truck load test assessment (strain gauges) reporting

8/12/2019 Truck Load Test Assessment (Strain Gauges) Reporting

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BACKGROUND STUDY

The temperatures fluctuation was found that contribute to the strain magnitude, the first loadingtest carried out in the morning time 11am and completed at 4pm afternoon time. The strain

reading after unloading to none vehicle was observed not return to zero. Therefore the

temperature deduction was taking into the consideration in the strain calculating.

In order to get the differential changes of the strain reading due to the thermal influence the

strain reading was collected after the traffic was diverted to the new bridge. The strain data

without traffic was observed at 24th

to 26th

February 2013.

Thermal influences are complex because it is not only the gauge that is affected but the element

to which it is attached and whole structure that is affected. The rate of temperature change and

the distribution of the thermal gradients also play a major part in influencing the actual strain(load) at any point and its effect on the gauge itself and its readings.

Consequently, in order to apply any correction for temperature it is necessary to first establish

the effects of the temperature changes on the Strain Gauge and the medium in/on which it isinstalled.

The second loading test was carried at the different loading and unloading sequence. In stead ofloading the trucks from constant loads, the new loading sequence was arranged to load the trucks

at once loading pattern in order to get the change of strain. The trucks will loaded from zero

truck to 4 nos. of truck then unloading to zero truck for session 1 and session 2 the number oftrucks were increased from zero to eight trucks.

Temp.

FluctuationStrain

Reading


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Testing time selection for Second loading test.

The existing bridge tested was actually connected with the new Batukawa Bridge by sharing thesame foundation (pile cap) as shown below.

Therefore the traffic load might affect the strain reading by transfer the movement through the

pile cap.

To prevent the effect of the running traffic from the new bridge, then testing period must avoid

the peak hour for the daily traffic from morning 6-8am and evening time from 5 to8pm.

The advisable time to carry out the testing was 10am and complete in 30 minutes or shortest time

to reduce the thermal fluctuation to the strain reading

Trend of strain running due to the temperature

Morning 6-8am

peak hour

Evening 5-7pm

peak hour

Evening 5-7pm

peak hour

New bridgeOld bridge


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As the preventive measurement to minimize the vibrating due to the new bridges traffic. The

readings taken moment, the traffic shall stop for several minutes to keep the bridge to stable and

stand still.

Refer to the strain reading below the selected strain 3, 13 and 14 was mounted at the steel girders.

Strain 3 and 4 located on top and 13 and 14

The strain data was collected from 17th

to 25th

February the old bridge traffic was diverted to the

new bridge on 23th February 2013. By comparing the graphical data obtained. The daily trafficstrain reading was increasing as the temperate was increasing but with the larger fluctuation

magnitude compare to the strain reading without traffic.

Daily traffic trend No traffic


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FIRST LOADING TEST (26TH

JANUARY 2013)

Field testing was carried out before and after upgrading works and in each case this consisted of a load

test carried out in a single day, and strain and acceleration monitoring exercise lasting approximately one

month. Only a summary of the results of the change of strain and subsequent analytical model updating is

given here.

C-concrete

S- Steel girders

The bridge monitoring program involved measurement of Vibrating Wire strain gauge at the

bridges mid-span and support using a purpose made bridge monitoring system. The monitoring

system consists of 28 nos weldable VW strain gauges which (8 located at the top and soffit of

girders beam and 16 nos located at the concrete slab), and a data acquisition system.

