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Connection of Guardrail on Departing End of Existing Concrete Structures

Question
State IL
Description Text

Tracy visited here yesterday to discuss the new Tollway Standards in Illinois, and to compare notes on how we handle various issues.  During our discussion we looked at our "Type 10" connection of guardrail to a concrete structure.  This is intended to anchor a rail to the departing end of a concrete structure on a one-way facility.  The connection consists of an end shoe for w-beam rail connected to the structure by four "epoxied" anchor bolts.  The reference to epoxy here is probably a misnomer, as other adhesives are used.  Our spec refers to a chemical adhesive resin system.  This is referenced at 1027.01 in our Standard Specifications:

 

SECTION 1027. CHEMICAL ADHESIVE

 

1027.01 Chemical Adhesive Resin System. The chemical adhesive resin system shall consist of a two part, fast-setting resin and filler/hardener. The system shall meet the requirements of the Illinois Test Procedure for Chemical Adhesives and be listed on the Department's approved list for Chemical Adhesives.

 

Regardless of the adhesive, we are concerned about the strength and long term durability of this connection.  In some cases it may be feasible to drill through the structure and provide backing with washers and nuts.  However there always seem to arise cases that do not allow this, or where it is impractical.

 

We would appreciate your comments on our concerns, and guidance to crashworthy or best practices.  We also wonder if this might be a good topic for a research and development effort?

Keywords
  • Guardrail
Other Keywords none
Date January 28, 2010
Attachment 211-631011-06_TrafBarTermType2.pdf
Attachment 211-631046-04_TrafBarTermType10.pdf
Attachment chemicaladhesives.pdf


Response
Response

A W-beam guardrail is often used to shield hazards beyond the end of the bridge. These guardrail systems can be attached to the downstream end of a bridge rail or parapet (i.e., upstream end of guardrail segment) and are also anchored with a trailing end terminal (i.e., downstream end of guardrail segment) when opposing traffic cannot occur. When errant vehicles impact these W-beam barriers, vehicle redirection largely comes through post-soil resistance during rotation, rail bending, and membrane action. The membrane action imparts tensile forces into the rail and transmits these forces to both ends " the upstream end anchorage (four bolts/anchors and W-beam end shoe) and the terminal end (anchor cable and post(s)).

 

The magnitude for the anchor loading on the upstream end is not well defined. In an extreme case, the anchor hardware would be designed to develop the ultimate tensile capacity of the rail section, which may be nearly 120 kips. On the lower bound, the typical downstream cable anchor assembly has a ultimate capacity of approximately 40 kips where the end anchorage is allowed some movement in the soil. The connection on the upstream end (i.e., attachment at downstream end of parapet) would experience little to no movement and need to withstand a higher dynamic load. Although the precision for this design load value in unknown, a reasonable rail design load (concrete attachment of trailing end guardrail only) may be 75 kips in tension. This design load would result in a shear requirement for each of the four anchors of nearly 19 kips per anchor. Additionally, each anchor would need to provide some pullout resistance due to the prying action of the end shoe away from the parapet when the guardrail is pushed laterally backward.

 

In your attached details, you noted that four 7/8-in. diameter anchor bolts are used to attach the end shoe to the parapet. At noted, through-bolts would seem to be the preferred choice. Other drill-in, mechanical anchors may also be acceptable. However, the details do not denote the grade or specification for the anchor bolts. Regardless of whether A307, A325/SAE Grade 5, or other bolts are used, the chemical adhesive could be selected to allow for the development for at least the shear capacities of the anchor bolts with considering the effect that anchor spacing, edge distance, and embedment depth have on reducing capacity.

 

Previously, MwRSF conducted a Pooled Fund research study which evaluated the capacities for various mechanical anchors. The MwRSF report no. is TRP-03-182-07. Currently, MwRSF has a limited WsDOT research study to evaluate the use and capacity of chemical anchor adhesives for attaching concrete parapets to bridge decks. Until this research study is completed and the results known, I would not yet recommend further research for your specific situation.

Date February 2, 2010


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