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06/11/2023 12:24 pm
In Auckland,it is common to see two or three storey timber buildings on top of a common concrete podium over either retails and/or carpark. These timber buildings are typically designed with GIB bracing wall system. If you need to assess the seismic rating of these buildings, what approach will you take? If you use part and protion, it is very likely that you will get a very low seismic rating, and in my opnion is too conservative. The flexible timber buildings are likely to have a very different dynamic behaviour to the rigid concrete podium’s below.
It is my opinion that these timber building could be assessed not with part and portion. Any comments from others on this?
11/11/2023 11:50 am
Hi I agree that the “parts and portion” approach isn’t suitable. The appropriate method aligns with ASCE 7 (12.2.3.2), utilizing the two-stage analysis procedure, which separates the upper and lower portions.
The two-stage equivalent lateral force procedure is applicable to structures with a flexible upper part over a rigid lower section, provided they meet specific criteria:
- The stiffness of the lower portion must be at least 10 times that of the upper.
- The overall structure’s period shouldn’t exceed 1.1 times the fixed upper portion’s period at the base.
- Design the flexible upper section separately using appropriate R and p values.
- Design the rigid lower portion separately, employing respective R and p values, with reactions from the upper portion amplified by the R/p ratio, not below a specified value.
This approach allows for the consideration of flexible upper structures on stiffer bases, a common construction type. Lower level vertical seismic considerations are based on the weights of the concrete levels. Then, the seismic base shear of the upper levels at the podium is added as an amplified force using the ductility ratio or overstrength factor.
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