The general consensus on when to start worrying about uplift forces in earthquake retrofits is, "It depends." This is the best answer possible, indicating that we rely on engineering judgment rather than some arbitrary force level.
Another question came up: If tie-downs are used, what is the effect of slack in the tie-down system? Having watched about 1/2-inch of slack develop in my own home addition project, where the only components that are not engineered lumber are three thicknesses of 2x plates, I will be even more demanding about shrinkage take-up devices in the future.
But a bigger question came up, one that I have not seen explored to my satisfaction yet. Engineers often use the self-weight of the building to resist overturning in shear walls. As an example, consider a shear wall that just happens to have exactly the amount of dead load on it that is required to resist the overturning. <<hoping the ASCII-Cad drawing below shows up...>>
Let's say 4k of seismic or wind force is acting from Right to Left at the top of an 8-ft tall, 16-ft long wall. We have a 4k reaction at the mudsill acting from Left to Right. To balance the overturning, we have 250plf dead load along the wall. This gives us an upward reaction of 4k at the left shear wall end-post, and zero reaction at the right end post (because that's how we originally defined the conditions....)
w=250plf
| | | | | | | | | | | | | | | | | |
-------------------- <-- 4k
| |
Ht. =8 ft. | |
| | No vertical reaction
4k --> -------------------- at this end post
^ L=16 ft
|
4k
If you move the force vectors around on the above diagram, you get the same shear force diagram as you do for a cantilevered beam 16 ft. long, supported at the left end, carrying a uniform load of 250plf. The shear distribution in a cantilevered beam is NOT uniform.... For the shear wall illustrated above, we have a shear of 500 plf at the left end and 0 plf at the right. When the earthquake forces reverse, the shear diagram is reversed. This results in "shear slosh" (the shear force diagrams look like water sloshing back and forth in a rectangular tank....) where the ONLY place the shear is 250 plf is at the middle of the wall.
For a shear wall with a loose tie-down system, something like the above will also occur; this could certainly lead to reduced capacity of the shear wall. Of course this is all "in theory," ignoring all kinds of things that occur in reality. But what IS occuring in reality? and what has the biggest effect(s)? Any masters students out there who need a good research project testing shear walls with loose tie-downs? (or NO tie-downs?)
Long ago I heard of a code provision (in Great Britain?) that allowed INCREASING the allowable shear in a wood panel shear wall if it had a uniform load on it. This seems to contradict the above discussion.
If anyone has more information or thoughts on any of the above, it would be interesting to kick around a little. I'm on digest mode and have foreign visitors arriving tomorrow for a few days, but expect this to be completely figured out by the time I check back ;-)
Thor Matteson, SE