Cantilevered Vierendeel Truss

[tdsull]

I am brain storming on a new project. The architect is a moving target so the details are wide open. Essentially there is a 20' to 24' cantilevered vierendeel truss carrying residential deck (60psf LL) and a low roof (40psf LL). The dead load is conventional light frame and a significant portion of the deflection is from live and snow loads.

The truss backspan is continued into the main house and a question about vibration has been raised, both from human activiy and wind.

Does any have down and dirty numbers and or a resource regarding a similar situation. See attached pic for clarity.

Thanks

 

[hokie66]

For moving targets, wide open choke is best.

 

[MiketheEngineer]

I think some heavy duty RISA or other software analysis is in order......

You are out on the edge a bit here.... Can it be done - sure - but a LOT of analysis will be required.

Wind uplift, snow, moving people, vibrations, deflections, etc....

Good Luck

 

[kslee1000]

Can you set the trusses on a slope to off-set the dead load deflection, or even partial anticipated live load deflection, at the tip of the cantilever?

For regular residential building with carefully selected flooring material, unless a dancing hall is included, I don't think vibration is a problem simply from human activities. But, depending the region, wind could be a concern if the structure is too flexible. Check out articles on wind engineering should help.

Beware of stress reversal on the trusses, and uplift on the connections.

 

[slickdeals]

Does it have to be a vierendeel? Can an exposed truss be used as an architectural feature? You could use all your diagonals in tension.

Your floor is probably 12' deep and your cantilever is 25' and all you really need is continuous chords.

Again, I don't know if door openings are a concern.

 

[msquared48]

Considering drainage, I would allow the deflection to go away from the house and not try to compensate for it. Most people don't like water in the living room - unless they're Olympic swimmers.

You are going to need some horizontal sway bracing too, not just the

Hokie - think Phalinx... screw the choke.

Mike McCann
MMC Engineering

 

[msquared48]

Sorry - that's ... not just the floor and roof diaphragms.

Mike McCann
MMC Engineering

 

[hokie66]

Mike,

I didn't even know what that was until I looked it up. I take it you mean Phalanx. But I think that's overkill (sorry, I mean excessive) even for an architect. An old fashioned scattergun should do just fine.

 

[msquared48]

You're right...

Mike McCann
MMC Engineering

 

[tdsull]

All your input is appreciated and falls along my current line of thinking.

I have a few types of preliminary models running some in RISA some in IES.

The Vierendeel truss is not the only option. The client was surprised with some of the initial sizes so we are presenting a few other options today. One option is a diagonal brace down from the 1st floor deck (out near the end of the catilever) and the other is a ‘V’ shaped cantilevered truss.

Back checking model results with a hand approximate method will be my challenge.

 

[slickdeals]

Wl^2 and M/d will give you a very good approximation. You can use 0.85 * 2 * (area of chord) * (distance to N.A of truss)^2 to get an approximate moment of inertia for deflections.

 

[csd72]

slickdeals, no that is a very inaccurate way of estimating deflection, vierendeels have a lot of shear deflection, usually more than flexural.

The beam is afterall basically a multi story portal frame on its side.

 

[slickdeals]

csd,
You are right, I was giving approximations based on a regular truss, however it will not be appropriate for a vierendeel.

A 0.85 factor is used to account for web shear deformations in a regular truss (i think i have seen that recommendation in joist catalogs and composite joist publications by AISC)

 

[plasgears]

Vierendeels imply moment resisting connections, and the sketch doesn't appear to reflect this. Go conventional.