Structural Engineering Research Projects:

Field Testing and Analysis of CRC Deck Girder Bridges

Large numbers of conventionally reinforced concrete (CRC) bridges remain in the national inventory that are lightly reinforced for shear. One of the most common types is the deck girder bridge, widely used during the highway expansion of the late 1940' s through the early 1960' s. Bridges of this type have girders cast integrally with the slab and may be single span or continuous over multiple supports. Many of these bridges are reaching the end of their originally intended design life and the combined effects of over-estimation of the concrete contribution to shear resistance at design, reduced anchorage requirements for flexural steel, increasing service load magnitudes and volume, as well as shrinkage and temperature effects, may contribute to diagonal tension cracking in these bridges. Inspections of approximately 1800 vintage CRC deck girder bridges in Oregon revealed over 500 with varying levels of diagonal tension cracking (ODOT 2002).

Two bridges described were used in the field tests and subsequent analytical modeling:

-The Willamette River Bridge is on Oregon Highway 219, located near Newburg, Oregon. The deck girder bridge was designed in 1954 and built in 1956. The concrete approach spans exhibited significant diagonal cracks.

-The McKenzie River Bridge crosses the McKenzie River on Interstate 5 in Lane County, Oregon. The bridge has four reinforced concrete deck girder approach spans at each end and was constructed in 1960. The south approach spans of the northbound lane of McKenzie River Bridge were selected for testing.

Field tests were performed using controlled truck loading. An ODOT (Oregon Department of Transportation) maintenance truck filled with gravel was used for tests on the two bridges as shown in Fig. 1.

Fig. 1. Axle spacing and weights for test trucks (length units = mm).

The axle weights and spacing were determined before the test as shown in the figures. Traffic was temporarily stopped or slowed with the use of a rolling roadblock so that the control truck would be the only vehicle on the bridge during data collection. The control truck passed over the bridge at several designated speeds and lane positions. Test speeds varied from 8 to 105 km/hr (5 to 65 mph). Lane locations included placing the truck in the truck lane, in the passing lane, and with the passenger side tires located on the fog line. For the Willamette River Bridge, eight truck passages were performed. For the McKenzie River Bridge, twelve truck passages were carried out. During each pass of the test truck, stirrup stresses and crack displacements were recorded. Stress ranges in the stirrups for the tested bridges are shown in Fig. 2. The measured stress ranges shown in the figure were produced by the control trucks traveling at 5 mph.

Gage No.

Fig. 2. Measured stress ranges at instrumented locations for truck traveling at 5 mph.

Conclusions

Strains in the stirrups crossing diagonal cracks were measured under a known test truck. The impact effect was measured based on trucks traveling at highway speeds compared with slow speed tests. Finite element models of the bridges were developed using linear elastic shell elements. The shear response under simulated truck loading was predicted for each bridge. Estimation of distribution factors for shear were made and compared with those prescribed by the AASHTO LRFD and Standard Specifications. Based on the field test and finite element analysis results, the following conclusions are presented:

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