when do I use the ASCE7 Special seimic load? 3

[pcoiron]

Reference the ASCE7-02 equations 9.5.2.7.1-1 and 2 The omega factor is a killer me, but ah ha it says to use this load "Where specifically indicated in the standard". Where is it used? Do I have to worry about it when designing a 4 story hotel?

 

[JAE]

This is a factor used within the code where specifically called for. You particularly see it in steel connections of brace members. Read through the IBC code and you can see various areas where the special load cases are required. I'm away from my office so I can't give you specific example sections.

 

[broncosfan]

I'm also confused by the wording of the code in regard to the omega factor. IBC section 1605.4 is specifically mentioned in section 1620.1.6 for the design of collector elements. However, the omega factor is also specifically mentioned in section 1617.6. From the wording in section 1617.6, it appears that the omega factor should be used in determining "element design forces." Section 1617.6 basically applies to every type of seismic resisting system known to man listed in table 1617.6 (including R=3 not detailed for seismic). So for instance, in a braced frame, "elements" would be the brace connections as well as the braces themselves. The columns and beams and their connections in the frame are also "elements" of the frame. So, basically the entire frame needs to be designed for the omega factor. Correct? Why did IBC go through the trouble of creating this maximum seismic load effect and omega factor, since basically everything needs to be designed for it? Wouldn't it be easier to just change the R factors to give a higher base shear? I'd appreciate a clarification of this omega factor from someone. Maybe I'm interpreting the code the wrong way. Thanks.

 

[JAE]

The key is that [Ω] is used per 1605.4: "where specifically required by Sections 1613 through 1622 or by Chapters 18 through 23.

1620.1.6 is an example of a "specifically required" condition....collector elements, their splices, their connections etc.

A simple element of the structure is not required to use the load combinations of 1605.4 unless 1605.4 is specifically referenced for that element.

 

[broncosfan]

Thank you for the response. After a quick read of the code, I only see a few areas where the maximum seismic load effect, Em, and the system overstrength factor is specifically required. The only areas I see are for collector elements, 1620.1.6, elements supporting discontinuous walls or frames, 1620.1.9, and certain components of light framed cold formed steel walls, 2211.7.2. That's all I can see. I was originally thinking that sections 1617.6 and 1617.1.2 were specific requirements to use the overstrength factors. This is apparently incorrect. So if I have a braced frame in design category C with R=3 (not detailed for seismic), then I can design the frame braces, beams, and columns, and all of their connections for the seismic load E (not Em). I can completely ignore the overstrength factor listed as 3.0 in table 1617.6 unless I run into the specific cases mentioned above. I guess the overstrength factor, even though it is listed prominently in table 1617.6, really only gets used in a few rarely encountered conditions as listed above.

 

[JAE]

Yes...but also refer to the AISC Seismic Design Provisions which are requried to be followed if you don't use R=3. These have a number of cases where the overstrength combo is required.

The idea (and I know my good friends in California here will step in if I step in it).... the idea is that you are trying to create a structure where the connections, brittle elements, and key brace elements are detailed to force a more ductile or energy-absorbing type of failure.

One good analogy is to remember how bars of steel are tested in a lab (remember your materials classes?) where the bar is large at each end and tapered down in the middle to avoid failures where the machine grips the bar. The larger ends avoid brittle, non-ductile failures and allow the test to proceed through the plastic zone of the material to get a good stress-strain diagram.

A brace is then similar, you don't want the welds, the bolts, or the gussets to fail in brittle fashion, so they are upsized to make the brace itself be the weak link in the chain of limit states along the load path.

So as you are designing for seismic, keep an eye out for those rigid, brittle areas along your lateral force resisting system and consider using the Omega load combo for them.