Timber Framers Guild Forums Tools Timberframe Design Post and Beam joint design to resist wind loading.

Been a while since I've posted but I've just got back into hewing.

I'm planning a new construction and need some help with the design.

Specifically how to design joints associated with a knee brace to resist wind loads.

In a simple post and beam frame the knee braces help the structure resist lateral loads and prevent the frame from wracking.

Under a strong wind load the knee brace on the windward side is generally discounted because it ends up under tension. This means that the knee brace on the lee ward side is assumed to be doing all the work.

This also means that the lee ward joint between the post and the beam has to be designed to resist the forces pulling it apart that are transferred there by the knee brace.

How big those forces are obviously depends on the force of the wind but it also depends on the placement of the knee brace. The shorter the knee brace the greater the force that the joint connecting the post and beam has to with stand.

Once I know how big that force is I know how to calculate the required size of the tenon on the post and the relish on the mortice of the beam.

The problem is I don't know how to calculate those forces

Hi Stuart,
See Ed Levin's and Tom Nehil's Frame Engineering articles in the Guild's journal, Timber Framing, for good summary articles on this topic, particularly issue #87, March 2008. Ed's articles include calculating the joinery design capacity once you know the load.

The only practical way to calculate lateral loads on frames and the resulting stress on joinery that I know of is through Finite Element Analysis (FEA) and that is done by a qualified engineer with computer simulation.