Timber Beam Capacity

[tclat]

Dear All,

I'm conducting a structural assessment of an existing building. The roof structure consists of double 2x6 rafters at 2 foot centers. Once of the rafters is a full length while the other has a joint at different locations but on either side of the joint there are two bolts 6 inches away. The two members are nailed together along the rest of the length with 2 nails every 6 inches. I'm looking for some help in analysing the moment capacity of a beam like this.

I was thinking of analysing it such that each member had a tributary width of 1 foot. I would then consider that the full length member supported the spliced member and as such would induce a point load at the location of the joint.

Any thoughts?

 

[Canuck67]

Read the recent post by ajk1 - "Built up Sawn Lumber 2-spand Continuous Beam". It has much information related to your question.

Why not analyze 2 ply 2x6 built up beam based on a tributary width of 24"? Then look at localized moment and shear at the splice locations. At each splice location, you only have the shear and moment resistance of 1 ply. Following that, interply connections (bolts/nails) would have to be looked at.

 

[jayrod12]

Agree with Canuck67.

I would bet if you did your original train of thought that the single 2x6 picking up the point load from the spliced member would fail miserably.

 

[BAretired]

Splices are not permitted by code in a single span rafter. The moment capacity of two 2x6 members may be adequate at midspan, but the shear capacity of a single 2x6 is likely inadequate.

BA

 

[msquared48]

Except in the case of high snow loads, and high snow load areas on the roof such as valleys and parapets, shear rarely governs for rafter design. It's usually bending or deflection. So, a shear diagram would be helpful here to see where the shear drops below the capacity of one rafter in order to properly analyze the capacity at the splice.

Mike McCann, PE, SE (WA)