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IBC special seismic load combinations for steel design

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Hello everyone:

IBC 1605.4 indicates the IBC  special seismic load combinations [combos with
Em] are required where required by Sections 1613 through 1622 or by Chapters
18 through 23.  Chapter 22 does not specifically require the special load
combinations.  IBC Section 2212.1.2 requires steel structures to be designed
and detailed in with AISC Seismic Part I or Part III and there are no
amendments to AISC Seismic.  AISC Seismic Part 1 Section 8.2 indicates to
use the AISC "additional load combinations" for column design when

IBC 1617.1.2 special seismic load combinations Em include a vertical
component of + - .2*SDS*D.  The AISC Seismic Part 1 Section 4.1 OMEGAo*Qe
load combinations do not include the + - .2*SDS*D and the section indicates
Qe is the horizontal seismic affect.  

Is this an oversight that the IBC adoption of the AISC Seismic Provisions is
not amended to specifically require the use the IBC special load
combinations?  Why-why not? The IBC special load combinations require the
additional .2*SDS*D vertical effect and this can be significant  for a
multistory building.

PS.  IBC 1605.1 indicates buildings need to be designed for the load
combinations indicated in 1605.2 and 1605.3 and the load combinations in
chapters 18 through 23.  IBC 1605.1 also indicates buildings need to be
designed for the special seismic load combinations when required by Section
1620.1.7 or Section 1620.3.5.  This appears to be a misreference and it
appears 1620.1.6, 1620.1.9 and 1620.3.4 are intended.


Scott M Haan P.E. 
Plan Review Engineer
Building Safety Division
Development Services Department
Municipality of Anchorage
phone:907-343-8183  fax:907-249-7399