basednot only on the work of the senior author at the Univ

14、ersity ofMinnesota,5-12,21but also on that of many researchers through-out the U.S. and Europe.11,13-19The extensive experimentalwork required in the development of these connections isdiscussed elsewhere569and will not be repeated here.Part I of this design guide is organized as follows. First,some

15、 discussion of partially restrained connection behaviorThe label PR-CC is meant to encompass the connections previously labelled semi-rigid composite connections (SRCC) by the senior author.1Fig. 1. Partially restrained composite connection (PR-CC).will be given to put PR-CC design in its proper con

16、text.Second, the advantages and limitations of PR-CCs are dis-cussed in the context of simplified or code-oriented design.Third, the assumptions and theory applied in their design aredescribed. Fourth, detail recommendations for the connec-tions under both gravity and lateral loads are given. In Par

17、t IIa step-by-step procedure is presented in outline form followedby corresponding detailed calculations for an example prob-lem in Part III. The 1993 Load and Resistance Factor Design(LRFD) Specification1is used in the design and ASCE 7-9320is used for load determination. Tables and design aids are

18、included in Part IV to facilitate the design.2. CHARACTERIZATION OF CONNECTIONBEHAVIORThe behavior of structural connections can be visualized fordesign purposes with the aid of moment-rotation curves(Figure 2). These curves are generally taken directly fromindividual tests or derived by best-fit te

19、chniques from theresults of multiple tests.22,23All design specifications requirethat the structural engineer have a reliable curve for thePR connections to be used in design since such curves syn-* 2003 by American Institute of Steel Construction, Inc. All rights reserved.This publication or any pa

20、rt thereof must not be reproduced in any form without permission of the publisher.the size the connections main characteristics: stiffness,strength, and ductility.6The application of PR-CCs to designimplies that reliable relationships have been developedand are simple enough to use in design. The eq

21、uationsdeveloped for SRCCs will be discussed in detail in Section 4.In Figure 2(a), the stiffness of the connection correspondsto the slope of the curve. For most connections, such asPR-CCs, the slope changes continuously as the moment in-creases. The real stiffness of the connection at any stage of

22、the curve corresponds to the tangent stiffnessHowever, for design purposes it is customary toassume a linear approximation for the service rangegenerally in the form of a secant stiffnessThis stiffness is generally less than the initial stiffness of theconnections (Ki), and corresponds closely to th

23、e unloadingstiffness (Kunloading).Based on the initial (Kior service stiffness (Kconn), connec-tions can be classified as fully restrained (FR), partiallyrestrained (PR) or simple depending on the degree of restraintprovided (Figure 2(b). The current approach in design is toassume that for members f

24、raming into relatively rigid sup-ports, if the connection stiffness is about 25 times that of thegirder (i.e, 25), the connection can be consid-ered rigid. Conversely, if the connection provides a stiffnessless than 0.5 times that of the girder, then it should beconsidered simple.* The classificatio

25、n by stiffness is validonly for the service load range and for connections which donot exhibit significant non-linear behavior atInsofar as strength is concerned, joints can be classifiedeither as full strength (FS) when they are capable of transfer-ring the full moment capacity of the steel beam fr

26、aming intothem or as partial strength (PS) when they are not (Figure2(b). The schematic moment-rotation curve for a PR-CCshown in Figure 2(b) does not reach the full capacity, andthus is a partial strength connection. Partial strength is desir-able in seismic design because it permits a calculation

27、of themaximum forces that a structural element will be required towithstand under the uncertain ground motions that serve asan input. If the designer knows what is the maximum momentthat a connection can transmit, he/she can insure that otherkey elements, columns for example, remain elastic and suff

28、erno damage even when the seismic input far exceeds the codeprescribed forces. This design philosophy, known as capacitydesign,24is employed in this design guide. Capacity designrequires that any hinging region be carefully detailed todissipate energy and that all other elements in the structurerema

29、in basically elastic when the maximum plastic capacityof these regions is reached. Following this design philosophy,the detailing of the PR-CCs is driven by the need to providea stable, ductile yielding mechanism such as tension yieldingof the angle legs rather than a sudden, brittle failure such as

30、bolt shearing.Ductility is required in structural design so that somemoment redistribution can occur before the connection fails.In applications for unbraced frames, and particularly if seis-mic loads are important, large ductilities are required. Duc-tilities can be defined in relative terms or ult

31、imaterotation capacity divided by a nominal yield one, see Figure2(a) or in absolute terms 0.05 radians, for example).The required ductilities are a function of the structural systembeing used and whether large cyclic loads need to be consid-ered in the design. In general cyclic ductilities greater

32、than 6(relative ductility) or 0.035 radians (absolute ductility) aredesirable for frames with PR-CCs designed in areas of low tomoderate seismic risk. Demands in unbraced frames for areaswhere wind governs the design or for braced frames are lower.The values of 25 and 0.5 selected here were chosen a

