Special seismic load combinations

[Usimo]

In IBC 2003, the maximum seismic load effect Em (1617.1.1.2) is used with load combinations in sec 1605.4. That section further says, this Em is required for the design of vulnerable elements critical to the stability of the structure. But, It didn’t spell out specific elements crictical to stability?
Then, in sec 1620.2.9, the code says, This Em is required for elements supporting discontinuous walls and frames. ( according to IBC commentary) That means, if we have a discontinuous shaer wall or bracing, we have to design the columns above which we have the discontinuous shear wall or bracing.
But, some of my collegues use this Em for all the bracing elemnts and the columns.
Could you please spend some times and clarify this Sections?
Also, Please try to explain the use of this special seismic load combinations.
Thanks in advanc
Tharani

 

[ajh1]

I would agree that the code is not particularly clear on when you use these provisions, other than the example you cited. There are also some Em requirements in regard to collector elements.
For steel design, the higher level seismic requirements get defined by the AISC Seismic Provisions (2002 edition for IBC 03). There are specific requirements in that book in regard to the use of Em type seismic combinations. For concentrically braced frames, Section 14.2 specifically states that the Em type combinations be used for design of both members and connections.
This will be changing with IBC 06, ASCE7-05, and AISC Seismic 05 to the more normal E for members, Em for connections, but they have drastically reduced the R factor as part of the change.

 

[Usimo]

Ajh1!
Yes,from 2002 AISC Seismic Provisions to that of 2005, They changed the requirement of Em for members. Originally, FEMA 302 does not require Em for all members. According to FEMA 302, the special seismic combinations are for special situations, example collector/chord, column in discontinuous braces and connections.
thanks