Improved load rating of reinforced concrete slab bridges.

648008868PDFTech ReportNM05STR-02CO4759Concrete bridgesReinforced concrete bridgesLoad limitsFinite element methodStrain (Mechanics)SlabsNew MexicoNew Mexico. Dept. of Transportation. Research BureauJ\uc3\ua1uregui, David V.Licon-Lozano, AliciaKulkarni, KundanNew Mexico State University. Dept. of Civil EngineeringNew Mexico. Dept. of Transportation. Research BureauUnited States. Federal Highway AdministrationNTL-HIGHWAY/ROAD TRANSPORTATION-Bridges and StructuresNTL-HIGHWAY/ROAD TRANSPORTATION-Pavement Management and PerformanceUS Transportation CollectionIn New Mexico, many reinforced concrete slab (RCS) bridges provide service on interstates I-10, I-25, and I-40. An accurate strength evaluation of interstate bridges is essential to avoid unnecessary load restrictions. The AASHTO load rating factor for this type of bridge largely depends on the live-load moment per foot of slab width. As a result, the main objective of this study was to determine a more accurate value for the equivalent strip width (using higher level evaluation techniques including diagnostic load testing and finite element analysis) for use in the AASHTO rating.A continuous, RCS bridge located in Las Cruces, New Mexico was evaluated in this study. An AASHTO load rating analysis based on the Load and Resistance Factor Rating (LRFR) approach was first performed using code-prescribed equations for the equivalent strip width to determine the live-load effects. A diagnostic load test was then conducted to measure the strain response at selected points in the positive and negative moment regions of an exterior and interior span. The measured response showed that the slab stiffness fit within cracked and gross section behavior. Furthermore, bending moments from finite element analysis agreed reasonably well with those derived from the experimental strain data (using the average of the cracked and gross section modulus).Using refined analysis, it was shown that the equivalent strip widths for positive moment were 26.1% and 22.1% greater than those calculated by the AASHTO approximate method for the exterior and interior spans, respectively. Furthermore, the refined widths for negative moment were greater than AASHTO by 13.1% for the exterior span and 11.1% for the interior span. This increase in the equivalent strip width reduced the live-load effects, which proportionally increased the rating factors. Accordingly, the inventory and operating rating factors for the bridge increased from 0.84 to 0.93 and 1.08 to 1.20, respectively. The factors increased by just 11% (rather than over 20%) since the rating was controlled by negative moment