built up wood beam, 4x6 conn. to bottom of a 4x10

[keyPitsimplE]

A client wants to strengthen a 16' long roof beam by adding section to the bottom (aside from all the other ways it can be done). 7' roof trib. w/ 15 psf DL and 25 psf SL. Existing 4x10 DF2 is under-designed, so placing a 4x6 DF2 under it will create an adequate section. I = 936 in4.

I found the location (2' from the end) where the moment capacity of the orig. 4x10 exceeds the design moment. I'll start my built up section there. Starting at 0' seems unnecessary and the shear flow there is really high. The shear in the beam is 1754 lb at 2'. Q = 3.5" X 5.5" X 4.625" = 89 in3. VQ/I = 167 lb/in.

I move to 4' from end, V=1169, so VQ/I = 111 lb/in. At 6' it is 56 lb/in. At 8' it is 0. I can acheive these loads with split rings spaced as needed. I'll jack the beam to 0 defl. prior to installing.

Are my numbers and methodology correct? Not sure about Q.

If we decided to use a steel plate on the bottom instead, with SDS screws, could someone share the proper way to do the calcs with transformed the sections?

I know this is a lot, but I also wondered about using a steel tension rod on the bottom, in a 'V' shape, anchored at the ends and held off with a steel post at the center. I would love to see a design example of this old school bridge technique.

Thank you very much in advance.

 

[Lion06]

I didn't run the numbers on your 4.625", but that should be the distance between the NA of the new 4x6 and the NA of the composite section. Additionally, I needs to be the composite I.
Additionally, you need to extend the reinforcement past where it is needed. That extra force needs to be in the reinforcement at that point (fully developed - not just starting to develop). I liken it to rebar. You wouldn't start the rebar at the critical section, it needs to be fully developed there.

 

[keyPitsimplE]

StructuralEIT,

What I used for Q and I is as you described. Great, one hurdle down.

I do understand development of rebar, but it seems to me this is different. The split ring is an anchor, not needing to be developed.

If I have a single split ring located at 2' from the end and it was designed for 167 lb/in X 12" = 2004 lb, I would be conservative because 167 is the highest load along this 12" length. It decreases as I approach the center of the beam. (Of course, the new 4x6 would need to run past this to maybe 12" from the end to keep the split ring from tearing out the end.) My next connector would be at 3' back from the end and designed the same way. And so on.

From 0' to 2', the original 4x10 is sufficient for the shear and bending.

Am I missing something?

Thank you very much for being willing to discuss this in depth.

 

[Lion06]

I definitely see the point, but one thing that is left out of this kind of thing, however, is the stress concentration that occurs when you have an immediate change in section. There is a shear lag effect that will not allow the full section to be utilized as soon as it is physically seen.
I have to admit my ignorance, as I've never dealt with a shear ring, so I can't speak too intelligently on the mechanics of it. That being said, if I design a WT to be welded to the underside of an existing WF to reinforce that existing WF I would determine where I actually need the additional section and then extend it beyond that point as much as required to develop the full tensile strength of the WT via the weld. I think if you follow that same kind of logic, it should be fine.

 

[keyPitsimplE]

Got it. Thank you for hashing that out with me.

You can see split rings in Ch. 12 of the 2001 NDS. There is an example solution on p. 153. They are an old, heavy timber connector. Great when you need a large amount of shear in a small location, and you can't get double shear for a bolt.

I wish I could find double sided nail plates with published load values to use in the same manner, but I can't. They would be much easier to install.

 

[civilperson]

Glue or epoxy adhesive can attach the two pieces of wood with sufficient shear strength. I would temporarily prop up the beam load prior to attaching the added piece. Nails will hold until adhesive sets.

 

[keyPitsimplE]

civilperson,

That would be wonderful! However, my lack of use of structural adhesive has always been predicated on insufficient info from the manufacturer. I have to prove to the building dept. that it will work as I say it will.

Can you recommend an adhesive with published load values and sufficient specifications for the gluing operation such as moisture content, temperature, clamping force, type of wood, volume of glue, and other things that greatly affect the performance of the adhesive?

Thank you.

 

[IFRs]

I would jack it past level and then let it settle down so the floor above is level when you are done. I also agree that the addtional wood should start before it is needed if only because the joint efficiency is less than 100% (if not now, then in the future)

 

[HouseBoy]

Perhaps adhesives such as those used in the glu-lam beam industry would be suitable.
I don't know what controls are used in fabrication (and therfore might not be available to you here) but it seems the behavior characteristics would be the same. It's just that the thickness and number of plys is different.
I'm not sure i'd want to rely on adhesives alone but I'm just speculating if such material would be well suited for your needs.

 

[keyPitsimplE]

Yes, I have speculated on adhesives many times on many different applications where it seemed to fit just right. I would certainly use some reliable redundant connectors. However, I have yet to find one with adequate specs. that is easy enough for the average contractor to achieve with real field conditions. I think glulam glues are probably just like manufactured truss plates in that they are not approved for use by the general public.

I have already solved the intial problem above with the client in a completely different and much simpler way. Imagine that.

Again, if anyone knows of a good adhesive with complete and useable specs, I would appreciate it for future use.

Thanks to all who posted comments.

 

[strWennie1]

I would use an epoxy. An unfilled on will not be thicksotropic (sp?). Epoxies get good strength even with some gaping. You can put a micro-sphere filler in the resin.
Not that I would use this system- but google on "west epoxy system". They have many examples on how their system has been used.


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