Seismic R factor for pole buildings

[keyPitsimplE]

I see that ASCE 7 requires an R=1.5 for cantilevered columns. This job is on an 85 mph Exposure 'D' wind site, and the structure is really light, but seismic still controls over wind because Cs = 0.488, even with taking 0.7E as a load combo!. It does not seem right that wind should not control here. Is it proper to use R=1.5 for a little 24'x24' pole building? Am I missing something? It is an enclosed structure.

Thanks in advance.

 

[msquared48]

Is this Cs still less than the maximum you need to use?

Mike McCann
MMC Engineering

 

[msquared48]

And don't forget that if you can tolerate more than 1/2" ground movement for the structure that you can double the allowable lateral bearing pressures for the poles.

Mike McCann
MMC Engineering

 

[msquared48]

Frequently, for these structures, if the lateral loads are ridiculous, going to a plywood diaphragm at the roof and plywood shear walls might be be the better option with the R=6.5. However, I would not expect to have to do this for a 24X24, omly in larger pole structures.

Mike McCann
MMC Engineering

 

[MiketheEngineer]

I have done many of these and have never found that seismic controls - but I suppose it is possible.

What is the use group - storage or horses??

To be honest - I just kind of ignore it and check for wind only..
I live in a fairly active seismic zone - Missouri - and if we have an earthquake large enough to take one of these down - nobody will be around to care any way.

 

[msquared48]

You have to remember too that these strutures were developed in Poleland, so how complicated can they be?

Mike McCann
MMC Engineering

 

[keyPitsimplE]

Sorry for the delay, Mike. For some reason my email notification was turned off, so I did not see that there were any replies. I ended up reducing my DL to actual intead of rough estimated weights and got the seismic down to about what wind load was. If this was Exp. 'B' which is more common, that would not have worked.

Yes, I knew about the 1/2" allow. defl. rule.

I dont know what you mean by your first question "Is this Cs still less than the maximum you need to use?" Due to R=1.5 it is much higher than I am used to seeing, thus making seismic govern over Exp. 'D' 85 mph wind.

This is a 24'x24' addition to the end of a "non-engineered" residential work shop pole bldg that is already about 80 x 24, so in order for the new addition to be self supporting and ignore the old, pole bldg design made sense with new poles just outside of the existing to start the add'n.

Yes, I think most people ignore seismic, including myself until I looked closer. Even a pole bldg manual I found said wind almost always controls. If we are going by the IBC and ASCE codes, we see that this R=1.5 is inconvenient, and will govern wind much of the time. My thought was I was missing some exeption or something.

I want to keep it simple, but its hard to just ignore such a large increase in code required load. I think this R=1.5 makes sense for buildings where you might have high axial loads that contribute to the P-delta affect, thus needing to keep the posts REALLY rigid to avoid collapse when a small amount of lateral load is applied. However, ASCE limits you to 15% of max. axial capacity AND limits bldg height to 35' for cantilevered columns. The combination just seems way too conservative for a light wood pole building.

I may have to ignore it in the future and just go with common sense, claiming "industry standard" if ever questioned. The poles and footings already end up quite large just considering wind.

 

[MiketheEngineer]

Not knowing the height of the building - I am guessing you are using a cantilevered post design - which is great but very conservative. I tend to use the walls as shear diaphragms. There are numbers for these. It actually makes sense and works in the real world.

Go to NFBA or google on Post Frame - there is a lot of info on these buildings on what does and doesn't work...

 

[msquared48]

I have seen seismic control where there was a high snow load on the struture, driving up the seismic forces. In that circumstance, I convinced the owner to do what Mike said - go to shear wall construction and standard framing to lower the costs.



Mike McCann
MMC Engineering

 

[HouseBoy]

MxM,
I don't think Mike the Eng was saying use "conventional" shearwalls as you wrote. I think he is saying use the shear wall values that are published (by NFBA) for typical pole building wall construction (frequently in the range of 100 plf)