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Flitch plate beams

ironsmit's picture

Does anyone know where I can get information on designing a flitch plate beam ?

I'm building a nearly flat patio roof off of the side of my garage. One end of the 2x6 rafters will be sitting on the top plate of the garage wall ( the soffit has been cut back to allow for this ), and the other end will sit on a beam supported by posts 14 Ft away. Since a 2x6 rafter 24" OC can't span this distance I want to see if a double 2x6 with a 3/16" or 1/4" flitch plate will do the job.  I would like to find a "cookbook" type solution so I don't have to relearn all of the college math I have forgotten.  My understanding is that the bolt pattern\schedule is important so I would need this info too.

I do ornamental ironwork and was thinking that the flitch plate could actually be a decorative element if allowed to protrude beyond the 2x6 at the end of the roof. For the overhang I could weld in a much wider plate and torch cut a nice design.



(post #120751, reply #1 of 4)

IMHO a flitch plate  is a waste of steel.

That said, since all elements are rectangles, use M=sigma *Z

M= your total max load moment on the beam - if you don't know how to figure that, quit now and go buy a 6" I or H beam or get the yard to calculate a glued lam for you.

sigma (2 values of these, one for the wood, the other for the vbertical steel plate) is the ksi rating of the material (e.g. 1200 for wood depending on species and grade, 90000 for normalized 4340 <G> , but I'll bet not using that, are you, so look up A36 or whatever you are using)

Z is the section modulus = (b*H^2)/6, b = actual thickness, H= actual height. Simply sum the 3 pieces.

Bolt pattern relatively inimportant. 

If you are not talking flitch plate but a composite beam in the classic sense, e.g. bolting a flat steel plate to the top and/or bottom of the 2x6, that's a different story, very effective use of steel but more complex calculations. Best refer to a text such as Timeshenko and Young "Strength of materials" or similar structural text, as the bolt pattern is critical and there are usually LOTS of bolts needed, and the spacing IS NOT even. (I once used nearly 100 3/8 lags thru a 3/16 plate on a 16ft 6x12 to double the strength)

(post #120751, reply #2 of 4)

Junkhound:  What the ##### do you do for a living?  I recognize 4340 steel, and it is not common.  Slightly modified, it is used for high strength large caliber artillery tubes.  I vaguely recall it is the steel used for the piece that holds the tail of the B-1 on the rest of the plane.


The GlassMasterworks - If it scratches, I etch it!
The GlassMasterworks - If it scratches, I etch it!

(post #120751, reply #3 of 4)

Don: Since you ask, I are an eng. at the major aerospace company. BTW, most the US commercial jets use 4340 forgings for engine mounts, excellent fatigue resistance in addition to high strength.

Got a 4"  by 4 ft dia blank of the stuff at surplus once to make a "model" BP cannon. Gave up after getting only 2 feet of 1" dia hole started on lathe, have tried to hand forge it also, about 10X as hard to forge as 1090.  

(post #120751, reply #4 of 4)

JH:  Now you know why it was/is used for cannon tubes.  The tube for the 175mm gun was made out of this stuff.  It was 35 ft long.  They trepanned out a rough bore while it was un heat treated.  The final bore & rifling was done on a special gun lathe and was a real bear.  The effective life was only 300 full charge equivalents.  Erosion and fatigue got to it, as good as it was.  I saw the very first production item for the B-1 being hogged out at Marquardt, back in 1986 or 87.  Quite impressive!


The GlassMasterworks - If it scratches, I etch it!
The GlassMasterworks - If it scratches, I etch it!