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Please see my responses below. Thank you for you assistance.

Hi Brian,

I am trying to get the installation straight in my head.

1.       It appears that you have temporary barriers on the far upstream and far downstream ends. Are these free-standing or anchored?  

I understand they are not anchored to avoid holes in the bridge deck and they are not close to a drop off.

2.       Next you have two types of “special barrier “ sections. Are these temporary barriers as well? Are they anchored or free-standing? What are the connections between the barrier sections.  

As above, I think these are not anchored.  They are temporary barriers.  One section is right in advance of the spanned are and the other one is outside the spanned area.  I assume they are similar to our standard F shape barriers.  I can ask for connection details if you like.

3.       On page 2, the special barrier sections appear to hang of the edge of the road surface? Is this correct or is the road surface only removed at the 6 ft opening?  

he road surface is only missing for the 6 foot section through the finger joint and only for half of the roadway. 

Thanks 

Thanks for the responses.

I have reviewed the detail you sent. In general, I think that the proposed solution can be made to work. I have a some comments and thoughts.

1.       TTI recently designed and tested a median barrier gate that uses a tubular structure that is hinged to protect an opening in a permanent concrete median barrier. This system is somewhat related to what you are proposing, but it was for permanent barrier. The sizing and connection details may be useful.

a.       http://tti.tamu.edu/documents/9-1002-2.pdf

2.       There are other gate systems such as the Armor Guard system that could be applied. However, it sounds like you need some clear area under the opening that these systems will not provide.

3.       The tubes in your system are 8”x8”x5/8” tubes. These have slightly lower bending capacity than the TTI design which used 12”x12”x1/4” tubes. However, that is not believed to be an issue due to the shorter span length in your design.

4.       The attachment of the tubes near the end of the concrete barrier appears to be done using a bent plate over the front of the tubes and some bolted brackets. It appears that this might be able to be simplified and made safer. The current bent plate bracket would have potential for vehicle snag on the vertical edge of the bracket. I have proposed a revised detail with at bent plate behind the tubes. The tubes would be welded to this plate and the plate could attach to the barrier at several locations. This would reduce vehicle snag and provide for a more positive attachment to the barrier.

5.       There will likely need to be additional attachments from the tubes to the barrier than the two shown adjacent to the opening. In order to prevent the tubes from flexing or prying off of the face of the barrier we would recommend additional attachment of the tubes near the tapered ends. You may want to have an additional set of attachments near the start of the tube taper. It is best to be conservative in the attachment scheme given the system is not crash tested. You could use the an attachment similar to the one shown above. Alternatively, you could through bolt through the tubes and barrier in the tapered region as shown below.

6.       The current configuration shown has tubes on only the impact side face of the barrier. While this does provide the redirective surface for the impacting vehicle, it is not optimal in terms of developing continuity across the barrier opening. For a system like this, you want to have the barrier act like a continuous unit across the gap. This means development of shear, tension and compression loads. Placement of the tubes on the front side only will handle the shear and compression, but may not be as effective in development of the tensile bending stresses between the barriers. The tubes you have are very strong, so their capacity along may be sufficient to develop continuity as long as they are very effectively anchored to the concrete barriers. However, it may be better to place tubes on the front and back side of the installation or a steel plate across the backside of the installation in order to create a stronger span that engages more effectively with the TCB on each end.

7.       Another concern would be snag of the vehicle on the tow of the concrete barriers you have shown. Currently you are transitioning from the sloped face TCB to at partially vertical face for mounting the tubes. However, the barrier tow that remains can be a significant snag hazard that can cause rapid vehicle deceleration and instability. We would recommend removal of the barrier tow and conversion to a purely vertical shape with the tube offset front the barrier sufficiently to prevent snag on the end of the concrete barriers.

8.       The steel tubes are currently tapered down at the ends to prevent snag. The taper shown is approx. 4:1. We would taper it more gently. An 8:1 or shallower taper is more appropriate.

9.       A simpler option for the design may be a specialized concrete barrier segment in lieu of the steel tubes. You could place the vertical cutout needed at the base of the barrier and not have to deal with all of the attachment concerns with the steel. The concrete section would need to have flared back sections on the ends of the vertical opening to prevent snag on exposed concrete. We do this on open concrete bridge rail posts and approach guardrail transition parapets.

10.   Depending on the type of connection used, the size and weight of the barrier segments, and the potential speeds and impact angles in this area, we would expect this type of system to deflect a significant distance when impacted. TL-3 displacements have been over 2 meters for MASH tests of F-shape TCB systems. Thus, you will need to consider the barrier displacement and worker exposure and positioning in the design. If sufficient displacement distance cannot be achieved, one would need to consider anchoring of the barrier system.

Take a look at these comments and let me know if you have questions or want to discuss things further.

Thank you very much.  You touched on all of the things that were giving be concern.  I will pass this along to our consultant.   If they have additional questions I will send you another note.  The quick turnaround is greatly appreciated.