Anchor Bolts, Base Plates 2

[umreng]

We are installing a new 60 ton overhead crane runway, and the structural engineer recommends a leveling nut be installed beneath the base plate to achieve a level structure and hold the structure in place prior to tightening the anchor bolts and grouting the base plates. I always thought it was best to tension the entire anchor bolt by using blocks of steel to support the leveled structure prior to grouting. The specified 1" dia. anchor bolt material is ASTM F1554, 36ksi. The grout thickness allowance is 2" There are four anchor bolts per base plate. The concrete strength is minimum 4000 psi. The base plate thickness is 1.5"
Please give me some general opinions on this approach. I am aware that the structural engineer's approach is quicker and less costly, but am most interested in the best installation.

 

[dik]

The proposed levelling nuts with grout is a good solution... using levelling plates would be a better, albeit, more costly solution and not likely warranted.

Dik

 

[evelrod]

umreng...In my 38 years of practice I guess I've seen my share of leveling nuts...Keep in mind that leveling 'plates' are often simply bar stock cut in short lengths and centered under the base plate. Leveling nuts are a pain on small base plates but are the best, IMO, for a 1.5" x ~18" square base plate or greater. The grout, properly installed carries the majority of the load, not the leveling nuts or plates.
Again, IMO, it doesn't hurt to go a little overboard on an OH craneway...I've seen them fail!

dik---Cost is relative...Leveling nuts on small columns is a time consuming pain... but is very cost effective on larger base plates not due to material cost, but labor cost!!!

Rod

 

[dik]

Yup...

Dik

 

[umreng]

Thanks for two great responses! I have a couple of follow-up questions for dik and evelrod.
1. You both agree, as I understand your responses, that the leveling nut approach is advantageous in project cost and project schedule. (I agree with this.) I get the feeling that you both feel that baseplate/foundation joint integrity improvements obtained by tensioning through leveling plates (the hard shim approach) would not be worth the additional cost and time in most cases. My concern is that a substantial uplift at an anchor bolt would tend to allow baseplate movement (cracked grout) because the anchor bolt is not in tension prior to the uplift. With the leveling nut method, the only portion of the anchor bolt in tension is the short portion engaged by the two nuts and baseplate. This may be an unfounded concern from a misguided mechanical engineeer. Please comment on the pros and cons of the leveling nut approach in the instance of a moment load at the baseplate.
2. The engineer has verbally told our field people to snug the anchor bolt nuts after leveling the structure (on the leveling nuts) prior to grouting. I understand that the term "snug", when used in connection with threaded structural fasteners, roughly means a full one-man effort on a 12" long wrench (or to take the slack out of the joint). I am in process of requesting this instruction in writing from the engineer, because I question the value of just snugging. I was surprised that the instruction was not to snug in sequence and then tighten to some level beyond the snug condition. Please comment. And thanks again!

 

[evelrod]

I'll start with #2
Yes, I can accept your defination. It varies a bit from job to job and inspector to inspector, but generally, "snug" is as you describe and is the common practice. You cannot normally tighten "in sequence" if your plumbing a column.

#1...As I said in my first post...worth repeating...It doesn't hurt to go a little overboard on the design as it relates to loading of craneway columns/baseplates, I've seen them fail!
If you have a problem that causes enough torque at the base to lift and crack the grout, you have exceeded design capacity. A second look at the numbers, 'beefing up'of support structure or a substantial reduction of the craneway capacity is in order, IMO. ---Alternately, it may also be a poorly installed grout problem (rare)...Certainly other events may cause grout failure, but my money is on an 'overload' or 'undersize' condition!

Keep in mind that 'shimming' large baseplates is a very time consuming and labor intensive procedure. Leveling nuts are simply a much better solution. On small plates, a central shim pack is a quick way,e.g., setting multiple columns of a bldg to grade...shoot them all in and them plumb by hand. On really big plates as in the core columns of a 'high rise', then leveling nuts may be insufficient and instead, leveling bolts will often be used (e.g., a 10" x 6' for a 20 ton column).

Rod