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Foundation design in high seismic category

Foundation design in high seismic category

Foundation design in high seismic category

(OP)
Hi,

Steel base plates needs to be designed for over-strength or mechanism based on type of lateral force resisting system used as per AISC 341. But, foundation to which base plate is attached is designed for straight seismic forces (no over-strength or mechanism). Why is this? Is this because of so much of redundancy in the foundation? I have never clearly understood this.

RE: Foundation design in high seismic category

The concept is: the foundation doesn't see the full seismic force because it is "lost" by the deformation incurred (from moment frames, shear walls, etc) during a seismic event (represented by the Response Modification Factor).

As to why other components get the overstrength factor when logically they shouldn't be seeing a great deal of this force (because it is after a point where the energy should be lost)......the fact is: seismic design still isn't a exact science. A lot of times the overstrength factor has been introduced for components whose failure was observed in a seismic event. (Not because it was predicted on paper.) It's still somewhat trial and error. Sometimes structures behave somewhat differently than anticipated.

RE: Foundation design in high seismic category

WARose covered it but to add to the overstrength discussion:

Another purpose of overstrength is to force failure where you want it. When you design a member for overstrength, it will remain elastic (in theory) and the yielding will occur in a more favorable place.

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