Revision and Errata List, March 1, 2003 AISC Design Guide 9: Torsional Analysis of Structural Steel Members The following editorial corrections have been made in the First Printing, 1997. To facilitate the incorporation of these corrections, this booklet has been constructed using copies of the revised pages, with corrections noted. The user may find it convenient in some cases to hand-write a correction; in others, a cut-and-paste approach may be more efficient.
Transcript
Revision and Errata List, March 1, 2003
AISC Design Guide 9: Torsional Analysis of Structural Steel Members
The following editorial corrections have been made in the First Printing, 1997. To facilitate the incorporation of these corrections, this booklet has been constructed using copies of the revised pages, with corrections noted. The user may find it convenient in some cases to hand-write a correction; in others, a cut-and-paste approach may be more efficient.
2.3 Avoiding and Minimizing TorsionThe commonly used structural shapes offer relatively poorresistance to torsion. Hence, it is best to avoid torsion bydetailing the loads and reactions to act through the shearcenter of the member. However, in some instances, this maynot always be possible. AISC (1994) offers several sugges-tions for eliminating torsion; see pages 2-40 through 2-42. Forexample, rigid facade elements spanning between floors (theweight of which would otherwise induce torsional loading ofthe spandrel girder) may be designed to transfer lateral forcesinto the floor diaphragms and resist the eccentric effect asillustrated in Figure 2.3. Note that many systems may be tooflexible for this assumption. Partial facade panels that do notextend from floor diaphragm to floor diaphragm may bedesigned with diagonal steel "kickers," as shown in Figure2.4, to provide the lateral forces. In either case, this eliminatestorsional loading of the spandrel beam or girder. Also, tor-sional bracing may be provided at eccentric load points toreduce or eliminate the torsional effect; refer to Salmon andJohnson (1990).
When torsion must be resisted by the member directly, itseffect may be reduced through consideration of intermediatetorsional support provided by secondary framing. For exam-ple, the rotation of the spandrel girder cannot exceed the totalend rotation of the beam and connection being supported.Therefore, a reduced torque may be calculated by evaluatingthe torsional stiffness of the member subjected to torsionrelative to the rotational stiffness of the loading system. Thebending stiffness of the restraining member depends upon itsend conditions; the torsional stiffness k of the member underconsideration (illustrated in Figure 2.5) is:
= torque= the angle of rotation, measured in radians.
A fully restrained (FR) moment connection between theframing beam and spandrel girder maximizes the torsionalrestraint. Alternatively, additional intermediate torsional sup-ports may be provided to reduce the span over which thetorsion acts and thereby reduce the torsional effect.
As another example, consider the beam supporting a walland slab illustrated in Figure 2.6; calculations for a similarcase may be found in Johnston (1982). Assume that the beam
Figure 2.2.
Figure 2.3.
Figure 2.4.
4
where
(2.5)where
(2.6)
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H
H
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Case 3
Case 3 Rev.3/1/03
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Pinned Pinned
Concentrated torque at = 0.1 on member withpinned ends.α
Pinned Pinned
Concentrated torque at = 0.1 on member withpinned ends.α
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Case 3
Case 3
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Pinned Pinned
Concentrated torque at = 0.1 on member withpinned ends.α
Pinned Pinned
Concentrated torque at = 0.1 on member withpinned ends.α
