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W-beam attachment to Culverts

Question
State NE
Description Text The post within the weak-post w-beam attached to culverts (TRP-03-277-14) is specified as 44" long.  A few situations may call for variable post lengths. What post lengths will function properly within this system
Keywords
  • Guardrail
Other Keywords Culvert w-beam weak post curbs
Date May 6, 2015


Response
Response

Without doing more analysis, we are not too comfortable with changing the dimensions of the post, as post bending is the main provider of the systems lateral support.  We could live with +/- 1”, but aren’t really big fans of recommending further changes without additional evaluation.

I'm not too sure why you would want a longer post anyway.Headwall and/or slab dimensions can change, but the socket has a defined length of 16.5” and a top mounting height of 2” above the top of the culvert. Thus, a 44” post will provide the specified rail mounting height of 31” above the ground/headwall.  Variations to the culvert structure will not affect this rail mounting height. The only thing that would affect mounting height would be if the top of the culvert is not at ground line.  However, when we reviewed each pooled fund States’ culvert standards, it was common that the top of the culvert headwall was even with the groundline.  Subsequently, the need for a longer post would only occur if your headwall was below the ground line.  I don’t know how this would occur as nothing would be preventing erosion/runoff if the headwall was not at ground line.

 Note, within the conclusions section of the report it states that the system is not recommended for use with approach slopes greater than 10H:1V as the system (and the original bridge rail) were never designed for, or evaluated  with, an approach slope.  A steep approach slope could significantly affect the performance of the barrier system.

Date May 6, 2015


Response
Response

If the parapet is above the shoulder or If this is used on a bridge with a curb: is a curb over 9” too high?

Date May 6, 2015


Response
Response

This design has not been evaluated with a curb.  As such, we do not recommended the system to be installed in combination with a curb or a headwall that extends more than an inch or two above the ground line / shoulder. 

Date May 6, 2015


Response
Response

Is the 1’ W-beam Backup Plate required at the splice joints?

Date May 6, 2015


Response
Response

Yes.  As shown at the meeting last week, rail tearing has been evident in multiple systems utilizing S3x5.7 posts and w-beam guardrail – typically at splices.   Thus, the 12” backup plate is recommended for use on all similar systems – including the original bridge rail which was only tested with 6” plates.  The 12” plates will not fit between the splice bolts, so they will have to be modified to fit over/around the splice bolts.  Oversized slots should do the trick.

Date May 6, 2015


Response
Response

So, I am stuck with only one length of post … if so, I will need to adjust most headwall parapets to meet the 4% to 6% slope of the shoulder, either cutting it to lower it or extending it to raise it?  Even a few inches?

But we expect the MGS leading into this area to work at both 31” & 27.75”? why don’t we think this will work properly?

Date May 8, 2015


Response
Response

This weak-post w-beam system was designed and tested for level terrain applications without curb. Thus, we recommend it be installed in such a manner.  We have recently shown that the addition of a curb below an otherwise acceptable system can lead to rail rupture and failure (the MGS stiffness transition to thrie beam AGT). So until the system is evaluated with the addition of a curb, we don’t recommend installation of the system with a curb.  If the culvert headwall extends more than an inch or two above the ground, it should be cut down or the a different system should be considered. If the roadway is significantly higher than the headwall, that would mean that there is a roadside slope leading into the headwall/barrier system.  Steep roadside slopes may cause vehicle instability issues and negatively affect the performance of the system.  Since this system has not been evaluated to use on steep roadside slopes, it is not recommended for installation on slopes greater than 8:1.

 

What we are left with are culvert sights were the roadway and the headwall are level with each other or near the same elevation.  I understand that there may be installation sites were the adjacent w-beam guardrail is at a height of 27.75”.  However, TTI has conducted recent testing of w-beam treatments for culverts and short bridge structures with this lower height w-beam and has observed rail rupture and testing failures with the MASH vehicles.  As such, we recommend keeping the installation height of this system at 31” and transitioning the guardrail adjacent to this weak-post w-beam up to 31” over a distance of 25 ft on both sides of the installation.
Date May 8, 2015


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