Problem statement:-

Why there is difference

between Vibrating Wire

Strain Gage and Load

Cell readings during

pre-loading

How does Vibrating

Wire Strain Gage

Work ?

VW gage

Pluck Coil

Vibrating Wire Strain Gage (VWSG)

Functionality

Strain is proportional to Frequency

x Frequency.

A 5% error in Frequency Reading

is a 25 % Strain calculation

= (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

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)

Esti

mat

ed

Str

ain

(u

E)

Series1

Linear (Series1)

Tritech

Instrument

Laboratory

Test Results

Linear

Regression of

Measured vs

Calculated Strain

m = 1.092

C = 11.065

Welding can cause

Vibrating Wire Strain

Gage Errors

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

How does Vibrating

Wire Strain Load Cell

Work ?

+/- 20% is actually 40% possible difference

Why use Vibrating

Wire Strain Gage?

A comparison

• US $200 each

• Measures less than 1% of strut cross sectional area

• US $4000 eachmeasures 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.

Known Documented

reading differences

between VWSG and

Load Cell readings

Comparison of Results between

Load Cells and Strain Gauges

Average 30%

difference @ low

load levels

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

Strain Gage installation

Load Cell installation