Minimum Seismic Base Shear- ASCE 7-2002

[JStephen]

Section 9.14.4.1 requires minimum seismic lateral force per Section 9.5.5.2. Section 9.5.5.2.1 says "When the fundamental period of the structure is computed, the seismic design coefficient Cs shall be determined in accordance with the following equation" and gives Eq. 9.5.5.2.1-1 for Cs. So the question: If the fundamental period is NOT computed, is that equation no longer applicable?

It seems odd because the period doesn't actually enter into the equation in question- simply the fact that it's been calculated gives me a higher seismic load. If I don't calculate a fundamental period I fall back to 9.14.5.1-1, which gives me a much lower shear. Does this sound reasonable?

Also, in Section 9.14.5.1, Note 7 says that for certain structures, "the minimum seismic design force shall not be less than that required by the approved standard for that system". Does this mean that structures covered under Note 7 do not need to meet the requirements of 9.5.5.2?

Thanks for any input!

 

[scottiesei]

Hmmm, my 9.14.4.1 refers to section 9.6. The T enters the capping equation for Cs, 9.5.5.2.1-2.

 

[scottiesei]

Did you mean that you're in 7-05? Your title has 02.

 

[JStephen]

The reference is Equation 9.14.5.1-1, which is in Section 9.14.5.1.

After looking at this some more, I think the intent of it all is that for structures that fit under that Note 7 in 9.14.5.1, none of the 9.5.5.2 stuff is applicable- Note 7 is one of the "following additions and exceptions" in 9.14.5.1.

 

[JStephen]

No, it is in -02.

 

[JAE]

I think that you have section 9.5.5.2.1 outlining the basic seismic factor Cs which is based upon four equations (1-4).

Section 9.14.5.1 says that you must use 9.5.5.2

EXCEPT

substitute the two given equations (9.14.5.1-1 and -2) for the 9.5.5.2 equations 3 and 4. These are just the minimum values.

Exception 7 seems to refer to Table 9.14.3 (via 9.14.7.3.1)where specific standards have been develop for particular types of non-building structures. If you appear to fall into those categories, then non of ASCE 7 seismic directly applies - you should go to those specific standards.