Geotechnical Investigations for Water Tank Foundations, pre-AWWA-96

[Liadain76]

My firm is currently analyzing a water tank's geometry & foundations, and are running into some insanely high working percentages for the leg and riser foundations. The water tank is a multi-column (12) design with a central riser, and we have recent ultimate soil bearing values for both. The AWWA is pretty conservative in their requirements for allowable bearing values, requiring a SF = 3.0 for multi-column tanks subject to lateral wind loading (we have a pad/pier design for each pedestal). Assuming that the design, being a municipality-owned structure, was fairly heavily scrutinized at the time, we are not able to find a reasonable answer as to why these foundations are exploding besides the fairly low bearing capacities.

In short, my question is: Is anyone aware of differences between geotechnical testing methods/capacity calculations that are used for water tanks vs. the rest of the structural world (or may have been used in 1994 or before)? We are currently looking at approximately $500,000 worth of modifications for micropile installation, and that's analyzing the foundations with the existing structure, no additional equipment installed (i.e. antennas). It seems like we are missing something significant, and it appears to be with the bearing capacity.

Any help at all would be appreciated.

Thanks-
Ashley

 

[Ron]

You indicate that this is an existing system. It has apparently been performing under gravity loading for its life thus far. I would suggest that you get some weather day, find out the maximum wind load it has experienced since built, apply that to your calculations and see if your foundations fail under those criteria (obviously no actual failure has occurred). If so, you have a significant bust in your calcs, not in the system.

Are you sure you have an ultimate bearing capacity given? That's unusual. Generally the allowable bearing capacity is given and that already has a 3 or 4 FS in it.

 

[Liadain76]

Ron-

Thanks for the reply. We are not failing under gravity load alone, but gravity + wind. The wind loading is based on AWWA requirements of a 3-second gust of 90 mph, so we are restricted to code for that data. The tank itself will likely never see the full 90 mph, but since code requires it, we have to apply it, and it appears to be the same wind speed used for the original design. We are being given the net ultimate bearing capacity for the soil, and back-calculating gross for our calculations. Most geotechnical reports we see are giving an allowable bearing with a 2-3 SF, but the firm we use for our geo investigations does not like to apply a safety factor given that the line of work we are in has varying required safety factors based on whichever code we are using.

 

[Ron]

Liadain76....I understand your need to follow the code mandate; however, what I was suggesting was to ground-truth you calculation process by back-calculating from a point that you know has not failed (i.e., checking weather data for max wind the tank has experienced). This will tell you whether your assumed bearing capacities are reasonable or not.

One item that is likely not being considered is the influence of vertical restriction of the tension side of your system when computing the effect of the compression side only (bearing capacity driver)..assuming your primary bearing pressure for gravity and wind load is the overturning moment. I'm also not sure how you are doing your column load distributions, as they are not equal under wind loading.

 

[BigH]

Without getting into it, it used to be normal practice to lower the SF from "3" for short term live load applications - i.e, use 2/3 of 3 or a live load safety factor of 2. Does normal practice deviate from this? Too, have you carried out your computations according to LRFD (brrrrrr) - for their various loading conditions? You might find some relief in this.