Diaphragms and Rigidity question
Diaphragms and Rigidity question
(OP)
Hi everyone. I was working on a problem from the IBC Structural /Seismic Design Manual and I have two questions I am wondering if anyone know, your answer would be appreciated. here goes:
1) when considering torsional and direct shear , is the torsional moment always assumed counter clockwise? The code and this example do not say anything on this. The example in this book uses counter clockwise and the only explanation it offers is that "the seismic force V applied at the center of mass CM is equivalent to having a V applied at the CR center of rigidity together with a counter clockwise torsion T." I don't know if its always counter clockwise though? it matters to because it affects the signs of a lot of the calculations I have to do after, like when calculating the amplification factor.
2) when calculating the torsional shears (the ones for the amplification factor. because there seems to be 2 sets of torsional shear equations, one for when calculating amplification factor and one for when calculating the actual torsional shears), it says we use the e+ e accidental only for both walls in question. NOT the e-e accidental. they use the e- e accidental when calculating the actual torsional shears. but my question is, do we always use the e+ e accidental only when calculating amplification factor ?
thank you I would really appreciate some feed back if anyone knows
1) when considering torsional and direct shear , is the torsional moment always assumed counter clockwise? The code and this example do not say anything on this. The example in this book uses counter clockwise and the only explanation it offers is that "the seismic force V applied at the center of mass CM is equivalent to having a V applied at the CR center of rigidity together with a counter clockwise torsion T." I don't know if its always counter clockwise though? it matters to because it affects the signs of a lot of the calculations I have to do after, like when calculating the amplification factor.
2) when calculating the torsional shears (the ones for the amplification factor. because there seems to be 2 sets of torsional shear equations, one for when calculating amplification factor and one for when calculating the actual torsional shears), it says we use the e+ e accidental only for both walls in question. NOT the e-e accidental. they use the e- e accidental when calculating the actual torsional shears. but my question is, do we always use the e+ e accidental only when calculating amplification factor ?
thank you I would really appreciate some feed back if anyone knows






RE: Diaphragms and Rigidity question
Suppose we have a beam rotationally fixed at each end and loaded with a concentrated load P at eccentricity e from the center of the beam. The applied torsional moment is P.e and can be clockwise or counter clockwise depending on the direction of eccentricity. The shear at each end, neglecting beam weight is P/2. The torsional moment at each end is Pe/2 and is opposite in direction to the applied moment because it is a reaction. Torsional moment adds to beam shear on the side where the eccentricity lies and subtracts on the opposite side.
2) I will pass on this question as I am not familiar with the IBC Structural/Seismic Design Manual.
BA
RE: Diaphragms and Rigidity question
that makes sense but look at how they have it ( I inserted picture of it). If we are following that logic of looking at it like a beam with an eccentric load, then wouldn't it be clockwise ? (based on the picture). I notice they have the arrows of the shear going with (instead of against, like reactions) the torsional moment.
I understand the whole subtracting and adding it based on these directions. I just dont understand why they are going the same as the torsion moment and why that torsional moment is going counter clockwise because its on the left side from the center mass so it would tilt the diaphgram clockwise wouldn't it?
I feel like the wall rigidities and difference in length has to do with it maybe but in this particular example the wall rigidies were given already pre-calculated.
RE: Diaphragms and Rigidity question
In the current example, the lateral force on the building is directed northward (toward the top), based on the diagram showing the direct shear contribution. Torsion is counterclockwise because the center of the applied force is east of the CR (presumably half way between wall A and B).
If the seismic (or wind) force were reversed i.e. pointing south, the direct shear would be reversed and the torsion would be reversed, i.e. counter clockwise.
If the applied force is F and it occurs 20' east of CR, the moment is 20F, counter clockwise if it's directed north, clockwise if it's directed south.
BA
RE: Diaphragms and Rigidity question
RE: Diaphragms and Rigidity question
BA
RE: Diaphragms and Rigidity question
The accidental eccentricity could be either positive or negative (the real distance between mass center and stiffness center can be larger or lesser than the distance calculated), you have to calculate the torsion based on real mass to stiffness center distance is larger than calculated distance