Point load on Water Tower Shell
Point load on Water Tower Shell
(OP)
How would you determine the allowable point load on a water tower shell?
1.) Ask a manufacturer (do these exist?)
2.) FEA shell analysis or something along these lines?
Thanks in advance!
1.) Ask a manufacturer (do these exist?)
2.) FEA shell analysis or something along these lines?
Thanks in advance!






RE: Point load on Water Tower Shell
RE: Point load on Water Tower Shell
Currently I don't need to perform the analysis (not in our scope of work) but I could see needing to do this in the future.
EIT
www.HowToEngineer.com
RE: Point load on Water Tower Shell
Also, I'd be worried about welding to a tank. Any coatings, inside and out, will be damaged by welding and have to be repaired.
Our company has turned down this type of work in the past. Cellular phone companies pay big money to attach their stuff to water towers. And the municipalities are very anxious to harvest any dollar they can. But we figured our risk wasn't worth the (small) fee.
RE: Point load on Water Tower Shell
RE: Point load on Water Tower Shell
JStephen- thanks for the reference.
EIT
www.HowToEngineer.com
RE: Point load on Water Tower Shell
Jstephen - I started looking for WRC-297 and WRC-107 what are these and where can I find them. I have started searching and have found numerous threads on them but I'm not exactly sure what they are, let alone where to look in them.
Thanks again!
EIT
www.HowToEngineer.com
RE: Point load on Water Tower Shell
http://www.forengineers.org/store.php?crn=55&r...
WRC 297:
http://www.forengineers.org/store.php?crn=65&r...
The basic issue is analyzing pressure vessel shells for external loads applied at piping nozzles. So the shells in question may be cylindrical or F&D heads (treated as spherical), and the attachment may be a pipe or a rigid plug. Assuming some sort of reinforcing pad is used, you then get back to the rigid-plug analysis for a point load. For the full-blown case where you have 6 loads (3 forces, 3 moments) with a nozzle, you wind up doing a lot of graph-reading and interpolating using the older versions. I haven't used he WRC 537, and don't know how easy it is to use. With a water tower, you may run into issues with the t/R ratios being outside the range covered- pressure vessels are usually thicker relative to diameter than a water tower. The data all relate to either cylinders or spheres, and if youj're actually dealing with the knuckle area, wouldn't be strictly applicable. The loading covered is just from external loads, and the stresses due to internal pressure are not covered.
There are pressure vessel design handbooks by Bednar and Megyesy that have simplified versions of this analysis. There is another common pressure vessel handbook by Moss, which I have not used, and I don't know if it includes similar data. Some of the pressure vessel design software has the WRC 107 analysis built into it as well.
The local stresses allowed for external loads are usually fairly high- in excess of yield- assuming that in most cases, the loads are self-limiting, and due to thermal expansion/contraction, etc.
RE: Point load on Water Tower Shell
EIT
www.HowToEngineer.com
RE: Point load on Water Tower Shell
RE: Point load on Water Tower Shell
I've seen some calculations come across my desk that give what appears to be an 'allowable point load' in range of 2.5k-3k, but I'm not sure how they derived that or if they are even referencing the shell material.
I'm wondering if I could even use a handbook formula from Rorak or Pilkey for point load on a circular plate. But there are many questions as to what to assume for the diameter and end conditions.
I'm thinking if I could create a model; put some point loads on it; see what stress there are; then try to correlate this to a 'handbook' formula, it may be of some use.
The only problem is - with so much 'approximation' when is this useful. Meaning by engineering judgement and experiences you can say < 3 kips is probably ok for 0.25" thick steel. If its 6 kips well now you may be pushing things. But at the same time I'd "like" to go through the exercise.
EIT
www.HowToEngineer.com
RE: Point load on Water Tower Shell
RE: Point load on Water Tower Shell
I think I may need to crack open Timeshanko's Theory of Plates and Shells and start learning how to model plate elements.
TLHS - I'm curious to know more about how you modeled this or atleast the program you used. Did you use many flat rectangular plates?
EIT
www.HowToEngineer.com
RE: Point load on Water Tower Shell
Dik
RE: Point load on Water Tower Shell
I had an overall system FEM plate model using STAAD, which wasn't brilliant for localized stresses. I did some test cases of partial shell high element count models to see the stresses locally. I don't really trust STAAD that much for this kind of thing, though, so to confirm I did some hand checks of block shear failures and local bending on some roughly equivalent flat plates and also using a method for local loads on pipes by M.W. Kellog (the WRC stuff would fill this same general void).
At the end of the day, I believe I used the allowables in API 650 plus some general engineering judgment to come up with an allowable local stress that I was comfortable with.
RE: Point load on Water Tower Shell
RE: Point load on Water Tower Shell
RE: Point load on Water Tower Shell
Dik
RE: Point load on Water Tower Shell
anchor bolt design per ACI 318-11 crane beam design
http://www.civilbay.com
RE: Point load on Water Tower Shell
If you do venture down the analysis route, a word of caution with FEM, have an actual look at the tower. I once spoke with an old engineer who used to work at one of those outfits in his early years, he recalled some being designed as bladders, not shells. A weak point may be at the cradle supports of the bladder.
HTH
VoD
RE: Point load on Water Tower Shell
You’ve kept us guessing long enough, why not show us a sketch of the vessel and the actual loads, with some dimensions, etc. Otherwise, I’m guessing one billion tons on a 3/16" plate, 18' high and 200' in dia.
RE: Point load on Water Tower Shell
BA
RE: Point load on Water Tower Shell
@AMEC - I intend to or atleast try, however I'm not sure I have the software to do so accurately. Any idea on if RISA 3D could give me something reasonable? What program would you use?
DH- I hear ya. I don't mean to keep you guessing, but I don't have a specific project that this is currently applicable. I am anticipating that I will need something in the near future. I will however give some better info. Typically I'd say the point loads are in the range of 1kip - 3kip vertical reaction and 0.5k to 1.5k horizontal reaction. Usually the 'column / leg support' is an angle with a base plate about 10"x10"x1/2" maybe. As for the water tower the tower is usually 40'-80' in the air. Usually domed.
See attached crude sketch.
EIT
www.HowToEngineer.com
RE: Point load on Water Tower Shell
A suggestion on your arrangement: If you have multiple loads in a circular ring, then check total loading on the whole spherical cap of that radius rather than dealing with the point loads. If desired, you could add a circular stiffener around the base of the ring.
Normally, loads applied tangentially to a dome would have low streses and aren't really an issue.
Assuming this is antennas or something similar, you're also increasing the wind loading in the structure and foundation.