Built Up Wood Beam - Beam Stability Factor 4

[codySTR]

Given a 3-ply sawn lumber bending member (face width oriented for strong axis bending), what width value, "b", would you use when evaluating the Beam Stability Factor, CL, in NDS 2015 3.3.3?

For example: Let's say you have a 3-ply 2x10 bending member. Ignore the possibility that you may have continuous lateral restraint or that you may have something preventing rotation at the member's ends (so CL doesn't default to 1.0). Do you evaluate the CL factor using b=1.5" for one 2x10 and multiply your design capacity by 3? Or rather, do you take b=4.5" and determine the bending capacity of the overall 3-2x10?

Thanks, team.
-Cody

 

[Archie264]

Well, it does say, "...in terms of resisting torsional buckling." I wonder what it is otherwise?

 

[Archie264]

A couple of hopefully-related questions on this topic:

1. I often see built-up “beams” of the type described by Oldestguy. That is, “[a] builder using several 10 ft. planks for a built up beam measuring 18 feet long, with a few joints near the middle.” I frequently see them as header beams above garage openings in residential construction (not on my projects). I can't see how it would work and yet, that's how LVL beams are constructed, though, granted, not nailed together. But what say ye esteemed folk?

2. In the past I’ve seen existing lumber joists strengthened by installing a 2x6 flange at the bottom of them, gluing and screwing them to the existing (designed using VQ/It, as KootK noted) making inverted T-joists. Per the research referred to by Timberworks it sounds like gluing makes the difference. But what say ye esteemed folk?

 

[Canuck65]

Archie,

1a. I cannot count the times I have come across multiply wood beams with incorrect splice locations.

1b. The prescriptive code I work under (OBC) allows a splice within 6" of the quarter points of the spans.

1c. If I need to splice, I generally adopt an approach where, if a 3 ply beam is required, than I spec a 4 ply beam and make sure I stagger the splice locations by a reasonable distance, plus include a nailing/bolting pattern (of course, then you get to site and find out the contractor did whatever they wanted and are surprised when you show them the drawings).

2. I have also increased the depth of multiply wood beams or full size timber beams by installing a 2x flange on the bottom. I specify the number of fasteners required by calcs, then throw in the glue as an extra.

 

[Woody1515]

On a somewhat related note, let's say you have a beam with joists attached to it's side via joist hangers. Is the beam laterally supported along the top and bottom, or not at all?