|TTI Research Supervisor:
Texas A&M Transportation Institute
Texas A&M University System
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:
Taya Retterer, P.E.
TxDOT Bridge Standards Engineer
Texas Department of Transportation 125 E 11th ST.
Austin, TX 78701
Bridge parapets are used on bridges as barriers to protect errant vehicles from departing the bridge into a hazard. Some parapets are mounted directly to the deck or on a raised sidewalk. Wherever the location is, the placement conditions are important since they can determine safety performances. Well-designed parapets can protect errant vehicles more effectively. Therefore, there have been many studies regarding the safety performance of bridge parapets. However, most of the studies were related to parapets mounted to the deck directly, and only a few have covered a sidewalk raised on a curb. Since a curb often needs to be installed for such functions as drainage, right-of-way reduction, and sidewalk separation (Hancock and Wright, 2013) more relevant research is necessary.
This study focuses on both test level two (TL-2) and test level three (TL-3) impact conditions as given in the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH) (AASHTO, 2016). The impacting vehicle are the pickup truck (Test vehicle 2270P) and the small passenger vehicle (Test vehicle 1100C) impacting the roadside device at 62 mi/h and 44 mi/h for TL-3 and TL-2 respectively. Both test levels are to be conducted at 25 degrees impact angle.
The research objective is to develop placement guidance for bridge parapets placement on sidewalk given MASH TL-2 and TL-3 impact conditions. This study will investigate a sidewalk with an eight inch curb height and placement position of the applicable MASH TL-2 or TL-3 compliant bridge rail system on sidewalks.
The research outcome will provide guidelines to help state agencies identify proper placement of bridge parapets on sidewalks with 8 inches curb height under both MASH TL-2 and MASH TL-3 impact conditions. The effect, if any exists, of the sidewalk slope will be provided in the guidelines.
TTI will provide a report documenting the vehicular trajectory tests, simulation cases and the guidelines for parapet placement on sidewalks.
The guidelines outcome of this project are envisioned to be ready for implementation by states DOT’s into their guidelines/standards.
TTI will conduct two crash test per MASH TL-2 conditions. These tests will be performed for identifying the trajectory for both the small car and the pickup truck vehicles after they impact the curb and travel across the sidewalk. The TL-3 tests from the Caltrans study cited earlier will be studied for insight of vehicular interaction with curbs at 62 mi/h speed.
According to MASH, up to two tests are recommended to evaluate longitudinal barriers to TL-2. Details of these tests are described below.
These tests will be instrumented to quantify the trajectories of key points on the vehicles. High speed cameras as well as targets on the vehicles will be used to capture the displacement time history of different points on the vehicles to better quantify the overall trajectory of the vehicles across the sidewalk.
The research team will perform vehicular trajectory simulations on the sidewalk-curb to validate the trajectory calculated using the finite element simulations. Vehicle models corresponding to MASH vehicles are to be used in the finite element analyses to validate their trajectory and their interaction with the curb (Figures 4 and 5).
Figure 4. Tracing of Hood Ridge Point of Car Impacting MnDOT 8-inch Curb and Sidewalk.
Figure 5. Tracing of Hood Ridge Point of Pickup Truck Impacting MnDOT 8-inch Curb and Sidewalk.
The research team expects that one of the critical interactions between the vehicle and the curbs to be the interaction of the tire and the curb face. As such, TTI researchers will include a detailed model of the vehicle tires to assist with accurately capturing the interaction between the vehicle tire and the sharp edge of the curb-sidewalk. TTI researchers have developed a high fidelity light pick up tire model that was constructed based on scanning and material properties testing and calibrated using FMVSS 139 plunger and bead unseating tests (Figure 6). A typical pickup truck tire and its corresponding finite element model is shown in Figure 7. Similarly a
Figure 5. A Dueler H/T 684 III Tire (Left) and the Corresponding Finite Element Model.
TTI researchers modified the existing pickup truck model of a quad cab Chevrolet Silverado to incorporate the new detailed tire model as shown in Figure 8.
Figure 6. Default Simple Tire and Detailed Tire Models Mounted on the Pickup Truck Model.
Therefore, the new model of suspension/tires of both vehicles will be further modified as needed to improve the trajectory predictions of the vehicle models used for these curb impacts.
Once a reasonably valid behavior from the simulation study of the vehicle trajectory is obtained, the research team will conducted finite element simulations of parapets placed on the sidewalk at different positions from the curb. The parapets are assumed to be rigid (no strength issue) so the analyses are to quantify their performance in terms of vehicular stability due to their position from the curb edge.
TTI researchers will tabulate the results from Task 2 and prepare a document summarizing the curb tests and the parametric simulation study to develop the guidelines for parapet placement on sidewalks given the test level, curb height (8 inches and 6 inches) and sidewalk slope.