Base Shear Calculation For Seismic Load

[foxview11]

What is the reason behind in excluding live load from calculating base shear for seismic load of a building? Any expert, please share your opinion on this based on IBC, UBC or any applicable codes. Thank you so much.

 

[Trenno]

Probability. Very unlikely you'll reach the ULS gravity load condition at the same time you have an ultimate earthquake. That's why you only consider a fraction of the design live load in the building's seismic weight.

Also the fact that some live loads won't contribute to the seismic mass due to not being 'fixed' to the structure. If the building shakes left and you loose your footing and are sliding back and forth on the ground - are you contributing the seismic weight of the building?

EDIT: Can't find it now, but there was a great discussion a few months back about considering the full weight of chickens in a multi-storey farm shed contributing to the seismic weight.

 

[foxview11]

Trenno, you have a good point, but much better if you can provide technical logic and how much percentage of live load is required based on applicable building code. Please explain further. Thank you

 

[Trenno]

I think it's prudent that one understands the logic and reason behind a particular design standard or code. So as to not follow it blindly.

However I'm not too familiar with the codes you mentioned in your original post. But the seismic weight is defined as G + 0.3Q in the Australian Standards.

 

[mike20793]

It has to do with the fact that most live load isn't actually attached to the structure in a way that it contributes to the inertial forces of the structure. In cases where it can add to the inertial forces(heavy storage, etc) you account for a percentage of the live load, but not all of it, for seismic mass. A good example to look at is partitions. By code (IBC/ASCE 7) we include the partition load as a live load, since they can be set up and rearranged. However, they are often attached to the structure in such a way that they contribute to the inertial forces. Therefore, we include partition mass in calculation of the seismic mass. I agree with Trenno regarding the chicken thread, but can't find it either. Try to find the one discussing chickens as a live load. There is great discussion on it since the cages were attached to the structure.

 

[KootK]

Chickens: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[foxview11]

Gentlemen/Sirs, thanks for your opinions. But what about importance factor for each type of occupancy, does it consider live load to account in computing base shear? For example, for a 4-story car parking building or a 15,000-capacity stadium. Rejoinder from any experts, kindly give your comments on this..Thanks.