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Consruction tolerances for Single-Slope Barrier

Description Text

We have a multi-billion dollar project going on for the
Gothals Bridge and the contractor’s slip form machine varied off course and we
have a barrier that in very isolated places is off by  1 ½ “ in height
over 20’ of length.  Also the batter is also less of a concern for me
since one of the last times we worked together we proved through FIE that a
barrier that was out some 15 degrees was still acceptable for crash
performance.   Would you let me know if you have any guidance on
acceptable construction tolerances?  I have looked through MASH, the RDG,
and the Green Book and find nothing.  TO make this a bit more complex, it
meets the construction tolerances in NJ, but not in NY.  I suspect it is
more than good for crash performance but want a second set of eyes.  Your
expertise is appreciated.   

  • Bridge Rails
  • Permanent Concrete Barriers
Other Keywords none
Date February 15, 2017


While I cannot make specific comments regarding the state DOT tolerances on construction, I can comment on the safety performance of the barrier as it pertains to these deviations.


Based on the detail you sent it appears that the barrier in question is a 42” tall single-slope barrier. These barriers have been tested and evaluated at various heights with a 10.8 degree and 9.1 degree sloped face. The 10.8 degree barrier is typically referred to as the Texas single-slope and the 9.1 degree barrier is typically referred to as the California single-slope.


The two tolerance issues you note are barrier height and slope angle. Both of these can affect safety performance.


In terms of barrier height, the sensitivity of the barrier height depends on the Test Level that the barrier was warranted or developed for. For a Test Level 4 (TL-4) or lower test level barrier, height variations of 1.5” would not be an issue. These barriers have currently met TL-4 testing under MASH at heights of 36”. For a TL-5 barrier, the height may be more of an issue. These systems have only been successfully tested at heights of 42” or greater. Thus, reduced barrier heights may increase the potential for reduced or compromised capture of the tractor-trailer type vehicles. The exact reduction in height at which containment is compromised has not been determined.


The effect of variation of the barrier slope is more difficult to ascertain. To date, single slope barriers have not been tested with slopes shallower than 11 degrees. Steeper slopes, including vertical faces, have been successfully evaluated. The concern with shallower slopes is that the face of the barrier can promote increased vehicle climb and instability. The point at which the slope angle increase becomes an issue is not fully defined. As such, I cannot say definitively whether or not the slope tolerances pose an issue or not. One would think that minor variations from the 10.8 degree nominal slope would not be sufficient to induce vehicle instability, but the point at which the shallower slope becomes a stability problem has not been fully defined. 

Date February 16, 2017

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