Cantilevered canopies
Cantilevered canopies
(OP)
I have recently designed 2 similar residential wood-framed canopies carried by 2 cantilevered steel beams. They are extensions of flat roofs out in front of the overhead doors on garages; the 2 beams carry wood joists. The first canopy was very soft & I had to design stiffening for the beams after it was built. The second one is framed & roofed now & it also feels soft, although I increased the beam stiffness more than I really thought necessary. The cantilever is about 12', the bending capacity of the beams is 65% more than the maximum load, and the calculated deflection under live load is L/360. However, the canopy is easily deflected by walking on it & bouncing a bit; much more than I would expect from my calculations. Is there something I am missing?






RE: Cantilevered canopies
When you bounce, might you be on a natural frequency?
A small deflection can feel big to someone standing on it.
Does your deflection under LL include LL on the backspan? How much does it deflect on twice your weight (allow for impact) as the only LL.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Cantilevered canopies
Tata
RE: Cantilevered canopies
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Cantilevered canopies
RE: Cantilevered canopies
BA
RE: Cantilevered canopies
RE: Cantilevered canopies
msquared48 I take it you didn't care for being told personally that your canopy was stiff (too expensive?) but I'll bet I care less for being told mine is too soft!
RE: Cantilevered canopies
RE: Cantilevered canopies
RE: Cantilevered canopies
Try calculating deflection on the assumption that the backspan is not continuously held down. Nailing joists to the top plate is likely not adequate to prevent upward deflection of the span.
Also, as pointed out by Lowlax, the beams should extend to the end of the canopy. You have a fairly flexible structure beyond the tip of the cantilever.
BA
RE: Cantilevered canopies
Any suggestions for a method if I have to stiffen it? The architect wants the beams to remain exposed. The last one I did was covered so I welded channels inside the sides of the beams on the backspan & about 1/2 of the cantilever.
RE: Cantilevered canopies
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Cantilevered canopies
RE: Cantilevered canopies
What is the design snow load on your roof? What size beam are you using?
I agree with hokie that tying the backspan down at midspan would be helpful.
BA
RE: Cantilevered canopies
It is easy to tie down the backspan so I will try that before anything else.
RE: Cantilevered canopies
Don't forget to check lateral torsional buckling. It could be critical unless you laterally brace the bottom flange appropriately.
BA