Strain gages are used in Temporary struts in Deep excavation sties. Load cells are also used for validation purposes. Regularly there is need to cross check their performances. The presentation explains the differences in the tow methods and possible differences between the two methods. The approach is a Cost balance with Acceptable Risk Management method

1. Problem statement:-
Why there is difference
between Vibrating Wire
Strain Gage and Load
Cell readings during
pre-loading

2. How does Vibrating
Wire Strain Gage
Work ?

3. VW gage
Pluck Coil
Vibrating Wire Strain Gage (VWSG)
Functionality

4. Strain is proportional to Frequency
x Frequency.
A 5% error in Frequency Reading
is a 25 % Strain calculation

5. = (0.03% of FSD)
In VWSG, we are monitoring the change in load over
time. Sensitivity or Resolution is more critical than
absolute level as we zero it

6. y = 1.0921x + 11.065
-2000.00
-1500.00
-1000.00
-500.00
0.00
500.00
-1800 -1600 -1400 -1200 -1000 -800 -600 -400 -200 0
Computed Strain (uE)
EstimatedStrain(uE)
Series1
Linear (Series1)
Tritech
Instrument
Laboratory
Test Results
Linear
Regression of
Measured vs
Calculated Strain
m = 1.092
C = 11.065

7. Welding can cause
Vibrating Wire Strain
Gage Errors

8. 0
500
1000
1500
2000
2500
00:00:00 12:00:00 00:00:00 12:00:00 00:00:00 12:00:00 00:00:00 12:00:00 00:00:00 12:00:00 00:00:00 12:00:00
Time
Load(kN)
SG Welding side
SG Non Welding side
Construction Impact - Welding
Load increase (600kN)
due to welding
Welding
Sudden decrease in load (400kN)
due to EMI effect of welding
SG on the side of
welding of strut
SG on the non welding
side of strut
Welding side

9. How does Vibrating
Wire Strain Load Cell
Work ?

10. +/- 20% is actually 40% possible difference

12. Why use Vibrating
Wire Strain Gage?

13. A comparison
• US $200 each
• Measures less than
1% of strut cross
sectional area
• US $4000 each
measures the full
load
Strain Gauges Load Cells
LTA : 25% of all struts for deep excavations shall
be monitored for load by strain gauge and/or load
cell, and of those monitored by strain gauge 15%
shall also be monitored by load cell.

14. Known Documented
reading differences
between VWSG and
Load Cell readings

15. Comparison of Results between
Load Cells and Strain Gauges
Average 30%
difference @ low
load levels

16. Actual VWSG and Load Cell
installations.
Only under ideal single strut
loading (without bracing) can
the readings be correlate
better
VWSG gives localize strain
while Load Cell gives global
strain

18. Strain Gage installation
Load Cell installation