PEStructural
Structural
- Oct 17, 2002
- 128
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 ). AWWA D-100 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?
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 ). AWWA D-100 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?
