When to Pre-tensioning skirt anchor bolt

[jtseng123]

Dear all,

I was hardly asked to consider pre-tensioning anchor bolts for tall tower. We just have a slender tower with nature frequency less then 1 Hz, and our Civil engineer asking pre-tentioning anchor bolt with the following reasons:

Quote "we will have to consider anchor bolt pre-tensioning when a tall vessel is mechanically designed flexible.
In the vertical vessel foundation design, we normally do not consider the secondary moment for foundation and anchor bolt design due to eccentricity of vessel mass under wind load. However this tower has C.G. located at very high elevation, 7" deflection for 130 ft tall, fundament frequency is 0.54 Hz. the most flexible and tall, we request to pre-tension the anchor bolts"

Does it make sense to pre-tension the anchor bolt from the above ?
(Note : this tower is ready for purchase and we do not want to make changes in the last minute unless it is technically wrong design. If pre-tensioning the anchor bolt, the entire anchor chair becomes no good and requires re-design due to higher bolt load)

 

[TomBarsh]

There have been several other discussion threads on preload of anchor bolts for pressure vessels in this site over the past 10-15 years (I myself have participated in some of them). I suggest using the search feature to locate these. Some may be in the "structural" forums as well.

There is a little publication from ASCE, "Anchorage Design for Petrochemical Facilities"; it briefly discusses preload of anchor bolts.

So does the text "Pressure Vessel Design Handbook" by Bednar; also briefly, and more from the stand point of reducing the cyclic stress in the skirt and bolts from varying wind (and seismic) loads. Not much commenting about foundation/soils design.

If it was me, I would listen to my foundation engineer. It is "cheap" to modify the design now (even if in fabrication at the shop) but totally another ball game if the anchor bolts or chairs fail in the field.

I would have your foundation engineer explain this to you very carefully so that you can understand the calculations and considerations. Take time to mull them over. It's likely going to permit a much cheaper foundation design if you can modify the base ring design...and really, the base ring/chairs are cheap relative to the vessel and foundation. Then discuss with your project manager, or whoever is writing the checks for the project.

 

[Duwe6]

Construction note: Pretensioned anchor bolts loosen up after a couple of days. Best Practice would to add a note that the pretension needs to be tested/reinspected 1-week(+) after the initial tensioning.

Don't understand the mechanism, but have seen it in action many times over several decades, at 100% of the places that were tested.

 

[jtseng123]

I found PIP STE05121 "Anchor bolt design guide", section 10 talking about pre-tensioning anchor bolt. It lists advantages and disadvantages. I passed it to our Civil engineer to make the final call.

 

[r6155]

1)Lubricate all threads on nuts and bolts, and nut face contact
2)Use 2 nuts / bolt
3)Hand tight plus slight impact wrench

Anchor chair not depend of pretensioning.

Regards
r6155