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TCB Transitions to Permanent Barriers

Question
State IA
Description Text

I have the following questions regarding TCB transitions to permanent concrete barriers/bridge rails:

1. In the tested median configuration, the thrie beam rails were attached to the first TCB section such that the front-side end shoe was located 45-1/4” from the end of the TCB section. Regarding the general placement of these thrie beam rails, could you provide a minimum and maximum recommended overlap distance for the rails to be installed onto the TCB section? i.e., how close to each end of the TCB section can the thrie beam rails be attached? Consider that Iowa specifies the use of 6'-3" long thrie beam rails in some cases.

2. When is it necessary to install additional anchor bolts at the midpoint of the nested thrie beam rails? These bolts were present in the median testing but not in the roadside testing. If these bolts are necessary, must they engage the TCB section, or can they engage the permanent barrier instead? Must these bolts be located at the midpoint of the thrie beam rails, or could they be shifted upstream or downstream some distance (to the next set of rail slots)? Would these bolts be necessary when using 6'-3" thrie beam sections?

3. For the median transition, would it be acceptable to use a 2-inch thick asphalt pad that extends 24 inches behind the TCB instead of the as-tested 3-inch pad? Thanks!

Keywords
  • Temporary Barriers
Other Keywords none
Date June 10, 2013


Response
Response

 

I have the following questions regarding TCB transitions to permanent concrete barriers/bridge rails:

 

1.       In the tested median configuration, the thrie beam rails were attached to the first TCB section such that the front-side end shoe was located 45-1/4” from the end of the TCB section. Regarding the general placement of these thrie beam rails, could you provide a minimum and maximum recommended overlap distance for the rails to be installed onto the TCB section? i.e., how close to each end of the TCB section can the thrie beam rails be attached? Consider that Iowa specifies the use of 6'-3" long thrie beam rails in some cases.

 

In the design and testing of the TCB to median barrier transition, MwRSF selected a 12’-6” long thrie beam to connect across the joint to provide increased stiffness and reduced the potential for vehicle snag on the edge of the concrete barriers. The 12’-6” section was chosen because it was more common and would fit across a wide variety of barrier sections.

 

With regards to placement of the thrie beam rail, we would prefer that it be centered over the joint when using the 12’-6” thrie beam section. This provides for more even load distribution over the joint between barrier types. 

 

We do believe that a 6’-3” thrie beam section can be used in lieu of the 12’-6” section. The shorter thrie beam should not adversely affect the joint stiffness and may improve it slightly due to placement of the end shoe anchorage closer to the edges of the barriers. The 6’3” section may prove more difficult to flex to meet the TCB on the backside of the transition, but it should still be achievable. Because of the reduced length of the 6’-3” thrie beam, MwRSF believes that the section could be mounted with the end shoe closer to the edge of the permanent barrier and extending onto the TCB.  The end shoe anchorage would need to be placed inside the outermost stirrup on the permanent barrier. This would help reduce issues with flexing the beam to attach to the back of the TCB. We don’t believe that we would want to offset the thrie beam closer to the end of the TCB section.

 

2.       When is it necessary to install additional anchor bolts at the midpoint of the nested thrie beam rails? These bolts were present in the median testing but not in the roadside testing. If these bolts are necessary, must they engage the TCB section, or can they engage the permanent barrier instead? Must these bolts be located at the midpoint of the thrie beam rails, or could they be shifted upstream or downstream some distance (to the next set of rail slots)? Would these bolts be necessary when using 6'-3" thrie beam sections?

 

We believe that the anchor bolts are needed at the midpoint of the thrie beam rails to increase the stiffness of the joint across the barrier types. Recall in the roadside testing, we evaluated a system that had similar shapes and a TCB connection pin in place. Thus, the stiffness of the joint was not as critical. For the median transition, we have differing barrier shapes between the TCB and permanent barrier and no connection between them other than the thrie beam.

 

We would recommend that the anchorages engage the TCB section as it is the more flexible part of the system.

 

We would not recommend shifting these bolts away from the tested location a significant distance. Changing the location from the joint between the barrier systems would change the stiffness of the joint. We would recommend that they be used with the 6’-3” section. Thus, this may require an additional hole to be fabricated in the shorter thrie beam section.

 

3.       For the median transition, would it be acceptable to use a 2-inch thick asphalt pad that extends 24 inches behind the TCB instead of the as-tested 3-inch pad?

 

Two inches of asphalt should be sufficient rather than the three inch pad. Previous component testing of the asphalt pins found little difference between the forces developed in 2 or 3 inches of asphalt.

Date July 1, 2013


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