Truck Load Test Assessment (Strain Gauges) Reporting

BACKGROUND STUDY The temperatures fluctuation was found that contribute to the strain magnitude, the first loading test 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.

Temp. Fluctuation Strain Reading

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 is installed. The second loading test was carried at the different loading and unloading sequence. In stead of loading 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 of trucks were increased from zero to eight trucks.

Testing time selection for Second loading test. The existing bridge tested was actually connected with the new Batukawa Bridge by sharing the same foundation (pile cap) as shown below. Therefore the traffic load might affect the strain reading by transfer the movement through the pile cap.

Old bridge

New bridge

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

Evening 5-7pm peak hour Morning 6-8am peak hour Evening 5-7pm peak hour

Trend of strain running due to the temperature

As the preventive measurement to minimize the vibrating due to the new bridge’s 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

Daily traffic trend

No traffic

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 traffic strain reading was increasing as the temperate was increasing but with the larger fluctuation magnitude compare to the strain reading without traffic.

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 bridge’s 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

Loading Procedure for Traffic Load Test Time Maintaining Number of Trucks (nos) Loaded Weight (Kg) Load Applied Load

0

0

2

76460

4

153890

6

229820

8

291790

10

354220

12

417050

10

340580

8

263160

6

187230

4

125260

2

62830

0

0

Start time

End time

25 mins

11:05 am

11:30 am

20 mins

11:35 am

11:55 am

15 mins

12:05 pm

12:20 pm

22 mins

12.:28 pm

12.50 pm

21 mins

12:58 pm

1:19 pm

17 mins

1:28 pm

1:45 pm

20 mins

1:50 pm

2:10 pm

10 mins

2:12 pm

2:22 pm

10 mins

2:24 pm

2:34 pm

10 mins

2:37 pm

2:47 pm

10 mins

2:47 pm

2:57 pm

10 mins

2:56 pm

3:06 pm

10 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.

Deflection Check for Traffic Load Test (Site Measurement) Date :26th Jan 2013 Location : Batukawa Existing Bridge(Span 2) Applied Load Number of Trucks (nos)

Loaded Weight (kg)

Deflection (mm) Time

SP1

SP2

SP3

SP4

SP5

Initial reading 0 11:05am 352 355 448 353 345 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 353 331 419 332 355 Deflection (+) 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 Deflection (+) uplift (-) +5 +13 +27 +18 -1 6 187230 2:37pm 350 336 422 333 347 Deflection (+) uplift (-) +2 +19 +26 +20 -2 4 125260 2:47pm 352 338 426 337 347 Deflection (+) uplift (-) 0 +17 +22 +16 -2 2 62830 2:56pm 350 343 435 340 346 Deflection (+) uplift (-) +2 +12 +13 +13 -1 0 0 3.05pm 350 351 446 347 346 Deflection (+) uplift (-) +2 +4 +2 +6 -1 No increment of load shall be applied until and unless the average of the strain gauge readings were stable. 0 2

Deflection (mm) VS No. of Trucks for Traffic Load Test (Site Measurement) Date :26th Jan 2013 35

30 28 30 29

30 24 25

Deflection (mm)

26 25

24 23

20

25 21

22

19 17

15

15 13

13 12

20

21 20 19 18 16

15

13 12

10

10 6

5 0 0

0.5

1

4 2 0 -1

4 2 0

0 1.5

2

2.5

3

0 3.5

4

4.5

3 1 0 -1 -2 5

5

5.5 0

-5 -5 -10

-10 -15

-5

-10

2 trucks 10 trucks

4 trucks 8 trucks

8 trucks 6 trucks

10 trucks 4 trucks

12 trucks 2 trucks

The graph below shows that strain reading increasing after each loading sequence with the thermal fluctuation during the testing period from 11am to 4pm for Strain 13

6 trucks

8 trucks

10 trucks

8 trucks

4 trucks

10 trucks 12 trucks

6 trucks 4 trucks

2 trucks 2 trucks

Change of strain = 345.6- 321.4 = 24.2 µε No truck No truck

Thermal Gradient

Strain reading at SG13 due to trucks load, the change of strain in micro, µε 2 trucks (334-320) =14 µε 4 trucks (341.6-320) =21.6 µε 6 trucks (345.4-320) =25.4 µε 8 trucks (345.6-320) =25.6 µε Maximum Change of strain due to load added

The graph below shows that strain reading increasing after each loading sequence with the thermal fluctuation during the testing period from 11am to 4pm for Strain 14 (Mid Span)

6 trucks

8 trucks

10 trucks

8 trucks

4 trucks

10 trucks

12 trucks

6 trucks

4 trucks

2 trucks 2 trucks

Change of strain = 265- 241.6 = 23.4 µε No truck No truck

Thermal Gradient

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. (-) compression

Truck’s Load

(+) tension Strain 3

Strain 4

(-) compression

Strain 13

(+) tension 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

Thermal Gradient

2 trucks

6 trucks

Change of strain = 258.35- 256.95 = 1.4 µε

2 trucks 8 trucks 10 trucks

4 trucks 6 trucks

4 trucks 8 trucks

No truck

12 trucks 10 trucks

No truck

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

Converting Hz to Microstrain If your readings are in Hz, convert them to microstrain (με). με = A(F2) + C Where, F = Reading in Hz A = 0.0007576 C= -2030.1 Calculating the Change in Strain The 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. Δμε = με current – με initial Positive or Negative Strain Due to its design, the strain gauge reports larger numbers as the structural member lengthens and smaller 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 can then 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 see large 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. Δμε corrected = Δμε – (TC m– TC g) x (Temp 1 – Temp 0) Where

Δμε is the change in strain, TCm is the thermal coefficient of the member TCg is the TC of the gauge: 10.8 με/°C or 6με/ °F Temp 1 is the current temperature Temp 0 is the datum temperature

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.

http://www.supercivilcd.com/THERMAL.htm

COEFFICIENT OF PER DEGREE CENTIGRADE

THERMAL

EXPANSION

Cement Concrete - Quartzite

1.2 to 1.3 x 10^-5

Cement Concrete - Sand Stone

0.9 to 1.2 x 10^-5

Cement Concrete - Granite

0.7 to 0.95 x 10^-5

Cement Concrete - Basalt

0.8 to 0.95 x 10^-5

Cement Concrete - Lime stone

0.6 to 0.90 x 10^-5

Aluminum

.0000230

Brass

.0000188

Bricks/Brick Work

.0000055

Cement

.0000144

Glass

.0000081

Granite

.0000085

Iron

.0000110

Lead

.0000290

Marble

.0000110

Sand Stone

.0000125

Steel

.0000120

R.C.C.

.0000117

Tin

.0000125

Zinc

.0000325

Wood

.0000110