API 650 - Steel Tank Roof Deflection 1

[PEStructural]

What deflection requirement would you recommend for the rafters of a supported cone roof on a steel water tank (for DL + LL)? I know UBC states L/240, but this really isn't a building. Another point to ponder, the only time the full live load of 15 psf would be applied is under a snow load, so then the deflection wouldn't be seen by the eye.

I have a 87 foot diameter by 27 foot high tank with a column in the center of the tank. I tried a W8x10 rafter. The stresses are OK, but the deflection is 4.477 in ( L/100 ). The code states that the friction between the rafters and the roof deck can be used as full lateral bracing of the compression flange (although there is no welding between them). The W8x10 beam deflection is 1/2" under its own dead weight for this length. If there was a wind that applied an uplift force on the top deck, the unbraced length of the rafter would greatly increase and therefore overstress the beam.

Anyhow, any deflection requirement suggestions?

 

[chicopee]

WOULD YOU BE BETTER OFF UNSING OPEN WEB STEEL BAR JOISTS FOR THE LENGTH IN QUESTION? AS FAR AS THE DEFLECTION, YOU MAY HAVE TO USE YOUR OWN ENGINEERING JUDGEMENT. YOU MAY ALSO HAVE TO DIG INTO THE WEB SITES FOR A WATER WORKS ASSOCIATION WHICH I BELIEVE HAVE DESIGN CRITERIA FOR SUCH TANKS.

 

[JoeTank]

I have always understood that the 3/4:12 minimum slope of API-650 was based upon expected deflections of the roof rafters. The intent is to assure a positive drainage slope at all times.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[Tankman650]

I sounds like you have one center pole with the rafters running radial to the side wall. AWWA has a max. rafter spacing of 84" thus you need 40 rafters. This is a lot to come together at a center pole.

Generally, you will have a ring of columns and girders midway between the shell and center ploe to support the ends of the rafters and there by reduce the number of rafters that rest on the center pole. When you do this you will save a lot of rafter steel and reduce the deflections.

 

[JoeTank]

In API-650, the use of friction for lateral bracing is permitted only for the DL + LL condition. For the DL only condition, lateral bracing by friction may not be assumed. Your concern for wind loads and loss of friction would be for the DL only condition.

Regarding the 4+ inches of deflection... the roof rise is excess of 32 inches (assuming a typical roof rise of 3/4" per foot), so the deflection would not be an obvious problem to me.

Frankly, the W8 sounds quite small. I would seriously doubt the ability to safely eresct these long, floppy beams. You may need to look a some form of lateral bracing just for their own dead load.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com