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Several states have been approached regarding the use of alternative connection designs with the F-shape PCB developed by the Midwest States Pooled Fund.

I am writing in response to some questions you raised regarding the use of alternative connection designs with the F-shape PCB developed by the Midwest States Pooled Fund. When looking at this issue, one has to consider the use of the barrier in both its free-standing and tie-down configurations. Currently, the F-shape PCB has been tested to NCHRP 350 and MASH in its free-standing configuration and has also been tested in several different tie-down configurations including a steel strap tie-down, and asphalt pin tie-down, and a concrete bolt tie-down. These tie-down systems have been applied to develop approach transitions between the F-shape PCB and rigid hazards on both the roadside and the median.

When we consider the use of alternative connections and the free-standing PCB design, it is likely that many different connections will perform acceptably. The main function of the connection in free-standing PCB's is to develop tension and moment at the joint during impact with the barrier. To a lesser degree, the joint needs to have the ability to resist torsional loads along the barrier axis and shear loads at the joint. When the free-standing barrier is impacted, the barrier segments deflect and are held together based on the tension in the connection. When the barriers have deflected sufficiently, the corners of the barrier segments come into contact creating a compressive load that is combined with the tensile load in the joint connections to create a moment. This is the main load on the free-standing barrier connection, and it is the main force providing the continuity of the PCB system. Because the critical loading of the joint is a tensile load, there are several connection designs that may work adequately for a given barrier section. I believe that the FHWA has generally approved alternative, free-standing barrier connections to be used on previously tested PCB designs as long as the reinforcement of the barrier is equal to or greater than the tested barrier and that the development of the connection reinforcement is sufficient. I think that this is a rational approach for free-standing barrier given the loading conditions. However, it should be noted that it is difficult to infer the performance of alternative barrier connections without more analysis and full-scale testing. I would recommend using a connection with shear, tensile, moment, and torsional capacities equal or greater to the connection you are replacing.

When consider the use of alternative barrier segment connections with the tie-down and approach transition applications, the loading of the barrier connection is significantly different, and the use of alternative connection designs with the F-shape PCB becomes more hazardous. Tie-down barriers have some form of constraint on the barrier. In the case of the tie-downs used in the F-shape PCB, the tie-downs consist of anchors that pass through the toes of the barrier and constrain lateral an longitudinal movement. This greatly affects the joint loading. When a tie-down barrier segment is impacted, the lateral and longitudinal translation of the barrier is limited by the anchors. Thus, the barrier edges do not generally contact and develop high tensile and moment loads at the joint. The majority of the tensile loads are developed by the tie-down. Because of the constraint and the lack of tension developed between the barrier segments, the behavior of the barrier system is such that the impacted barrier tends to deflect laterally and rotate back along its longitudinal axis. The barriers downstream of the impact have that motion transferred to them based on the shear and torsional loading of the barrier connection. Thus, as the first impacted barrier segment deflects laterally and rotates, the constrained, downstream barrier segments do not move until the shear and torsional loads are transferred through the joint. This creates a potential for vehicle snag on the end of the downstream barrier segment as it is exposed by the deflection and rotation of the impacted barrier unless the barrier connection can effectively transfer the shear and torsional loads to cause the downstream barrier to begin moving as well. This can be seen schematically in the attached Figure 1.