timber header fish-plate design 4

[bplech]

An unusual construction for a timber header for a room addition to an older duplex residential building. The header is built from two 2x8's and a 3/8" steel plate between the boards. Is it possible to ratio the modulus of elasticity for steel and wood to get an equivalent area of wood and then check the allowable stress and deflection? I am using a timber design computer program to solve the problem.

 

[PSlem]

I always do flitch plates based on steel only. A 3/8" plate at 24,000 PSI is 3 times stronger than 3" of wood at 1000 PSI so the contribution from the wood is not going to help that much other than bracing the compression side of the plate. Especially when you factor in the different moduli of elasticity.

 

[ERV]

bplech,

I agree with PSelm, in addition, use the properties of the wood for lateral support of the plate. In affect what you have is an I-Joist, with the steel being the web and the wood being the flanges.

 

[SlideRuleEra]

bplech - Yes, for a flitch plate as mentioned by PSlem. You can ratio the modulus elasticity to "convert" the steel into an equivalent amount of wood. For example:
if the steel plate and the 2x8's are both 7.25 in high and
E wood = 1.5 x 10^6 psi
E steel = 29 x 10^6 psi
Then the the 0.375 inch steel plate can be assumed to act as a 7.25 in (high) x 7.25 in (wide) wooden member. Of course the composite member that you would analyze would include the 2x8's to make the eqivalent (wood) composite member 7.25 inch (high) by 10.25 in (wide). Also as PSlem mentioned you have got to limit the actual stesses to acceptable values for both the wood and the steel respectively.
Chances are you cannot ignore the wood for shear stress calculations, it often provides almost all bearing since the steel plates bearing area is very small and is usually negligible.
All of this assumes adequate connection (bolting?) thru the wood / steel plate.

 

[Stress02]

I was never convinced that flitch plated wood beams bearing on wood stud walls was a good idea due to the fact that the magnitude of reactions generated by the fact that you needed steel in the first place is quite high when considering that it's usually only the wood portions that bear on the wall. Bearing stresses get large.