huangyhg

1961 laminate wood beams
i have some existing 5 1/4" x 11 1/2" laminated wood beams in a project built in 1961 that need to be reinforced for increased loading. i am considering applying 3/4" structural plywood to the laminate beam sides and bottom for added width and depth. can anyone in engineering cyberland suggest a source for information on reinforcing the beams; glue types and nail patterns for the applied plywood to make it composite with the existing laminate beams in place? the original design drawings called for fb = 2,400 psi for the laminate beams, but i have no shop drawings on the actual beams to verify wood species and stress rating. beam spans are 20' with 3' cantilever.
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i know this can work, and looks good on paper, however i have never really believed in upgrading laminate beams with timber. i don't think the glue can be appropriately applied in the field, i would never want to guess at the grading of an existing beam with specific drawings (and would probably still hesitate even then!), and i normally just believe in applying a steel plate to the outside surface of the top of bottom flange.
where that plate is held against the timber by loading i have used timber rivets to secure it. if this is not the case, i am really not sure how you might fasten it if your timber section is not large enough for a through bolt. perhaps you could have a custom channel made and screw through the sides?
if you plywood is certainly a higher grade, and you are comfortable with the fastening, there is no reason i can think of not to pursue your original plan. just don't count on the glue!
oh, and by the way, occasionally i hear engineers argue that nail built-up side plate secured beams were very common 100+ years ago. the problem is that in those beams the side plates were just to make two smaller beams composite; you're taking about the side plates being part of the structural
plywood is not very efficient, only half + 1 of the plys are effective, it may be better using lvl. another advantage of lvl is the long lengths that can be used, minimising the end butt joins to deal with.
built up timber beams such as box beams typically only use ply for the webs, the flanges are solid or lvl.
i agree that glue shouldn't be relied on for strength in your application, but it can be used to minimise slip, as long as the correct glue is used.
here is a little general information from a 1970 book.
glue laminated beams
the span tables have this note:
total live load 40 psf
fiber stress in bending 2400 psi
modulus of elasticity 1,800,000 psi
deflection limitation 1/360 of beam span

thanks for your thoughts. i also checked with the american institute timber construction (aitc) which has a few technical bulletins on some aspects of the question on their website.
did you mean 3 'feet' camber or 3 'inches'?
at a large enough curvature, radial reinforcement may become necessary.

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