| TTI Research Supervisor: Sofokli Cakalli Assistant Research Scientist Texas A&M Transportation Institute Texas A&M University System TAMU 3135 College Station, Texas 77843-3135 (512) 998-6039 [email protected] | Pooled Fund Technical Representative: Mary McRae, P.E. Engineer I Design and Construction Standards AK Deptartment of Transportation & Public Facilities 3132 Channel Drive Juneau, Alaska 99811-2500 (907) 465-1222 [email protected] |
Buried-in-backslope (BIB) terminal designs for beam guardrails, by burying the end
terminal in the backslope, significantly reduce the risk of vehicular intrusion behind the barrier
upon end-on impact. The BIB terminal design for 27¾-inch high guardrail systems was originally
developed under National Cooperative Highway Research Program (NCHRP) Report 350 (1).
The design has since been adapted by several states for 31-inch high guardrail systems. A
previous project funded by the Roadside Safety Pooled Fund Group evaluated the crashworthiness
of the modified BIB terminal design for 31-inch high guardrails systems through full-scale crash
testing under Manual for Assessing Safety Hardware (MASH) Test Level 3 (TL-3) criteria for
non-gating terminals (2)(3). The modified BIB terminal design satisfied the crashworthiness
standards for MASH Tests 3-34 and 3-35.
However, the MASH TL-3 crash tests on the modified BIB terminal design for 31-inch
high guardrail systems were conducted for only a specific site configuration. This configuration
placed the BIB terminal system on a V-ditch with a 4H:1V foreslope and 2H:1V backslope.
The purpose of this research is to investigate the crashworthiness of the modified BIB
terminal design for 31-inch guardrail systems for a range of site conditions and if possible, make
recommendations for MASH compliant limits on foreslope, backslope and ditch parameters.
A survey will be conducted to identify the common applicable ranges of foreslope,
backslope and ditch configurations used by the states for the modified BIB terminal design. An
engineering analysis will follow to determine the most critical BIB terminal design configurations.
The critical configurations will be further evaluated under MASH TL-3 conditions through fullscale finite element (FE) computer simulations. Results from these simulations will show how
adaptable the BIB terminal design is to site variations.
Recommendations for MASH compliant limits for installation site parameters will be
provided if the number of configurations evaluated is large enough to yield confident guidelines.
Buried-in-backslope terminal designs for beam guardrails significantly reduce the risk of
vehicular intrusion behind the barrier upon end-on impact by burying the end terminal in the
backslope. The BIB terminal design for 27¾-inch high guardrail systems was originally
developed under the NCHRP Report 350.
After the acceptable performance of the modified G4(1S) W-beam guardrail system with
timber blockouts, the Federal Highway Administration (FHWA) decided to evaluate two terminal
designs for the steel-post W-beam guardrail with similar modifications as the timber blockouts.
Texas A&M Transportation Institute (TTI) evaluated two BIB end terminals for use with modified
G4(1S) W-guardrails (4). Crash tests were conducted in compliance with NCHRP Report 350
criteria, which involved a 2000P vehicle impacting the terminal at a nominal impact speed and
angle of 62mi/h and 20⁰, respectively. The BIB terminal design for the W-beam guardrail was
crash tested in two configurations: one with a ditch and the other with a drop inlet. The top of the rail was 27 inches high, as measured from the shoulder grade, and the guardrail end was anchored
to a concrete block buried in the backslope.
For these tests, the ditch configuration consisted of soil graded away from the shoulder at
1V:10H slope for 6ft, followed by a 3-ft wide ditch, then by a 1V:2H backslope, while the drop
inlet configuration consisted of soil graded away from the shoulder at a 1V:10H slope for 9ft,
followed by a 1V:2H backslope. Both configurations satisfied the evaluation criteria for NCHRP
Report 350 test designation 3-35. Insignificant deformation and no intrusion into the occupant
compartment were observed. The occupant risk factors were all well within the recommended
limits.
The BIB terminal design was also tested and performed acceptably on installations over a
1V:6H foreslope forming a V-ditch with a 1V:2H backslope and over a 1V:4H foreslope forming
a V-ditch with a 1V:2H backslope (5). Both configuration were tested for NCHRP Report 350 test
designations 3-35, with a 2000P vehicle impacting the terminal at a nominal speed and impact
angle of 62mi/h and 20⁰, respectively. The primary goal of these tests was to evaluate the
structural adequacy criteria of the system, the ability of the device to safely contain and redirect a
2000P pickup truck.
