Bolt head and washer bearing capacity in a wood member (tension). 1

[pelotoner]

This idea was brought up by DaveAtkins in this thread: thread507-22707.

I have a configuration in a wood wind frame that will cause direct tension to act on a bolt (see attached file). NDS lists perpendicular bearing capacities for So. Pine at 565 psi. However, in the Dowel-Type Fasteners Chapter, the capacity for dowels in shear (same general mechanical force orientation) for perpendicular to grain between 2550-5150 psi, based on their diameter. There is nearly an order of magnitude difference.

I equate the difference to block shear rupture strength in steel. Can anyone give NDS reference or theoretical concept for design basis?

 

[woodman88]

You need to look at the equations/tables for dowels to see all the reductions that are being applied to the dowel bearing strength. Also look at the increase (bearing area factors) that is allowed for bearing width for the compression perpendicular to grain. For 1/2" bearing it is 1.75, for 1" bearing it is 1.38.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[focuseng]

Post your calcs, I don't quite get all your ref and question. The bearing of a washer for a bolt in tension has little to do with dowel bearing failure modes. It is a straight compression failure with all NDS modifiers. Don't forget the washer has a little hole in the middle. In anchor bolt situation, the hole is also quite often oversized by carpenters who like to make life easier on themselves (so add a factor of safety there). Also I believe the new ICC codes stipulate square washers in sill plate anchorage in some cases.

 

[pelotoner]

Ah, in Chapter 3, Design Equations (Table 3.10.4); not in the Fastener chapter. It specifically mentions washers and plates.

Thanks Woodman88

 

[dhengr]

I think you are headed in the right direction. You need a larger washer to distribute your bolt tension into the wood member. You must check compression perpendicular to the grain for the tension load, with all seven hundred modifying factors +/-.

 

[shobroco]

A dowel in shear is entirely different. It induces tension perpendicular to the grain, & the wood will typically split in line with the dowel long before the dowel will exceed the compressive capacity of the wood perp. to the grain, (with possible exceptions when there is an extremely large edge distance).

 

[racookpe1978]

In compression a steel washer under a steel nut under tension by tightening can be pulled enough over time to pull the nut and washer completely through the wood "surface" so both the nut and washer are "under" the original wood's surface.

A wood dowel doesn't behave that way (in part because a wood dowel will have a lengthwise (axial) grain structure) and the wood structural members will be perpendicular to that grain in one o=r the other holes. A wood dowel, under almost all reasonable construction techniques, will be driven firmly into the hole between both wood structural members, and (usually) wedged tight. Further wetting/drying tightens a dowel joint, rather than loosening as in a steel-through-wood-hole joint.