Tieback Testing w/ Load Cell
Tieback Testing w/ Load Cell
(OP)
I'm dealing with a situation on a DOT project where the State does not want to accept several anchors due to differences between the jack calibration and load cell readings. There are significant descrepancies - the load cell showing as little as only 65% of the test load in the worst case.
These are large (260kip - 375kip) strand anchors supporting a sheetpile wall, performance tested to 150% DL with a 10-minute creep hold. All were tested with the same jack and load cell on the same day. The tests met the criteria for elongation (above PTI min elongation) and creep (under 0.040" in 10 minutes).
Of course, the DOT is taking the load cell readings as gospel and recommends retesting the anchors unless I can provide an adequate explanation for the differences.
Does anyone have any experience with this situation? I'd appreciate any suggestions.
These are large (260kip - 375kip) strand anchors supporting a sheetpile wall, performance tested to 150% DL with a 10-minute creep hold. All were tested with the same jack and load cell on the same day. The tests met the criteria for elongation (above PTI min elongation) and creep (under 0.040" in 10 minutes).
Of course, the DOT is taking the load cell readings as gospel and recommends retesting the anchors unless I can provide an adequate explanation for the differences.
Does anyone have any experience with this situation? I'd appreciate any suggestions.





RE: Tieback Testing w/ Load Cell
RST Instruments Ltd., per
Chris Bray, EIT
Sales Engineer
Tel: 604.540.1100
RE: Tieback Testing w/ Load Cell
Load cells are more affected by weather, moisture, temperature, etc. than are pressure gauges. In addition, the contractor should have two calibrated gauges, one being a spare. If you have a problem with a pressure gauge, it can be switched easily. You can't switch a load cell easily. The test would have to be stopped and then be rerun. The load cell is usually used only to check if the jack is losing load during an extended hold. If the load cell's digital counter is decreasing, the jack can be pumped to take the load back up to the desired pressure gauge reading.
RE: Tieback Testing w/ Load Cell
Consider recalibrate the jacks and the load cell. We use Dudgeon in Bridgeport CT. They ship all over, very good calibrations. One sticking point is always the seating load, but I don;t think that would account for that much dicrepancy.
RE: Tieback Testing w/ Load Cell
I don't think it's a calibration problem. The test setup was calibrated just days before being used. (I use WB in Wood-Ridge, NJ - they're every bit as good as Dudgeon)
I'm thinking more along the lines of PEInc that load cells are finicky and affected by things like weather and especially can give funky results if loaded slightly eccentrically as can be common for multi-strand tiebacks. I was looking for some backup that the pressure gage readings, and not the load cell, should be relied upon to indicate applied load.
DRC1, what do you mean by seating load?
RE: Tieback Testing w/ Load Cell
RE: Tieback Testing w/ Load Cell
RE: Tieback Testing w/ Load Cell
You said, "I was looking for some backup that the pressure gage readings, and not the load cell, should be relied upon to indicate applied load." The FHWA reference I quoted above should be the backup you want.
RE: Tieback Testing w/ Load Cell
This problem might not be revealed by a calibration in a testing machine that allows very little extension of the jack during the test.
Call me conservative, but faced with two conflicting pieces of data, I am slow to adopt the optimistic one.
RE: Tieback Testing w/ Load Cell
Also - I don't have 2 pieces of data, I have 3. Jack pressure, load cell readings, and anchor elongation. 2 of them say I had the loads right.
RE: Tieback Testing w/ Load Cell
RE: Tieback Testing w/ Load Cell
If the first gauge's readings look good, forget the backup readings and use the first set. That's why you have a "backup" gauge - in case something happens to the other gauge. If you really needed two sets of readings, they would need to specify three gauges, with the third being the backup gauge.