Pre-Loaded (Torqued) Cast in Place Concrete Anchors for API 650 and AWWA D100 flat bottomed tanks

[JGard1985]

Greetings

I am evaluating revised wind load on an existing vertical storage tank using API 650 & AWWA D100. Both codes contain equations to determine the maximum bolt load and allowable tensile stress. They also specify provisions for preloading the anchor bolts to a percentage of the yield stress of the bolting material.

When assessing the anchor bolts using API 650 & AWWA D100 does one need to consider the applied preload and a reduction in the available bolt capacity?

In my experience with the AISC steel design manual, equations for bolt allowable shear and tensile loads for high strength fasteners are provided but are not as a function of applied bolt preload.

I seem to remember that the steel to steel bolted connection is extremely stiff and applied external loads to the joint has little effect on the tension in the preloaded bolt.

Does anyone have a code reference as to how to assess the change in allowable anchor capacity as a function of applied preload, or is this not normally assessed? Is a preload deduction included in the published allowable stresses for anchorage material?

Looking forward to your thoughts, thank you

 

[IFRs]

I'm not aware of preload in API 650, just basic wrench tightened nuts. There is no need for preload, I think.

 

[JStephen]

In AWWA, it is only the high-strength bolts that require preload, normal anchor bolts don't. That would mainly be in high seismic areas. I don't recall if API has a similar requirement or not. I would have to look into the issue a little more, though.

 

[IFRs]

API limits the allowable anchor bolt stress the lower of its yield or 55ksi / 320MPa, with factors for each load case.

API further limits the tightening as follows: "Any anchor bolts shall be uniformly tightened to a snug fit (nuts hand tight in contact with anchor chair top plate plus maximum of 1/8 turn with wrench)"