Using leveling nuts instead of grout?

[MM904]

Hello,

I'm working on the baseplate/anchor design of a sub-scale engine test stand. The test stand has three legs - it looks exactly like a teepee if you can imagine it - and the engine to be tested is mounted in the center of the "teepee", firing upward (obviously). I have attached a picture of the test stand for reference.

The baseplates are 24" x 24" x 3/8" thick.The test stand is rated for engines capable of up to 4500 lbf of thrust.

It is important for the test stand to be level, but also is "movable", as in it can be loaded into a truck and moved to a different test location. Because the loading is not compressive during the operation, can I get away with using leveling nuts to get the plate level, and not use grout? I have seen grout is necessary for transferring the bearing load, but in this case the dead load of the structure due to it's own weight (about 1500 lbs fully loaded) is all the "bearing" it will experience.

Is there any example of this in practice? How could I determine if it is feasible? I used the AISC Design Guide 2 in the design of the baseplate and anchor bolts.

 

[jayrod12]

I think in an application like this, you wouldn't want to use grout. If you have to up the anchor bolt diameter to have them act like short columns for the little gravity load it will see, then I think that would be more tolerable to the end user.

 

[MM904]

The concrete pad is a sticking point here. It is a thin pour, only 3.5" thick. Have to work with an old pad, so must assume it is cracked when designing fasteners. I'm limited a lot by this, but so far I have spec'd 1/2" drop in anchors, 4 per plate.

 

[WARose]

People do it all the time......sometimes without letting the engineer know!

While it's typically not advisable.....for light loads, it might make it. (You'll need to do the calcs to be sure.)

 

[BridgeSmith]

We use leveling nuts on all of our pole bases, from the small luminaire poles to the 120' tall high mast towers. If there's shear on the bolts, there will be moment due to shear, but we ignore it if the unsupported length of the bolt (bottom of leveling nut to top of foundation) is less than a bolt diameter.

 

[BridgeSmith]

From what I understand you're describing, I'd be more concerned about the durability of the concrete pad than the nuts or bolts. Do you know if the pad is reinforced to survive the handling stresses when it gets moved?

The headache involved in getting holes for the anchors drilled in the right place may prove to be more trouble than setting up the stand with bolts secured in the baseplate holes and casting a new (reinforced) pad around them.

 

[MM904]

Sorry for the confusion: The idea of it being movable is that we would remove the baseplates from the post-installed anchors, take it to some different concrete pad, and then install it there.

HotRod10, why are you able to ignore the moment due to the shear? I was under the assumption that by having the baseplate grouted then I could ignore the moment acting on the bolts, but then with this idea of just using the leveling nuts, that I would have to account for it.

 

[dauwerda]

The utility industry (transmission towers and substations)typically uses leveling nuts without grout as you describe. ASCE 113 Substation Design Guide provides a method for determining base plate thickness when installed in this fashion. I believe there have been a few posts on this in the past. As HotRod10 mentioned, ASCE 113 also makes the allowance to ignore bending in anchor rods if the space between TOC and bottom of base plate is less than 2X anchor diameter. Keep in mind, with no grout, the anchor must also be designed for compression/bearing, and with that thin of slab you may have an issue with it punching right through the concrete.

 

[BridgeSmith]

If the bending length is short enough, the moment is small, and can be ignored. The 'one bolt diameter' limit is essentially a rule of thumb.

 

[MM904]

Dauwerda,

For the compression and bearing area calculations I was going to assume that the total projected area of the four nuts would be bearing those loads. Since the engine fires up into the structure there would be no compressive load other than the dead weight of the structure

 

[MM904]

Also, thank you for the reference to ASCE 113, I am looking into that.

 

[racookpe1978]

Isn't all of the engine net thrust ending up in the test stand? So total thrust (down) = gravity weight (engine + test stand + fuel) + engine thrust (up into air)?

Add a "washer" under the nuts to increase bearing area of each leg on to the concrete upper surface. (Actually, clamp that washer between an upper and lower nut on each of the three legs.)

 

[SwinnyGG]

so far I have spec'd 1/2" drop in anchors, 4 per plate

Careful there- keep in mind that mechanical anchors are typically designed to be loaded in tension. Some anchors will not support any significant negative load.

If your intent is to install your leveling nuts on the actual anchor studs, this is probably not advisable. The utility installations others are talking about are using cast-in-place anchor bolts, not a post-installed anchor.

Machines which have significant uplift and require leveling but can't use cast-in-place anchors can still be mounted a number of ways- you can use leveling screws, and then anchor a bracket to the pad which is then attached via a field weld; you can use pre-configured leveling jack screws which are available from many manufacturers, which are designed to handle the leveling with a concrete anchor through the center bore to provide uplift resistance, etc.

 

[MM904]

jGKRI,

You beat me to the point - I just finished that section of ASCE 113 and was going to bring that up. I will look into the leveling screws.

Many thanks to all!

 

[MM904]

Also, racookpe1978 I believe you are correct, my apologies.