DESIGN INNOVATION: Deflecting praise

By: Chris Pourteau

Although agencies do their best to avoid affecting traffic flow when repairing roadways and bridges, inevitably some of that repair work must be accomplished while drivers are using the system.

Predictably, this raises safety issues, both for the crews in the work zone and the drivers who pass through it.

Lateral move

Typically, barriers in a work zone are used to protect workers while minimizing hazards to drivers. The amount of working space available is one of the key factors in determining the kind of barrier to use. The space needed to allow the barrier to deflect (or move laterally) in case of a vehicle impact, as well as the amount of space desired for traffic and construction operations, determine the suitability of a barrier for a specific work zone.

Different maintenance situations require different kinds of barriers. Free-standing barriers are generally preferred, because they are easy to install and relocate. However, these barriers can result in large lateral deflections on vehicle impact. When work-zone space is limited, such as in bridge repair or expansion projects, restrained temporary barriers are required. These barriers are anchored, pinned or bolted to the bridge deck or pavement to limit lateral movement in the event of a crash.

Yet, the very characteristics of restrained barriers that help them protect workers and motorists also have a downside. Due to the need to anchor the barriers on-site, special installation procedures are required, and those cost more and take more time to install than free-standing barriers. Even when restrained barriers do their job perfectly, crashes usually mean damage to the bridge deck or pavement. Repairing damaged decks is costly, time consuming and can result in significant traffic congestion and motorist delays while a bridge is closed for repair.

Temporary concrete barriers are commonly used in construction zones for their cost-effective ability to provide protection in a work zone. They shield motorists from extreme drop-offs on bridges, for example, and protect workers and equipment from traffic incursions. In general, the stiffer the barrier, the less space is needed, because there is less deflection, or lateral movement, of the barrier as a result of an impact.

“Most barriers will move laterally as they absorb and redirect energy from the impact,” explained Texas Transportation Institute (TTI) Assistant Research Engineer Nauman Sheikh. “So a certain amount of space is necessary behind the barrier to allow for this movement.”

Given that work zones are carved out of existing traffic lanes in a highway corridor or on a bridge, the space available for concrete barriers (and their need to account for deflection) is very limited. In the case of bridge-replacement projects, this is particularly the case, where drop-offs are sheer and usually into underlying traffic or a body of water. Sheikh and his research team sought a solution to the dueling needs of safety (proper anchoring) and cost effectiveness (setup time, resulting damage to decks following a crash) presented by current barrier options. Conducted for the Roadside Safety Pooled Fund Program, the project aimed at creating a barrier for use in bridge repair that was easy to install, easy to remove or relocate, needed as little deflection space as possible and caused minimal damage to the deck if hit.

The key to simultaneously addressing safety and cost effectiveness lies in the barrier’s tie-down mechanism. Because of the damage to the bridge deck from a crash, through-the-deck bolting was undesirable. Related to this issue are installation procedures. They needed to be simplified, since through-the-deck bolting requires access underneath the bridge during installation and removal. Epoxy anchors, an alternative to through-the-deck bolting, also need to meet certain standards regarding proper installation to achieve the desired strength requirements. Inspection of these anchors is difficult.

Temporary restraining

A Roadside Safety Pooled Fund project sought a design for a temporary concrete barrier restraint system with the following characteristics:

Doesn’t require through-deck bolting or epoxied anchor bolts;
Easy to install, inspect and remove or relocate;
Segment length of 12.5-15 ft;
Meets NCHRP Report 350, TL-3 requirements; and
Has a safety-shaped profile.
The general concept couldn’t be simpler: The new design uses pins dropped in at an incline through the barrier and deck to restrain deflection and rotation of the barrier. This solution eliminates through-the-deck bolting, as well as the associated deck damage that can occur in the case of a crash. Researchers used finite-element analysis and crash testing to determine the most effective restraint design. Variables considered in the finite-element simulation analysis included drop-pin diameter, drop-pin angle, barrier profile (NJ and F shape) and offset distance of the barrier from the edge of the deck. Simulation yielded a final design with the following characteristics:
- F-shaped barrier profile;
- 1.75-in.-diam. drop-pin passing through the toe of the barrier and extending 6.25 in. into the deck at an angle of 40°;
- Two drop-pins per barrier segment;
- Diagonal U-bar reinforcement around the drop-pin hole to offer resistance to drop-pins if concrete fails in the vicinity; and
- Longitudinal rebar underneath the drop-pin hole to provide additional resistance to barrier roll in case of concrete failure.
- The simulated design was then crash-tested. The test installation had a total length of 100 ft. The barrier, which used a pin-and-loop connection, was placed at the edge of the drop-off to maximize the use of deck space for traffic operations. The researchers used the precast holes in the toe of the barrier as a guide to drill the holes into the concrete deck. The drop-pins were then placed into these holes to restrain the barrier.
  
  The principal purpose for the test was to ensure that the new design met NCHRP Report 350 requirements for a test-level 3 barrier. A 4,400-lb pickup truck impacted the barrier 1.2 meters upstream of the joint between the third and fourth segments of the installation. The truck hit the barrier traveling at a speed of 62.7 mph and an angle of 25.4°. (For the crash-test video, visit the Roadside Safety Pooled Fund website at www.roadsidepooledfund.org/acb/405160-3-2a_pan.wmv.)
  
  Contained and redirected
  
  The crash test proved the efficacy of the simulation modeling, with the new design meeting all the NCHRP Report 350 criteria. The barrier successfully contained and redirected the vehicle. Regarding performance, the maximum dynamic- and static-barrier deflections during the crash test were 11.5 in. and 5.8 in., respectively. While the barrier sustained some damage that would require repair, there were no detached barrier elements, fragments or other debris that penetrated (or showed potential to penetrate) the occupant compartment of the pickup truck. Likewise, no wayward fragments resulting from the crash presented undue hazards to others in the area.
  
  Other than the two drop-pins adjacent to the impact joint, none of the drop-pins were deformed. The two deformed pins could be removed using a cutting torch. The crash caused no significant damage to the concrete pavement. In short, the new design passed with flying colors.
  
  “Our research found what we were looking for a cost-effective and easy-to-use restraint method that minimizes damage to the concrete deck and maximizes the use of space in a work zone,” explained Sheikh.
  
  Specific benefits of the new design are:
  - Minimal damage to the underlying concrete deck or pavement occurs as a result of a crash;
  - Ease of installation and inspection and barrier removal and relocation;
  - During installation, the barrier’s precast, inclined holes can be used as a guide to drill holes in the underlying concrete pavement or deck;
  - The pins used to secure the barrier can be removed by hand to untie the barrier; and
  - To inspect for proper installation, simply verify that the drop-pins are flush with the barrier surface.
  - “TTI’s new restraint design has applications wherever there are bridges in need of repair,” explained Paul Fossier of the Louisiana Department of Transportation & Development. “This research will provide added safety for drivers and maintenance workers.”