33、rbitrarily; ranges from 18 to 25 for the FR limit and 0.2 to 2 for the simple limit are found in the literature. The selection ofspecific values is beyond the scope of this guide. These values are cited only for illustrative purposes.Fig. 2. Characterization of connection behavior.2* 2003 by America

34、n Institute of Steel Construction, Inc. All rights reserved.This publication or any part thereof must not be reproduced in any form without permission of the publisher.The PR-CCs described in this guide meet the criteria for areasof low to moderate seismic risk and can be used for the otherdesign co

35、nditions described above.It is important to recognize at the outset that for designpurposes an exact, non-linear moment-rotation curve such asthose shown in Figure 2 may not be necessary. In fact, onlytwo important points need to be known for design. The firstcorresponds to the serviceability level

36、where the stiffness,Kconn, must be known for deflection and drift calculations. Thesecond point is the ultimate strength (Mult) and rotationachievable by the connection to insure that adequate plasticredistribution of stresses can occur.3. ADVANTAGES AND LIMITATIONSThere are several practical advant

37、ages to PR-CCs. By usingreinforcing in the slab the need for a top angle or top plate iseliminated. This provides a more economical solution forseveral reasons:(a) The top force and moment arm are increased resultingin either (1) a reduction of the forces in the connectionfor a given design moment,

38、or (2) an increase in theconnection moment capacity. The difference in strengthcan be substantial because the ultimate capacity of aseat angle in tension is only about one-third of itscapacity in compression (area of its leg times its yieldstress). Thus an A 36 -in. top angle 8-in. wide (totalforce

39、= 8 x 0.5 x 36 x 0.33 = 48 kips) can be replacedwith four #4 Grade 60 reinforcing bars (total force = 0.2x 4 x 60 = 48 kips). The capacity of the connection canthen be controlled by the amount of steel in the slab. Inaddition, in a floor system with shallow beams (sayW14s or W16s) the increase in mo

40、ment arm (Y3) canadd 20 to 25 percent additional capacity.(b) In gravity design PR connections result in an efficientincrease of the end moments. For a composite section,the strength in positive bending is typically on the orderof 1.8 times that of the steel beam alone (Mp). Under auniformly distrib

41、uted load, if simple connections areused, the structural efficiency of the system is lowbecause the large capacity of the system is required onlyat the centerline; most of the section strength is wasted.Similarly, if rigid connections are used the efficiencyof the composite system is considerably re

42、duced be-cause the end moments (wL2/12) are large where thesection strength is small (Mp), and the midspan mo-ments are small (wL2/24) are small where the sectionstrength is large (1.8Mp). Only the use of semi-rigidconnections and composite action allows the designerto “balance“ the connection such

43、that the demand (ex-ternal moment) is balanced by the supply (section ca-pacity).(c) The use of PR-CCs reduces the required beam sizeand/or reduces deflection and vibration problems be-cause of the composite action provided by the slab. Theuse of reinforcing bars, as opposed to the common steelmesh

44、used for temperature and shrinkage crack control,is neceesary to achieve these benefits. The use of dis-tributed steel reinforcing bars around the columns con-siderably reduces crack widths over beam and columnlines.(d) From the construction standpoint the need to cut andresupport the steel decking

45、around the support is elimi-nated. The placement of some additional reinforcingbars in the slab should not represent significant addi-tional costs.Connection research on PR frames until recently consideredonly bending about the strong axis of wide flange columns.In this guide some preliminary recomm

46、endations for extend-ing their use to the weak axis of columns in braced frames aregiven. When used on the weak axis the web angles aretypically not used and the connection strength is reducedslightly. In general a stiffened seat is used to help carry theshear force in this situation.Because of its

47、increased flexibility relative to rigid (Type1 or FR) connections, the system is most applicable in struc-tures that are ten stories or less, and it should be limited to usewith lateral wind forces or seismic loading with groundaccelerations less than or equal to 0.2g only, pending furtherresearch.I

48、t should also be clear that PR-CCs cannot, in general, beused as substitutes for rigid connections on a one-to-one basis.This implies that more connections will have to participate inresisting the lateral loads in a SRCC frame. The key to theeconomy of the system is that it allows the designer to tu

49、rnsimple connections into semi-rigid ones by adding only slabsteel. The latter is inexpensive and is already being used bymany designers to control cracking over column lines. Thusthe additional costs for material and labor will be small. Thegains in structural efficiency and redundancy will far out-weigh the additional construction costs. The recent experi-ence with the Northridge earthquake clearly points out theneed for redundancy and ductility in steel lateral load resistingsystems. PR-CCs clearly provide a superior level of perform-ance in this respect and can be adopted as a s