Composite Section 5

[dik]

Has anyone used an adhesive to attach steel strap to dimensioned lumber wood joists to improve the load carrying capacity? I'm thinking of gluing 1/8" x 1-1/2" straps on existing 2x10 joists and maybe pre-flexing the floor joists to offset some of the present dead loading.

Dik

 

[valleyboy]

I'm sure that this is feasible - but is there any reason you can't simply screw the steel straps to the timber rather than use adhesives?

I'm not sure what your application is, but I would have concern about deterioration of adhesives if exposed to moisture etc. (although this could be overcome with correct specification of the adhesive).

 

[Lion06]

valleyboy-
My guess is that because the strapping is so thin, you would have SERIOUS problems with buckling of the strapping unless you space them at something ridiculous, say 2" or 3" o.c.

dik,
Can you use adhesive just as good practice and to help with using fasteners at a reasonable spacing? That strap seems awfully small, could you just use a 2x12 with a better grading?

 

[valleyboy]

Hi Structural EIT

I think Dik is proposing to use the strapping as a tension member applied to the bottom face of the joists, in which case buckling should not be a concern.

This type of strap (38x3mm) is commonly used in the UK as tension members to aid resistance to wind uplift for timber roofs.

 

[COEngineeer]

I would at least use nail @48". I am sure it will work. Now that you said it, maybe I'll use it for my future remodeling project. Dik, can you still nail something easily to the bottom of the composite members when you finish the ceiling? It is a little hard to make sure you go trough the provided holes. Thats the only concern I have. You could also use flat 2x4 at the bottom of 2x10 maybe?

 

[dik]

I was thinking of fasteners at about 12" centres to 'clamp' the metal strap to the wood... tension for sure and was thinking of using for compression at the top. Essentially the shear would be developed in the glueline.

I've a query in to 3M regarding one of their contact adhesives.

Looking at treating the steel and wood as an elastic composite section using an equivalent n value for Ew/Es. Existing electrical/mechanical issues make 'sistering' of floor joists impractical.

Dik

 

[JAE]

dik, you mean you don't want to consider anti-gravity paint?

I think your approach sounds reasonable. But I just never felt comfortable using glue for structural applications within a load path.

 

[dik]

Thanks, JAE... and I agree about the adhesives for long term... the application is converting a residence to a commercial use with an increase in LL from 40 psf to 100 psf. Somewhat smoke and mirrors since the 'real' LL will likely be less than 20 psf, but the reinforcement should have a high confidence level. My query to 3M:

m looking for an adhesive to reinforce existing wood floor joists with steel straps to increase the load capacity and was wondering if 3M™ Scotch-Weld™ Neoprene High Performance Contact Adhesive 1357 would be suitable.

The glueline must be capable of supporting long term shear stresses. If the above is suitable, can you provide an indication of the ultimate shear capacity and if a safety factor of 4 would be adequate? or a reasonable safe working load? Does the adhesive maintain its strength over an extended period of time when subject to repetitive loading?

Are there any other adhesives that would be better suited?

thanks, Dik

I'll check with them about moisture issues... I didn't think this would be an issue with a neoprene based adhesive, but will check.

One reply that I hadn't considered, but would have if the application works is the attachment of the ceiling... have to scab something on the side or glue a nailer to the bottom.

Dik

 

[COEngineeer]

Or maybe use a hat channel for ceiling finish attachment.

 

[dik]

I've heard from 3M and they suggested I contact LePage's... Recommendation from LePages is to use PL Premium for gluing two non-porous surfaces and glueline strength will be much greater than wood. If one surface is non-porous, PL 400 can be used. I'll be using PL Premium...

Dik

 

[PMR06]

LL increase from 40psf to 100psf on existing 2x10's is tough, no doubt!

I'd make sure to watch out for the bending stress on the compression side also. You've added steel to the tension side doubling Sbot, but Stop only increases 30%.

And don't forget about shear!

 

[dik]

Thanks PMR06... fortunately most of the joists are in good shape (60 years old) and spans are short... only a couple of areas that require reinforcing. Shear, no doubt, will be a bit of a problem, but looking at transferring load near top of beam and very near support... will sort it out somehow... There has also been an increase in allowable shear values from the ones that were likely assumed 'back then'. I've nearly completed an excel file that calculates new section modulii and shear based on initial DL + preflex load + new DL and LL... It allows me to tinker with the various reinforcing straps...

Dik