ASCE 7-10 12.10.2.1 - Collector elements

[ilsose]

ASCE 7-10 12.10.2.1 Exception 1 states:
"The forces calculated above need not exceed those calculated using the load combinations of Section 12.4.2.3 with the seismic forces determined by Equation 12.10-3"

Section 12.4.2.3 contains Seismic Load Combinations (without the overstrength factor) and Equation 12.10-3 states Fpx=0.4*Sds*Ie*wpx.

Is this a misprint? Should the exception reference 12.4.3.2? The pdf in the following link contains a comment about this exception on page 17.

http://content.seinstitute.org/files/pdf/ROPC-Ch12SeismicDesignReqforBldgStructures.pdf

From the building I'm designing
Sds = 0.31
SDC = C
R = 3 (steel not specifically detailed for seismic resistance)
Omega = 3
Cs = .103
(Ex*Omega)/wpx (0.4*Sds*Ie*wpx)/wpx Ratio
2nd 0.04*3=0.12 0.12 1.00
3rd 0.05*3=0.15 0.12 0.80
4th 0.09*3=0.27 0.12 0.44
5th 0.10*3=0.30 0.12 0.40
6th 0.15*3=0.45 0.12 0.27
7th 0.20*3=0.60 0.12 0.20
8th 0.25*3=0.75 0.12 0.16

Using Eqn 12.10-3 and 12.4.2.3 decreases the lateral load used for collector design to a level below the load determine using the equivalent lateral force procedure of 12.8.

 

[JoshPlumSE]

I don't think this is a typo. Though the logic gets a bit confusion:

The collector elements must the largest of items 1, 2 and 3:
1) Overstrength load combinations using ELF or RSA design forces.
2) Overstrength load combinations using diaphragm design forces (eqn 12.10-1)
3) Regular load combinations using the minimum diaphragm desig force equation 12.10-2.

So, the basis of collector design starts with overstrength load combinations. Then they introduce some exceptions (I'll call these e1 and e2 to avoid confusion). The collector forces need not exceed the following:
e1) Regular seismic LC's (i.e. without the omega overstrength factors), but combined with the "need not exceed" diaphragm forces of eqn 12.10-3.
e2) For light framed structures only, you need not exceed the regular seismic LC's in conjunction with the diaphragm forces. Effectively, this takes the omega overstrength out of the basic force cacluation in items 1 and 2 from above. But, this is only allowed for light framed structures.

 

[ilsose]

I took the exception to mean substituted and not combined with. Combining with would result in lower loads on the 2 upper levels.

(Ex*Omega)/wpx Ex/wpx+(0.4*Sds*Ie*wpx)*Omega/wpx
2nd 0.04*3=0.12 0.04+0.12*3=0.40
3rd 0.05*3=0.15 0.05+0.12*3=0.41
4th 0.09*3=0.27 0.09+0.12*3=0.45
5th 0.10*3=0.30 0.10+0.12*3=0.46
6th 0.15*3=0.45 0.15+0.12*3=0.51
7th 0.20*3=0.60 0.20+0.12*3=0.56
8th 0.25*3=0.75 0.25+0.12*3=0.61

Anymore thoughts on this? I'm not convinced either is definitely correct.

 

[ilsose]

Combining with yields the following, not what I computed in my last post:

(Ex*Omega)/wpx Ex/wpx+(0.4*Sds*Ie*wpx)/wpx
2nd 0.04*3=0.12 0.04+0.12=0.16
3rd 0.05*3=0.15 0.05+0.12=0.17
4th 0.09*3=0.27 0.09+0.12=0.21
5th 0.10*3=0.30 0.10+0.12=0.22
6th 0.15*3=0.45 0.15+0.12=0.27
7th 0.20*3=0.60 0.20+0.12=0.32
8th 0.25*3=0.75 0.25+0.12=0.37