Seismic loading - very high Cs
Seismic loading - very high Cs
(OP)
I'm a British structural engineer and have never had to consider seismic loading until now...
...the job in question is in Algiers Algeria, it is a 5 storey block of flat constructed from light steel framing with low-strength lightweight concrete poured between the wall studs. Considering the seismic loading as per IBC I get Cs = 0.875, so I have to resist nearly 90% of the seismic weight as a lateral force! This seems too high, I was expecting the lateral load to be comparable with the wind loading but my seismic load is nearly 13 times greater.
I arrive at Cs = 0.875 as follows.
S1 = 0.56 for Algiers, from various websites
Sm1 = Fv x S1 = 1.5 x 0.56 = 0.84 (Site class D since unknown at this stage)
Sd1 = 2/3 x 0.84 = 0.56
R = 2 considering the lightweight concrete as plain masonry similarly to how you could consider masonry infill to assist the stability of a frame.
Ta = Ct h^x = 0.02*41^0.75 = 0.32
Cs = Sd1 / Ta*(R/Ie) = 0.56 / 0.32*(2/1) = 0.875!
My gut feel says this can't be right but I don't know where I'm going wrong. Any help on the matter would be greatly appreciated.
...the job in question is in Algiers Algeria, it is a 5 storey block of flat constructed from light steel framing with low-strength lightweight concrete poured between the wall studs. Considering the seismic loading as per IBC I get Cs = 0.875, so I have to resist nearly 90% of the seismic weight as a lateral force! This seems too high, I was expecting the lateral load to be comparable with the wind loading but my seismic load is nearly 13 times greater.
I arrive at Cs = 0.875 as follows.
S1 = 0.56 for Algiers, from various websites
Sm1 = Fv x S1 = 1.5 x 0.56 = 0.84 (Site class D since unknown at this stage)
Sd1 = 2/3 x 0.84 = 0.56
R = 2 considering the lightweight concrete as plain masonry similarly to how you could consider masonry infill to assist the stability of a frame.
Ta = Ct h^x = 0.02*41^0.75 = 0.32
Cs = Sd1 / Ta*(R/Ie) = 0.56 / 0.32*(2/1) = 0.875!
My gut feel says this can't be right but I don't know where I'm going wrong. Any help on the matter would be greatly appreciated.






RE: Seismic loading - very high Cs
First let me say I have never designed a building in Algeria and I have no clue as to whether or not the IBC even applies. I assume you did your research on that.
-Usually there are 2 terms that you need to check Ss and S1. With my projects, Ss controls the design (typically Ss = 0.24 S1 = 0.06) but I am not in a high seismic area. The equation you used to calculate Cs is a "need not to exceed" formula. Using Ss may give you a smaller Cs.
-Using your numbers (Sd1>0.2g) you fall under a seismic design category (SDC) = D
-If your S1 is that high I would stay away from plain masonry (remember all those buildings in Haiti?). In fact, ASCE 7-05 (the one I dug out to look at this) does not allow plain masonry with SDC = D. Using a better wall system will give you a higher R and a lower Cs.
-You are getting a 5 story building in 41 feet. People in Algeria must be small.
-Getting a geotechnical report for the site may lead to a better site class and a smaller Fv and therefore a smaller Cs.
Other than that, I think your numbers are correct. The answer is to pick a better system with a higher R and you will end up with a smaller Cs.
RE: Seismic loading - very high Cs
I see you have an S1. In the US under ASCE 7 we have both S1 and Ss to include in the derivation of Cs.
There are also maximum limits on Cs.
RE: Seismic loading - very high Cs
RE: Seismic loading - very high Cs
Mike McCann
MMC Engineering
RE: Seismic loading - very high Cs
In response to some of the points raised:
- I'm using IBC 2009 section 1613 which refers me to ASCE 7.
- Ss = 1.24. I didn't include that in my post since S1 gave me the worst case Cs.
- The wall system is actually light weight steel frame with flat strap X-bracing. The walls are infilled with lightweight concrete, since I calculated such high seismic force I wanted to consider the contribution of the infill, conservatively I considered the lightweight concrete as plain masonry since it is low strength and un-reinforced. If interested this link is a youtube video of the proposed construction method http://www.youtube.com/watch?v=TLW2FTbfiNs
RE: Seismic loading - very high Cs
This is with a TL of 12 - which may not be correct.
RE: Seismic loading - very high Cs
I have to say I'm very impressed with the speed and quality of the help given on this forum so many thanks again.
RE: Seismic loading - very high Cs
RE: Seismic loading - very high Cs