Lag Screw Connection for Timber Rafter

[keyPitsimplE]

I have been allowing my timber frame guys to use single vertical lag (or GRK or Torx Screw) thru 4x or 6x timber rafters as a hidden uplift and shear connection over a beam or top plate bearing. I do not see comments on this type of connection in wood books regarding shrinkage cracks. See attached detail for example which the builder requested.

Has anyone had a problem with this type of connection? It seems very common in timber framing. I can see it 2 ways - either the screw restrains the timber across the grain and causes cracks as it shrinks, or it partially holds these cracks together sort of like reinforcing steel in concrete. After all, some checking is to be expected in heavy timber.

(Of course, the wood is ideally kiln dried, but that does not always happen. As in most wood connection examples, please assume the wood will have some shrinkage.)

Thanks,

 

[woodman88]

In this kind of connection I usually double the lag screw embedment as my safety factor. I am concern that your detail does not show bearing for the intermediate 2x6's, e.i. no birdmouth cut at bearing. Also, as the 4x6 tails you have created are required to be regraded (per the NDS) as a 4x6, I hope you have this done or have enough of a safety factor in your design.

Garth Dreger PE
AZ Phoenix area

 

[keyPitsimplE]

Yes, the embeddment is more than adequate for my loads and the tail is designed as a 4x6. The 2x6 is carrying only ceiling board and insulation for an 11' span, however my intention was to have them cut a min. 1" level bearing and use LUS24, but I have not yet modified the detail.

Regarding my question: It sounds like you have not had trouble with cracking, or the beam bearing raising out of the seat as a result of shrinkage?

Thanks again for your input.

 

[msquared48]

"It sounds like you have not had trouble with cracking, or the beam bearing raising out of the seat as a result of shrinkage?"

From my experience with log structures, I would definitely be concerned with that aspect and overdrill the depth of the hole in the rafter vertically to allow more penetration in the column and less in the rafter. i.e., Bear deep in the rafter depth and the shrinkage problem will be minimized. Two lags would work better here too.



Mike McCann
MMC Engineering
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