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18/11/2023 10:43 am
Is there a definite way to assess a moment resisting concrete frame with uplift under pad footing? Say for example, you have a very long stiff moment frame (period<0.4s) with bearing on one end and uplift on the other due to lateral loads, how much will the footing actually uplift by before reversal happens? Will it be enough to cause collapse? Is there any guide to design the frame then for rocking effects and how such phenomenon can affect the frame’s local ductility?
If I am designing a new building, I will size my footings to prevent any uplift at the base to ensure my moment resisting frames always have a fully supported condition to perform reliably and be within the bounds of conventional structural frame behaviour. However, how would you approach this if it were an existing building?
Similarly for sliding in a building. I always design my structures to resist sliding at the base under seismic actions, but is this necessary from a life safety criterion? I believe there will be potentially other issues with not designing for sliding, for example collection of soil at one end with loss of soil below footing due to sliding effect, where we may have to specially assesses overturning due to rotation and passive pressures.
What are your thoughts on the above?
30/04/2024 10:57 pm
Hi Rajeev,
I prefer to carry out the FEM design of the pad or raft foundation in SAFE using NLA (non-linear compression only subgrade modulus). If there is any uplift, then the contour(heat diagram) will show a dark blue colour, and you have to increase the foundation size or introduce piles. For it is worth it, FEM analysis is the best way to do the NLA of the foundation with uplift.
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