Design of Cantilevered Sistered Member

[Chengineer808]

Project has an existing Glulam Beam 5.25" wide x 17.75" deep spaced at 15'-0" on center. This Glulam Beam is supported over Glulam beams 7" wide x 37.25" deep. The GLB 7x37.25 are spaced 28'-0" on center. This gives the GLB 5.25x17.75 a span of 28'-0" and it has a 3'-6" cantilever at one end supporting a mansard type roof. (see attached snippet of existing conditions)

The cantilevered ends of the GLB 5.25x17.75 are damaged and not repairable. The plan is to remove this cantilever portion of the GLB 5.25x17.75. This means that the mansard type roof is now unsupported. I am planning to sister on a new GLB 3.5x16.5 to allow proper support of the mansard type roof. I will be using a 11' long GLB 3.5x16.5 to sister to existing GLB, 3'-6" of that new GLB will be cantilevered and the remaining 7'-6" will be fastened to the existing GLB 5.25x17.75 with Simpson SDS screws (SDS25600). I wanted to see if you guys had any input on if this design approach seems reasonable?

I have calculated that the mansard roof puts 500 lbs of dead load and 1350 lbs of roof live load on the end of the cantilever. I have also included a tension strap at the end of the sistered member to resolve any uplift loads that occur at that end.

 

[KootK]

I wanted to see if you guys had any input on if this design approach seems reasonable?

In general, I dig it.

I have also included a tension strap at the end of the sistered member to resolve any uplift loads that occur at that end.

As you've shown it, I think that strap will be pretty useless as it will tend to just peel off of the top of the existing GLB. If you're going to do the strap, I'd do it as shown below. That said, I wouldn't bother with the strap personally. I'd get all of your connecting done with two discrete groups of fasteners as shown in the second sketch below. Then just do some sport fastening in between.