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We are currently developing MASH-compliant chain-link fence systems for NJDOT standards for use on bridge parapets. We are seeking clarification on whether the MASH design philosophy requires these systems to withstand structural impact or if a "breakaway/sacred" failure mode is the intended standard.

Key Points of Analysis:

• Failure Mode: Based on MwRSF Report No. TRP-03-434-22, it appears that vertical posts are intentionally designed to fail in bending rather than shear, with anchors designed to outlast the post members. This suggests that post-failure during a vehicular impact is acceptable.


• High-Level Impacts (TL-4 & TL-5):  It is typical DOT industry standard to utilize 2” to 4” diameter round or square posts. These sections are structurally incapable of resisting the transverse impact forces (Ft) associated with TL-4 (54 kips) or TL-5 (124 kips).


• Safety Concern: If the system is designed to "resist" the impact, the resulting anchor and parapet reinforcement requirements would be massive. Conversely, if designed to break away, we must ensure the fence components do not become secondary projectiles or snagging hazards for high-profile vehicles.

Questions for MwRSF:

1. Does MASH philosophy for bridge-mounted pedestrian fencing prioritize controlled failure (breakaway posts with wire-based debris retention) over structural resistance to the impact?


2. If structural resistance to TL-4/TL-5 impacts is required, are there known design configurations or material specifications that can accommodate these loads without compromising the parapet's structural integrity?

In answer to your submission, see responses in red below.

Let me know if you have further questions.

We are currently developing MASH-compliant chain-link fence systems for NJDOT for use on bridge parapets. We are seeking clarification on whether the MASH design philosophy requires these systems to withstand structural impact or if a "breakaway/sacred" failure mode is the intended standard.

MASH is not really a design philosophy. It is a guide document for evaluation of hardware. As such, it does not provide rationale for design of hardware devices. Instead, it focuses on performance parameters for devices to ensure that they do not pose a hazard when impacted by errant vehicles. However, there are relevant evaluation criteria that could help guide the design of debris fences.

Key Points of Analysis:

• Failure Mode: Based on MwRSF Report No. TRP-03-434-22, it appears that vertical posts are intentionally designed to fail in bending rather than shear, with anchors designed to outlast the post members. This suggests that post-failure during a vehicular impact is acceptable.

The post elements were offset to the backside of the rail to limit the interaction of passenger vehicles with post elements. This fence design was specific to MASH TL-3 impact conditions with passenger vehicles. You are correct that the fence in that report was designed with posts that would plastically hinge prior to overload of the parapet. This was done to reduce the potential for damage to the parapet during an impact. The elements were also designed to be retained during impact so that portions of the fence structure did not disengage and create a debris hazard below the bridge.  

That said, one could use different design philosophies and still meet MASH. For example, the post and fence elements could be more robust but offset even farther to eliminate the potential for contact and damage from passenger vehicles. Alternatively, a much more robust fence structure could be designed with lower, horizontal rail elements that provide redirection and shielding of the fence posts. Either approach could potentially be made to meet MASH criteria.

• High-Level Impacts (TL-4 & TL-5):  It is typical DOT industry standard to utilize 2” to 4” diameter round or square posts. These sections are structurally incapable of resisting the transverse impact forces (Ft) associated with TL-4 (54 kips) or TL-5 (124 kips).

I would agree with that. We have noted to DOTs in the past that debris fencing to resist MASH TL-4 or higher impact conditions require much more structure to contain the box of the heavy truck vehicles.

TTI developed a sacrificial TL-4 debris fence with smaller posts, but the design exhibited a lot of fence damage and debris during testing.

TTI initial design – failed TL-4 - https://www.roadsidepooledfund.org/wp-content/uploads/2022/04/TRNo613131-03-1-2-FINAL.pdf

TTI revised design – https://www.roadsidepooledfund.org/wp-content/uploads/2024/06/TRNo.-617741-01-Final.pdf

There was a problem in the Midwest Pooled Fund Program in FY2027 to develop a TL-4 structural debris fence that could withstand a TL-4 impact condition, but it was not selected for funding. I can send the proposal to you if you are interested. We thought it was a really good proposal and need.

• Safety Concern: If the system is designed to "resist" the impact, the resulting anchor and parapet reinforcement requirements would be massive. Conversely, if designed to break away, we must ensure the fence components do not become secondary projectiles or snagging hazards for high-profile vehicles.

I would generally agree with that statement, but I do think it can be done. We have developed TL-4 noise walls based on that principle years ago that worked well – see attached FHWA letter. Note that this system had more structure than we would need for a debris fence due to the use of large acrylic sound panels.  

Questions for MwRSF:

1. Does MASH philosophy for bridge-mounted pedestrian fencing prioritize controlled failure (breakaway posts with wire-based debris retention) over structural resistance to the impact?

As noted above, I believe it can be done either way for TL-3. In terms of cost and structural efficiency, TL-3 applications may be better served by a controlled failure approach.

2. If structural resistance to TL-4/TL-5 impacts is required, are there known design configurations or material specifications that can accommodate these loads without compromising the parapet's structural integrity?

There is not a standard design configuration for TL-4/TL-5 debris fencing. However, as I noted above, we have done similar work for structure mounted noise walls, so we believe it can be achieved.