Connection Design in the New AISC Manual

8/9/2019 Connection Design in the New AISC Manual

1/3STRUCTURE magazine November 2006

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Connection Design in the New AISC Manual

This is the rst in a two-part series of articles exploring the manychanges to the design of connections in the 2005 AISC Specication andits accompanying Manual of Steel Construction, 13 th Edition. In this rstarticle changes to the Specication are discussed. The second part willdiscuss changes to the Manual.

Design Philosophy (B3.3 and B3.4)The most sweeping change in the 2005 Specication is that it

incorporates both Load and Resistance Factor Design (LRFD)and what is now termed Allowable Strength Design (ASD). Byincorporating both design philosophies, the 2005 Specicationreplaces both the 9th Edition ASD and the 3rd Edition LRFD.The same nominal strengths are calculated regardless of whetherthe designer chooses to use LRFD or ASD. However, theresistance factors (factors of safety) are different to accommodatethe different load combinations. In LRFD, the nominal strength

is multiplied by a f factor, as was done in all previous LRFDspecications. In ASD, the nominal strength is multiplied by1/W. In all cases the ratio off to 1/W is 1.5. In most cases, 1/W

will be the inverse of the safety coefcients ASD designers areaccustomed to.

Bolts in Combination with Welds (J1.8)Signicant changes have been made in this item, which was

J1.9 in the 3 rd LRFD and J1.10 in the 9 th ASD. The changes arebased on recent research, which showed that sharing the loadbetween welds and slip critical (SC) bolts, which was previouslyallowed, can be unconservative.

The new J1.8 does not allow sharing of load between bolts

and welds in new work, except as follows: The bolts must beinstalled in standard holes or short slots perpendicular to theload, and the llet welds must be loaded longitudinally. Thebolts, which can be any bolt permitted in A3.3 (i.e. A307, A325,

A490 and others) shall have an available strength not greaterthan 50% of their available bearing strength. Note that the boltsneed not be considered to be slip critical, so A307 and othernon-pretensioned bolts can be used. Note also that, in all earliereditions, if slip critical bolts were used, holes size and orientation,and weld orientation, werenot limited.

In old work (alterations to existing structures), there is nochange from previous editions.

Minimum Loads (J1.7, 3rd

Edition)The 2005 Specication does not contain any limitations onminimum loads. The 3rd Edition LRFD, and the 2nd EditionLRFD Specied 10 kips in J1.7, and the 9th Edition ASD, 6kips in J1.6. These are not considered to be design requirementsand have therefore been eliminated. Requirements such as mini-mum loads, maximum slenderness ratios, and the old 50% rulefor truss internal members (9th Edition ASD J1.5) have beengradually eliminated because they are not design requirementsand have led to abuse (i.e. a strict interpretation of these require-ments without engineering judgment has led to expensive andunnecessary details.)

Compression Members withBearing Joints (J1.4)

Previous specications have required joints to be designed for 50% of the requcompressive strength of the member actintension. The 2005 Specication has a morealistic requirement as an alternative. Considtransverse load acting at the joint location

at the center of a truss compression chord panel. The transvload is taken as 2% of the required compressive strength ochord member. The chord between panel points is considto be pinned at the panel points, and the resulting shears moments at the joint location are used to design the splicthe bearing joint. This is a much more rational approach todesign. The old 50% rule is still allowed, if one wishes to u

Edge Distance (J3.10)This is not new in the 2005 Specication. It was new in

3rd Edition LRFD Manual Specication, but was not incluin the 9th Edition Manual Specication. Those accustomeusing the 9th Edition Manual are in for a shock, because the requirement, in most cases given by formula J3-6a as:

R n = 1.2LctFu Equation where, Lc is the clear edge distance

t is the plate or member thickness Fu is the tensile strength

can result in signicantly reduced capacities with the usual cof hole (Le) edge distances of 1: , 1 2 , and 1 3 inches. Tlimit state represented byEquation (1) is referred to as tear-obecause it involves a tearing out of a slug of material betwee

bolt hole and the edge of material. Equation (1) replaces9th Edition ASD and 2nd Edition LRFD requirement that whLe 1.5d, where d is the bolt diameter, edge distance is ndesign consideration for 2 or more bolts in line of force. necessity to consider edge distance in every case has a proeffect on connection design, particularly for bolts thateccentrically loaded.

