Pre-tensioning anchor bolt

[jtseng123]

All,
I normally look at the critical wind velocity or wind induced vibration to decide if pre-tension is required. However, my company's civil engineer told me that if nature frequency is less than 1Hz (flexible structure), anchor bolt shall be fully pre-tensioned to the larger of (1/3 of bolt tensile strength, maximum bolt tensional stress due to full wind). He said this is a new recommendation from API or whatever sources.

This is going to impact the skirt and anchor chair design with more cost. What is your opinion or practice ?

 

[r6155]

See API 650
5.12.11 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) and any anchor straps shall be welded while the tank is filled with test
water but before any pressure is applied on top of the water. Measures such as peening the threads, or adding
locking nuts, or tack welding nuts to chairs, shall be taken to prevent the nuts from backing off the threads.

Regards
r6155

 

[jtseng123]

Attached from Compress run showing nature frequency is less than 1 Hz that our Civil engineer asking this tower to be pre-tensioned. Does anyone use the same criteria to pre-tension anchor bolt ?

 

[ConstantEffort]

I've gone back and forth with civil and structural engineers on this one without arriving at a good (in my opinion) answer. Some say pretension every anchor bolt up to 100% Fy... even on my little pots and pans that weigh less than 1 ton installed indoors. Other say don't worry about pretensioning anything... even the 200' tall towers in hurricane-prone regions. I'm interested to know what others in the industry actually practice.

But with what seems like a ~12ft diam x 220ft tall tower in a 150mph wind zone, I wouldn't follow the API 650 recommendations offered above.

 

[r6155]

The initial anchor bolt tension does not increase the maximum bolt tension caused by wind or earthquake. This initial tension will only clamp the base ring to the concrete.
See more in Pressure Vessel Design Manual 4th edit. , by Dennis Moss.

Regards
r6155