Slide 4 of 13
Notes:
- Weakness of the Construction
- The posts and their top and bottom connections provide very little resistance to horizontal forces caused by an earthquake.
- Without a bracing system, the posts will topple over and the house will collapse to the ground.
- What has been tried in the past?
- Sometimes wood bracing installed in an “X” or “V” shaped pattern may interconnect the existing posts.
- These braces are typically nailed or bolted to the top and bottom of each post.
- This type of bracing may have been used as part of a repair of previous earthquake damage or as an attempt to provide some earthquake bracing.
- Although this type of bracing does add some resistance when compared to posts without any braces, it has too little strength to prevent damage and possible collapse at this level when subjected to strong earthquake shaking.
- Why doesn’t this work?
- Weakness in braced system is primarily at pier pad level.
- The connection between the bottom of the post and top of the pier must resist horizontal loads transmitted by the braces.
- If a traditional toe-nailed connection is all that is present, the bottom of the post will slide off the top of the pier pad.
- This results in a loss of vertical support for the post and leads to a collapse similar to that for an unbraced post system.
- Adding stronger connections at the bottom of the post (such as a metal post base that is embedded in the pier pad and nailed or bolted to the post) is insufficient.
- The forces generated by the braces will then push on the pier pads and can cause them to slide or possibly overturn.
- Either effect will induce damage into the post and brace system causing it to degrade and possibly fail
