Flitch Plate Design

[jopalu]

When designing a wood/steel section(beam) with flitch plate, is it reasonable to determine a "transformed" Allowable Bending Stress, Fb, by reason of propotionality?

i.e.
A 4-ply 2x12 SYP #1 (Fb=1,062 psi)(wet-service adjusted)
w/1-ply A36 0.25x11 (Fb=21,600 psi)

A(SYP)=67.5 si
A(A36)=2.75 si
A(combined)=70.25 si
Therefore:
Fb(SYP)=(67.5/70.25)1,062=1,019 psi
Fb(A36)=(2.75/70.25)21,600=864 psi
Fb(combined)=1,019+864=1,883 psi

Please advise... Thank you...

 

[UcfSE]

You do not "transform" stresses. If you transform the steel to wood, use the allowable stress for wood. If you transform the wood to steel, use the allowable stress for steel.

 

[jopalu]

How do you transform one material to the other?

 

[CROWN06]

Modular ratio = n = E steel / E wood . Look into any area or section transformation method in a book or simply type it on internet.

 

[woodengineer]

In a nutshell, here is the process:

1. Run your analysis in the software of your choice knowing that the wood alone will fail. The analysis will give you the shear(V), moment(M), and deflection(D).

2. Decide what wood depth you will use as well as species, which includes LVL.

3. Choose an initial steel thickness, starting with 1/4" plate. Fy=36 ksi (Assume the depth of the steel to be 1/4" less than the outer wood members depths.

4. Calculate the EI for the steel and the EI for the wood separately. Then calculate the total EI by adding the two EI values: K(Total)=K(Steel)+k(Wood), whe K=EI

5. Develop your distibution factors:
Wood Distribution factor = K(Wood)/K(Total)
Steel Distribution factor = k(Steel)/K(Total)

6. You can take your deflection(D) you initially calculated with the wood alone and multiply it by the wood distribution factor. This will result in the flitch beam's deflection.

7. Once you are satisfied with that check the stresses in both the wood and the steel (Both bending and shear) by muliplying the forces and moments by the appropriate distribution factors. Increase the steel in increments of 1/4".

This process is iterative and writing a spreadsheet is helpful, but here is the process.

Good Luck.



Jordan

 

[Lutfi]

jopalu, I am shocked that you are asking this question if you are an engineer!!!

You need to go back to your statics and strength!!!

I would suggest that he does all manual calculations. Using software is not a good idea since he does not know how to transform one material to another.

Sorry if I am being harsh.


Regards,
Lutfi

 

[UcfSE]

I totally agree with Lutfi. That is an early junior year or sophomore year skill.

 

[ctcray]

Jopalu,

This site has great walk-thru examples of what you are trying to do....explaining each step along the way. Specifically, click on "Bending" and then "Non-homogenous"

http://web.umr.edu/~mecmovie/index.html