Uplift Capacity of Sill Plate Anchor Bolts

[DaveAtkins]

How does one determine the uplift capacity of an anchor bolt which holds down a wood sill plate? I have been limiting the capacity of a 1/2" diameter bolt to 315#, based on the bearing capacity of the 1" diameter washer (minus 9/16" diameter hole) against S-P-F wood. However, I think this is too conservative.

 

[JAE]

I think you are correct that the capacity is based on the bearing area of the washer calculated with the value of the allowable compression perpendicular to grain.

That said...where do you get such a condition? Most of our wall designs are based on the shear on a shearwall where the anchor bolts are used to take lateral shear ONLY and the uplift at the end of the shearwall is contained by steel holddowns. Do you have a net uplift on a non-shearwall wall?

 

[redhead]

The leeward wall of a gable roofed structure will usually be subject to net uplift from wind forces on the roof, without the presence of shear forces.

 

[DaveAtkins]

Every one story wood framed building must be designed for uplift on the roof. Per the IBC, this uplift will almost always exceed 0.6 times the dead load of the roof and walls, resulting in a net uplift along all walls which support roof trusses.

 

[sven]

Dave,
Timber has a very high short-term strength and therefore under wind load you can generally use a significant "overload" factor. In the Australian code, a factor of 2 is allowed for this. Otherwise, your approach sounds correct.
Sven.

 

[JAE]

sven - Same thing in the U.S. The NDS spec allows a duration of load factor = 1.6 for wind on wood.

 

[Ron]

DaveAtkins...you have two capacities to check in this case...1) Pullout resistance of the fastener, and 2) "Pull-over" resistance of the wood. Most of the "conversation" has been regarding item 2. It is not always the controlling factor.

The value you gave is very conservative. If you consider the shear of the wood (that is ultimately the failure mode in uplift failure, even though initial "failure" is in compression parallel to the grain)under the shear plane of the washer, you will find a relatively high value. As an example, for your 1" diameter washer, you will have a shear area of 3.14xthickness. For nominal 2x lumber, the shear area will be 3.14x1.5=4.71 sq. in. At nominal 125 psi allowable shear stress, the load becomes 589 lbs. Obviously, the larger the washer, the greater the allowable load (up to a point!)

Compare this to about 950 lbf. allowable pullout of the anchor in normal concrete. (This varies significantly with anchor manufacturer and type, so check their tables)

 

[inspectorjeff]

sometimes plate bending stresses will dictate bolt spacing.
And here in Fl. Engineers use anchor bolts for overturning at SWS ends...a no no.
jeff