PSI - Issue 62

Alberto Gennari Santori et al. / Procedia Structural Integrity 62 (2024) 339–346 Gennari Santori A. et al. / Structural Integrity Procedia 00 (2019) 000–000

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Table 1.XRD method - Longitudinal stress in the bars with and without compensation for residual stress.

Compensated

Average Axial

Standard

Span No.

Bar No.

Treatment

Average Axial Stress (MPa)

Stress (MPa)

Deviation (MPa)

7

1B

NO

321

56

446

7

2A

YES

253

27

336

9

1A

NO

207

9

331

9

4A

YES

241

23

324

9

4A

NO

181

26

306

9

10A

YES

347

18

430

9

10A

NO

268

4

392

Fig. 6. XRD applied on Dywidag bars.

For each bar mentioned above, measurements of the stress state were performed on multiple points (from a minimum of 3 to a maximum of 5). It is important to notice that the bar 9A of span 7 was unloaded and the measurements carried out revealed a tension equal to -83 MPa, using the electrolytic surface treatment, and -109 MPa without the same treatment. These measurements were used for the estimation of the residual surface stress and the value was considered on all the measures performed on site. The estimated values of the axial stress acting on the analyzed Dywidag bars with and without compensation for the residual stress are summarized in the following Table 1.

4.3. Summary of measurements performed on Dywidag bars

The estimated average axial load was equal to 370 kN but the highest values up to 620 kN have been measured. These axial load values are higher than those designed on the original project which was equal to 430 kN and close yield load of the bars, equal to 845 kN. These findings made necessary to evaluate that variations in axial stress due to traffic loads could not cause the special-joint system failure. In the next pages results of further instrumental surveys and tests aimed at evaluating the capability of the described system under different load patterns.

5. Static load test on span No. 7

A static load test was executed to evaluate the structural behavior of the midspan special joint located on span No. 7. The test was divided into 3 different configurations using up to 3 trucks simultaneously. These have been defined so as not to cause torsional stresses on the deck. For each configuration, the following parameter has been evaluated: • Deck vertical displacement, using high precision digital levelling measurements on 22 points. • Relative displacements between the upper and lower part of the special joint on both side of the deck by using Linear Variable Differential Transformer (LVDT). • Axial load variation acting on three Dywidag bars using strain gauges and the dynamic approach using Equation 2. Since the unloaded configuration measurement was performed, the first configuration was carried out using two tracks positioned with the back axle in correspondence to the expansion joint and symmetrically arranged to the longitudinal direction of the deck. The total applied load was equal to 33.5 ton. Successively, for the second configuration a machine mounted on a standard truck (characterized by a weight equal to 26,5 ton) was positioned centrally between the first two trucks used on configuration 1. The total weight applied was equal to 60 t. Eventually, the third and final configuration included the use of the same three vehicles of the configuration 2 but mirrored respect to the expansion joint. After the execution of the three load configurations, a final reading without loads applied on the deck was performed to evaluate any permanent deformations.