Shear in Flitch Plate Beams
Shear in Flitch Plate Beams
(OP)
I have designed a flitch plate beam to be used in a wood framed house, but am baffled by a phrase in an article, which I found trough this forum.
THe clear span is 13'1", loaded at 1470#/lf for a total of 19,228#. THe beam is (3)2x12 with (2)1/2" x 11" steel plates, held together by (2) 5/8" bolts 16" o.c. plus 4 bolts at each end. The ends will be supported by 4" of wood posts the full width of the beam.
THe article "Flitch Plate Beams Design Guide" from STRUCTURE, includes a similar beam in a table of beam section properties. The phrase which worries me is "The shear capacity indicated in table 1 is the shear strength Of the wood alone since the full beam reaction must be transferred through the wood side pieces at end bearing supports".
This qualification would make my beam fail. As I understand it, shear in the wood will be shared with the steel through the bolts, and so not be a problem. The bit of wood which extends below the depth of the steel plates would be subject to compression perpendicular to the grain, which it passes, so this beam is OK for the application.
Am I missing something?
THe clear span is 13'1", loaded at 1470#/lf for a total of 19,228#. THe beam is (3)2x12 with (2)1/2" x 11" steel plates, held together by (2) 5/8" bolts 16" o.c. plus 4 bolts at each end. The ends will be supported by 4" of wood posts the full width of the beam.
THe article "Flitch Plate Beams Design Guide" from STRUCTURE, includes a similar beam in a table of beam section properties. The phrase which worries me is "The shear capacity indicated in table 1 is the shear strength Of the wood alone since the full beam reaction must be transferred through the wood side pieces at end bearing supports".
This qualification would make my beam fail. As I understand it, shear in the wood will be shared with the steel through the bolts, and so not be a problem. The bit of wood which extends below the depth of the steel plates would be subject to compression perpendicular to the grain, which it passes, so this beam is OK for the application.
Am I missing something?






RE: Shear in Flitch Plate Beams
I would also consider the d/2 allowance for shear reduction, as with concrete.
You can also install a steel bearing plate between the column and beam of sufficient length to negate the shear problem.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Shear in Flitch Plate Beams
Mike, how do you get to double the shear capacity if there are no checks, splits, or wanes? Isn't that accounted for in the grading?
RE: Shear in Flitch Plate Beams
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Shear in Flitch Plate Beams
However, in the old 1994 UBC, Note #5 to Table 23-I-A-4, there is a factor listed for "No Splits" and "No Shakes" for the shear stress factor Ch. This is what I am talking about.
I have specified "no splits or shakes" on the drawings for years for beams in a shear-critical condition to avoid having to oversize the beam in bending.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Shear in Flitch Plate Beams
I don't understand the phrase "The shear capacity indicated in table 1 is the shear strength Of the wood alone since the full beam reaction must be transferred through the wood side pieces at end bearing supports".
The flitch plates share the load with the wood members in accordance with their stiffness. If the flitch plates resist moment, they must also carry shear. The end reactions of the flitch plates must be transferred to the wood members by fasteners and the wood members must deliver the reaction to the support. This does not stress the wood members in shear if the end fasteners occur over the support. It stresses them in compression perpendicular to the grain.
I believe the quoted article is wrong and I do not think you are missing anything. Just make sure that you account for how the end reactions of the steel plates get into the support.
BA
RE: Shear in Flitch Plate Beams
RE: Shear in Flitch Plate Beams
I thought these were steel flitch plates...
Oh, and from your calcs, if the Fv on this is 95 psi or greater, and if the joists have no splits or shakes, you are on the line of not needing any flitch plates.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Shear in Flitch Plate Beams
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Shear in Flitch Plate Beams
BA
RE: Shear in Flitch Plate Beams
For wood side plates then it seems the configuration is wswsw and not swwws. Correct?
If it is wswsw, then this must be a new beam and not a retrofit beam.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Shear in Flitch Plate Beams
RE: Shear in Flitch Plate Beams
RE: Shear in Flitch Plate Beams
Also, I am concern about your required bearing. reaction/((compression perpendicular to grain)x(beam width))= bearing length gives for DF-L 19228/(625x4.5)= 6.84" required bearing length for the beam. What species of 2x are you using?
Garth Dreger PE
AZ Phoenix area
RE: Shear in Flitch Plate Beams
The construction is new. The beam design is WSWSW. THe species under consideration is hemlock #2. If the shear is carried by the wood only, as the article indicated, it ends up being 285 PSI, so even doubling the 85 PSI I was using won't save me.
I was thinking along the lines of what BA retired wrote. It seems like my options are to increase the bearing at the ends so I can put enough bolts there to carry the load from steel to wood - which will also eat away at the amount of wood and so weaken it - or to put in a steel bearing plate in direct contact with the flitch plates, so the wood only has to carry it's own proportion of shear. I had speculated about that before, and seems the easiest thing to do.
THanks again!
RE: Shear in Flitch Plate Beams
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask