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)
Estimated
Strain
(uE)
Series1
Linear (Series1)
Tritech
Instrument
Laboratory
Test Results
Linear
Regression of
Measured vs
Calculated Strain
m = 1.092
C = 11.065
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
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.
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