Determination of Maximum Safe Gap in Permanent Concrete Barrier
situations where there are longitudinal gaps located within permanent concrete
barrier. For example, small gaps ranging from one to four inches may occur when
accommodating an expansion/contraction joint between permanent concrete barrier
and concrete bridge rail. Larger gaps of up to 4 feet may occur due to
accommodation of drainage features, such as an existing storm sewer catch basin
or a manhole access point. Relocating these types of features is typically
discouraged as the process would add significant costs and delays to the
project. However, gaps in permanent concrete barrier can create a potential
safety hazard for errant vehicles due potential vehicle snag on the downstream
end of the gap and corresponding risks for rapid vehicle deceleration, increased
occupant risk, and vehicle instability. Gaps in these barriers may also pose a
structural issue due to the discontinuity in the barrier section. Currently
there is limited guidance on the maximum, unshielded gap that meets current
MASH safety performance criteria, which limits agencies ability to effectively
address this issue.
Based on these
concerns, a need exists to determine the maximum allowable unshielded gap in a
permanent concrete barrier system under MASH TL-4 safety performance criteria. This
issue directly addresses goals of the AASHTO Strategic Highway Safety Plan and
the FHWA Roadway Departure Strategic Plan, including the importance of reducing
the incidence and severity of roadside crashes. The AASHTO Technical Committee
on Roadside Safety (TCRS) has also prioritized the combined use of in-service
performance evaluations (ISPEs), computer simulation, and full-scale crash
testing in its strategic plan.
objective of this research is to determine the maximum allowable unshielded gap
for permanent concrete barriers under MASH TL-4 impact conditions. The research
should provide guidance for the maximum allowable gap for a range of permanent
concrete barrier geometries.
hardware is critical for reducing severe crashes on the nation’s highways. There is an urgent need to develop guidance
for the agencies. Knowing the maximum
allowable unshielded gap and development of crashworthy barrier hardware would
provide agencies with increased safety through protection of vehicle snag posed
by the interruption of the permanent concrete barrier system. This research will provide a measure of
assurance in the placement of permanent concrete barrier and a higher level of
safety for the traveling public.
Transportation Agencies continue to strive to reduce the frequency and severity
of run off the road crashes while balancing community and environmental needs,
accommodation of utilities, and effects use of limited transportation budgets. Construction costs and delays can be reduced
by not having to relocate drainage and utility structures located within
alignment of the new concrete barrier system.
State Transportation Agencies may utilize the results of this research
for policy development for the placement of permanent concrete barrier. It is anticipated that the results of this
research will be incorporated into the AASHTO Roadside Design Guide.
In the absence of
this research, needless severe injury and fatal crashes will likely continue to
occur due to the hazard posed by excessive gaps within permeant concrete
barriers. Continuing without such guidance will result in the inconsistent and
potentially unsafe practices by user agencies.
Limited research has been conducted regarding
gaps and shielding of gaps in permanent concrete barrier. Most of the existing
research has dealt with evaluation of expansion gaps in concrete bridge rails.
The Midwest Roadside Safety Facility (MwRSF) tested a Nebraska open concrete
bridge rail with a 4.5-in expansion gap under the AASHTO PL-2 criteria. The
barrier was evaluated at the gap location with a 5,399 lb pickup truck at 61
mph and 20 degrees and a 18,000 lb SUT at 51.9 mph and 16.8 degrees. Both of these
tests were successful, but snag was evident in both tests. Additionally, the
pickup truck test was conducted at a lower angle than prescribed NCHRP Report 350
or MASH TL-3 and TL-4, and no small car testing was conducted. More recently,
TTI tested the Texas T224 bridge rail to MASH TL-4. As part of that evaluation,
they successfully tested the 10000S vehicle across a 2” wide expansion gap. The
passenger vehicle tests were not conducted across the expansion gap.
Similarly, temporary barrier designs in
free-standing and anchored configurations have had gaps as large as 4-in. that
have been successfully traversed by vehicle in MASH TL-3 crash tests. For
example, the Midwest States F-shape PCB was tested to MASH TL-3 when anchored
to a bridge deck with a 4-in. barrier gap. This test did exhibit some snag
across the PCB joint, but the vehicle was safely redirected. The snag may have
even been exaggerated in this type of system as compared to a permanent
concrete barrier as the upstream barrier translate laterally to some degree prior
the vehicle traversing the gap, thus increasing the exposure of the barrier
face at the downstream end of the gap.
Additional research has been conducted
for shielding of larger barrier gaps. MwRSF developed a steel cover plate for
spanning a large expansion joint in a TL-5 bridge rail developed for Manitoba
Infrastructure and Transportation. In this effort, a steel cover plate was
incorporated to span a 6 5/8-in. gap in a 49 ¼-in. tall, narrow single-slope
concrete bridge rail, and it was crash tested according to MASH test
designation 5-12. It may be possible to develop a similar reinforced steel
cover plates for bridging other type of barrier gaps. The bridge rail in this
research was also designed with increased barrier reinforcement adjacent to the
gap to compensate for the lack of continuity in the barrier section.
While this research provides some
insight into the issue of gaps in permanent concrete barrier, it does not
provide guidance on the maximum allowable unshielded gap for commonly used
permanent concrete barrier profiles in accordance with AASHTO MASH TL-4.
research contractor will require the following major tasks:
review of previous research related to barrier gaps and gap spanning systems
state DOTs for permanent concrete barrier geometries and permanent concrete
barrier gap configurations and scenarios, including expansion joints, drainage
existing barrier geometries to determine a potential maximum unshielded gap
length for permanent concrete barriers and determine a critical gap length and
barrier geometry for evaluation through full-scale crash testing
the critical gap configuration through full-scale crash testing to MASH TL-4
generalized guidance for allowable gaps lengths for permanent concrete barrier
a summary report to document all design, analysis, testing, conclusions, and
The results of this research will assist
state and local standards engineers to update the standard drawings and design
manuals to provide guidance on the installation of permanent concrete barrier. With proper review, the AASHTO TCRS could
assist implementation through adopting the research results into the future
update of AASHTO Roadside Design Guide and MASH. Implementation could also be supported
through a discussion of the findings at a TRB AKD20 meeting and the conduct of
the TRB webinar.
The desired outcome of this research will be guidance for incorporation in the AASHTO Roadside Design Guide. The final product will include guidance to agencies to avoid placing permanent concrete barriers with gaps exceeding the maximum allowable limit.
|Sponsoring Committee:||AKD20, Roadside Safety Design
|Research Period:||24 - 36 months|
|RNS Developer:||Hung Tang, Bob Bielenberg, Rod Troutbeck|
|Source Info:||AKD20 Summer Meeting 2020|
|Index Terms:||Barriers (Roads), Joints (Engineering), Concrete structures, Highway safety, |
Safety and Human Factors