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IaDOT Questions

Question
State IA
Description Text
  1. Do you have a recommendation regarding the partial use of an existing w-beam bullnose installation?  Specifically, we have a situation where an existing bullnose is protecting the open area in the median between two bridges.  One of the bridges is being replaced and traffic will be head-to-head on the remaining bridge.  So some of the bullnose will need to be removed in order for the contractor to replace the bridge.  Question: how much, if any, of the remaining bullnose installation can we use as a standalone guardrail installation for the head-to-head traffic during the construction period?  This would not be used long-term following construction.
  2. I have attached a drawing of a bracket used to connect high tension cable to a concrete barrier or bridge end.  Our intent is to use these brackets on the trailing end of bridges, instead of a ground anchor, to provide continuous protection for errant vehicles.  For continuous median installations, this bracket would be located beyond the clear zone for opposing traffic.  The manufacturer has informed me that the bracket is the same one that is used on a ground anchor, and it is almost identical to a low-tension bracket used by South Dakota (detail attached), but with larger bolt holes.  This cable attachment portion of the bracket would be located behind the bridge rail where it cannot be hit.  However, part of the bracket is exposed on the front side of the bridge rail and could possibly be a snag point.  I was hoping you could give me your opinion on this design in general.  Also, I would like to know whether you feel this setup would require additional crash testing.  The manufacturer has told me that since this is an anchor, and not a terminal, that crash testing is not required.
  3. I have also attached a drawing of what we call our "Permanent Road Closure Barricade."  The design is based on that of a Type III Barricade.  However, this design has more than two posts and the rails extend entirely across the width of the road.  We are in the process of updating the drawing, and I am questioning the crashworthiness of this design.  Specifically, I was unable to find any crashworthy Type III Barricades that were wider than 8 feet.  Or is this barrier not subject to that restriction since it is not used in a work zone?  Additionally, the change we are making to the drawing is a result of the reflective sheeting peeling off some of the installations.  To combat this, we are proposing to have the sheeting applied to thin aluminum sign stock and that, in turn, would be bolted to the rails of the barricade.  Would this change the structural integrity of the barricade and if so, the requirement that it be crash tested?
Keywords
  • Approach Guardrail Transitions
  • Bridge Rails
  • Guardrail
Other Keywords none
Date September 30, 2008


Response
Response
  1. Please send us details of the W-beam bullnose system along with a description of how much of the system is desired to be removed and/or detached from the bridge. Once we have this information, we will review the design and test reports to determine if your proposed changes would affect the results of those observed in testing.
  2. The cable bracket is depicted on the front face of the parapet near the downstream end. This type of bracket would appear to provide concerns for vehicle snag on the bracket and anchored cables. You noted that it would be on the back-side face even though it is shown on the front face. It would be cleaner to attach the bracket on the back side. You note that the manufacturer stated the bracket is nearly identical to the SD DOT bracket used on low-tension designs and identical to their own ground anchor for HT or LT designs? The manufacturer plans to use this on high-tension designs as part of a system. If it already has been evaluated in HT cable barrier testing, then I do not see an issue with structural capacity. If it has not, then the manufacturer would need to provide some assurance that it would also work with HT systems through calculations, components tests, full-scale test, etc. The downstream cable anchorage scenario would not likely need to be re-tested as it is a rigid barrier transitioning into a flexible barrier.
  3.  I am not aware of any crash testing on such a design but will have a staff member review the designs and then get back to you.
Date October 3, 2008
Attachment s62930.pdf
Attachment ere03a.pdf
Attachment CCP_Cass-TL4_2008-06-24.pdf


Response
Response

I have attached details of our W-Beam bullnose system. We would like to be able to utilize half of the existing installation (shown as the "T" distance on RE-67) when traffic is head-to-head on the side of the roadway near the top of the page. I am unsure whether we would need to leave the 5-foot radius end section attached, or if we would need to replace that with some other type of end terminal.

Date October 7, 2008
Attachment RE-67 Bullnose.jpg
Attachment RE-53 Bullnose.jpg


Response
Response

I have reviewed the materials that you have provided. Based on this review, I offer the following comments. First, I understand that two-way traffic will be utilizing the lanes provided at the top of the page (page RE-67), while the bridge at the bottom of the page is being replaced. As such, the hazard between the twin bridges still requires shielding. As I see it, you have two basic options.


Option 1 consists of removing the bullnose buffer end and guardail sections that connect the bullnose barrier to the lower bridge end. Once that material is removed, a crashworthy guardrail end terminal and anchorage system would be connected to the guardrail system shown at the top of the page. The flared guardrail length would be selected such that the system provides adequate shielding of the median hazard.


Option 2 consists of removing a portion of the lower approach guardrail transition (i.e., the section connecting the bullnose guardrail to the lower bridge end). The "STS" segment, measuring approximately 18.75' in length, could be removed to allow construction of the new bridge. Then, in the first two spans of remaining rail, an approved anchorage system could be installed such that anchorage is provided in both directions, thus simulating a rigid attachment to the bridge end. If this option is desirable, we could assist with this detail.


The basic design consists of a standard foundation tube with soil plate at the last two wood BCT posts. A standard steel channel strut is connected between the two posts/steel sleeves. No impact head would be needed in this region as no crashes would be expected at this far end and since the bridge/road is closed. Standard anchor cable hardware would be placed between the two wood posts and in both directions (reverse cables in first span). You would need to drill an extra set of holes to place the second cable anchor bracket on the rail close to the top of post 1, similar to that near the top of post 2. Now the rail would be anchored in both directions " tension and compression, thus simulating a rigid attachment to the bridge end. We have done this in our thrie beam bullnose testing as well as in recent box beam testing when we were unsure with load direction would occur.


I am enclosing CAD details for a new MnDOT bullnose R&D project currently within MwRSF. For this effort, we have placed both the standard and reverse direction cable anchorages on the downstream end. In prior bullnose testing efforts, we switched the direction of the single cable anchorage from one test to another. In future testing, we will place two anchor cables on the downstream end " one in each direction.


It should be noted that this double anchorage should be used on any bullnose design that incorporates a free end that requires anchorage.


Our CAD details show the use of a 6-ft long tube without a soil plate but with a channel strut between two tubes. An alternative would be to use the shorter BCT tubes that incorporate soil plates and use the channel strut.

Date October 30, 2008


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