SEISMIC CALCULATION

[FiniteElement2012]

Hi, I am attaching my calculation for calculation of in-plane seismic shear loads to parallel wall. Effective Seismic weight of building comprised of 18" reinforced concrete walls, 10" masonry walls and non-composite roof is 266 kips. Assuming the structure to be rigid, I am getting 88/wall of in-plabe shear load at roof. Just wanted to make sure that I am crunching numbers correctly as the loads seem to be on higher side.

 

[msquared48]

The center wall you have referred to as both a 18" concrete wall and a 10" cmu wall. Which is it?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[FiniteElement2012]

The one below imaginary reference line is 10 and the one above is 18". Half the length is 18" whereas the remaining half is 10".
For the two parallel walls (West and East) I am coming up with approximately 216 kips for 18" and 50 kips for 10" front wall.

 

[FiniteElement2012]

Height of the walls is 32'-0".

 

[DST148]

Ignoring eccentricity......the three walls in the north-south direction will share lateral load equally.
For the two walls in the east-west direction, I have made the following assumptions: f'c = 4 ksi, f'm = 2 ksi, Eff. M.I. for concrete = 0.35 Ig, Eff. M.I. for masonry = 0.30 Ig
Given the squat nature of the walls, the distribution of the lateral load is governed by shear rigidity of the walls. 18" thick concrete wall takes about 80% of the lateral load, and 10" thick masonry wall about 20% of the lateral load.
Your sketch shows 266 kips as the effective seismic weight. You will have to calculate the lateral load at the roof level based on seismic parameters for your region. Moreover, for single story buildings, the mass of the structural walls in the direction of the load may not be included in the calculation of the base shear. You may add the effects due to the mass of the walls separately. (the seismic load generated due to the mass of the structural wall will be absorbed by the wall itself without being transferred to the diaphragm)

 

[FiniteElement2012]

Thanks DST148, your input is very helpful. This is indeed one story building.
Effective Seismic Load of 266 kips is for the entire building that includes all walls and roof. For the direction of the seismic force shown in the sketch, I will calculate effective seismic weight (base shear) due to two parallel walls (North and South Walls)and tributary roof. Dividing this load equally among the three walls 1, 2 and 3 will give me in-plane lateral load at roof level for these 3 walls. Similarly, if the load is appllied on East Wall, effective seismic weight will be based upon 3 parallel walls and trib roof. Distributing this load based on wall rigidities will give me in-plane lateral force at roof level. Please correct me if I am wrong in understanding the concept.