Background
A culvert is a structure that allows water to flow under a roadway. A culvert may comprise multiple concrete pipes or box sections creating a steep embankment along the edge of the roadway.
The installation of a road safety barrier over a culvert can become problematic:
- The height of material above the culvert to the road level may be insufficient to accommodate the embedment depth of a guardrail system featuring driven posts.
- The available distance between the edge of the roadway and the steep culvert embankment may be narrow and insufficient to accommodate the dynamic deflection of a conventional guardrail system.
Together with bridges, these structures present a high risk for an errant vehicle that departs the laneway of the roadway.
Problems with Traditional Guardrail Designs
Guardrail barriers typically feature a steel beam, either w-beam or thrie-beam, supported by posts that are driven directly into the ground. The surrounding ground material provides lateral support to the posts, allowing the guardrail beams to deflect as the road safety barrier system absorbs the impact energy of the errant vehicle.
Installation of these crash barriers on a bridge or culvert represents a significant departure from the crash tested configuration which is typically undertaken on level terrain with the posts installed in well compacted soil.
Modifying these systems to accommodate attachment to a bridge or culvert structure may alter the crash worthiness of the system. Whilst many roadside guardrail barriers provide a baseplate option, these variants are designed for installation on strip footings where an underground hazard prevents the supporting posts from being driven into the ground. Sufficient space behind these barriers must still be provided to accommodate the dynamic deflection of the system when baseplate posts are installed.
In some instances, rigid post designs have been developed for direct attachment to the bridge or culvert structure. However, the strength of these designs to absorb vehicle impact loads are typically limited to the capacity of the anchor bolts. Once the anchors reach capacity, their failure often causes significant damage to the bridge or culvert structure. Repair of these damaged systems is problematic, and the life span of the bridge or culvert structure may be comprised as steel reinforcement becomes damaged or exposed to atmospheric contaminants and water.
The HammerBeam® Advantage
Developed by Safe Direction, Hammerbeam® uses standard components eliminating the need for bespoke designs, reducing supply costs and lead-times, and facilitating rapid repair.
Hammerbeam® combines the strength of thrie-beam guardrail with the yielding behaviour of the RamShield® W-Beam post for longitudinal barrier installations along the edge of elevated bridge and culvert structures.
The RamShield® post achieves a controlled redirection of errant vehicles by releasing the thrie-beam guardrail from the post at an optimal load to retain rail height, limit dynamic deflection and to allow the post to collapse without tripping the vehicle.
The RamShield® post is designed to collapse upon impact distinguishing it from heavier, rigid post systems which become problematic when attached to concrete bridge and culvert structures. The yielding performance of the RamShield® post reduces forces transferred to the bridge or culvert structure preventing damage and facilitating ease of repair.
HammerBeam® may be installed with side mount or surface mount (baseplate) posts.
Side mounting aligns the face of the thrie-beam guardrail with the edge of the elevated structure maximising usable bridge and culvert width, reducing the potential for centreline crowding. The side mount bracket features a recess for insertion of the RamShield® post which is cut to length on site to the required length. This feature provides the contractor with the ability to adjust the vertical position of the bracket and avoid conflict with service conduits or reinforcement.
The bracket design and anchoring is designed to exceed the bending capacity of the RamShield® post. Damaged posts are simply removed from the bracket and replaced. This feature ensures high-risk sites are repaired rapidly using standard components.
The baseplate option positions the barrier on the top of an elevated structure for sites where access to the side of the structure may be restrictive. The baseplate design and anchoring exceeds the bending capacity of the RamShield® post. Following a vehicle impact the damaged post is simply removed from the anchors and replaced.
Crash Test Compliance of HammerBeam®
HammerBeam® has been fully crash tested and evaluated according to the specifications for Test Level 3 (TL3) of the AASHTO Manual for Assessing Safety Hardware (MASH). The MASH specification is an update to and supersedes NCHRP Report 350 for the purposes of evaluating new safety hardware devices.
MASH is also the basis of testing procedures for safety barrier systems as stated in AS/NZS 3845.1: 2015 Road Safety Barrier System and Devices and AS 5100.1:2017 Bridge Design.
The crash test evaluation of HammerBeam® to MASH TL3 (156.4kJ) significantly exceeds the MASH TL2 impact energy (76.6kJ) nominated for Low Performance Barriers as described in AS 5100.1:2017
HammerBeam® was crash tested when positioned on the edge of a concrete beam representing a bridge or culvert installation. Material behind the concrete beam was excavated preventing the test vehicles from contacting the ground behind the barrier. This ensures vehicle redirection behaviour is thoroughly evaluated as the barrier deflects beyond the edge of the elevated concrete beam. Crash testing was performed with and without a 200mm high hob installed along the edge of the elevated concrete beam representing real-life scenarios and assessing the potential for vehicle vaulting during containment and redirection.
The crash testing of HammerBeam® when positioned on the edge of an elevated concrete beam distinguishes HammerBeam® from roadside barriers which have been assessed with posts supported laterally in soil and/or evaluated on flat terrain. Conventional guardrail systems, including their baseplate variants, should be restricted to sites with sufficient clearance behind the barrier to accommodate dynamic deflection. This makes these systems unsuitable for installation on the edge of elevated structures.
RamShield® Post Capacity Analysis
HammerBeam® features RamShield® posts designed to yielding during impact. The RamShield® posts yield at lower forces when compared to rigid posts preventing damage to the bridge or culvert structure during a vehicle collision.
The mounting bracket, baseplate and anchors are designed to exceed the bending capacity of the post as demonstrated through MASH TL3 crash testing. When retrofitting HammerBeam® to an existing bridge or culvert structure, the structural capacity of the elevated structure may be unknown or unreliable.
To assist designers, Safe Direction has undertaken RamShield® post capacity analysis using LS-Dyna. Analysis was undertaken for side-mounted posts and baseplate posts using four (4) load conditions acting in varying directions to ensure the maximum loading condition is identified.
Regulatory Acceptance of HammerBeam®
HammerBeam® has been recommended for acceptance by the Austroads Safety Barrier Assessment Panel (ASBAP) which assesses the crashworthiness and suitability of road safety barriers, systems and devices for deployment on roads managed by Australian/New Zealand transport agencies.
ASBAP undertakes a technical and risk assessment to determine the suitability of the road safety barrier. The assessment includes analysis of full-scale crash testing and supporting documentation provided by the proponent to ensure correct and safe deployment of the system. ASBAP supports the Safe System philosophy and the minimisation of harm for all road users.
