Formula1251
Member
Hopefully this is a quick one. (I feel I need to mention I'm a bridge guy, because I feel the answer should be relatively straightforward for someone more familiar with the material.)
On p. 13, Table 1A-3, in the SEAOC Structural/Seismic Design Manual 2009 IBC Vol 3: Building Design Examples for Steel and Concrete, Example 1A, they are calculating the accidental torsion for each of the braced frames as a percentage of the total shear load. Due to symmetry (rectangular building), there is no inherent torsion load, only accidental. For design of the braced frame in question (BF-2 oriented in the building-transverse direction, for seismic loads in the same building-transverse direction), the additional shear due to accidental torsion, calculated as %-total shear, is calculated using the equation: |e_acc * R_i * d_i / sum(R_i * d_i^2)|. I can follow the calculation for the transverse frames, where:
However when calculating for the percentage to the longitudinal braced frames, I cannot arrive at the number shown in the table (0.7%). I am using the same formula to calculate, and replacing only the longitudinal frame stiffness, R=1.00, and the offset, d=75' (I am keeping the same eccentricity, since it is asking for the accidental torsion due to the same moment), and I get 0.9%. I'm not sure if I'm looking at this incorrectly or not.
If I try also replacing with the accidental eccentricity for forces in the longitudinal direction (e_acc_long=7.6'), I get 0.64% -- and I do not believe this to be correct, because it is due to a different loading.
Can anyone please shed any light on this and confirm if I'm on the right track leading to 0.9%?
Thank you!
On p. 13, Table 1A-3, in the SEAOC Structural/Seismic Design Manual 2009 IBC Vol 3: Building Design Examples for Steel and Concrete, Example 1A, they are calculating the accidental torsion for each of the braced frames as a percentage of the total shear load. Due to symmetry (rectangular building), there is no inherent torsion load, only accidental. For design of the braced frame in question (BF-2 oriented in the building-transverse direction, for seismic loads in the same building-transverse direction), the additional shear due to accidental torsion, calculated as %-total shear, is calculated using the equation: |e_acc * R_i * d_i / sum(R_i * d_i^2)|. I can follow the calculation for the transverse frames, where:
- e_acc_trans=10.6' (n-s)
- d=105'
- R=1.25
- sum(R_i * d_i^2) = J = 88,980
However when calculating for the percentage to the longitudinal braced frames, I cannot arrive at the number shown in the table (0.7%). I am using the same formula to calculate, and replacing only the longitudinal frame stiffness, R=1.00, and the offset, d=75' (I am keeping the same eccentricity, since it is asking for the accidental torsion due to the same moment), and I get 0.9%. I'm not sure if I'm looking at this incorrectly or not.
If I try also replacing with the accidental eccentricity for forces in the longitudinal direction (e_acc_long=7.6'), I get 0.64% -- and I do not believe this to be correct, because it is due to a different loading.
Can anyone please shed any light on this and confirm if I'm on the right track leading to 0.9%?
Thank you!
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