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We have some questions reqgarding a vandal protection fence mounted on 32" barrier. Florida DOT has this fence in their specifications.

Can you provide some information on the background and testing of the vandal fence?

I have located and briefly reviewed the report which contains the crash test results for a vandal protection fence attached to a New Jersey safety shape concrete bridge railing. The report is titled, Crash Testing and Evaluation of Retrofit Bridge Railings and Transition. The report date is January 1997, while its number is FHWA-RD-96-032. Some of the key design parameters included:

-          32" tall parapet

-          6" wide parapet at top

-          2.875" OD Schedule 40 pipe posts at 10' spacing

-          three 1.66" OD Schedule 40 pipe rails at 3' spacing

-          8.7' system height

-          1"x1" wire fabric (11-gauge core wires with PVC coating)

-          5/8" diameter anchor bolts

-          ¼" thick bent clamping plates

-          back-side post mounting to parapet

-          6" lateral offset from upper corner parapet to face of vandal protection fence

-          AASHTO PL-2 pickup truck test was successful

From this review, several of these parameters are different than those later shown in the Florida DOT plans, including:

-          variable-height concrete parapet

-          unknown parapet width " F-shape and vertical

-          3" (3.5" OD) Schedule 40 pipe posts with 0.216" wall thickness at 10' spacing

-          two 2.5" (2.875" OD) Schedule 40 pipe rails with 0.203" wall thickness at 5'-5" spacing

-          6' system height plus parapet height

-          2"x2" wire fabric (9-gauge wires with zinc coating)

-          5/8" diameter anchor bolts

-          ¼" thick bent clamping plates

-          back-side and top-side post mounting to parapet

-          unknown lateral offset from upper corner parapet to face of vandal protection fence " larger for F shape and smaller for vertical

Further, some changes may result in degraded safety performance as compared to that observed for the crash-tested system. No comparisons were made to end regions. Thus, we may want to continue discussion over the phone in near future.

We discussed checking with Florida regarding their NCHRP 350 TL3 modifications.  Would you look into that as part of this discussion?

I have emailed and spoken to Mr. Charles Boyd, Structures Section, Florida DOT, regarding the vandal protection fence and its evolution. From our conversation, Charles noted that changes were implemented to accommodate a few concerns when considering its use under TL-3 pickup truck impact events according to NCHRP Report No. 350 where the approach angle is increased to 25 degrees. Years ago, Mr. Boyd noted that these changes were discussed with Dr. Dean Alberson, TTI, and Mr. Mark Bloschock, former bridge engineer with TxDOT and now with NTTA.

From their discussions, they eliminated the horizontal pipes and replaced them with tensioned cables due to concerns with dislodged pipes becoming hazardous projectiles for motorists and nearby pedestrians. I would assume that interior cables, longer pipe brackets, and/or insert bolts/pins could be used to mitigate these concerns as well. At any rate, the cables were used to address concerns raised after considering the 25-degree pickup truck test condition.

The vertical pipes were sized up to accommodate increased wind load conditions near Florida coastal regions. For Midwestern regions, Charles noted that the original sizes should be adequate and thus likely could be retained. In addition, the 2" wire mesh was used to more closely comply with a FDOT standard pedestrian mesh. However, he noted that some newer installations have used the 1" wire mesh as well and may reduce any concerns for increased vehicle snag into the larger mesh openings.

Charles clarified and reiterated that the top-mounted detail with vertical-face parapet is not for use on vehicle barriers. Instead, only the back-mounted configuration attached to safety shape parapets is to be used as a combination vehicle-pedestrian-vandal protection barrier. In the future, FDOT personnel will try to clarify the details to help ensure that it is not misused. As I recall, the AASHTO TF13 bridge guide and the 2005 FHWA Bridge Railing Guide may have inappropriately shown the system as top-mounted and on a vertical-face parapet.