Countersunk Bolts and Connection Strength

[tclat]

Hi All,

Where I am, it is common for contractors to countersink bolts and fill the hole in with automotive body filler. In such a case, would you consider the timber member thickness to be limited by the length of the bolt (inclusive of nut and head) or limit it to just the shank of the bolt that is in direct contact with the wood.

Alternatively, is it plausible to deduce that countersinking has not impact on connection strength since full thickness of the wood comes into play very quickly.

Having said that, would you countersink in a 2x??? timber member which is only 1.5" thick?

Thanks

 

[shobroco]

When you say thickness, it depends what you mean. For lateral load on the bolt, the full length of the bolt/width of the timber could be still engaged if the head bears against the side of the hole (bondo is stronger than most wood species in compression). However for tension in the bolt, the timber member thckness is reduced. This may or may not be significant, depending on the thickness. The bolt will likely pull through a 2X with a countersunk hole, but in a 12X the governing stress will either be the tension in the bolt or the compression perpendicular to the grain under the washer. Not very helpful, maybe, but I'd say the answer to your question is "It depends..."

 

[tclat]

shobroco,

The connection is in shear. I agree with you in the sense that for shear, the bolt head and nut will bear onto the bondo which is stronger than the wood. So there should be no reduction is shear capacity for countersinking.

Thanks

 

[woodman88]

Not a good practice, especially for shear connections. The bolt holes in the wood should be larger than the bolt. Unless the wood member has some loading applied to it before the bondo is hard, the wood, under design load, will bear on the bondo before the bolt. If the wood is not "dry" before the bondo hardens, then you have the possibility of the wood splitting.
I would design the shear connection for the reduce length of the side member, due the the countersinking. Plus a overcut (ever though I would note "Do not overcut the countersink depth") safety factor.

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.