Seismic zone 3 and the IBC 2

[mrMikee]

I have just received a project where I will redesign a modular cement silo and agg bin for seismic zone 3. This is a standard product that could be shipped anywhere in the US most of which is now IBC, so site specific values are not available at this time. The problem is that the customer has thought about seismic in terms of zones for many years and the new IBC maps are somewhat confusing. Well, me too. I need to come up with a Ss and S1 to use for my design, or at least something to make a recommendation on.

For quick calcs and preliminary work with the UBC in the past I used to use the max value of CS=.14 in the ZIKCSW equation which came to approximately:

Zone 1, V=0.525W
Zone 2a, V=0.105W
Zone 2b, V=0.14W
Zone 3, V=0.21W
Zone 4, V=0.28W

Has anyone come up with a similar rule of thumb for the IBC spec?

Thanks,
-Mike

 

[mrMikee]

So far I have come up with the following:

Using IBC 1617.5.1 for the simplified procedure for seismic design:

Assume Ss=1.46g

Sms=Fa*Ss=0.9*1.45=1.314

Sds=Sms*2/3=0.875

V=1.2*Sds*W/R=0.21W

where 0.21W is the same as the UBC approximation for zone 3.

This makes sense to me? I am looking for an approximation or 'rule of thumb' to put an engineering perspective on the old versus new regarding seismic requirements. This is clearly an approximation and not for final designs.

Any questions or comments?

Thanks,
-Mike

 

[SacreBleu]

For Ss values (latitude and longitude):

http://eqint.cr.usgs.gov/eq/html/lookup-2002-interp.html

You can't get a rule of thumb because Ss varies so much, even every few miles.

The simplified method you cite above yields a higher seismic base shear than the "non-simplified" method. You might as well not even bother with the simplified method, as I have learned.

 

[aggman]

Mike,
I really don't think you can generalize the IBC into zones. My recommendations would be as follows:

A) Design the structure to UBC and state that the standard product is designed as such. If the customer needs a product to adhere to the IBC then you can do a check to see if it adheres to the later code. If it doesn't then the product will have to become a non-standard application and be modified as such.
B) The IBC has tables which define the seismic design category. You could pick a class C which will give you ranges of Sds and Sd1, choose values of Sds and Sd1, and design the structure. Then you can state what the silos are designed for on the drawing. As long as the local area falls under your design range then you are ok.

I have found that the simplified method is basicly useless for any type of silo or similar structure.

 

[mrMikee]

Thanks aggman and SacreBleu for the comments.

I agree that looking at it this way is of limited usefulness. What I wanted was a 'rule of thumb' to put a perspective on the two codes. When I designed a product that could be shipped anywhere in the US I needed a way to categorize the designs. A 'standard' design was zone 1, and anything else was a heavy or special. I think many people are still thinking in terms of zones, and this is a place to start with general discussions.

I think the way to do this now is to use the IBC Seismic Design Categories as aggman suggests. This gives us a method to specify structures in 5 or 6 groups based on Ss and S1, with each group having its specific requirements in the IBC. I like this approach, thanks aggman.

With that said, here's what I came up with the zone versus Ss rule. The simplified procedure does overestimate the seismic base shear by 20% or more which is about what the CS=0.14 did. With site class D and R=5 (and K=2 in the ZIKCSW equation) I came up with the following approximate correlation.

zone 1: V=.0525W ... Ss=.20
zone 2a: V=.105W ... Ss=.47
zone 2b: V=.14W ... Ss=.73
zone 3: V=.21W ... Ss=1.31
zone 4: V=.28W ... Ss=1.75

This is not a substitution for engineering, always do the calculations.

Regards,
-Mike

PS: Don't show this to anyone in the sales dept.