Sill plate question 1

[FSS]

Posted this in Structural forum with no response. This forum is actually more appropriate.

Have a basic question regarding wood sill plates for which I can't seem to track down an exact source. According to NDS Table 8.5.3 the minimum edge distance for a bolt loaded perpendicular to grain is 4D for loaded edge and 1.5D for unloaded edge. For a wall subjected to wind pressures (positive and negative directions), this 4D requirement technically prevents the use of 1/2" diameter bolts in a 2x4 sill plate, since half of 3.5 inches is less than 4D distance. Of course, 1/2" and even 5/8" bolts are commonly used as anchor bolts. Can anyone help clear up the apparent conflict?

 

[jheidt2543]

I don't have an answer, but I do have a suggestion. Try contacting the US Forest Products Lab in Madison, Wisconsin. They performed many of the original testing programs to determine structural wood stress levels. I think their webaddress is

http://www.fs.fed.us

If not, search for it on the web.

Just a thought!

 

[IFRs]

Isn't 2x4 a pretty small sill plate? I seem to remember seeing 2x6 or 2x8 when walking through new construction sites.

 

[FSS]

For residential construction, 2x4's are common in one-story structures.

 

[boo1]

Sill plate loads are primarly shear, which are parrell with the grain.

 

[boo1]

Typically framers drill the sill plate to 5/8" for 1/2" and 3/4" for 5/8" anchor bolts.

 

[FSS]

boo1, my concern is when wind blows on wall being considered. Load then gets transferred perpendicular to sill plate.

 

[boo1]

If the bottom plate of this shear wall sits directly on top of a foundation stem wall, bolting the sill down for lateral loads. If the bolts that will have at least four diameters of edge distance on both sides the bottom plate so these same bolts can resist perpendicular forces. This means we should use a bolt diameter less than (5.5" / 8) or 5/8" for 6" wall or (3.5" / 8) or 7/16" for a 4" wall. Since 1/2" bolts are typically required specify with 4 inch wall systems consider adding a 2 1/2" or 3" square washer to spread the loads and decrease the stress concentrations.

 

[Miper]

The information in your initial question indicates that a 1/4 inch bolt would have the required edge distance in a 2x4. Are 1/2 inch bolts required? If so, you should be able to get a variance on the basis of equivalent strength by using four times as many 1/4 inch bolts.

Alternatively, you could nail a 2nd 2x4 on top of the sill plate to provide a doubler at each bolt. Drill through the sill plate and the doubler board. You will now have twice the shear strength at the connection, so 1/2 inch bolts should be OK.

Another solution would be to buy or make a metal "C" section to fit down onto the sill plate at each bolt. I have seen these for sale at hardware stores in the area where joist hangars and rafter ties are sold. If you can't find them, they would be easy to bend from flat sheet metal of medium gauge. Drill a hole in the center so it fits down over the bolt and the sill plate. The "C" takes out most of the side load and backs up the sill plate to help prevent it from splitting out under side loads.

 

[Trussdoc]

As I read it, the initial inquiry was about how the use of a !/2 inch diameter bolt in a 2 x 4 nominal sill plate was justified when the prevailing NDS requires 4 diameters on the loaded side. The use of this connection is grounded in prescriptive code acceptance over a majority of the USA and I believe Canada. However, where shear or extreme lateral load on the wall becomes critical, or where the question of overturning of the wall must be addressed, other systems need to be used. This is why residential construction is frequently moving towatrds use of 6" nominal lumber.

 

[FSS]

Trussdoc, your point is well taken. Problem I have is that another engineer has questioned use of the anchor bolts which have been OK prescriptively for years, but can't be calculated due to the firm 4D limit in the NDS standard. Don't you just love litigation?

 

[keithsines]

The one thing I haven't seen anyone ask is what kind of wind pressure and/or wind speed are you concerned about?

 

[boo1]

Why can't you calulate the allowable pressure?
The NDS is a recommended guide not a code requirement.

 

[burnaway]

I think boo1 had most of the answer earlier...according to Table 8.5.3 of the '91 NDS, when the direction of loading is parallel to the grain, the edge distance is 1.5D. The spacing of anchor bolts is determined by the shear load in the wall, which acts parallel to the grain.

Consider a 20psf wind load perpendicular to the wall. The shear on the anchor bolts, based on an 8' tall wall, would be 80 plf.

If the same wall had 1/2" ply nailed with 8d @ 6" oc, the shear on the anchor bolts could be as high as 260 plf, 3.25 times the load from out-of-plane loading.

So, since the primary function of the anchor bolts is to resist in-plane shear, you only need satisfy the 1.5D requirement.