Block Shear (J4.3)This has changed from all previous formulations due to re

research which has shown that failure always occurs withsion fracture in the tension net area, followed by shear yielon the gross shear area. But, to cover the cases where the net area is signicantly reduced from the gross shear area, the of the shear yielding and the shear fracture strengths is useconcise way to represent the new block shear requirement i

R n = UbsFu A nt + min{0.6Fy A gv ,0.6Fu A nv }Equation (2)

where, Fy is the yield strength Fu is the tensile strength A nt is the net tension area A gv is the gross shear area A nv is the net shear area Ubs is 1.0 when the tension stress is uniform,

0.5 otherwise

Part 1

By William A. Thornton and Larry S. Muir

Changes to the 2005 Specication



Equation (2) is a simplication of AISC 2005 formula J4.5. Ubs is ablock shear coefcient which is introduced to handle double columnsof bolts and other cases with larger than normal eccentricities.

Effective Throat of Partial-Joint-PenetrationGroove Welds (J2.1a)

Several changes have been made to the determination of theeffective throat for partial-joint-penetration groove welds in TableJ2.1. These changes primarily are concerned with the welding processand positions used.

Fillet Weld Directional Strength (J2.4)This was new in the 2nd Edition LRFD Manual and was also in the 3rd

Edition LRFD Manual. It has been in AWS D1.1 in ASD form sinceAWS D1.1-2000, so its use with the ASD 9th Edition ASD Manualwas possible when project specications included both AISC 9th Edi-tion and AWS D1.1-2000. All of the eccentrically loaded weld groupcharts in the 2nd and 3rd Edition LRFD Manuals in-clude the llet weld directional strength.

A llet weld loaded transversely has 50% morestrength than one loaded longitudinally. Unfortu-nately, the ductility of the transversely loaded lletweld is much less than the same llet longitudinallyloaded. Therefore, when transverse and longitu-dinal llets are in the same group, their strengthscannot be directly added. For eccentrically loadedgroups, this is addressed by the equations of J2.4(b).But for concentrically loaded llet groups, manyengineers and some AISC publications have sim-ply added the transverse and longitudinal lletstrengths. This is unconservative. The 2005 Speci-cation has prohibited this practice in J2.4(c) forgroups that include both transverse and longitudinalwelds by means of formulas J2-9a and J2-9b, whichcan be written as:

R n = max{R w l + R wt , 0.85 w l+1.5R wt}

Equation (3) where,R w l is the strength of the longitudinal welds

R wt is the strength of the transverse welds

R w l and R wt are calculated as 0.6FEXX A w l and0.6FEXX A wt , respectively, where FEXX is the electrodeclassication number (tensile strength) and A w isthe weld area.

Equation (3) allows the old method of using thesame llet weld strength for both the longitudinaland transverse welds without consideration ofductility. This is the rst term in the brackets. The

second term uses the increased transverse weldstrength, 1.5R wt , but recognizes that when thisstrength of the transverse weld is reached, only 85%of the longitudinal strength has been achieved.This result can be derived from the equations ofJ2.4(b), recognizing that for a concentrically loadedweld group the instantaneous center of rotation isat innity. Also Figure CJ2.13 of the Commentarycan be used to derive a formula similar to J2.9b.

For concentrically loaded cases containing longi-tudinal, transverse, and oblique welds, the formulasof J2.4(b) or the Figure CJ2.13 can be used.

Fillet Welds Minimum Size (J2.2b )Previous AISC Specications have always matched minimum

sizes to thethicker of the parts joined. The 2005 Specication chanthis to the thinner of the parts joined. This brings AISC into line w

AWS which has used the thinner part requirement for some time. Commentary gives some reasons for the change, which is primarilto the prevalence of low hydrogen electrodes.

