The effect of Dead Load on Base Shear 1

[eng.chix]

The interpretations I’ve assumed for seismic design is that ALL structural weight is to be considered for seismic design, namely Base shear. What I struggle to grasp is the effect gravity loads that are being supported on the foundation only (ie. Masonry walls spanning from u/s of 2nd floor to main floor) have on the base shear value.

I’ve attempted this 2 ways:
1. Hand calculation of the full mass of the structure and used it to calculate base shear
2. Model in SAP 2000

The two methods are identical if I take the base shear/weight for each and compare.

But what I’ve noticed is that SAP 2000 only uses the weight that is seen by the structure. It does not seem to use the weight of anything that is supported on foundations only. So it's base shear is significantly smaller than the base shear I calculate.

Now, I could manipulate the weight in SAP and force half of the weight of the foundation only supported structure ‘up’ to the storey above similar to what you would do for wind. But I struggle with this, since the mass of the structure is not a live load. It is a dead load and as much as you may try to force it to, dead load will always do what dead load does and force ‘down’.

Does anyone have any insight on this topic they’d be willing to share?

 

[bones206]

So the definition of "base" with respect to seismic loading is the elevation at which the motion of the ground gets imparted to the structure. So for simplicity, if you consider the ground motion to be imparted at the top of foundation, the seismic base shear includes all of the tributary mass above that level.

 

[eng.chix]

@bones206 Understood.

So that leads me to my next question:

For foundation supported systems such as infill walls and exterior walls who's support conditions are essentially pinned/fixed at the base and then roller/pinned at the top, do you need to include the weight of these systems in your calculations of the seismic shear at the roof or second level, or do you concentrate those to the foundation/base level only?

 

[bones206]

If a wall is laterally supported both top and bottom, the top half of the wall mass is tributary to the second level diaphragm and the bottom half of the wall mass is tributary to the foundation level.