Both BIB terminal designs performed acceptably for NCHRP Report 350 test designation
3-35, as the pickup truck was successfully contained and redirected by the device. The maximum
occupant compartment deformation for the BIB configurations with a 1V:6H and 1V:4H forslope,
were 1.77 inches and 4.9 inches, respectively. The occupant risk factors were within the
recommended limits.
The satisfactory performance of the BIB terminal tests in different ditch configurations
culminated with an FHWA letter of acceptance #CC 53A (Acceptance Letter, 1998).
The guardrail heights have since increased to 31 inches in many states and hence, the BIB
terminal design has been adapted by several states for the higher guardrail systems. A previous
project funded by the Roadside Safety Pooled Fund Group evaluated the crashworthiness of the
modified BIB terminal design for 31-inch high guardrails systems through full-scale crash testing
under MASH TL-3 criteria for non-gating terminals. The modified BIB terminal design satisfied
the crashworthiness standards for MASH Tests 3-34 and 3-35.
However, the MASH TL-3 crash tests on the modified BIB terminal design for 31-inch
high guardrail systems were conducted for only one specific site configuration. This configuration
placed the BIB terminal system on a V-ditch with a 4H:1V foreslope and 2H:1V backslope. The
purpose of this research is to investigate the MASH crashworthiness of the modified BIB terminal
design for 31-inch guardrail systems for a range of site conditions.
The BIB terminal design for 31-inch high guardrail system has been successfully tested
under MASH TL-3 criteria for only a specific site configuration. States have different design
standards for field parameters and adapting the device for site variability, while maintaining
MASH compliance, would increase the number of locations this device could be installed.
Also, if acceptable limits for site slopes are established, wider installations can be
implemented to reduce risk to the traveling public.
This project will provide BIB terminal designs for 31-inch high guardrail systems
adaptable to site variations from the previous crash tested field conditions of a V-ditch with
4H:1V foreslope and 2H:1V backslope, while maintaining MASH TL-3 compliance.
Also, the project will recommend MASH TL-3 acceptable limits for site parameters when
installing the BIB terminal design for 31-inch high guardrail systems, if the number of
configurations evaluated is large enough to produce confident guidelines.
The proposed work plan consists of three tasks as described below.
Task 1: Literature Review & Survey
The TTI research team will perform a literature review of the BIB terminal designs and
installation site conditions for 31-inch guardrail systems that have been investigated and/or
tested. In addition, the TTI research team will conduct a survey among Roadside Safety Pooled
Fund members to identify the common applicable ranges for the installation site parameters
subject of this study, as well as best practices for installation and implementation of the BIB
terminal design for 31-inch high guardrail systems. Examples of site parameters and common
practices are the degree of backslope, type of ditch configuration, foreslope width, use of rubrail
in the design, etc.
Based on the Task 1 results, the TTI research team will determine the most critical BIB
terminal design configurations for further evaluation.
Task 2: Engineering Analysis & Finite Element Investigation
Based on the poll results of engineering evaluation and members, the TTI research team
will determine the critical BIB terminal designs and the respective characteristics of soil grading
at foreslope and backslope to be further evaluated under MASH Test 3-34 and 3-35 conditions
through full-scale FE computer simulations.
MASH Test 3-34 consists of a 1100C vehicle impacting the terminal at a nominal speed
and impact angle of 62mi/h and 15⁰, respectively. MASH Test 3-35 involved a 2270P vehicle
impacting the terminal at a nominal speed and impact angle of 62mi/h and 25⁰, respectively.
The number of critical cases evaluated will be dictated by the project resources and how
much effort will be needed to develop FE models and perform reliable simulations on them. The
TTI research team will use the FE model developed for the BIB terminal design configuration
under TTI Project 405160-39 as a base model (6). However, due to the scope of this project, the
TTI research team will make significant changes to the base model to study the different site and
design variations.
The TTI research team will work closely with the project Technical Representative and,
if necessary, will coordinate with the Pooled Fund DOTs members to investigate details of the
BIB terminal design and site parameters that better address their needs.
Task 3: Report & Recommendations
The TTI research team will generate a final report of the research findings. The report
will indicate MASH compliance of the BIB terminal design for 31-inch high guardrail systems
on select installation site conditions.
The number of BIB terminal designs and installation site variations studied will
determine if confident guidelines for MASH compliant limits of site parameters can be provided.
REFERENCES