Data acquisition can be set at equal time intervals (every minute) or triggered if the response

exceeds a user-defined threshold. A major advantage of the system is that the data acquisition

system is powered by 12 V batteries, facilitating use in remote sites. The strain gauges were

mounted on the bridge soffit to Span 2 which located on the land side and longest span of 38.5m,

before and after upgrading works, with each monitoring program lasting at least 20 days. Data

acquisition was triggered by ambient traffic at selected levels of strain. The data acquisition unit

was set to record dynamic strain time series and the peak strain value for a particular event

At the same time, the deflection of the bridge was taken by the survey work with the indicator

leveling instrument at the remarked point as required. Each loading with 2 trucks the deflections

and time were recorded


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Loading Procedure for Traffic Load Test

Applied Load Time

Maintaining

Load

Start time End time

Number of Trucks (nos) Loaded Weight (Kg)

0 0 25 mins 11:05 am 11:30 am

2 7646020 mins 11:35 am 11:55 am

4 15389015 mins 12:05 pm 12:20 pm

6 22982022 mins 12.:28 pm 12.50 pm

8 29179021 mins 12:58 pm 1:19 pm

10 35422017 mins 1:28 pm 1:45 pm

12 41705020 mins 1:50 pm 2:10 pm

10 34058010 mins 2:12 pm 2:22 pm

8 26316010 mins 2:24 pm 2:34 pm

6 18723010 mins 2:37 pm 2:47 pm

4 12526010 mins 2:47 pm 2:57 pm

2 62830

10 mins 2:56 pm 3:06 pm

0 010 mins 3:05 pm 3:16 pm

No increment of load shall be applied until and unless the average of the strain gauge readings

were stable. Time strain reading shall refer to the CR1000 data logger with related to the loading

time.

Load weight shall refer to the transporter Delivery Order.


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Deflection Check for Traffic Load Test (Site Measurement) Date :26th

Jan 2013

Location : Batukawa Existing Bridge(Span 2)

Applied Load

Time

Deflection (mm)

Number ofTrucks (nos)

LoadedWeight (kg) SP1 SP2 SP3 SP4 SP5

Initial reading

0 0 11:05am 352 355 448 353 345

2 76460 11:35am 352 342 433 341 345

Deflection (+) uplift (-) 0 +13 +15 +12 0

4 153890 12:05pm 352 336 420 332 353

Deflection (+) uplift (-) 0 +19 +28 +21 -8

6 229820 12:28pm 348 331 420 332 350

Deflection (+) uplift (-) +4 +24 +28 +21 -5

8 291790 12:58pm 353 331 419 332 355Deflection (+) uplift (-) -1 +24 +29 +21 -10

10 354220 1:28pm 353 334 419 332 345

Deflection (+) uplift (-) -1 +21 +29 +21 0

12 417050 1:50pm 353 331 423 335 342

Deflection (+) uplift (-) -1 +24 +25 +18 +3

10 340580 2:10pm 353 332 418 334 344

Deflection (+) uplift (-) -1 +23 +30 +19 +1

8 263160 2:20pm 347 335 421 335 346


6 187230 2:37pm 350 336 422 333 347

Deflection (+) uplift (-) +2 +19 +26 +20 -24 125260 2:47pm 352 338 426 337 347

Deflection (+) uplift (-) 0 +17 +22 +16 -2

2 62830 2:56pm 350 343 435 340 346


0 0 3.05pm 350 351 446 347 346


No increment of load shall be applied until and unless the average of the strain gauge readings were

stable.


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0

13

15

12

0-1

24

29

21

-10

-1

2425

18

3

-1

23

30

19

12

19

26

20

-2

0

17

22

16

-2

2

1213 13

-1

2

4

2

6

-10 0 0 0 0

4

24

28

21

-5

-10

-5

0

5

10

15

20

25

30

-15

-10

-5

0

5

10

15

20

25

30

35

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Deflection(mm)

Deflection (mm) VS No. of Trucks for Traffic Load Test (Site Measurement) Date

:26th Jan 2013

2 trucks 4 trucks 8 trucks 10 trucks 12 trucks

10 trucks 8 trucks 6 trucks 4 trucks 2 trucks


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The graph below shows that strain reading increasing after each loading sequence with the thermal fluctuation during the testing

period from 11am to 4pm forStrain 13

Strain reading at SG13 due to trucks load, the change of strain in micro,

2 trucks (334-320) =14

4 trucks (341.6-320) =21.6 6 trucks (345.4-320) =25.4

8 trucks (345.6-320) =25.6 Maximum Change of strain due to load added

No truck

No truck

2 trucks

2 trucks

4 trucks

4 trucks

6 trucks

6 trucks

8 trucks

8 trucks

10 trucks

10 trucks

12 trucks

Thermal Gradient

Change of strain =

345.6- 321.4 = 24.2


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The graph below shows that strain reading increasing after each loading sequence with the thermal fluctuation during the testing

period from 11am to 4pm forStrain 14(Mid Span)

