01-19-2010, 08:51 AM
StillHope:
By slab I mean a continuous slab of concrete large enough to tie the whole house together and support it and with enough rebar and also thick enough to not crack unacceptably. If you are looking under your house and seeing concrete but don't know how thick it is or how much rebar there is then I don't think you can claim the full benefits of having built on a slab.
There are some parts of the country that have really expansive soil where pre-stressed steel is installed in the slabs. In these cases the soil expands and contracts several inches over the course of the seasons. The foundation must literally be able to be supported at odd random points without cracking. These slabs are highly engineered to hold up while the ground drops away beneath them. The pre-stressing allows the slabs to do this without cracking and with a minimum thickness. There are big hydraulic rams that are used to stress the steel and expertise is required so usually this is too expensive to do on a single house. Developers are able to take advantage of prestressing when they are doing several houses. Otherwise you can make the slab thicker and put in lots of closely spaced rebar. There will be cracks but they will be many small cracks that are too small for bugs to get through.
I think that when people think about slabs they are picturing older thinner slabs where the effort was not made to get everything you could out of the slab. A badly cracked slab certainly sticks in your mind. Decent slabs that are holding together OK are out of sight and out of mind. I have also heard repeatedly that an amateur built slab is easy to screw up so there will be a certain percentage of crummy slabs out there.
I have been reading about traditional techniques of building stone houses out of rubble stone and timber in seismically active areas of India and Pakistan and there are some remarkable houses that have stood through hundreds of years of earthquakes. They are usually on built-up stone foundations a few feet high. I think this forms a sort of base-isolation, but at any rate a tremendous amount of effort is put into the foundation before they even get to the rest of the house. I'm no expert of course but looking at it that way I can see that if you choose building on piers because then you can perch the house on a rough slope where you can't put an extensive foundation, well that skips step #1 in the traditional earthquake resistant house construction book.
By slab I mean a continuous slab of concrete large enough to tie the whole house together and support it and with enough rebar and also thick enough to not crack unacceptably. If you are looking under your house and seeing concrete but don't know how thick it is or how much rebar there is then I don't think you can claim the full benefits of having built on a slab.
There are some parts of the country that have really expansive soil where pre-stressed steel is installed in the slabs. In these cases the soil expands and contracts several inches over the course of the seasons. The foundation must literally be able to be supported at odd random points without cracking. These slabs are highly engineered to hold up while the ground drops away beneath them. The pre-stressing allows the slabs to do this without cracking and with a minimum thickness. There are big hydraulic rams that are used to stress the steel and expertise is required so usually this is too expensive to do on a single house. Developers are able to take advantage of prestressing when they are doing several houses. Otherwise you can make the slab thicker and put in lots of closely spaced rebar. There will be cracks but they will be many small cracks that are too small for bugs to get through.
I think that when people think about slabs they are picturing older thinner slabs where the effort was not made to get everything you could out of the slab. A badly cracked slab certainly sticks in your mind. Decent slabs that are holding together OK are out of sight and out of mind. I have also heard repeatedly that an amateur built slab is easy to screw up so there will be a certain percentage of crummy slabs out there.
I have been reading about traditional techniques of building stone houses out of rubble stone and timber in seismically active areas of India and Pakistan and there are some remarkable houses that have stood through hundreds of years of earthquakes. They are usually on built-up stone foundations a few feet high. I think this forms a sort of base-isolation, but at any rate a tremendous amount of effort is put into the foundation before they even get to the rest of the house. I'm no expert of course but looking at it that way I can see that if you choose building on piers because then you can perch the house on a rough slope where you can't put an extensive foundation, well that skips step #1 in the traditional earthquake resistant house construction book.