Equivalent Lateral Force Procedure 1

[DaveAtkins]

I am confused by Equation 9.5.5.2.1-1 in ASCE 7-02. This equation states that Cs = SDS/(R/I), "when the fundamental period of the structure is computed." First of all, fundamental period does not enter into this equation, so why does it matter? Second of all, does this mean that in order to use this equation for Cs, I must limit T to Cu*Ta, as specified in Section 9.5.5.3?

I have an actual situation I am dealing with--in Seismic Design Category B. Cs as computed above is less than
SD1/(T(R/I), if I assume T = Ta. So I would like to use Cs as computed above. But I performed a dynamic analysis on RISA-3D, and T for some of my braced frames is greater than Cu*Ta. So I think I need to stiffen up my braced frames, but I am not sure.

The other thing that confuses me is that I believe in ASCE 7-98, it did NOT have that statement "when the fundamental period of the structure is computed."

DaveAtkins

 

[MarcbSE]

Dave,

This equation is now in section 12.8.1 in ASCE 7-05. It gives the default maximum value for Cs that you need to use. I have found that it almost always controls in the designs that I do.

The section goes on to say "The value of Cs computed ... need not exceed the following:

Cs = Sd1/[T(R/I)] for T < TL
or
Cs = Sd1*TL/[T²(R/I)] for T > TL"

Section 12.8.2 - Period Determination, basically states that IF you calculate T based on the structural properties from analysis then your calculated T can not exceed Cu*Ta. I don't think that means your actual T can not exceed Cu*Ta, just that you can't count on the higher T to reduce the seismic coefficient.

 

[DaveAtkins]

Thanks, MarcbSE,

I came to a similar conclusion this morning. I don't do much seismic design, but based on the research I did, I found that a stiffer structure can actually be a BAD thing--it won't dissipate as much energy. The maximum T used in that other equation is a safeguard which keeps applied seismic forces above a minimum threshold.

In fact, I think my copy of ASCE 7-02 has a typo--I think it should read, "when the fundamental period of the structure is NOT computed..."

DaveAtkins

 

[MarcbSE]

Bingo!!

I only have a partial copy of the '02 version that does not include the chapter 9 so I'm not sure about the typo, but it sounds logical. I'm not familiar with any statement like that in the '05 or '98 versions.