Cantilevered canopies 1

[shobroco]

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?

 

[paddingtongreen]

What is the length of the backspan?
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.

 

[corus]

My guess is that the flat roof it's connected in to isn't a rigid structure. A flat panel geneally has a low bending stiffness so this will flex as you impose load on to the end of the cantilever.

Tata

 

[msquared48]

I did one for a structure about five years ago, but used L/1000 for my criteria with no problems. It was very stiff and the contractors let me know that, personally.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

are the wood joists in the perpendicular direction part of the deflection you are feeling?

 

[BAretired]

Provide a framing plan of your roof structure. Deflection calculations should not be difficult.

BA

 

[shobroco]

The joists span the width of the garage and the canopy; beams extend back to the rear wall, 24'. They are 1' away from the side walls, and the joists are fastened down to the top plate of the side walls, so the beams are held down continuously on the 24'. The beams sit on steel columns in the front & rear walls. One question that just came up yesterday: I am in Canada & designed with W350 steel, the standard strength (it used to be W300). The fabricator used an American beam, would it be different?

 

[shobroco]

I should have answered the other questions in my last post but I'll try this time: The joists feel quite stiff & don't seem to be part of the bounce. There is no live load on the backspan when I bounce on the cantilever but the beams are held reasonably well.

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!

 

[hokie66]

The yield strength of steel makes no difference in deflection calculations (until it yields, that is).

 

[LowLax]

From your picture it looks like the steel beams don't extend to the end of the cantilever. That could be a problem.

 

[BAretired]

The modulus of elasticity is approximately 200,000 MPa (29.000,000 psi) for 350W and 300W steel.

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

 

[shobroco]

The architect did not want the beams extending to the end of the cantilever for the look of it. The trimmer joists are doubled and the last 3 joists (beyond the beams) are in joist hangers. There is no deflection in the wood, it seems to be all in the steel. I used the weight of the roof beyond the ends of the beams as a point load on the ends of the beams in my calculations, and the rest of it as a UDL. I had the architect (210lb) bounce up & down on the end while I checked the beams at midspan inside the garage. I could not see upward deflection but I could hear movement in the wood framing.

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.

 

[msquared48]

I agree with BP here in that you should look at this scenario with no load on the backspan, and only with two supports for each steel beam - i.e., not continuously supported along the length of the backspan. The resulting deflection will be markedly different. Trust me.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Providing another steel column halfway along the backspan would be a lot less work than welding channels to the beams. Might not solve all the problems of a 12' cantilever, but would help.

 

[BAretired]

shobroco,

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

 

[shobroco]

The roof snow load is 1.2kPa (approx. 24 psf). The beams are W310X45 (W12@30).

It is easy to tie down the backspan so I will try that before anything else.

 

[BAretired]

If the beams cantilever 12' and the roof extends another 3', then the effective cantilever is 15'. A W310x45 seems pretty limber.

Don't forget to check lateral torsional buckling. It could be critical unless you laterally brace the bottom flange appropriately.

BA