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Re: "Standard of Practice" for wood-frame earthquake retrofit tie-downs

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Well, with that sort of personal invitation.....

This raises the whole question of what to do where the new meets the old, both in calculating loads/forces and in evaluating those forces, AND in accommodating those forces with new or existing construction.

I mean, what do we REALLY know about how the EQ forces will gather and route thru the structure above and down into the lower structure that we're reinforcing.  It's not like in a new structure where we pretty much ignore the "nonstructural" interior walls and consider just the "pure" structural elements that we're designing.  So, IMHO, we really don't know how the forces are coming down to the lower shear walls that we're building.  If we only know force distribution to ONE significant figure, why figure overturning/uplift/etc. to 3 or more?  GIGO.  Call it rationalization but I don't see the sense of worrying too much about a little (theoretical calculated) uplift.

To the subject at hand:  If a wall is too short in height to allow installation of a hold-down, and assuming that it has a reasonable length (say several times its height MINIMUM) I find it hard to worry too much about uplift, no matter what the numbers say.

OTOH I do consider it essential to have a reasonable number of hold-downs sprinkled throughout the structure, for general attachment of wood to concrete.  (I'm thinking here primarily of improvements to the earthquake strength of the crawl space and basement and perhaps garage level of a single-family house, my usual project.)  Which is why I include hold-downs at the ends, and at major openings, of ALL shear walls where it's possible to include them, regardless of the numbers  (the exact opposite of the "perforated shear wall" philosophy.)  The code has traditionally given short thrift to vertical forces, and yet homes have bounced off their foundations all over the place (e.g., Coalinga, etc.). 

Another aspect of this is, What about the existing foundation, and its strength/continuity/etc.?  What good is a new hold-down installed in a very old, deteriorated trapezoidal foundation??  Lacking rebar the foundation probably has little to offer other than the weight of a few feet of its length.  Makes the effort of installing a new HD pretty much wasted. 

All of this is addressed to VOLUNTARY seismic upgrades where we're trying to improve the strength/safety of the most vulnerable parts of an older home at a reasonable cost.  If you're adding a story to an existing home the code requires upgrading the existing lower parts to current standards.  In that instance you do whatever you have to do to make it work, including whatever foundation improvements are necessary to accomplish that.  And providing whatever hold-downs are called for by the calcs.

Good question, Thor.

Ralph Hueston Kratz
Structural Engineer

In a message dated 6/11/09 7:42:46 AM, thorm(--nospam--at) writes:
This is a question for SF  Bay Area engineers who do residential or light commercial earthquake retrofits for wood-framed buildings.
For cripple walls that are not very tall (say 18 inches or less), tie-down installation becomes a problem.  Most engineers will have some degree of confidence that the self-weight of the building will resist overturning up to a certain level, without doing extensive calculations to determine tributary loads.
What calculated "raw" uplift (overturning moment/shear wall length, ignoring self-weight) do you categorically ignore?   500 pounds?   750 pounds?  1000 pounds?   Or do you consider each case individually, and use your experience and judgment to decide whether you need a tie-down to resist 751 pounds?
Feel free to e-mail me off-list.   If I get enough off-list responses to average, I will post the result with any highlights.  I'm counting on you, Ralph....   ;-)
Thor Matteson

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