Seismic lateral Story Forces Cs Vs Fx for Design Category A 1

[Bala0404]

Hi Friends,


Suppose your seismic coeff is more than 0.01 but you are getting seismic category as A as per ASCE7-2005, how will you calculate story shears?
I have seismic coeff, Cs=0.0211 and category A.
If I consider eqn 11.7-1 which is for category A, then it is like seismic coeff I am considering is 0.01 which is much less than 0.0211.

I gave equation 11.7-1 reference because in seismic area with category A, buildings have to be designed for minimum of 1% of gravity loads .
But my main question was, if Cs is more due to my building properties, but as per clause 11.7.1, if I use only 1% instead of higher Cs, am I not under estimating EQ forces?
Cs changes when we change R value (if I keep SDS & I values constant). If I choose smaller R, then automatically Cs increases. (Eq 12.8-2)

how does equation 11.7.1 differ from eq 12.8.11. Should i use which ever value is higher?

Please clarify if any one understands and can clear my confusion.

 

[WinelandV]

11.7.1 Applicability of Seismic Requirements for Seismic Design Category A Structures.
"Structures assigned to Seismic Design Category A need only comply with the requirements of Section 11.7. The effects on the structure and its components due to the forces prescribed in this section shall be taken as E and combined with the effects of other loads in accordance with the load combinations of Section 2.3 or 2.4. For structures with damping systems, see Section 18.2.1."

 

[Bala0404]

Thanks for your quick response. i agree with you on following section 11.7 but still little confused.

So even my Cs is 0.211 i still have to use the 0.01 value as section 11.7 governs. will i be under estimating the lateral forces?

 

[WinelandV]

After reading "need only comply with the requirements of Section 11.7", I wouldn't have turned the page into chapter 12.

You're not underestimating the forces, since you are designing the building to meet the minimum loads required by code. Now, there could be a second conversation about whether the code minimum is enough or not. But one could get drawn into a discussion about the assumptions that the ELF method is based on (Soil Site Class, response coefficient R, approximate period calc, vertical distribution of lateral forces, etc.).

Cutting through all of that, will the Cs = .021 make the seismic control the lateral design over wind? My guess is probably not.

 

[Bala0404]

Thank you WinelandV. Understood.