Load Rating Bridge Substructure Units

摘要

The adequacy of bridge substructures is usually based on information from as-built plans, design calculations and inspection results. Bridge substructures should be checked to verify that they have at least the capacity of the lowest rated superstructure member. If this information is not available, substructure adequacy is left up to the engineer's judgment. For massive wall piers and abutments this may be acceptable. Multi-column bents and piers and other similar substructures where a component failure could potentially cause collapse should be load rated if they show signs of distress such as structural cracking, excessive concrete spading, excessive reinforcing steel corrosion, reinforcing bars not engaged by concrete, are classified as structurally deficient due to primary flexural or shear reinforcement, show signs of movement, or exhibit other distress. Load ratings should then be performed based on operating levels for reinforced concrete members as stated in the current AASHTO Specifications. Load rating of bridge substructure can also be effectively done by the strut and tie approach. The strut-and-tie method generally uses lower effective concrete strengths and lower resistance factors than traditional analysis methods and therefore, will generally predict conservative member capacities. The strut-and-tie model may be beneficial where deficient shear reinforcement is compensated by the reserve capacity in the flexural reinforcement or vice versa forming alternate load paths and in cases with concentrated loads close to supports. The strut-and-tie method is also a simple method to use once understood, and therefore does not require significant additional effort to produce substructure load ratings. Load rating bridge substructures provides additional value to the bridge owner rehabilitating a bridge to ensure that the required design life is met.