12-05-2009, 06:06 PM
The reason for individual posts/piers is economy and minimal use of materials. The reason not to use same is that there is nothing to ensure that all the posts will move in the same direction at the same time. If they do there is a much better chance of the house surviving. If they don't and they move in different directions, the house will fall apart. By fixing the posts to the underlying rock you are tying the posts together so that they all move with the underlying bedrock. This is beneficial to some degree but has the drawback that in an earthquake it is the underlying rock that is the source of the damaging motion, plus you are counting on the bedrock to not have a crack in it that runs under your house. So, the underlying rock is a poor man's slab that you assume is solid but may not be, that has no steel in it, and that is an intimate part of the largest shaker table known to mankind. Keep in mind that while tofu block plantation style houses have a certain charm, like many facets of local Hawaiian culture, they are not necessarily the best design. They were built that way because they were cheap. Some people make excuses like "Yea, after a quake you can walk around with a sledge and knock each post back into place a little at a time." Wouldn't it be better to not have the house sprawling all over like a klutz on roller skates in the first place? I say, and I have talked to builders who agree, put it on a good well reinforced slab with a foot of crushed rock underneath. Remember, you don't WANT it to move during a quake.