On a project on the interstate, a designer has indicated that the barriers need to be pinned to the concrete when traffic is on both sides of the barrier. Given that this is on the interstate (high speeds and ADT) the designer does not want the barrier to be deflected near the already narrow roadway lanes. I happen to think that the designer is making the right call in this situation.
The contractor has asked if they need to pin the barrier and install the connection pin. I don't there is much of an option about installing the connecting pin between barriers. I also, given the importance of the facility (main highway between Milwaukee and Madision and in the Milwaukee Metro area), I don't want to see the facility completely shut down because a barrier deflected too close to a lane.
Currently, WisDOT has only directions to pin the barrier when traffic is on one side of the barrier (see attached SDD sheets). Would the pinning requirements be the same for traffic on both sides of the barrier? If MwRSF could provide guidance on pinning the barrier in both directions it would be appreciated.
I have some recommendations/guidance with respect to your TCB installation issues.
1. Currently, we do not recommend pinning or anchoring both side of the TCB in a tie-down application. Anchoring the barrier on the both sides creates a pivot point on the non-impacted side of the barrier that promotes barrier rotation and tipping and thus promotes vehicle instability. We cannot eliminate this concern without further analysis and/or testing, so we would strongly recommend against it.
2. We do not believe that you need to tie-down the barriers in the installation shown. The installation in the detail has three 11' wide lanes separated by a 6' space between the opposing traffic lanes for the separating TCB. In MwRSF Research Report No. TRP-03-113-03 (attached), we recommend that the pooled fund states TCB can be installed with the assumption of a 2' deflection based on the 85th percentile impact expected. The 2' deflection is based on modeling with the impact conditions for the 85th percentile impact severity based on accident data. The basic argument behind the recommendation is that most impact in the work zone do not generate barrier deflections as large as those observed in full-scale crash tests. You have room for the 2' of deflection recommended in your 6' separation area.
3. In addition, potential higher deflections are not believed to be as critical for several reason in the type of installation you have. First, you have 11' wide lanes and vehicles tend to drive near the middle of those lanes not near the edge. This is much more prevalent in work zones were drivers tend to be more cautious and shy away from the barriers when placed close to the travel lane. This has been proven in an accident study in Iowa where they looked at accident rates in work zones when the barriers were placed very close to traffic. The accident frequency was very low. Then they moved the barriers farther away from the traffic and the accident rate increased. The outcome of the study found that placing barriers within 6' of traffic reduced the impact frequency and severity. Because of this factor, you effectively have more than the required two feet need for deflection.
4. Although larger deflections could begin to intrude into the normal paths of oncoming traffic, the risk of an accident involving opposing traffic is still relatively low. Even when a vehicle in the opposing lane strikes a deflected barrier, the impact angle associated with any resulting crash would be expected to be extremely low. For this situation, the consequences of exceeding the deflection limit are not catastrophic.
Based on my comments above, we would recommend that you use free-standing PCB's in the installation you have shown. We believe that free-standing barriers will provide adequate protection without intruding into adjacent lanes in an unsafe manner.
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