Wood-steel composite beam...DF vs. PSL

[roberto222]

If i design sandwiched wood beam with steel plates, then i use the ratio of steel elasticity modulus over wood...
n=29000/1800=16...so steel converted to DF#1 will be 16x stronger.....and i will use fb of wood for this composite...plus/minus 1000psi

What about PSL beams?
n=29000/2000=14.5...so steel a little less stronger than in DF#1 scenario, but i am using fb of PSL....2900psi

So compare the same sandwich with DF and with PSL.....the sandwich with PSL is 2.9 times stronger than sandwich with wood....i would just expect that wood part plays little role here and that steel is the main member setting the strength.

Can anybody clarify this please?

 

[msquared48]

First off, you do not use the wood to reinforce the steel.

You use the steel to reinforce the wood.

So steel flitch plates are usaually used to reinforce an existing wood beam where increasing the existing wood beam depth is not a viable option for whatever reason.

What is your site condition?

Mike McCann, PE, SE (WA)

 

[roberto222]

That is correct, but i don't understand that if i "convert" steel to DF, i use DF strength (1000psi).
If I convert steel to PSL, I use PSL strength (2900psi), even if conversion ratio is almost the same as if converting to DF.

Site conditions is wood framed building, using this sandwich beam as substitution to wood or PSL beam.....but i would like generally explain, not applying to specific condition.

 

[msquared48]

The moments to the wood and the steel are apportioned according to the EI ratios, for they must deflect equally.

So for example, if the EI of the steel is 9 and the EI of the wood is 1, assign 10 percent of the total moment to the wood and 90 percent to the steel.

Figure out you material stresses from there.

You will have to figure out your shear flow to connect the members together so they act compositely.

Mike McCann, PE, SE (WA)

 

[jdgengineer]

When I design a flitch beam I typically design the steel to take 100% of the load and use the wood nailers just to provide compression blocking for the steel plate and nailing to the diaphragm. As you've noticed, the wood doesn't typically have a huge impact to the plate design. We most commonly only use flitch beams for bent beams under light loads where the wood would be spliced at the max moment anyways.

 

[Celt83]

That is correct, but i don't understand that if i "convert" steel to DF, i use DF strength (1000psi).

You use the material strength of the ply you are checking stress at. You are transforming the steel to be equivalent to either DF or the PSL for the purposes of determining the stress distribution in the various ply's of material. In each ply you will compare the stresses to the allowable stresses of that ply's material.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering