[Bridge Analysis Simplified By Bakht Jaeger.pdfl
Laurice Whack
Precast, prestressed concrete adjacent box beams are widely used in short- and medium-span bridges in the United States. However, a recurring issue with this type of bridge is the deterioration of shear key connections resulting in substandard performance of the overall bridge system. This research used full-scale structural tests to investigate four different shear key connection designs, including partial- and full-depth connections constructed with either conventional non-shrink grout or ultra-high performance concrete (UHPC). Quantitative measures to assist in evaluating the connection performance are suggested in the study. Parameters for the connection design were developed using the shear and tensile stresses in the connection. The behaviors of the connections using conventional grout and UHPC are presented. It was found that UHPC connections can be a resilient and innovative solution to prevent connection degradation in adjacent box beam superstructure systems, advancing the state of the practice in bridge construction.
This study, completed as part of the FHWA Structural Concrete Research Program, investigated two conventional non-shrink grout connections and two novel UHPC connections. Designs for partial- and full-depth connections were investigated for each connection material, and quantitative measures to evaluate the shear key performance are suggested. Design parameters, including the transverse post-tensioning force, the transverse shear strength of the connection, and the interface bond between the grout and box beam concrete, were also assessed. The full results of this study can be found in a separate report.(1) A peer-reviewed journal paper also presents the results of this study.(9)
Four connections were evaluated in this study. The first two used conventional high-strength, non-shrink grout in tandem with transverse post-tensioning. The partial- and full-depth connections are shown in parts A and B in figure 1, respectively. The surface of the precast concrete in the connection had a sandblasted surface finish. The other two connections investigated were new design details that take advantage of the advanced mechanical and durability properties of UHPC, as shown in parts C and D in figure 1. These were also investigated as partial- and full-depth connections. The UHPC connections included an exposed aggregate surface finish on the precast concrete and reinforcing steel that extended from the precast box beams into the connection to form a non-contact lap splice. No transverse post-tensioning was included. The exposed aggregate surface preparation is emerging as a preferred surface finish for both UHPC and conventional grout applications.(10,11) The enhanced mechanical properties of UHPC allow for reduced embedment lengths for embedded deformed reinforcement, which simplifies the design and construction. An embedment length of only 5.5 inches (140 mm) for the No. 4 (i.e., M13) bars used in these connections has been demonstrated to develop the yield strength of the bar.(12) The dimensions of the connections are presented in figure 2.
The conventional grouting material used in this study was a portland cement-based, prepackaged, non-shrink grout. It reached an average compressive strength of 7,800 to 8,120 psi (54 to 56 MPa) at the time of testing. The UHPC used is a commonly available prepackaged product with a steel fiber content of 2 percent by volume and an average compressive strength of 26 ksi (179 MPa) at the time of testing.
Thermal loading was simulated by pumping steam through copper tubes cast in the top flange of each box beam. A temperature gradient between the top and bottom flanges of approximately 50 F (28 C) was created. A total of 10 thermal cycles were applied to each test specimen, and visual inspection was conducted.
The cyclic structural loading was applied by four-point bending, as shown in figure 3. The loading was intentionally placed 6 inches (152 mm) off the centerline of the box beam to create a more severe torsional moment. This generated a greater transverse tensile force in the connection compared to centrically loading the beam.
The cyclic load was applied as a sinusoidal wave with a 180-degree phase angle between the beams. An analysis of a representative adjacent box beam bridge indicated that a loading range of 18 kip (80 kN) is the approximate distributed load on a single beam from a fatigue truck, as indicated in the AASHTO LRFD Bridge Design Specifications.(4) Based on this information, loading ranges of 18, 36, 54, 72, and 90 kip (80, 160, 240, 320, and 400 kN) were applied. Three different boundary conditions were used in this research: an unstiffened simply supported case, a partially stiffened case, and a fully stiffened case. The unstiffened simply supported case is shown in figure 3. A partially stiffened case was employed where the end transverse rotation was restrained by clamping the ends of beams (see figure 4). The last boundary condition was a fully stiffened case that restrained the deflection of one beam with extra supports at diaphragm locations (see figure 5) in addition to the end restraint of the partially stiffened case. The stiffer boundary conditions were intended to provide a more realistic representation of a full superstructure system. More details of the test setup and loading protocol can be found in the associated full report and in papers by Yuan and Graybeal.(1,9,13)
The transverse post-tensioning force used with the conventional connections varied. Post-tensioning levels of 8, 6, 4, 2, 0.8, and 0 kip/ft (117, 87, 58, 29, 12, and 0 kN/m) were tested. Figure 3 shows the transverse post-tensioning locations for the conventional grout connections. These connections received a small post-tensioning force prior to connection grouting with the full post-tensioning force being applied after the grout had gained strength. Transverse post-tensioning was not applied to the UHPC connections.
