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Guardrail Post in Concrete and other Urban Considerations

Question
State WI
Description Text

Is the following correct interpretation of the current state of research on guardrail? When guardrail posts are installed with in a sidewalk (see picture1 and 2) the concrete prevents the post from pivoting like it is suppose to. Therefore, a guardrail installed as in the picture does not meet 350 requirements. Would this also be true for asphalt installed around guardrail?

Second question:

If you have a narrow median, higher design speeds, high ADT, and urban section. How does a designer end a barrier with a crash cushion or EAT and not leave a large median opening (need mountable curb to EAT or cushion correctly see pictures: plan view and east London rd)?
Keywords
  • Guardrail
Other Keywords none
Date August 23, 2007


Response
Response

With regard to your first question on guardrail systems installed within concrete pads, we recommend that States do not place guardrail posts within rigid pavements for several reasons. First, wood and steel posts are designed to rotate laterally in the soil for a specified distance while providing a soil resistive force on each post. The work done by the rotating posts in soil allows for a portion of the impacting vehicle's kinetic energy to be dissipated as well as helps the guardrail provide for a smooth vehicle redirection. When guardrail posts are placed in rigid pavements, the expected post-soil interaction is altered. For example, wood posts placed in concrete may cause premature post fracture and a significant reduction in the post's energy absorption capacity. As such, guardrail installations requiring asphalt or pavement surfacing under the guardrail must consider special leave-outs around the posts where surfacing allows for the proper post translation in the soil. Both MwRSF and TTI have developed special guardrail systems and recommendations for these applications. First, MwRSF researchers developed design guidelines for placing steel posts in holes where subsurface rock is encountered. Second, TTI researchers later developed recommendations for the blocked-out region surrounding posts when mow strips are needed under guardrail systems. In both systems, consideration for the allowance of post rotation was deemed critical.

In your second question, the details of the median situation make it appear as though continuous barrier protection is likely warranted (i.e., narrow median, higher design speeds, and high ADT). However, you note that large median openings exist in combination with use of guardrail end terminals and crash cushions. From the photographs, it appears that these attenuation devices are located therein in order to shield rigid obstacles and not to prevent crossovers along the entire median length. You describe the need for openings in this median application, what are the specific needs - intersecting streets, etc.? It would be helpful to have additional information on this second topic in order for us to better assist you.

Date August 27, 2007


Response
Response

The problem I see is the need for end treatments (energy absorbing terminals, and crash cushions) and the cross sectional/operational problems the flat or near flat approaches can cause on urban projects (e.g. the picture with the crash cushion and the signal).  Would it be possible to to design some type of end treatment that could be used behind vertical curb?  For an example, (probably would not work in a median <14 feet), the MwRSF bullnose system behind vertical curb.

We are currently in the process of updating the guidance on beam guard to require an opening for the post to rotate.  Thanks for your help.  Has MwRSF had a chance to adapt Illinois's MGS drawings?  I would like to get some test projects using the MGS system in our state. 

Date August 28, 2007


Response
Response

From your clarification, it is my understanding that you desire a means by which you can effectively prevent vehicles from crossing the narrow, flat, paved medians in urban region and in advance of impact attenuation devices. You suggested using curbs to perform this roadway delineation and to prevent unwanted, intentional crossovers. The use of curbs in combination with and in front of crash cushions and guardrail end terminals is not recommended at this time due to the expected vehicular instabilities that would occur in advance of the vehicle to barrier impact. In the past, the Pooled Fund group has asked for a proposal for studying this problem. However, this project was not funded. At this time and following the summer TRB meeting, a NCHRP problem statement is being prepared on this topic. Hopefully, it will be selected and funded for an upcoming research project.

With regard to the MGS implementation project, I anticipate starting this effort in late September.

Date August 28, 2007


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