View Q&A

MoDOT regularly places permanent concrete traffic barrier with anchoring dowels omitted, as long as there are 1-3/4 inch minimum lifts of hot mix asphalt (HMA) abutting both barrier faces (Figure 1).  The DOT also places stepped (separating different grades) concrete median barriers, but only when they can be doweled into concrete pavement (Figure 1).

A curve’s superelevation on a recent add-a-lane project resulted in a hybrid of these two scenarios: a stepped barrier “pinched” in between lifts of HMA (Figure 2).  There is a concern that the increased moment arm on the upper side would have a greater propensity to overturn the barrier in a crash.

Please answer the following questions:

1.       Is the situation shown in Figure 2 likely to work?

2.       If not, what steps should be taken ensure adequate performance?

The layout shown in Figure 2 should be adequate to withstand most impacts.  Heavy vehicle impacts (e.g., a tractor trailer - TL-5 impact) may cause barrier failures in the form of severe cracking/fracture or rotation/overturning due to the increased load height for impacts on the high side of the barrier.  Judging by the reinforcement of the barrier (figure1), I will assume this not a TL-5 barrier.  Thus, it should perform as intended.  A few notes:

     1) the transvers steel stirrups should be extended for the entire height of the barrier, so the height dimension for the V1 bars would vary depending on the median step height and the total height of the barrier.

     2) Any barrier height change should not alter the strength of the top portion of the barrier, meaning the location of longitudinal bars should remain fixed with reference to the top of the barrier.  If the barrier height is increased significantly, additional longitudinal bars should be added to the extended lower portion of the barrier in conjunction with the extended stirrups.