low deck on piers – earthquake stable?
I’m building a low deck on 10″-high cement piers, 7″ square surface on top with 1 1/2″ wide steel hold-down straps and 11 3/4″ square base. SoCal location, frost is no issue, but we do have earthquakes.
Roughly 16′ x 24′ deck with four 2x6x16′ PT stringers sitting directly on 20 piers spaced 4′ apart; 2x6x8′ joists hung 16″ OC between the stringers; 5/4″ x 5 1/2″ T&G GeoDeck planks secured with SS trim head screws; no railings. Total weight probably 2000 lb?
So the question is, in a moderate earthquake will this thing topple those piers? Do I need some poured piers sunk a foot into the ground to resist lateral motion?
As the ground moves side-to-side, the inertia of the deck mass will make it tend to stay still as the earth’s movement tries to pull the piers out from under, but each pier will be supporting only about 25 to 150 lb (corners support 1/4 as much as center piers, sides 1/2) so maybe the tipping force on any one pier is not much?.
Replies
I can't give you the actual data you will need, but I can offer I have seen several houses "jump" off the foundations in Cali earthquakes------ that was before securing the house to the foundation was code.
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What does your building department say? I'm in the seattle area (still earthquake country) and my building department allows them.
I'm not a big fan of them, however.
My gut feel is that you're fine. The structure will hold the piers together so that either they all tip or none do, and it sounds like you have enough aggregate footprint and a light enough deck that they won't tip (at least not at in any quake that leaves your house standing).
Besides, worst case is you have to play pick-up-stix -- even if something does shake loose it won't be a total loss, mostly just a labor cost to set things right.
In fact, poured piers might make things worse by distributing stress unevenly, increasing the likelihood of structural failure.
Speaking of gut reads, I'd put in wide footings for the piers to sit on and make them bottom heavy. Use some bent rebar or a concrete mesh cylinder might be easier through the footing up the piers.
Building as thou art paranoid never hurt anybody.
Best to you and yours, Chris.
Some say I know too much? Can you ever?
Yeah, that was my only real concern -- that the pier bases should maybe be a bit wider for the height. But realistically speaking it's a very light load on the piers, and the strapped-on framing will provide some additional resistance to tipping. I can't see that it's worth doubling the labor and adding maybe another 50% to materials costs by adding footings. There's no life hazard represented by such a low deck, and, given the unlikelihood of damage in the first place, it would be far cheaper to rebuild than overbuild.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
I wasn't thinking that direction. But now that you bring it up, if it fails, it adds to the repair budget of what else failed when it did. Mute point granted if the whole house is lost.Best to you and yours, Chris.
Some say I know too much? Can you ever?
Remember that the rebuild costs are apt to be covered by insurance, but original costs are not.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Not a StateFarm customer? Good for you.
Well, maybe that wasn't funny.Best to you and yours, Chris.
Some say I know too much? Can you ever?