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R-values in seismic design
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- Subject: R-values in seismic design
- From: terrence turner <tnturner(--nospam--at)glen-net.ca>
- Date: Sat, 26 Sep 1998 15:29:29 -0400
Excuse me for not explaining myself fully when I introduced the R-factors without referencing their respective codes and location. The R-factors I was discussing were with respect to the National Building Code of Canada(1995). I will explain briefly how , we Canadians, obtain our elastic base shear and design base shear. Elastic Base Shear Ve= v*I*F*S*W v is the zonal velocity ratio ranging from 0 to 0.4. In my city Ottawa our zonal velocity ratio is 0.1 I is the importance factor of the structure which varies between 1 to 1.5 F is the foundation factor which varies from 1 to 2 S is the seismic response factor which is dependent on the natural period of vibration of the structure and the ratio of acceleration related to velocity related seismic zones. This factor varies form 4.2 to 1.5/T*0.5 and W is the weight of the building (includeing 25 percent of the design unfactored snow load. Design Base Shear V=0.6*Ve/R 0.6 is a factor representing level of protection based on experience. Our R-factor is dependent on the type lateral force resisting system used. Our R-values range from 1.5 to 4. To obtain a greater R-value more stringent detailing requirements are required. Some of the corresponding lateral force resisting systems include: STEEL R=2 BRACED FRAME WITH NOMINAL DUCTILITY MRF WITH NOMINAL DUCTILITY R=3 DUCTILE BRACED FRAME R=4 DUCTILE ECCENTRICALLY BRACED FRAME CONCRETE R=2 WALL WITH NOMINAL DUCTILITY R=3.5 DUCTILE SHEAR WALL R=4 DUCTILE COUPLED SHEAR WALL A R value of 1.5, which corresponds to any system which has no ductile detailing requirements, is permitted depending on location and the number of stories of the structure. Our load combinations from the NBCC 1995 are the following 1.0D + 1.0E or 1.0D + 0.5L + 1.0E My question was with regards to the detailing requirements for a steel braced frame with nominal ductility(R=2.0). Our requirements are governed by CAN/CSA-S16.1- M which states (clause 27.5.3) the connection must be designed to "In velocity or acceleration related seismic zones of 2 or higher the brace connections shall have a factored resistance at least equal to the axial tensile yield strength of the brace (AgFy) unless the designer can show that a lower resistance is adequate." I find this clause leads to excessive forces for connection detailing and that the final clause...(unless the designer can show...) is too open ended. Any suggestions? thanks Jon Turner
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