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["Carter, Charlie" <carter(--nospam--at)aisc.org>]

Title: Message

>IBC Section 1617.1.1.2 indicates that an allowable stress increase of 1.7 can

>be used with the amplified seismic forces, which is not a change for me...great.

>Except, this only applies when you are using the simplified analysis procedure,

>which is only allowed to be used for relatively small structures ... 

>When using the equivalent lateral force procedure, you are directed to ASCE

>7-02, which in turn only permits the use of a 1.2 allowable increase for the

>amplified loads!  Is this correct?... This is a whopping 42% increase in force.

 

1.7 is not really an allowable stress increase. I think the IBC and AISC Seismic Provisions have parallel and complementary provisions, which ensure that the ASCE/IBC load provisions and AISC steel strength provisions are properly used together. That is, strength-level loads are compared to design strength-level capacities, and ASD loads to working-stress-level capacities. The AISC Seismic Provisions are based upon strength-level loads whether you do LRFD or ASD, so the IBC always put you to the strength-level loads when it requires you to design in accordance with the AISC Seismic Provisions.

 

In AISC Seismic Part III (ASD), when you multiply by 1.7, you are offsetting the effects of the factor of safety buried in the ASD capacities -- you are multiplying the working-stress-level capacities by the value of the least factor of safety used in ASD (5/3, which is roughly equal to 1.7). In effect, this converts the ASD design equation from the working stress level to the nominal strength level. Then, the appropriate phi factor in Part III of the AISC Seismic Provisions is used to reduce the calculated strength to the design value. The variability of loading was already taken care of in the calculation of the loads and the phi factor takes care of the variability in the strength.

 

 

>A conflict is that the seismic provisions for steel structures (AISC 341-02) specifically

>indicate that the 1.7 increase may be used.

 

As explained above, I think they are coordinated. But please let me know if after reading that you still think they conflict.

 

 

>What has happened to the 1/3 stress increase for steel design

>when used with non-amplified seismic or wind loads?  The 9th

>Edition AISC had it in there, but it was then removed by Supplement

>#1 (December 17, 2001), and instead put the responsibility on the

>applicable building code.  IBC 2003 does not reference a 1.33

>allowable increase for steel design as far as I have seen (they in

>turn put the responsibility back on AISC), except in the case

>of the amplified seismic load.

 

The ASCE 7 load combinations, which form the basis for IBC (and NFPA) load combinations, specifically address how load reductions for combinations of multiple transient loads are to be addressed. Their treatment is more advanced than AISC's treatment of it. To remove the conflict, and in recognition that AISC does not have the authority to address load combinations (in LRFD or ASD) since that is the job of ASCE 7, the one-third stress increase is now deleted from the AISC Specification.

 

The one-third stress increase has evolved almost continuously from its early uses in material design specifications, through the days of ANSI A58.1, which is the load document that preceded and became ASCE 7, and now ASCE 7. My colleague Keith Mueller and I have written a summary of how ASCE 7 (1998 and 2002) and current model building codes (including IBC, NFPA 5000, UBC, BOCA, and SBC allow -- or do not allow -- the one-third stress increase. This summary will be published as an installment of the feature SteelWise in the October 2003 issue of AISC's Modern Steel Construction magazine.

 

Charlie