View Q&A

We have a couple of questions for you below.

Question 1:

When two bullnoses are placed in a median to shield bridge piers (i.e., symmetrical bullnose), can the same thrie-beam rail configuration that begins at Post 9 and continues to Post 12 on each bullnose be used to connect the two bullnoses? The configuration we are referring to is the thrie-beam system with a rail height of 31 5/8", standard blockouts, 78" long posts, and splice points located at the posts. We have not been able to locate a definitive working width value for this configuration.

In the Pooled Fund Q&A dated May 10, 2022, the response to Wyoming DOT:

“You can install the bullnose as a closed guardrail envelope. The bullnose uses 78” long line posts and thrie beam after post 9, note that it uses shorter blockouts. This thrie beam can be continued and connected to the far side bullnose and provide vehicle redirection…”

Is there a known minimum required offset from the face of the thrie-beam to the fixed object (e.g., bridge pier) within this envelope?

Question 2:

If the TTI MASH-tested thrie-beam system (Report No. 614341-01) is used in the area between two bullnoses in a median, is there a recommended method to transition from the 31 5/8" bullnose rail height to the 34" rail height required by the TTI system?

We’ve attached a proposed method that creates the vertical transition and relocates the splice to mid-span by introducing an additional post located 3'-1½" beyond bullnose Post 12.  One drawback of this approach is that on narrower bridges, the rail height would increase to 34" for a short distance, then drop back down again.

A Pooled Fund Q&A dated November 10, 2023, responding to Wisconsin, references the TTI report, but we are unclear on how to estimate the working widths. According to the report, the working width is 66.2" in the W-beam to 34" thrie-beam transition zone, and 55.2" in the main run of 34" tall thrie-beam. Could you clarify how to estimate the required working width for a configuration like this?

The flared installation shown in the manual and used by MnDOT was proposed by FHWA when we first developed and tested the bullnose under NCHRP 350. In order to deal with wider installations, we had proposed using larger nose radii and/or flaring at more moderate rates downstream of the nose section of rail. I believe these options are still listed in the MASH testing report under the implementation section.

FHWA use the high flare on the opposing traffic side and promoted that to states. As such, several states still use it. We had concerns that impacts of oncoming traffic on the far side of the nose of the system would be engaging a straight rail segment that would need to buckle, which was not preferred by MwRSF.

The odd flare rate was selected by FHWA at that time, but I don’t believe there was a strong rationale behind it. The difference between the flare shown and 1:5 is only 0.4 degrees. I highly doubt that this change in flare rate would significantly affect the performance of the system.

In terms of this configuration when you have oncoming traffic on both sides, we would have concerns about the straight rail segment causing the bullnose to performance differently than the as tested design in terms of both head on impacts and redirective impacts just downstream of post no. 1 or near the beginning of LON. As such, we would likely recommend shielding of that side during the constructing phasing.

Thanks!