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Re: AISC Seismic Provisions

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I'll address your first question.

> 1.  Regarding Sect. 9.2b of AISC EQ Prov.: If per 9.2b, Mu must be
> at least equal to Mp, why is Vu in 9.2c allowed to be limited to
> Vdl+Vll+Vs+Veq(factored loads).  The  Commentary indicates that
> lower combinations of end moments may be justified in some cases but
> 1) it is not clear as to how this occurs/when 2) makes it sound like
> this lowering effect is a rare occurance while 9.2b infatically
> states, " the required shear need not exceed the shear resulting
> from Load Case 4-1".  Why the limit on shear?

In lieu of an actual plastic analysis of the system (which would 
satisfy the rational analysis provision), the Provisions allow two 
simpler approximations.  The first is a "plastic analysis" of the 
member based on Vdl+Vll+Vsl+ flexural yielding at both ends of the 
member (1.1RyFyZ).  The second is a very rough (judgement-based) 
approximation for the system as a whole.  That is, the code writers 
felt that the system as a whole is unlikely to have more overstrength 
than omega_0.  The implication is that the code writers expect that 
if you actually performed a plastic analysis of the system, you 
wouldn't get more load than is indicated by the amplified load 


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Michael Valley                                   E-mail: mtv(--nospam--at)
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201