sdz
Structural
- Dec 19, 2001
- 562
Background: I'm actually designing water tanks in Australia but due to lack of relevant information I have turned to API-650 for some design criteria. API-650 uses Allowable strength design (ASD) while Australia used Limit state design (LSD). I'm trying to understand the API-650 provisions to formulate comparable LSD criteria for unanchored tanks.
Now I think I can more or less understand the first two. These assume a low probability that maximum overturning effects will occur concurrent with minimum restraining effects; i.e.a completely empty tank.
1) corresponds to an empty tank with 60% of maximum design wind load plus maximum internal pressure; with a 1.5 safety factor applied to the shell dead load (but why not to the roof dead load?)
2) corresponds to full wind load with a partially empty tank plus plus normal operating pressure, 50% full; with a 2.0 safety factor applied to the shell and contents (but why not to the roof dead load?)
But what does 3) correspond to? Why is there a roof dead load but no roof wind load contributing to overturning?
5.11.2.1 Unanchored tanks shall satisfy all of the following uplift criteria:
1) 0.6Mw + MPi < MDL /1.5 + MDLR
2) Mw + Fp(MPi) < (MDL + MF)/2 + MDLR
3) Mws + Fp(MPi) < MDL /1.5 + MDLR
Fp is the pressure combination factor, see 5.2.2; the ratio of normal operating pressure to design pressure, with a minimum value of 0.4. MPi = 0 in my case for a normally vented water tank.
Mf calculated for tank 1/2 full (5.11.2.3)
1) 0.6Mw + MPi < MDL /1.5 + MDLR
2) Mw + Fp(MPi) < (MDL + MF)/2 + MDLR
3) Mws + Fp(MPi) < MDL /1.5 + MDLR
Fp is the pressure combination factor, see 5.2.2; the ratio of normal operating pressure to design pressure, with a minimum value of 0.4. MPi = 0 in my case for a normally vented water tank.
Mf calculated for tank 1/2 full (5.11.2.3)
Now I think I can more or less understand the first two. These assume a low probability that maximum overturning effects will occur concurrent with minimum restraining effects; i.e.a completely empty tank.
1) corresponds to an empty tank with 60% of maximum design wind load plus maximum internal pressure; with a 1.5 safety factor applied to the shell dead load (but why not to the roof dead load?)
2) corresponds to full wind load with a partially empty tank plus plus normal operating pressure, 50% full; with a 2.0 safety factor applied to the shell and contents (but why not to the roof dead load?)
But what does 3) correspond to? Why is there a roof dead load but no roof wind load contributing to overturning?