No truck

No truck

2 trucks

2 trucks

Thermal Gradient

4 trucks

6 trucks8 trucks

10 trucks

4 trucks

6 trucks

8 trucks

10 trucks

12 trucks

Change of strain =

265- 241.6 = 23.4


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The composite design for the steel girder and Reinforced concrete slab which connected by the

shear stud was take into consideration for the testing.

The composite properties for the composite girders were found that not exactly as expected

during the design stage. The strain gauges attached underneath the steel girder (mid Span)

illustrate the strain at the bottom was increasing as expected and at the top flange was decreasing.

The strain gauge was mouthed perpendicular to the bridge at the mid span of span 2 both strain 3

and 4 was installed parallel to verify the properties of the composite design.

Based on obtained values of strain field analyses, the theoretical analyses of steel girders, was considered

at service conditions for the following 3 positions. Strain 3, 4 & 13

(+) tension

(-) compression

(+) tension

(-) compression

Strain 3Strain 4

Strain 13

Trucks Load

No truck

No truck

2 trucks

2 trucks

4 trucks 6 trucks

8 trucks

10 trucks

4 trucks

6 trucks

8 trucks

10 trucks

12 trucks

Thermal Gradient

Change of strain =258.35- 256.95 = 1.4


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Therefore the strain reading shows the top flange was compressive when then trucks was loaded

and the bottom flange was in tension with positive (+) strain reading


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Converting Hz to Microstrain

If your readings are in Hz, convert them to microstrain ().

= A(F2) + C

Where, F = Reading in HzA = 0.0007576

C= -2030.1

Calculating the Change in StrainThe reading from the strain gauge is now in microstrain, but it does not represent the total strain

in the structural member.

There was strain in the structural member before the gauge was attached, and there was strain in

the wire inside the gauge, since it had to be tensioned in order to operate.Therefore, a datum reading must be obtained after the strain gauge is installed. The datum is

subtracted from any subsequent strain reading to find a change in strain.

= current initial

Positive or Negative Strain

Due to its design, the strain gauge reports larger numbers as the structural member lengthens andsmaller numbers as the structural member shortens.

When a tensile load increases, successive strain readings will be greater than the initial reading,

and the change in strain will be positive. In the same way, if a compressive load increases,successive strain values will be lower, and the change in strain will be negative.

Temperature Effects

We recommend that you always record temperature when you record strain readings. You canthen use the temperature data in addition to strain data to characterize the behavior of the

structure.

The steel used for the wire in the strain gauge has a thermal coefficient of expansion similar to that of steel used in structures. Thus, if the gauge and the steel are at the same temperature, no

corrections for temperature corrections are required.

If the temperature of the gauge and the temperature of the steel are not the same, you may seelarge changes in apparent strain. This is usually not a problem with the spot weldable gauge.

If there is a steel that has a very different coefficient of expansion from the steel in the gauge,

the temperature correction might be appropriate.

corrected = (TC mTC g) x (Temp 1Temp 0)

Where is the change in strain,

TCm is the thermal coefficient of the memberTCg is the TC of the gauge: 10.8 /C or 6/ F

Temp 1 is the current temperature

Temp 0 is the datum temperature


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As shown of above graph, the Vibrating Wire strain gauges of the girder show the same periodic fluctuation as does

the temperature during the loading test on 16th Jan 2013. This is due to thermal expansion in the girder and the

concrete slab.


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truck load test assessment (strain gauges) reporting

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