Elevated Water Tank - Anchor Checks for Combined Forces 2

[GT_Structural]

When analyzing elevated water tanks, AWWA D100-11 does not explicitly state that anchor bolts should be checked for combined tension and shear stresses.

I have always taken the statement from 3.3.2 Bolts - "bolts that are subject to shearing and tensile forces shall be so proportioned that the combined unit stress will not exceed the allowable unit stress for bolts in tension only" and interpreted it as:

T/An + V/An <= (4/3)*Flt for anchor rods, and checked the interaction accordingly.

I will admit that I only analyze 2-3 tanks a year, so I'm not the most well-versed, but I recently was discussing with a fellow professional who stated that they only check anchor rods for tension or shear separately as there is no explicit direction.

Anyone with more experience that can comment as to which approach is correct?

 

[JStephen]

The section you mention is for general bolt loading, not necessarily for anchor bolts.
For anchor bolts, it could be done several different ways. For example, the anchor bolt design in ACI 318 includes an interaction equation that could be applied to steel strength as well (although with lower stresses for AWWA).
Also see Section 3.8.7.2. Note that depending on your anchor detail, the maximum shear and the maximum tension stresses may not occur at the same point.
Generally, the design methods for stuff like that in AWWA are either older, simpler, or more conservative versions of AISC, API-650, or other industry sources, so it's appropriate to borrow as required when details are not specified.

 

[WARose]

In AISC (13th ed.), on p. 16.1-109, they have provisions for combined tension and shear in bolts. I have typically used those provisions. (And combined the bending and tension together for a total (tensile) stress where bending in anchor bolts occurs.)