Skid Jackscrews

[aseeng]

I was asked to look at sizing some jackscrews for a compressor skid. Typical skids consist of a steel wide flange fully welded base about 35 ft long x 12 ft wide. As an example we would have 5 anchor bolts on each long side - (10) total with (10) jackscrews placed near the anchor bolts. The skid is lifted into place over the foundation and set down on the jackscrews. These would be screwed thru a welded nut attached to the bottom flange of the skid. The screws are then rotated to achieve a precise degree of level and then the skid is grouted into place. After the grout is hardened the greased screws are back out.

Question is how many jackscrews will carry load? Due to unevenness of the concrete foundation I can imagine that not all screws will be in contact at the same time. I have rationalized that a minimum of 3 screws is required for stability. If the CG is located close to a line between 2 of the screws then it may be possible for 2 screws to be carrying most of the load and I have settled on designing the screws for 1/2 the total skid load. I feel sure this will be sufficient for design and is likely conservative but is there another way to rationalize the maximum load to any one jackscrew? The screw design itself is not so much of a problem for this high load but other local design issues com into play.

Thanks in advance for any responses.

 

[dhengr]

Aseeng:
I’d look at designing the 10 A.B’s. to do double duty. Put leveling nuts on them and do a good job of leveling these before you even lower the skid into position, and you should be 95% of the way home. Then use a thin ‘pancake’ hydraulic jack to take some of the load off at any one A.B. for some final adjustment of the leveling nut. Then, finally install the top anchor nuts and grout the bearing pads at the A.B’s.

It would be good to see some plans and details of the skid, with dimensions, loads, C.G., member sizes, etc. etc. This would help us see what you are dealing with. If I were going to use jacking screws something like what you describe, I would look to try to make off-the-shelf jack screws of some sort work, and adjust my detailing on the skid side sills or cross members to make this work. The jack screws or even bottle jacks could be completely removed and used on the next skid installation.

 

[aseeng]

dhengr- thanks for the reply. An example skid is attached. We do not want to use leveling nuts on the anchor bolts because these bolts are post tensioned after grouting. The shop fabricated detail is pretty common in the industry. I often see 5/8" or 3/4" bolts used. A193 Gr. B7 material. If I assume total load divided by total # of bolts - no problem. Total load divided by half the bolts? Just trying to envision several guys with laser leveling equipment around the skid turning screws until its level. Some of them may not be touching concrete - correct? How many?

 

[racookpe1978]

But don't "overthink" it either.

What is the failure mode if a jackscrew is overstressed? Everything is already low down, over concrete, on a packaged frame of substantial steel members. (To only slightly exaggerate.) The "tension" is in the potential for half the load to be on only two jackbolts - but three is more likely by far. The failure would be not the frame bending (a problem!) but one of the welded nuts holding the jackbolts pushing through = stripping as the unit as a whole is leveled.

So, your calc of half the load pushing on one nut is more-than-adequate for the world it needs to hold up and the length of time it might need to hold that load under the conditions specified and assumed. And, if that one threaded nut fails, there are 4 more on each side remaining to hold the force and allow the leveling process to continue.

 

[dhengr]

Aseeng:
Shoot the 4 A.B. locations (on the conc.) nearest the 4 lifting trunnions and you should know about where to set those 4 leveling screws to end up pretty level. This is all done before you even start lifting. The end of the screw is set “x” distance below the bot. flg. of the side sill. Then, you will have the load on at least 4 points. By bearing on the 4 screws nearest the lifting trunnions, you shouldn’t even get too much frame deflection when the crane releases the load. By shooting the other 6 A.B. locations (before hand) you should be able to set those leveling screws pretty close too, but a little high. While the crane is still holding the load, but bearing on the first 4 leveling screws, bring these last 6 screws down and into good solid contact. If you have some levels sitting on the frame, this should go fairly quickly, to get all 10 leveling screws bearing to some extent. You still haven’t told us the wt. of this entire frame, but there will be some hellish conc. bearing and grinding stresses under those leveling screws, unless you put some sort of a small bearing plate under the screws.