Seismic loading - very high Cs

[Timbo9]

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.

 

[SteelPE]

I think you may have a few things wrong here..... but not what you might expect.

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.

 

[JAE]

Could you clarify the code you are designing under?
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.

 

[JAE]

....and SteelPE beat me to it.

 

[msquared48]

You need to calculate your TL FOR Algeria so you can set the upper limit of Cs to use per ASCE7 12.8-3 or 4. The tables in the IBC do not cover Algeria. Or you can calculate it based on Section 11.4.5.

Mike McCann
MMC Engineering

 

[Timbo9]

Thank you all very much for your help, I will work through your comments and hopefully come out with a more sensible answer.

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

 

[JAE]

With your S1 and Ss values and a five story building, 41 ft. tall, Site Class D, I get a Cs = 0.415.

This is with a TL of 12 - which may not be correct.

 

[Timbo9]

I have reworked my calcs and get Cs = 0.207 using R = 4 for light frame steel walls with x bracing. I hope JAE used R=2 and therefore my value is right. In trying to determine TL for Algeria I did plenty of googling and eventually found an English copy of the Algeria Earthquake code (RPA 99). I've worked through that code and the seismic load is very similar to my load using ASCE7 so I'm now designing the bracing to suit.

I have to say I'm very impressed with the speed and quality of the help given on this forum so many thanks again.

 

[JAE]

I used R = 2

 

[SAIL3]

if the seismic force is that high, I would not use unreinforced concrete walls....the seismic loads will initially be resisted by the stiffest elements(conc walls) untill they yield/fail and then ultimately by the more flexible system(steel)...depending on the connection between the steel and the conc infill walls, these conc wall may pick up more seismic load than that resulting from their own weight/mass...I guess what I am trying to say is that you may have a combination of systems here...conc shear wall and steel braced frames....I would consider the failure of these infil wall as a safety issue...you may have already addressed this in your design..just thought I would mention it as the trend of the post is focused on the correct magnitude of the seismic force....