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Title:Methodology for Load Rating Double-Tee Bridges
Authors:Sandip Rimal, Brian Kidd, Mostafa Tazarv, Junwon Seo, and Nadim Wehbe
Publication Date:Jul 2019
Report #:MPC-19-389
Project #:MPC-523
TRID #:01713854
Keywords:condition surveys, field tests, girder bridges, inspection, live loads, load and resistance factor design, load factor, methodology, precast concrete, prestressed concrete bridges, shear strength, statistical analysis
Type:Research Report – MPC Publications



The most common type of bridge on South Dakota local roads is a precast prestressed double-tee (DT) girder bridge. More than 700 DT bridges are currently in-service in South Dakota. Structural detailing, aging, traffic volume, and environmental conditions affect structural performance, integrity, and capacity of DT bridges. When a bridge is affected by one or more of the aforementioned parameter(s), estimation of the bridge safe live load is necessary to ensure safety of the traveling public and prevent excessive bridge damage and collapse. Load rating of damaged bridges is challenging because of a lack of information regarding the capacity and live load distribution of damaged components. In this study, quantitative definitions were first proposed to identify all damage types and condition states specific to double-tee girders. Subsequently, more than 370 inspection reports on South Dakota double-tee girder bridges were reviewed to determine the frequency of damage types and condition states, bridge span length, bridge number of spans, girder depth, and bridge skew conditions. The statistical database was then used to identify double-tee bridge candidates for field and strength testing. Using the database, 10 double-tee bridges were identified as suitable for field testing and inspected for further evaluation. Subsequently, two bridges were selected for field testing. Girder distribution factors (GDFs) and dynamic load allowance (IM) were measured. The field test results confirmed that AASHTO LRFD specifications can be used to estimate the moment and shear GDFs for South Dakota DT bridges with a longitudinal joint damage condition state 3 or less. For the calculation of moment and shear GDFs for a South Dakota DT bridge with a longitudinal joint damage condition state 4, GDFs were proposed to be the greater of: (a) the factor for the exterior girders, (b) the factor for the interior girders, and (c) 0.6. Furthermore, AASHTO LRFD specifications can be used for estimation of IM for damaged double-tee girders with no further modification. An accurate estimation of the capacity of damaged double-tee girders was critical in this project for a safe load rating. To verify the available moment and shear capacity estimation methods, two 45-year old double-tee girders, one 50 ft (15.24-m) long and another 30 ft (9.14-m) long, were extracted from a bridge located in Nemo Road, SD, and were strength tested at the Lohr Structures Laboratory at South Dakota State University. A four-point loading configuration was selected for the strength testing. Data was collected, and the methods were validated. Verified methods were then used to calculate the shear and moment capacities of 23 different double-tee sections, which have been used in South Dakota. Based on the statistical, experimental, and analytical studies, a load rating methodology was proposed for damaged double-tee girder bridges in which the rating may be performed similarly to the AASHTO Load and Resistance Factor Rating (LRFR) method that currently used in practice. Nevertheless, it was recommended to modify the capacity (C) and live load components (LL and IM) of the load rating equation accounting for different damage types and condition states.

How to Cite

Rimal, Sandip, Brian Kidd, Mostafa Tazarv, Junwon Seo, and Nadim Wehbe. Methodology for Load Rating Double-Tee Bridges, MPC-19-398. North Dakota State University - Upper Great Plains Transportation Institute, Fargo: Mountain-Plains Consortium, 2019.

NDSU Dept 2880P.O. Box 6050Fargo, ND 58108-6050