ASCE 7-05 Section 12.7.1

[ARLORD]

Has anyone ever used this section before. If so what does it mean and when does it apply. It states:

"Foundation Modeling. For purposes of determining seismic loads, it is permitted to consider the structure to be fixed at the base."

 

[apriley]

Modeling your footing as fixed will result in higher loads on the structure, which is conservative. Using soil springs typically increases your period, so you get lower displacements/loads. ASCE7-05 is essentially asking you to be conservative.

 

[ARLORD]

Higher applied loads or higher member forces? Do you then have to design the columns and foundation as fixed?

 

[apriley]

You need to design and model your columns as they actually are - fixed or pinned. This clause is addressing the foundation, meaning the soil. Soil isn't infinitely rigid, and doesn't act like it in an earthquake. But if you assume it's rigid, most likely your structure period will be shorter, which usually results in higher accelerations on your ARS curve, corresponding to higher seismic forces (consequently, higher member forces).

If you're new to seismic design, I would suggest discussing this with someone in your office (?) who may be knowledgeable, as it's a pretty important concept for seismic design.

 

[slickdeals]

Using soil springs typically increases your period, so you get lower displacements/loads
You mean higher displacements and lower loads?

 

[slickdeals]

Higher your period, more flexible the structure and lower your base shear.
But you still have to stay within the allowable drift limits, which may not be too much of a concern.

More often than not, your sections will be drift-controlled rather than strength.

 

[apriley]

Slickdeals: The structure itself should displace less with soil springs, as the soil will take some of the displacement, and lower seismic loads will be applied. The overall displacement may be greater, but the local displacement (ie. on a column) shouldn't be.