LIFT OFF TESTS

[augustus50]

We are having trouble determining the exact lift-off load on anchors. The bar is a 1.0 inch DSI grade 75. The lock -off load is only 10 kips. We have to wait 48 hours, then perform a lift-off test. The lift off load must be between 9-10 kips. We have a hand pump so it is difficult to pinpoint when the nut lifts off the plate and slows the rate of pressure gain as the entire unbonded length starts extending.

The unbonded length is only 5 ft.(bonded length=18 ft) and we have about 2 ft of free length between the plate/nut and the 120 kip hydraulic jack. I uderstand wire is sometimes used behind the nut, but I don't know the details of that method.

Thanks.

 

[Erdbau]

Load the anchor past liftoff, slip a short piece of wire between the nut and the bearing plate, lower the pressure back to alignment load, and then pump up again. Read the pressure gage value at the instant the piece of wire falls out (the nut lifts off the bearing plate, causing the wire to fall). Alternatively, you can place a dial gauge on the nut, and read the pressure when the dial gage starts moving.

I have never done a lift-off test at a load that low, but it seems like nailing the lift-off load down to within 1 kip would be tough no matter what method you use.

 

[PEinc]

The dial gauge on the nut works well as long as you can get the gauge positioned properly on the lock-off nut (not the jack's stressing nut). This method is much more accurate than using a wire or a piece of paper or looking for when the jack pressure gauge starts to flutter. Also, the 5 feet unbonded length is shorter than the 10 feet minimum that is recommended by PTI for bar tendons.

http://www.PeirceEngineering.com

 

[Ron]

With a 60 ton hydraulic jack and a 5 ton load, your gage will not likely have sufficient resolution to catch small increments. Use a two-stage gage system where low loads are recorded by a gage that goes from, say, 0 to 10 kips and the other gage goes from 0 to 120 kips.

As for a tell-tale, use a feeler gage and slide under the nut with each increment. Very accurate as feeler gages can be as thin as 0.001", with "off the shelf" gages down to 0.002".

 

[dcarr82775]

Use a dial gauge as PEinc recommended. And with a load that low you had better use a small jack or you will shot past 10 kips in a hurry