To compare the connection performance and efficiency under different conditions, quantitative measurements are needed. Deteriorated connections can compromise both the strength and serviceability of bridges. When the connection becomes cracked, the load distribution between the box beams is compromised, and the live load may remain concentrated in a few beams under the wheels. This can potentially exceed the allowable loads of the beams. Beams with failed connections will not deflect equally under live loads. Excessive differential displacements between adjacent girders may further degrade the connection and lead to reflective cracking in the overlay if one is present. Cracks can allow chloride-laden water to infiltrate the structure and can corrode the reinforcing bars and prestressing strands adjacent to the connection.
This study adopted two parameters to measure the performance of the connection. The first was the moment distribution factor that evaluated the ability of the shear key to transfer loads; it relates to the strength condition of the bridge. The second was the differential deflection that measured the relative displacement between the adjacent beams and corresponds to the serviceability of the bridge.
For the case of the fully stiffened support condition used in this study, the moment distribution could not be calculated using the ratio of carried moments because the boundary conditions on the two beams were different. In this case, the equivalent moment transferred through the shear key was used. This method calculated the moment using the additional strain in the beams rather than the recorded strain at the maximum load. This measure of moment approximated the amount of moment transferred through the shear key from the loaded beam to the unloaded beam.
With the pre-wetting and curing procedures adopted in this study, the four connections investigated were constructed successfully in the laboratory. Each test specimen was first thermally loaded and then structurally loaded. Note that the full-depth conventionally grouted connection cracked upon release of the small post-tensioning force that was used to stabilize the specimen during grout casting. This was likely due to differential sweep in the beams.
Each beam was thermally loaded to create a temperature gradient between the flanges of approximately 50 F (28 C) for 10 cycles. The thermal loading generated an upward deflection at the mid-span of 0.425 to 0.570 inch (10.8 to 14.5mm). The behavior of the beams used in the tests was generally the same. Visual inspection conducted during the thermal loading detected only minor, non-structural cracking in the partial-depth conventionally grouted connection. No debonding was caused by thermal loading for any of the connections.
For the partial-depth conventional grout connection, nearly 7 million loading cycles were applied. The cyclic structural load was not observed to propagate the minor cracks formed during thermal loading or initiate any new cracks. The uncracked connection effectively transferred the load and limited differential deflection regardless of the level of transverse post-tensioning force. The partial-depth conventionally grouted connection was intentionally cracked by applying a direct tensile force to the connection, as shown in figure 6. The interface between the grout and concrete was the weak link in the conventionally grouted connection, as cracking occurred primarily at this interface. Mechanical cracking of the connection ceased when about two-thirds of the connection was cracked. The partially cracked connection was then cyclically loaded and performed similarly to the uncracked connection in terms of moment distribution and exhibited a slight increase in differential deflections. The crack propagated as loading cycles progressed. Differential deflection and crack propagation increased as the level of transverse post-tensioning decreased. When the connection was mechanically cracked so the full length of the connection was cracked, the same observations were made. The distribution of longitudinal strains was not substantially affected, and differential displacements were seen to slightly increase, particularly when little to no post-tensioning was applied. The loadings imparted were not sufficient to significantly degrade the shear interlock across the cracked connection. Moving loads and/or water ingress combined with freezing temperatures, neither of which were applied in this study, would likely serve to widen cracks and commensurately decrease the ability of beams to share loads across connections.
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