Preload anchor bolts on tall pressure vessels?

[TomBarsh]

Related to thread507-96919,

Anyone involved in designing anchor bolts for tall pressure vessels do you typically preload the anchor bolts or not?

Applying a preload certainly benefits the fatigue life (and tightness) of the anchor bolts but aren't contractors often reluctant to perform the work necessary to tighten such bolts to preload?

If you do preload, what level do you preload the bolts to?

(okay, I am now going off to re-read the "pretensioning" chapter in ASCE's "Anchor Bolt Design for Petrochemical Facilities")

 

[jte]

TomBarsh-

Preloading anchor bolts would be done only in very particular situations such as trying to make a new or modified vessel work with existing anchor bolts. Typically we would expect the anchor bolts to be tightened to about 4 arm-grunts: Two hands on the wrench, tighten 'till you grunt twice. Any pretensioning would have to be very explicitly defined and Inspection or Engineering would have to personally witness the torquing to have any confidence in the installation.

jt

 

[TomBarsh]

Thanks for the comments jt.

I should clarify my original post. Preloading bolts would seem to be necessary or helpful only for tall towers subject to high wind loads. The change in base moment from 0 at no wind to maximum moment at full wind causes high stress variation in the bolts and they'll be subject to fatigue.

Or so the authors of a number of books and design guides would have us believe (which I do). But I am wondering how much this is done in the "real world" for those tall towers.

 

[unclesyd]

You should have the bolts preloaded so as to handles the maximum stress imposed by the loads. This became big problem when we changed the design parameters on several large stacks at our site. Since it is natural for tall skinny stacks to oscillate in the winds we had add additional to the support rings.
This became kinda iffy due to the fact that the anchor bolts were really and unknown quantity as far as embedment was concerned. This greatly affected the amount of tension that we wanted to apply to the anchor bolts.

The depth of embedment was resolved several years later by using Ultrasonics. We found out that they were print length and we assumed the reinforcement was as designed and the we were able to properly tighten the anchor bolts.

Anecdotal:
The anchors got a test by fire in 1988 when one stack was heated by a flaming relief valve to the point that it had bent over to a 40°- 45° angle at the 20' level and was in danger of toppling. When the fireman were appraised of the problem they applied water to the hot spot and received a lesson in flame straightening as the stack came back to a 15° list. There was no sign of deformation on the stack supports. The same stacks withstood a 125 mph wind gusts in 1991.

 

[unclesyd]

I forgot to mention that we have 3 towers on site greater than 140' that have the anchor bolts tensioned. A sister site has a 220' tower that is well anchored with tensioned anchor bolts. It has been hit with by a tornado twice.

 

[jte]

TomBarsh-

Preloading bolts is also useful for seismic design. Without preloading, the assumption (most simplistically, 1/2 of the bolts) is that some of the bolts resist the uplift while the bolts on the other side are not working as the baseplate is in compression. By adding preloading, you can place some additional bolts into tension. Even if its only 2 or 4 bolts, that can save the day if you have found a need to add say 20' to an existing column.

jt