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Tie Down Straps on thin HMA Pavements

Question
State MO
Description Text

Like most states, Missouri has been faced with an increasing need to tie down barriers on Portland cement concrete pavement (PCCP) that has been overlain with hot mix asphalt (HMA) pavement.  The MwRSF has developed methods to tie to PCCP, or HMA on base, but a solution to the composite pavement still eludes the industry.

An upcoming project in the St. Louis area will require that freeway traffic on an 8-lane freeway to be run head to head (separated by Type F concrete barrier) in 6 of the lanes.  This particular segment of highway has a 3-3/4 in. overlay of HMA.

Questions:

1. Would it be a reasonable variance of the tested and approved tie-down strap method to remove the asphalt pavement at each joint and pin directly to concrete by way of an elongated strap?  (see attached diagram)

2. If this is a possibility, what would be considered a practical limit as to the thickness of asphalt layer for which this anchorage would be feasible?

3. A similar proposal involves milling a 2 ft. wide trench the entire length of the barrier run and pinning directly to the concrete by way of the conventional length strap.  Would the Type F barrier still be functional if its effective height is decreased by 3-3/4 in.?


Portable Barriers
Sign Supports and Work-Zone Traffic Control Devices


Date December 14, 2012
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Attachment Type F Strap.jpg
Response
Response
(active) 		1. There are several unknowns with this type of installation of the tie-down strap. using the straps with a mill out as you have shown would prevent loading of the anchors through a layer of asphalt and provide similar anchor capacity to the tested design. However, doing so would require lengthening the sides of the strap. This change in geometry would likely affect the force-deflection and energy absorption of the strap to some degree and modify the loading of the anchors. The effect of these changes cannot be quantified without further study, but could potentially increase barrier deflections and the anchor capacity as compared to he tested system. Thus, while the potential exists for this type of modification to work, we cannot accurately quantify the impact performance without further study.

2. As noted above, changing the length/geometry of the strap would affect the force-deflection and energy absorption of the strap. The deeper the asphalt thickness, more prominent those effects would be and the greater the expected change in system performance. Determination of an acceptable asphalt depth limits would require further study.

3. We cannot definitively say that the barrier system will or will not work with the reduced height when anchored, but our experience in testing the F-shape PCB's in anchored configurations leads us to have concerns for vehicle stability at the reduced height. If you look at our testing of the anchored F-shapes with 32" heights, you will note the degree of instability present. Reduction of the height to 28.25" would be significantly lower than previously tested TL-3 F-shape barriers, and would not be recommended. 

 
Date December 17, 2012
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Thank you for the prompt reply to my inquiry.  I have shared your analysis with the project team and have the following questions by way of follow up.

 

  1. Your response mentioned the change in geometry of the strap leading to an (as yet) incalculable effect on the force-deflection and energy absorption of the strap as well as the loading of the anchors. Would a thicker strap, perhaps 3/8 to 1/2 in., be sufficient to allay that effect?

 

  1. Would my entire original inquiry perform sufficiently under TL-2 conditions?
Date December 27, 2012
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Responses in Red. 

  1. Your response mentioned the change in geometry of the strap leading to an (as yet) incalculable effect on the force-deflection and energy absorption of the strap as well as the loading of the anchors. Would a thicker strap, perhaps 3/8 to 1/2 in., be sufficient to allay that effect?

Increasing the strap thickness is not a viable option. During the development  of the strap tie-down, we investigated various strap thicknesses. It was observed that thicker straps tended to pry the anchors out of the concrete with very little energy absorption and made the system less effective. 


  1. Would my entire original inquiry perform sufficiently under TL-2 conditions?
There is increased potential for the proposed modifications working under TL-2 impact conditions. We believe that the first option  to remove the asphalt pavement at each joint and pin directly to concrete by way of an elongated strap has a very good chance of performing well under TL-2. With the lower impact energy of the TL-2 impact, the effect of modifying the strap geometry becomes much less critical.

We would not be as confident in the second option to mill underneath the barriers and effectively lower the height 3 3/4". There has been very little research done on reduced height barriers with sloped faces for TL-2. The majority of the reduced height sections for TL-2 have been vertical face designs. As such, we would be more wary of this option, especially when considering high CG vehicles and the potential for barrier climb. That is not to say that it cannot work, but that our confidence is lower than the first option. 
Date January 2, 2013
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