Standpipe water tower analysis question 2

[holttho]

Good afternoon,

I am doing analysis on an existing standpipe style water tower (simple 20ft diameter, 85ft tall) with telecommunications equipment mounted at the top. Typically in this scenario, we would check anchor bolts and overturning, but we are being asked by the jurisdiction to provide calcs that show the pressure in the wall of the tank from the water combined with the wind pressure from the top equipment. We don't typically do anything with pressure vessels, so this is a little outside our wheelhouse. What is the best way to go about this problem?

Thank you,

 

[r13]

The jurisdiction is asking you to perform a structural analysis on the complete structure - water tower plus the equipment under the design wind, with the water tower is either empty, or full.

 

[holttho]

Our typical analysis would assume the tower be empty, as that would provide the least overturning resistance. This analysis request is for when the tank is full and there is column water pressure inside the tank combined with the bending stress from the antennas down at the base.

 

[r13]

Yes, you got it. IMO, it shouldn't be too difficult to justify the stress increase is such small, compared to that without your equipment.

 

[JStephen]

If it's a welded tank, get a copy of AWWA D100-11.
For a typical tank like that, compressive loading is pretty negligible, so you can check it, but it oughtn't be a problem.
The water weight gives you hoop stress in the shell, but zero vertical load. Wind overturning gives you compressive loading in the shell, but zero calculated hoop stress. So the two stresses are considered separately, and both are just compared to allowable stresses, none of the Mohr's circle/failure theories, etc.
For tanks with seismic loading, even fairly low seismic loading, the compressive stress from that will normally be higher than from wind, but antenna loads oughtn't add appreciably to seismic loads.
For most areas, the current (and older) AWWA D100 wind loadings will be higher than the current ASCE 7 loadings. So if you can't make it work per the AWWA standard, consider evaluating wind load per ASCE 7 instead.
The axial stresses in the shell are just evaluated like a big beam, Mc/I + P/A. Allowable buckling is lower based on the thin shell and is covered in AWWA.
Note that AWWA D100 is all allowable-stress design, also that wind and seismic get a 1/3 increase.
I would anticipate any issues to be in anchor bolt size, anchor bolt chair design, and possibly overall foundation sizing.

 

[Geoff13]

The stress from wind is longitudinal (vertical), and is independent of whether the tank is full or empty.

The stress from the water is circumferential (horizontal), and is independent of whether wind is acting or not.

Just as the anchor bolt load is slightly increased by the additional antenna weight and wind moment there would be a slight increase in the bottom shell ring longitudinal compression stress. I suppose you could check that per AWWA D100 allowables to show them the tank can withstand the extra antenna loading.

 

[Geoff13]

Looks like JStephen types faster than me, and provided a more thorough answer.

 

[r13]

The two extreme cases for stress checks are:

1. Tower empty - maximum axial stress on wall elements in direction of wind, f_t/c = P/A ± M_w *r/I, in which, P/A is the stress due to weight of the wall.
2- Tower full - M_w is reduced by the stabilizing effect of water, but the wall element has biaxial stress acting on it - downward stress due to wind, calculated using M'_w*r/I, and horizontal radial stress due to hydrostatic pressure.

 

[HTURKAK]

holttho (Structural)
...... (simple 20ft diameter, 85ft tall) with telecommunications equipment mounted at the top. Typically in this scenario, we would check anchor bolts and overturning, but we are being asked by the jurisdiction to provide calcs that show the pressure in the wall of the tank from the water combined with the wind pressure from the top equipment.

.

Dear holttho , the jurisdiction is right to ask to check the stresses for hydrostatic and wind.
The following snap is from (NZSEE: Seismic Design of Storage Tanks: 2009) depicts the vertical bending moment due to hydrostatic pressure .

.


It is pity that the code ( AWWA D100 ) is silent for this case (the case where vertical bending moment may govern ).
In SECTION 14: ALTERNATIVE DESIGN BASIS ' 14.3.2.2 Analysis. The shell membrane hoop stress may be computed by the formula in Sec. 3.7 or by shell analysis theory.* Boundary conditions for shell analysis theory shall assume a fully plastic moment in a 1/4-in. (6.35-mm) bottom plate thickness regardless of the actual bottom plate thickness required and zero radial displacement deflection.'

Whis para. IMO, the COde is forcing for the FEM analysis rather than allowing...

I do not want to remind how to design the shell, roof, anchors , foundation etc but, advise to perform FEM analysis of the shell at least for the first two -three meters in order to see the combined stress effects (hydrostatic hoop tension+ wind bending (vertical +,- stress )+ hydrostatic bending ) and check the VON MISSES stresses, elasto -plastic buckling.

 

[holttho]

Thank you very much for the assistance on this. These have all been very insightful. Specifically JStephen, we figured that the stresses would be independent, but got ourselves thinking in circles about it. We definitely agree that the thing to check is the anchor bolts and foundation - which is what we typically do.

 

[oldrunner]

Checking the anchor bolts for an empty tank and lateral loads may become an issue. AWWA D100 utilizes base plate friction against the sand and maybe a concrete ring foundation for sliding. Depending upon the amount of windage of the equipment that is being placed at the top of the tank, you may be intruding on the factor of safety for sliding. The anchor bolts are usually connected to chairs some distance above the footing and the tank itself is allowed to thermally grow at the base plate, so the bolts are only resisting the overturning. There you may find another problem as to whether the bolts are to be the first failure or the concrete is to be the first failure.