02-19-2006, 11:47 PM
Attaching the house foundation solidly to rock will transmit the full effects of any earthquakes directly to the structure of the house. Most slabs are set on at least 6" of compacted gravel. Post and pier foundationss have the piers strapped to the posts but not usually attached to the ground. The piers are usually set on either "undisturbed" soil or on bedrock. Otherwise the soil needs to be compacted. In an earthquake, the piers may move in relationship to the ground, but they will hopefully still be attached to the bottom of the posts.
Actually, in the older houses (prior to 1930 or so), they just set the posts on rocks and eventually the rocks would wander. Then they upgraded to concrete blocks. For the longest time, it was stacked "tofu" blocks. One big block 16"x16"x8" with a smaller 8"x8"x6" block set on that. A square termite pan and the post just set on top of that. I used to nail the pan to the bottom of the post so the pan would hold it in place somewhat. Now you can choose between a precast concrete footing with either one or two straps. I find the two strap ones easier to move around since you can put a big nail through the holes in the straps and with a stick through the straps move the footing easily with two people.
The County now requires the 4' wide shear walls at the corners of the houses. That was in effect since several years ago. If you have an architect or engineer design the plans, then you can have a different foundation.
After Hurricane Ewa (or was it Iniki? I forget) anyway, after the hurricanes, then they started "wind load" concerns. You are supposed to have a "load path" every four feet around the perimeter of your home. This means some sort of either solid lumber or metal strap to attach everything together. The piers are strapped to the posts, the posts are attached with a simpson tie to the rim joists, then every four feet along the rim joist the stud has to have a simpson tie top and bottom. The top strap gets to attach the stud to the rafters as well. There is also more bracing at gable ends.
All this will help your house stay standing in a hurricane, but will do little to protect it from the 100 mph flying coconuts.
Actually, in the older houses (prior to 1930 or so), they just set the posts on rocks and eventually the rocks would wander. Then they upgraded to concrete blocks. For the longest time, it was stacked "tofu" blocks. One big block 16"x16"x8" with a smaller 8"x8"x6" block set on that. A square termite pan and the post just set on top of that. I used to nail the pan to the bottom of the post so the pan would hold it in place somewhat. Now you can choose between a precast concrete footing with either one or two straps. I find the two strap ones easier to move around since you can put a big nail through the holes in the straps and with a stick through the straps move the footing easily with two people.
The County now requires the 4' wide shear walls at the corners of the houses. That was in effect since several years ago. If you have an architect or engineer design the plans, then you can have a different foundation.
After Hurricane Ewa (or was it Iniki? I forget) anyway, after the hurricanes, then they started "wind load" concerns. You are supposed to have a "load path" every four feet around the perimeter of your home. This means some sort of either solid lumber or metal strap to attach everything together. The piers are strapped to the posts, the posts are attached with a simpson tie to the rim joists, then every four feet along the rim joist the stud has to have a simpson tie top and bottom. The top strap gets to attach the stud to the rafters as well. There is also more bracing at gable ends.
All this will help your house stay standing in a hurricane, but will do little to protect it from the 100 mph flying coconuts.
Kurt Wilson