MIDWEST STATES POOLED FUND PROGRAM
Length of Need for Free-Standing, F-Shape, Portable Concrete Barrier
Sponsoring Agency Code
TPF-5(193) Supplement 75, TPF-(295) Contract No. 16346 (SWDZI)
Portable concrete barrier (PCB) systems redirect errant vehicles through a combination of various forces and mechanisms, including inertial resistance developed by the acceleration of several barrier segments, lateral friction loads, and the tensile loads developed from the mass and friction of the barrier segments upstream and downstream of the impacted region. Typically PCB designs are evaluated and tested using 200-ft long system lengths. It has generally been assumed that this length of system provides vehicle redirection and resulting system deflections and working widths that are representative of longer PCB installations. Unfortunately, recommendations on minimum PCB system lengths have generally been limited to the 200 ft length or longer in order to preserve the as-tested system deflections and impact behavior. In addition, guidance on the beginning and end of the length of need of these systems is typically given as a minimum of 100 ft (i.e., 8 barrier segments of 12.5 ft long) in order to preserve performance similar to the existing crash tests.
There are many instances where state DOTs and other end users may wish to use shorter PCB installations to shield a hazard or work zone or limit the number of barriers required on the upstream and downstream ends to reduce overall system length. Shorter barrier lengths are associated with lower accident frequencies and provide improved cost and safety benefit as long as they retain their ability to safely contain and redirect errant vehicles. However, there are concerns with the performance of shorter PCB installations that must be considered. First, shorter PCB systems would be expected to have higher deflections and working widths than installations of 200 ft or more due to the reduction of upstream and downstream barrier mass and friction forces. Second, PCB systems may not develop sufficient longitudinal resistance at significantly shorter system lengths and may form a pocket in front of an impacting vehicle, which could lead to vehicle instability or excessive decelerations. Finally, no impact testing has been performed near the upstream or downstream ends of the free-standing PCB system to determine the limits of the length of need of the system. Impacts at or near the barriers at the ends of a free-standing barrier system may produce very different barrier performance than impacts near the center of the system, and the results may include the potential for gating of the vehicle through the system, pocketing, rapid deceleration, and/or vehicle instability.
Thus, there exists a desire to install PCB systems shorter than 200 ft and a need to more accurately define the beginning and end of the length of need for these systems. However, concerns for the safety performance of a shorter PCB system have limited their use. Further study on the minimum effective length of PCB systems, their associated deflections and working widths, as well as a determination of the length of need of these systems is warranted in order to provide more efficient and safe PCB installations.
MwRSF previously developed and full-scale vehicle crash tested a 12.5-ft long F-shape temporary concrete barrier system for use in both free-standing and tie-down applications. This temporary barrier design is currently used by the Nebraska Department of Roads (NDOR). FDOT refers to this barrier design as the â€œType Kâ€ in their standard plans. Full-scale crash testing of this barrier system was conducted under both the NCHRP Report No. 350 and MASH Test Level 3 (TL-3) safety requirements. During the MASH TL-3 full-scale crash test, test no. 2214TB-2, the F-shape PCB exhibited a dynamic deflection of 79.6 in. when impacting near the middle of a 16 barrier segment test system with an overall length of 200 ft.
As noted above, impacts on PCB installations shorter than the tested length would likely increase dynamic deflections as well as the potential barrier pocketing. It is believed that the potential exists for shorter runs of free-standing F-shape PCB to safely redirect errant vehicles. However, no research effort has been done to date to quantify the increased deflections and potential safety issues associated with shorter system lengths.
In order to effectively determine minimum system lengths and the required beginning and end of the length of need for the free-standing F-shape PCB system, analysis of three main factors must be considered. These factors include the number of barriers required on the upstream end of the system, the number of barriers required on the downstream end of the system, and the overall system length. A minimum number of barrier segments are required on the upstream end of the system or beginning of length of need to provide sufficient anchorage to safely redirect impacting vehicles with a reasonable dynamic deflection. Similarly, a minimum number of barrier segments is required on the downstream end of the system (i.e., end of the length of need). However, the number of required barriers may be different on the upstream and downstream ends. In addition, the number of barrier segments required on the ends of the system will likely be affected by the overall length of the system. For example, the number of barrier segments required on the upstream end of a long PCB installation (i.e., higher downstream barrier resistance) may be different than the number of barriers required for a short system length that allows increased PCB movement downstream of the beginning of length of need. Thus, determination of safe system lengths and beginning and end of the length of need requirements for free-standing F-shape PCBs will require consideration of all of these factors.
The objective of this research effort is to investigate and evaluate the safety performance of the previously developed F-shape PCB system in order to determine minimum system length and the number of barriers required for the beginning and end of the length of need. It is proposed that the system be evaluated according to the TL-3 criteria set forth in MASH. Two full-scale crash tests would be conducted to evaluate the performance of PCB system in order to evaluate its safety performance and investigate its dynamic deflection.
Evaluation of the F-shape PCB minimum system length and the number of barriers required for the beginning and end of the length of need will provide Florida DOT with improved and validated guidance for their PCB system configurations. These guidelines will improve the safety of PCB installations and may potentially shorten the number of barriers used in these types of installations. This will improve the flexibility of the PCB systems and reduce the number of impacts.
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