Post loading on existing basement floor 1

[joep23]

We are building a new house over an existing basement. The new house will have a different placement for the support posts. But we would like to avoid tearing up the raised insulated floor and existing concrete to pour new pads for the 8 new posts. The print for the new house specifies a maximum load of 15168 pounds at each post. Would it be possible to place a thick steel plate on the floor under each post and avoid tearing up the floor to put down new concrete pads. We would obtain steel that is thick enough to not bend and keep the load uniform over the area of the plate. If so how big (area) of a plate would we need under each post so the existing 4" concrete can withstand the load? This basement was poured 12 years ago over course well drained sand.

 

[GeoPaveTraffic]

First of all, I recommend that you talk to/hire a local structural engineer to evaluate the situation. Some things to consider:

1. what is the strength/condition of the existing 4" concrete slab? Many relatively new floor slabs are not placed with very good concrete or on a very good base.
2. how thick is the sand and how was it compacted and what is below the sand? Much better support is required to support posts of the building than just a floor slab.
3. if you persist with this idea, concrete would be a better choice than a steel plate. While steel is very strong a steel plate is not stiff; and stiffness is what you need to keep the load uniform over the floor. What you are trying to due is create a column capital in reverse. Look at the top of a column where it meets the floor above in a parking garage or building. That is what you will need between the post and floor.

I expect that you will need to saw cut the floor and place new footings for each of the new posts.

 

[joep23]

Thanks GeoPaveTraffic. I can answer your questions but the more I think about this the more scared I get.
1. The existing concrete slab is in great shape.
2. I helped a contractor pour it over 6" of compacted sand. It was compacted with a compactor and I ran the compactor extra after the contractor left for the day.
3. I am concerned that digging down through this consistant sand layer in the footing pad areas to give the pad area enough deeper support will end up going into the clay underneath. This hole would then be filled with sand or gravel. This will create a place where water may accumulate in the sand or gravel and cause the surrounding clay to become soft instead of just draining away into the tile placed under and around the basement.

It seems like the best solution would be to totally rework the whole basement floor or leave it alone and do the steel plates. I will look for a structural engineer. But I really don't feel good about where this is heading. The local inspector has said that we just need a 16"x16" by 8" deep concrete pad under each post. But that is small compared to others I have heard of and it seems like that is their standard answer no matter what the soil condition or max. load from the house.

Finally, you say that the steel would not be as stiff as concrete. But there must be some thickness of high carbon steel that is as stiff as some thickness of concrete. I have looked in the phone book for a structural engineer. But I have no idea how to pick one. I guess I will just have to start calling around. Thanks again for your insight.