Built-Up Beam Moment Splice

[wcfrobert]

Hello all,

I am looking at installing a built-up beam that spans 18 ft. The problem is, I only have access to 2x6 lumber that is 12 ft long; so i have to splice them at the end somehow.

In a continuous beams, you would just splice the ends at the support post. But i do not have an intermediate support in this case.


Is it possible to create a moment connection using just nails such that the built-up beam behaves as if there is no splice in between supports?

 

[Shotzie]

Let's say you're doing a 3 ply built up 2x10 beam. I've wondered before if you could provide 1 additional ply in order to have the splices occur in an offset fashion, meaning that every 3' you'd have one of the 12' long single members ending. You'd have to design the fasteners to transfer load into the remaining three plies. I've never proved to myself one way or the other if this will work. See attached sketch.

 

[jayrod12]

Ug,

I've tried to justify this before, it's really not the easiest to do. Considering you're trying to do it with 2x6 depths make it even harder. You need a lot of nails to generate moment splices, 2x6 doesn't have a lot of room for nails.

 

[Shotzie]

Ah, I missed that you said 2x6 lumber. That would definitely be hard to do unless the demand on the beam was very low. I feel that deflection of an 18' long built up 2x6 beam would be quite bad as well, even if you got the moment capacity to work.

 

[wcfrobert]

Thanks Shotzie and Jay for the quick response.

Yea, using a 4-ply is adequate for ULS but im getting like almost 35 mm deflection which is a bit much.

I'm fairly confused about the this topic in general. The code specifies that the spacing between nails should be less than 4*d so that it deflects together, but if I calculate shear flow to find nail spacing, it is a LOT less.
In addition, with a lot of splices, it gets hard to visualize the load path, so I'm not really sure what forces nails are taking either...

At the splice location, if I just calculate moment assuming it is a regular beam --> then the stress due to moment My/I --> and calculate that resultant tensile force, i get a ridiculous amount of nails which i have to space over a long section. I feel like I am completely off , there's no way we need that much nails.

Any tips?

 

[dik]

6" deep for 18' is pretty shallow for wood, or even steel for that matter. Might consider a paralam or maybe steel beam...

Dik

 

[KootK]

The FBD is somewhat approximate but altogether suitable for a design office solution.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.