Experimental test is a powerful tool for researchers to investigate the structural behavior of bridges in the actual situation. Various types of instruments, like strain gage, string potentiometer, load cell, accelerometer, etc., are utilized by researchers to measure the bridge responses in static and dynamic tests. The observed results are not only necessary to get a knowledge of the structural performance, but also useful to validate the computational models. Several representative experimental studies on the structural performance in general and live load distribution behavior in particular of bridge superstructures are introduced in this section. The bridge types include prestressed concrete spread box beam bridge, steel girder bridge, solid slab bridge and prestressed concrete I-girder bridge.
Prestressed concrete spread box beam bridge was thoroughly investigated by Fritz laboratory at Lehigh University in the 1960’s. Five in-services bridges in Pennsylvania were tested under static and dynamic vehicle loads to determine the load distribution behavior (VanHorn 1969). Figure 2.8 shows the elevation and cross section of Dreherville Bridge, which is the first test bridge in this research project. Strain gages and deflectometers were installed in the bridge superstructure to record the responses of different girders in static and dynamic tests. The vehicle was parked at critical locations in the static test to generate the most adverse moment values. In the dynamic test, timer and lateral position indicators were utilized to determine the vehicle speed and location. The dynamic loads were applied by driving one or two trucks at various speeds (16-55 km/h) along different lanes. Based on the data recorded, moment distribution factors and
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impact factors were determined. From the test results, it was concluded that: 1) the magnitudes of moment distribution factors are relatively insensitive to the vehicle speed; 2) the experimental moment distribution factors were considerably less than code specified values for interior girders; 3) due to the extra stiffness contributions from the curb and parapet, the observed moment LDFs for exterior girders were larger than code specified values; 4) compared with the bridge response under crawling speed (3.2 km/h) trucks, the amplification factor induced by moving loads is smaller than the code specified value. This bridge test served as a pilot study, from which reliable instrumentation, load pattern and test procedure were determined and then applied to other bridge tests. Schaffer and VanHorn (1967) and Lin and VanHorn (1968) studied the effects of skew and diaphragms on the load distribution behavior of spread box beam bridges by conducting similar field tests on Brookville Bridge and Philadelphia Bridge, respectively.
Kim and Nowak (1997) investigated the load distribution behavior of steel girder bridges by conducting experimental tests on two simply-supported bridges in Michigan. For both bridges, the strain transducers were attached on the bottom flanges of steel I- girders at midspan location to measure the bridge responses under normal traffic loads without lane closure for two consecutive days. The girder LDF values and impact factors were determined by processing the recorded strain data. It was found that the effects of closely spaced diaphragms were negligible. Measured LDF values of the bridge with more sparsely spaced girders were more uniformly distributed than the other bridge. In terms of impact factors, the test data indicated that the increasing strain values reduced the impact factor and the measured values for large strains were smaller than the code specified value.
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(a) Transverse Section
(b) Elevation View Note: 1 inch = 25.4 mm, 1 feet = 304.8 mm
Figure 2.8. Transverse Section and Elevation View of Drehersville Bridge in Pennsylvania (Douglas and Vanhorn 1966).
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Eom and Nowak (2001) carried out an experimental research project on evaluating the conditions of existing bridges by testing 20 steel girder bridges. The strain gages were used to record bridge responses and further infer the experimental moment LDF values. It was found that the values specified by AASHTO LRFD Bridge Design Specifications (AASHTO 1998) and AASHTO Standard Bridge Design Specifications (AASHTO 1996) were always conservative as compared to test results. It was also noted that the existence of secondary components, such as sidewalk, railing and parapet, cause effects on the load distribution behavior due to the extra stiffness.
Amer et al. (1999) conducted experimental tests on three short-span solid slab bridges to investigate the equivalent width and load capacity of existing slab bridges. The strain gages were installed at critical locations to measure the bridge response. The moment values were determined by multiplying the strain values with the section modulus and concrete elastic modulus. It was found that the depth of edge beams had significant effects on the equivalent width because the edge beam moment raised with an increase of the moment of inertia. Conversely, observed results showed that the variation of slab thickness caused very little influence on the equivalent width. It was also noted that for bridge structures with considerable cracks, test results based on measured strains may not be realistic if the material nonlinearity is not taken into account.
Schwarz and Laman (2001) conducted field tests on three prestressed concrete I- girder bridges, with span lengths of 10.2, 23.3 and 31.1 m, to investigate the lateral load distribution behavior and the dynamic amplification factor. Strain gages were installed underside of each girder at the midspan of bridges to measure the strain values and further
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determine the moment LDF values. This arrangement is consistent with previous experimental work done by many researchers (Kim and Nowak 1997); (Laman et al. 1999); (O'Connor and pritchard 1985); (Paultre et al. 1995). By comparing with the values calculated from AASHTO Standard and AASHTO LRFD formulas, it was found that the code-specified LDF values were greater than those measured in the test for both one and two lanes loaded, thus the code equations are conservative for design usage. In terms of dynamic effects, the test results indicated that the amplification factor decreases with the increasing stress. With the increase of the vehicle speed, it was observed that the amplification factor increases.
From the literature described above, it is shown that the strain gage was normally utilized to measure the bridge response in the field tests and the strain value was the major parameter to determine the girder moments and further infer experimental LDF values. Most research work focused on the moment actions while very few studies dealt with the shear force measurements.
In order to investigate the shear distribution behavior of slab-on-girder bridge structures, Barr and Amin (2006) carried out a static load test on a full-scale steel I-girder bridge. This simply-supported bridge had a span length of 12.2 m with three steel I-girders. Three load cells were installed between the girders and supports at one end to measure the reaction forces under externally applied load. Based on the data recorded by load cells, the shear LDF values of steel I-girders were further determined.
Hughs and Idriss (2006) adopted embedded fiber-optic sensors to investigate moment and shear load distribution behavior for a prestressed concrete spread box-girder
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bridge. The sensors were arranged in different topologies to measure various structural responses. Crossed topology is used to measure the shear forces while the parallel topology is for bending moments, as shown in Figure 2.9(a). The elevation view and plan view of sensor layout for the test bridge are shown in Figure 2.9(b) and Figure 2.9(c). By comparing test results with those calculated from the AASHTO LRFD empirical formulas, it is found that the LRFD empirical formula would yield safe design values, although the distribution factors of exterior girders would be overconservative.
In this dissertation, an in-service spread slab beam bridge was tested to investigate the load distribution behavior of this new bridge system. A creative method that using bearing pad deformations to infer the shear LDF values was developed in the test process. Besides, the effects of the secondary elements (guardrail and sidewalk) and amplification factors were also evaluated based on the test results.
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Parallel Topology Crossed Topology
(a) Parallel and Crossed Topology for Moment and Shear Measurement
(b) Elevation View of Sensor Layout for Girder 1
(c) Plan View of Sensor Layout
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2.5 COMPUTATIONAL ANALYSIS FOR BRIDGE SUPERSTRUCTURES