seismic base shear 1

[temp]

I have a book that has the base shear calculated on a one story masonry shear wall building. It uses the total weight of the roof, plus the total weight of all four walls (full wall heights)in the V=C*W equation. I have another book that takes the weight(W) as the total roof load plus 1/2 of each of the four walls to get the base shear. Which one is correct?

 

[godoza]

Both of your books are correct. Depends of your assumptions. I sugest you to study the location of the inflexion point on the moments dyagram assumed. This is the application point of the base shear force. Usually this point is considered on the middle of the wall. The forces considered for base shear force are given by the weight of the elements located above this point. But, even you consider all the height of the wall you will see at last unconcludent difference. Florian Smochina

 

[ishvaaag]

If coming from the formula ZICW/Rw (I use UBC 94 as guide) W is defined the total dead load plus the probabilistically standing live load (a fractin of the nominal live load). Hence using the total weight is truer to the intent of the formula definition.

 

[JAE]

The base shear is "at the base" and should include the entire weight of the structure. But if you are designing various parts of the structure, you will not simply take the total base shear and use it throughout the structure.

Some examples -
If you are designing the roof diaphragm, you'd want to include the roof dead load and the upper half of the two walls which are perpendicular to the direction of loading as this is the load path that goes THRU the diaphragm.

If you are designing and exterior wall for seismic forces perpendicular to the wall you'd only include the wall self weight.

If you are designing the two shearwalls which are parallel to the seismic force, you'd include the upper half of the perpendicular walls (the lower half mass force goes straight down to the ground) and include the roof dead load as well as the shearwall self weight. The connection between the shearwall and the roof diaphragm would NOT include the shearwall self weight.

If you are designing the lateral resistance to building sliding you'd include everything.

Just be sure to correctly visualize the load path that each little "piece" of the building mass takes to get "down" to the ground and this helps to ensure the proper value to use.