Slip Critical Connections (J3.8)In past specications, all slip critical (SC) connections were des

to prevent slip at the service load level, whether they were designthe service load level (ASD) or the factored load level (LRFD). Thtor of safety against slip for these connections was generally 1.4 t

with respect to the service loads. Other modes of failure, such as ying, buckling, and fracture, have factors of safety of at least 1.7respect to the service loads. These factors of safety are the same w

ASD or LRFD methods are used for the design.continued on next



A D V E R T I S E M E N T F o r A d v e r t i s e r I n f o r m a t i o n , v i s i t w w w . S T R U C T U R E m a g . o

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From the above discussion, it is obvious that bolts in SC connectionswill slip before any other limit state is reached. In the case of long spanat trusses assembled with SC bolts in oversized holes or slots parallelto the load, excessive deections due to slip could cause ponding andultimately collapse. To rectify thus situation, the AISC 2005 Specica-tion introduces a limit state of slip at the required strength level. Thisyields a factor of safety of about 1.7 against service loads. The joint ina structure designed to this criterion will not slip until other failuremodes are also reached.

There are now two design levels for SC connections: (1) designagainst slip as a serviceability limit state, and (2) design against slip asa strength limit state. The latter is appropriate where oversize holes orslots parallel to the load are used, and slip into bearing could cause acatastrophic collapse.

Shear Lag Coefcients (D3)This was B3 in the 9th Edition ASD and the 3rd Edition LRFD. A

new table D3.1 is now provided that expands the number of cases con-sidered, and includes tubular member cases which were contained inthe HSS Specication. The HSS Specication has been incorporatedinto the AISC 2005 Specication and no longer stands alone. ChapterK of the 2005 Specication is dedicated to HSS Connections.

Column Stiffeners for Strong Axis

Moment Connections (J10.8)The 2005 AISC Specication requires that column stiffeners be at-

tached to the inside of the loaded ange for the difference betweenthe required strength and the available limit state strength. This is areturn to the practice of the 9th Edition ASD Manual. The 2nd and 3rd Edition LRFD Manuals both required that the stiffener be welded tothe inside of the loaded ange for the contact strength of the stiffener.The new requirement of J10.8, quoted above, will produce more eco-nomical designs.

Group 4 and 5 Shapes (A3.1c)This is not strictly a connection design issue, but ASTM A

longer identies W shapes by group. Therefore, the AISC 2Specication Section A3.1c replaces the requirements for Charpnotch testing for Group 4 and 5 members used as tension memor subject to tension due to exure and spliced with CJP welds, similar requirements for rolled shapes with anges having a thicexceeding 2 inches. The latest issue of ASTM A6 Supplemenprovides Charpy V-notch information that is now required.

Shear Yielding Limit State (J4.1, G2.1a)These two sections, the rst dealing with connections and the

ond dealing with members, return the specication to the histfactor of safety for shear in rolled beams and connection plates oThe 9 th Edition ASD has this factor of safety, but the interveningand 3rd Edition LRFD specications used a factor of safety of 1.6no reason other than academic purity. The factor of safety of 1.5stood the test of time since 1923.

Buckling of Short Members (J4.4)The 9 th Edition ASD Specication for column strength allow

factor of safety of 1.67 for very short members. The 2nd and 3rd EditiLRFD had a constant factor of safety of 1.5/0.85=1.76 for all colum

a much more conservative formulation. The 2005 Specicationthe 13 th Edition ASD/LRFD Manual allow the same factor of safethe 9th Edition ASD when kl/r 25, i.e. W =1.67. This is another sback to a time-honored factor of safety.

Proportions of Beams and Columns (F13)This was B10 in the 9th Edition ASD, and the 2nd and 3rd Editi

LRFD. The new requirement is based on research and is much meconomical than the old. The old requirement was not basedresearch but rather was a conservative rational approach to the eff

holes in tension anges of exural memFor A992 steel, the old requirement alloonly a 7.7% reduction in ange area bethe holes had an effect. The new requiremallows a 23% reduction in ange area beholes reduce the exural strength. This iobvious step in the direction of economy

Acknowledgement A paper and presentation by Bill

Thornton and Larry Kloiber for the2006 SEI Conference in St. Louis, MO

is the original basis of this article.

William A. Thornton is a CorporateConsultant for Cives Steel Company

Roswell, GA. Larry S. Muir is the Presof Cives Engineering Corporation.

Part 2 of this series will explore howthe changes in the 13 th Edition Manual

will affect connection design. It will alssummarize the economic effects of thechanges in both the 2005 Specication

and the 13 th Edition Manual.

Part 2

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Connection Design in the New AISC Manual

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