×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Lag Screw Connection for Timber Rafter

Lag Screw Connection for Timber Rafter

Lag Screw Connection for Timber Rafter

(OP)
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,  

RE: Lag Screw Connection for Timber Rafter

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

RE: Lag Screw Connection for Timber Rafter

(OP)
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.

RE: Lag Screw Connection for Timber Rafter

"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
Motto:  KISS
Motivation:  Don't ask

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources