Torque required for the anchor bolts of a process column 1

[lganga]

Hello friends
1) Can the anchor bolt be torqued after leveling with shims and without grouting?
2)Can the anchor bolt be preloaded with a torque, considering the bolt material condition (70% of yield? for example) or we have the risk of damaging the concrete foundation. As an example I have 3" 193B7 for anchor bolt and manufacturing bolt company writes in their catalogue, it can be loaded with torque of 23,000 Ft.Lbs
thanks
Luis

 

[TomBarsh]

1) Yes. This the customary mode of construction

2) Yes, you can preload the anchor bolts, this is advisable for taller vessels, which are subjected to higher wind loads. However, this will produce higher tension in the bolts (but possibly no higher than under the maximum wind/seismic loads...but then consider that these loads may be "factored", whereas the preload will not be); this should have been considered in the design of the foundation before this sort of decision was considered. If you wish to pursue this then carefully consider the capacity of the bolts and the concrete pullout. The benefit of preload is to possibly reduce deflection of the vessel from wind...by minimizing flexibility of the base, and it will reduce the cyclic stress variation so that fatigue life benefits.

 

[Duwe6]

Further, plan on retorquing to reestablish this preload. Wait at least 3-days after the grout has set, and at least a week after the initial torque-up. I do not understand the mechanism, but anchor bolts embedded in concrete always loosen up after a couple of days, and will need to be retensioned. Even if your spec is only 'snug tight', equipment anchor bolts always require a retensioning.

 

[TomBarsh]

And for that matter, don't try to use a "torque" measurement for the bolt tension, this is generally unreliable to obtain the desired tension, especially for anchor bolts. Use a "turn of nut" method, load indicating washers, or direct indicating instruments. Preloading (pre-stressing) anchor bolts for pressure vessels is not unknown, but is quite rare in my experience and observation. It will generally require a field crew experienced in this operation.

 

[TomBarsh]

The text "Pressure Vessel Design Handbook" by Henry Bednar discusses preloading anchor bolts.

Also see "Anchorage Design for Petrochemical Facilities" by ASCE (American Society of Civil Engineers) (or the previous Edition "Wind Loads and Anchor Bolt Design for Petrochemical Facilities") for practical information.

 

[TomBarsh]

also see

http://www.eng-tips.com/viewthread.cfm?qid=251704

 

[lganga]

Tom and Duwe
I have a partial calculation note from Compress . With a Summary of the loads,including Seismic and Wind loads. The bolt load, is 516,000 Newon without preload, area of the bolt provided is the double from required. 3" bolt A193B7. The engineering company wants to torque the bolt with the manufacturer data, of 23,000 Ft. Pound, that is a clamp load of 460,000 Pounds. It means about 230,000 Kgs, this load is almost 4 times the calculus.
Should the engineering company has to recalculate the concrete base for this condition?
Thanks for your comments

 

[Duwe6]

23,000 ft-lbs sounds reasonable for a 3" B7 bolt/stud. Do not worry about the concrete until you figure this loading spread across the base of your tower. Unless it exceeds 2,500 psi, the concrete will be fine. If it exceeds 2,500, just make sure that the concrete actually placed will sustain the loading. During a 'forensic' coring testing of recent placed concrete, there were several cylinders that went over 9,000 psi, and this pour averaged just above 8,000 psi.

 

[TomBarsh]

"Should the engineering company has to recalculate the concrete base for this condition?"

I would certainly think they should be consulted and/or informed. I don't think that just anyone should decide "oh, we're going to do 'this', even though the design and construction didn't consider 'it'. It'll be okay!". Just imagine if your pressure vessel was designed for some given pressure (and had a 'slight' amount of extra headroom for MAWP) but some other team or group decided "hey, we'll just crank this pressure up 30%, that'll really get the process flowing!", "Awesome, Dude!"

 

[lganga]

Thanks you two
Comment:
The construction company wants to apply the total torque load with the column seated on the 16 leveling plates and without grouting, that gives very punctual loads on the concrete base. Should they pour the grouting for a better distribution of loads before the torque? .
Does the applied torque can damage the grout?
Regards

 

[Duwe6]

I've seen the "no grout' design before. It appears to be something currently being taught to CivE's. Does not work. For it to work, the 'leveling plate' would need to be treated like a pump sole-plate. Plat needs to be flat within 0.005" across its span, in all directions. Then plate needs to be installed level within a few Minutes of Angle. Not gonna happen. Therefore it is a 'great' idea that cannot work, due to the realities of field conditions.

To properly pretension your anchors, the equipment needs to be set onto stacks of shims*, NOT near the anchors. Once you are satisfied with elevation & plumbness, immediately grout the equipment. Once the grout is fully cured per mfr's spec [usually 7 days for cementious grout], torque the anchors. Wait a week or 3 and come back and retorque them. Simple. Many decades of proven good history.

*Watch out for 'jacking nuts'. Some equipment riggers want to put adjustment nuts on the anchors, under the equipment's baseplate; makes their work a lot easier. Don't let them; when you torque those anchors the tension only goes as far as the nut, never reaches the actual anchorage.

 

[MJCronin]

Great practical advice Duwe6 ! Kudos !

MJCronin
Sr. Process Engineer