Seismic Forces at Raised Floor (Wood Builiding)

[omoreno1]

I have a wood building installed on a raised floor foundation. I know typically the assumption for the effective seismic weight in a single story building is to assumed that the lower wall height and floor are distributed directly to the foundation and are not accounted for in the seismic base shear calculation. This is probably the correct assumption since the slab on grade floor is being directly supported by the ground. But in the case of a raised floor, it is my instict to treat it as a story and appply the effective seismic wieght tributary to the floor and assume the weigh of the perimeter foundation was is effectively transfered to the ground. This effectively doubles my shear forces at the shearwall to foundation wall connection. I know this sounds a little redundant, but I would like to have some input so that I do no over design the 3x sill plate to foundation connection.

 

[msquared48]

Unless you have wood pony walls in the order of four to eight feet high above your concrete foundation stem wall, I would not worry about it.

If the floor joists rest on a sill plate that sits on top of a concrete foundation wall, I would also ignore the loads from the floor.

Mike McCann
MMC Engineering

 

[NKT]

I second Mike's "don't worry about it too much" sentiment.

There are typically very few gaps in the attachments between the first floor diaphragm and the foundation, so the attachment loads are usually very well distributed on the perimeter foundation, and are reduced to a level which is negligible.

If you're uncomfortable enough to be asking the question, include it in your analysis for the next few projects and take a look at the increase in the connection loads.