30 November 1998
Table D-3. Short-Direction Lateral Seismic Force Calculations for the Barracks Building.
due to Acc
hxS or hxL
The vertical distribution of lateral seismic forces in the short direction, FxS, induced at any level shall
be determined using Equation C-25. These values are determined based on the vertical distribution
factor in the short direction, CvxS, calculated in Equation C-26. Values for WxS, hx, wi, and hi used in
Equation C-26 are given in Table D-3. The short-direction lateral seismic forces, FxS, shown in Table
D-3 are the lateral force per frame in the short direction. There are nine frames in the short direction,
nS , so that lateral force per frame is calculated as follows:
The barracks building is very regular in plan, so the center of rigidity, CR in both directions should be
at the center of the building. The accidental torsion is accounted for by offsetting the center of mass,
CM, 5 percent in both directions in plan at each floor level (see Figure D-2). The total mass at each
floor level in each direction (long and short) is multiplied by the 5 percent of the building dimension in
that direction to calculate the accidental torsional moment, Mta at each floor level. Similar to the
lateral seismic forces, the accidental torsional moments, Mtax are distributed along the floors of the
building according to the vertical distribution factor given in Equation C-26, which is expressed as
Mtax = 0.5[VSCvxS (FloorLength) + VLCvxL (FloorWidth)]
CvxL = vertical distribution factor in the long direction.
VL = the base shear in the long direction.
Table D-3 gives values for accidental torsional moments, Mtax at each floor level.
The torsional resistance, Mtr (see Equation 3-3) is proportional to the square of the distance from the
center of resistance to the plane of each panel. The torsional resistance is also proportional to the
lateral stiffness of each panel. Therefore, because the barracks building is very long in one direction,
the shear panels in the short direction near the ends of the building will dominate the torsional
resistance. For this example it will be assumed that all torsional resistance comes from the shear
panels in the short direction. The torsional resistance from all shear panels, Mtr, in the short direction
can be expressed as follows (from Equation 3-3):
The symbol for the number of frames in the short direction, nS, must not be confused with the number of faces with diagonal
straps on a given shear panel, ns.