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As mentioned in
the Year 28 Problem Statements by Wisconsin, we frequently have to use TBR for
projects with less than TL-3 speeds. Up until six months ago, we had defaulted
to using full eight sections for an unanchored run before we could expect the
48” deflections used in our standards. In answering a unique design question
recently, I had to dig into that guidance to see what modifications we could
make to fit the project’s needs. Part of my reply was the following:

For 62 mph, previous guidance (page 6 of TRP-03-209-09 and
others) has suggested that 8 sections (100') upstream and downstream of the
impact point (or protection area) are needed to properly “anchor” a
free-standing section so that the deflection relates to the Unanchored column
in Table A of BA-401.
Using that information, we could presume that for similar deflections, a 50%
reduction in speed would allow for at least a 50% reduction in length of need.
As it turns out, the attached shows that it’s more like a 66% reduction in
length of need (35 mph = 33% of TL-3). The equation and values come from page
11, equation 2-1, in 2009 MASH.

My question to
you is, are the presumptions made above consistent with other available
information you’ve been able to find since TRP-03-209-09
and TRP-03-113-03
were published or in preparation for the Year 28 Problem Statements packet? The
above situation was focused on minimizing TBR length rather than deflection,
but perhaps the opposite approach could be used keeping the eight sections
upstream/downstream and focus on reduced deflection. See attached for
calculations.

Any additional
thoughts or comments will help guide our decisions until such time these
layouts are actually tested.

Thank you for
your time.

I have just finished writing up sections of a report on this subject for NDOR where we looked at reduced PCB lengths an potential deflections for TL-3 and the 85^th percentile impact conditions. That report is currently in draft review internally at MwRSF. It should address a lot of your questions.

The work for NDOR does not go down to TL-2 or 1, but does look at speeds around 51 mph. 

As we discussed in November at the Pooled Fund meeting, if states want to look at the lower speeds, we can run some simulations under a very small year 28 project to look at the lower speeds.