Max Span Length of Cantilever Canopy ?

[JoeH78]

Dear Colleagues,

I've been asked to design 40feet span cantilever canopy, I run a quick analysis and resulted in following P/M ratios as shownin in the attached image, cross section check seems fine but should I trust the results? Aside from those cross sectional check, where I more worry about the overturning moments, quick hand calaculation to the bottom of column(where I totally neglect balancing force of left sided truss, and only take vertical component of left side diagonal bracing) yields that overturning moment is almost equal to each other, that ratio is 1, normally it should be at least 1.5 as safety factor.

Am I correct in my overturning moment worries or the computer calculation is misleading?

Regards,

 

[KootK]

Super cool. I don't see why you'd need to neglect the effect of the left side truss it's tension tie. Just ensure that:

1) you're using the appropriate overturning load combination per your governing code.
2) you're using suitable and, where applicable, code appropriate assumptions regarding load patterning and partial loading.
3) you're using a reasonably conservative estimate of your loads where those loads resist overturning.


I think that stability and appropriate assumptions regarding bracing etc are your biggest concerns here. I also don't much care for the way in which your deep and skinny trusses meet. I'd introduce a k-webbing configuration in the deeper truss so that the bottom chord of the shallow truss comes in at a triangulated joint. That should lower the D/C ration for the vertical member that separates the two trusses.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

You may want to provide additional members at the main support so that the bottom chord on the LHS is framing to a panel point rather than mid member.

Dik

 

[Teguci]

"...but should I trust the results?"

Do a rough numbers hand calc on your 2 higher stressed elements. How does that compare to the model's results? Repeat and refine until you trust the results.

Now, take a step back and look at the design. Does it make sense? This is when you should fix that discontinuity. What about lateral wind loading? Any torsion on your columns? What kind of deflection are you expecting? Any thought towards harmonics?