EVALUATION OF THE CAUSE OF CRACKING IN BRIDGE #MEG-124-6.78

Bridge MEG 124-6.78 was a single span, composite, adjacent, prestressed box girder bridge with a 45 deg right forward skew. After being in service about 1 year, the three edge beams on one side of the bridge showed extensive bottom flange cracking. Diagonal and vertical cracks were visible on the sides of the fascia beam. Beam specimens were removed from the bridge for destructive testing. The destructive testing showed that the diagonal cracks occurred as soon as the first flexural cracks began to appear. As load increased, the diagonal cracks grew larger, but the beams held over 445 kN (100 kips) total. It was found that the cause of failure was construction error. These skewed boxes were detailed to have all the strand debonded from the end of the beam to the obtuse corner. The end of the beam has conventional reinforcing. This creates a straight (non-skewed) beam with a reinforced concrete triangle on the end. Unfortunately, the contractor debonded the strands past the obtuse corner to end of the mild steel, essentially creating an unreinforced section. This section failed under load, causing the diagonal cracks. The bridge was rebuilt and tested. It was found to behave as a single unit and there was reasonable distribution of loads between the beams. The flexural stresses in the girder were very low. No diagonal cracking was observed. The testing confirmed that, properly constructed, the bridge was adequate. Finally, finite element models of the bridge were constructed. The first model duplicated the "as built" condition of the original. This model confirmed the crack patterns shown in the actual test - diagonal cracking at the end of the girders. A second model was of the "as built" for the new bridge structure. The model confirmed that, properly constructed, the girders will not have diagonal cracking at the end. The last model was a combination of the first two. In this model, half the prestressing strands were debonded to the termination point of the mild steel. This model showed normal flexural cracking at the midspan, but also showed cracking at the ends.