A few amatuerish questions

[GalileoG]

Hello all,

During meetings I hear a lot of terms that I am unfamiliar with given that I am still green in the belly, I usually jot these unfamiliar terms down and try to discover what they mean referring to books.

One term that I did not understand and was not able to find anything on was "non-shrink" grout. What is the purpose of using "non-shrink" grout as opposed to normal grout (that I would presume shrinks?)

What about leveling nuts? What exactly are these leveling nuts, and when would you use them?

Sorry for the overly amateurish questions, but I'm learning. ;(

Thanks,

Clansman

 

[Ussuri]

I think in your context you are talking about column baseplates.

Non-shrink grout is typically used to fill voids that require the void to be 'full' after the grout has cured. For instance, under column base plates. The grout is used to fill the gap and provides continuity between the column base and the foundation allowing for the transmission of vertical loads through bearing, you can see how this wouldn't work if the grout shrank and left a void. The bolts are designed to take the shear and uplift (in reality shear would also be transferred through the grout by the bond between the grout and base plate).

Levelling nuts usually consist of a nut both above and below a column baseplate. They allow for the setting of the level of a column when setting out a building. The void being filled with the non shrink grout once the column base plate is set at the correct level.

 

[SlideRuleEra]

Non-shrink grout and leveling nuts have numerous applications. Another is for use under equipment installed at industrial plants. Typically, a concrete slab is cast a little low (on purpose) under the places where equipment will be installed. Threaded anchor bolt project up from the concrete. Leveling nuts are put on the anchor bolts to (temporarily) hold the installed equipment at the correct elevation. Often these leveling nuts are made from nylon or similar (weak) material. Then non-shrink grout is placed to fill the gap between the top of concrete and the bottom of the equipment. Since the grout does not shrink (very much) it can be counted on to stay at the proper elevation after curing.

Then the usual (steel) nuts are put on the anchor bolts to hold the equipment securely to the concrete. The threads on the nylon nuts under the equipment can be counted on to strip if there is any movement. That way you are sure the grout/concrete is carrying the load rather than the anchor bolts.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[COEngineeer]

I think they are talking about the base of a steel column? You will have tolerance when you fabricate steel column. So the column may be 1" short. So they use leveling nuts at the bolts sticking out of the concrete to make it level at correct height. Then they put non shrink grout to make sure the grout doesnt shrink and the column will not only sit on the bolts.

I hope i gave you the correct explaination.

Never, but never question engineer's judgement

 

[COEngineeer]

LOL.. everyone gave the same answer at the same time.

Never, but never question engineer's judgement

 

[GalileoG]

Thank you all. You guys are great!

Yes, this is about steel baseplates.

What do you folks think about this statement that I heard, "anchor rods can not be relied upon to transfer any significant shear given that the holes are oversized for erection purposes." The holes are about 1/2" greater than the diameter of the rod.

I have followed code procedure to size up my anchor rods for both shear and tension, but now I am considering using shear lugs wherever I have shear greater than 50 kips. Does that sound reasonable?

 

[Ussuri]

The link below is for a 'typical' non-shrink grout from a proprietary manufacturer.

http://www.arconsupplies.co.uk/pdfs/grouting_and_anchoring/conbextra g p.pdf

 

[csd72]

You guys wer 5 minutes too quick for me, I agree with all of the above.

Non shrink grout is also sometimes referred to as shrinkage compensated grout.

 

[Lion06]

To have anchor rods deliver shear, you can provide plate washers with normal holes that will slide over the anchor rods and get welded to the baseplate.

 

[JAE]

But AISC typically frowns upon using anchor rods for shear.

We've started using embedded plates in the tops of the concrete supports on either side of the base plate. Then add vertical shear tab plates welded to the embedded plate and to the side edges of the base plate.

 

[dozer]

Regarding your question about anchor bolts transferring shear, I don't know, depends what the person meant by "significant". AISC puts out a good design guide entitled "Base Plate and Anchro Rod Design". In it, the author recommends, in his words, ". . . a cautious approach, such as using only two of the anchor rods to transfer the shear . . .". He is referring to a base plate with four anchors.

Whoops, lunch is over. I need to wrap this up. I wouldn't want to advise you on an exact number for when to switch to shear tabs, but I would take the advice I cited and not count on all of the anchor bolts when they are in oversize holes. If you can lay your hands on that design guide I think you'll find it very helpful.

 

[WillisV]

I would add to dozer and StructuralEITs comments that even with welding of the washers (which I agree is necessary) or use of two of the bolts, you also have to take into acount the increased stress due to flexure of the anchor rods (between the top washer and the top of the concrete) which can add significantly to the required size of the rods. I generally don't use anchor rods for shear transfer when the load exceeds about 25kips as they just get too big.

I do what JAE does except I use post installed angles on both sides of the base plate anchored to the footing with screw anchors to prevent problems with the embeds missing or being misaligned.

 

[civilperson]

All substations and electrical components supported on vertical beams,(my name for columns with small axial loads and large moments), use anchor bolts to resist shear. Grout is optional so the bolts are designed for tension/compression, shear and if longer than one diameter between concrete and bottom of base plate also designed for bending. AISC rules against shear resisted by anchor rods are not followed in many types of design.