Hi,
I was wondering if anyone know why the wall weights in problem #19 of the Heiner Notes are divided by 2 for calculating center of mass? I would really appreciate the answer. Thanks
You need to add weight of the walls since they are tributary to the overall weight of the diaphragm but only require half the weight since the ground will resist the other half. Horizontal forces (wind, earthquake) are distributed along the height of the wall and resulting forces are split between the roof and the ground. You are determining the weight of the diaphragm, not the weight of the entire structure.
But you are doing the CM of the building, and it should have nothing to do with the load on diaphragm. CM is only one and it's independent of seismic context. You should take the whole height of the wall in my opinion. Other authors than Hiner do that.
Just looked at another problem from Mansour practice exam (Problems #22 thru 28) and saw the entire weight considered for calculating center of mass and then later used to find eccentricity and total torsional moment. The code says all tributory loads ...not sure which is correct.. In my own opinion.....If a question is asked simply to find the center of mass i would go with all loads but if the calculation of center of mass is used to find eccentrecity and torsional moment for a rigid diaphragm i think only half the wall should be considered...A correction needed for Mansour notes?
A building can't have more than one CM. Even if you go by Hiner version, it's is in practice that 1/2 wall heights are considered to load the diaphragm, but NOT stipulated by codes. I'd say Hiner need to do the correction, not Mansour. Man, and I thought Seismic is much less confusing than Survey
That has been one of the big confusions i have had in the seismic test. Baradar and Hiner take half height of the wall, Dr Mansour takes full height.
I agree with Silva, there is only one CM in a structure. Personally, I will take only half height for computing the CM.
