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Is crack substantial to the bridge's integrity?

posted Sep 9, 2018, 7:25 AM by jeffery jim

1. This is the Petagas Flyover which is located close to the Kota Kinabalu Airport. This is not a new issue as it was discussed and inspected by the PWD back in June 2013. The verdict; it is safe!

2. What are the loadings for bridges? It is good to understand loading before we go into bridge design. Recent bridges are designed to accommodate Medium Term Axle Loading (MTAL) since (Long) LTAL is quite expensive and usually forfeit the (Short) STAL and Sub-standard Axle Loading (SSAL). Even some of the major bridges around the state built in the 60s are still robust enough to withstand MTAL requirements under dynamic analyses conducted last year.

3. If you look at the superstructure, it is still robust enough to cater even heavy dynamic (traffic) load since the span of the affected area is relatively short. But is that the case?

4. What happened and causes or triggered this to happened? The design and the manufacturing processes! I will not disclose a sensational bridge case study which also involved the same element; pier (and the crosshead). If you look deep into bridge failure literature, we can say this is a common defect or delamination.

5. Assuming this is not an aesthetic crack, what causes such crack failures at the web of the box girder? First, it is not a tensile or flexural crack since the cracking mode is not concentrated with multiple crack lines at the soffit/flange of the box girder. Second, it is definitely not shear crack which formed parallel to the direction of the traffic. If we conduct a traverse analysis model, the affected sections have the lowest force and moment envelope value.

6. This is a possible result of thermal cracking during curing of the box girder section (as we know, non-uniform hydration often leads to cracking) and thermodynamic of the material when exposed to extreme heat or weathering throughout the years. The slender web means there is a rapid heat dissipation when compared to other sections which retain heat for longer period.

7. The other possible issue here is the moment distribution (from one bay to the other, not grids of each bay) at the pier and/or crosshead which leads structural members to be in non-equilibrium state and subsequently lead to cracking under compression at this section. The displacement of crosshead and bearing leads these box girders to be under constant compression relatively (although there is no tendon at this section but affected by the tendon at the bottom flange). This is due to the tensile strength of post-tensioned tendon which interacted in order to remain in equilibrium. This will reach natural equilibrium when experiencing tendon lost-effect through time due to creep and shrinkage.

8. Lastly, does this affect the bridge structure in the long run? Not really. The crack at side or the edge have no significant magnitude in regards to the robustness of the structure. Nevertheless, the crack at the bottom may pose some long term issues. Prestressed or post-tensioned concrete components are very sensitive to chloride attack. For that reason, aggregate with slightly higher chloride (<2%) content may be allowed for normal reinforced concrete structures but not when it involves prestressed or post-tensioned tendons (<1%). This will lead to corrosion of the tendon and the failure of the overall system since this bridge is extremely close to the shoreline.

9. If there is a need to conduct assessment, reliability test or even appraisal, parties can always reach me through my inbox.

Here is the original Link of the discussion.