Double Cantilevered Canopy

[Robbiee]

The attached sketch shows a canopy at the entrance of a 4 story building I am working on. Yes, it is architecturally required to look like that. I have done 12’ cantilevered canopy in the past, but not 17’ cantilever supported by 10’ other cantilever. So, I tried the framing shown, put 6’ of snow on top and with some W18 beams, the max. deflection under snow is about L( length of cantilever)/300. Compression flanges will be braced along the beams. Please provide your comments and recommendations.

 

[Teguci]

Looks good to me. Bolts at moment connection will need to be slip critical, but I'm sure you have that covered.

6 ft of snow sounds like Buffalo.

 

[msquared48]

Was the Architect torqued when he designed this? Perhaps he designed Chubby Checker's house too.

Do your results include the deflections due to torsional rotation?

Mike McCann
MMC Engineering

 

[csd72]

Ailmar,

A few things to check:

Firstly with that moment connection detail you have a long buckling length for the plate at the compression flange so you need to check that it has sufficient buckling capacity.

The uneven configuration will result in an uneven front under dead load so you may want to precamber the most highly loaded cantilever to make up for this. With things like this that people can look along even small deflection can be noticed.

 

[PUEngineer]

Is the beam on line 2 deflecting upward? That coupled with the downward deflection on line 1 will make the deflection of those channels 'worse'. Might think about some cambering.

 

[Robbiee]

Thanks all,
I am happy that every note mentioned so far was thought of. Torsional deflection, yes include, slip critical connection will be specified, and for buckling of the bottom plate, thinking of bolting to transverse flange or make it thick enough for the length. The building is in London, ON. Any other notes are welcomed and appreciated.

 

[JAE]

Ailmar - I'd also be concerned about wind loading - specifically whether this structure would begin to oscillate under winds.

I would imagine that the framing will be quite flexible and reaching resonance - or even minor oscillation with fatigue, might be an issue.

 

[Ron]

Agree with JAE...I've done these in aluminum and steel, so you have to check the wind condition. Since your condition is unbalanced, oscillation might be significant.

 

[hokie66]

Go back and check the main beam for torsion with unbalanced wind loading. I wouldn't depend on an open section to resist that type loading.

 

[hokie66]

Forget my comment. I thought there was only one main beam, but now see that there are two. But I agree with JAE and Ron about wind loading being an issue.

 

[Robbiee]

Thanks JAE, Ron and Hokie66. I checked its period and with the assumption of flexible diaphragm, the period of in-plane /torsional vibration is near 2.2 sec and oscillation period is 0.17sec. With the assumption of rigid diaphragm, its oscillation period is 0.15sec. Please note that the canopy is connected to the building at two columns. So, looks like I need to pour concrete on the steel deck or design the steel deck to be rigid.Your comments are appreciated as always.