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Downstream Bridge Attachment

Question
State MO
Description Text MoDOT usually leaves the downstream ends of bridge on divided highways unshielded. The fill slopes associated with many bridges, however, warrant guardrail on the downstream end. Several of these locations are older and consist of a W-Beam mounted directly to the concrete parapet (as shown in the attached photo). Since a crash in this situation would involve exiting a rigid structure and entering a semi-rigid environment, pocketing does not seem to be a concern. I do have a concern about rail rupture if a crash were to bend the rail around the sharp concrete corner.
Is it acceptable to leave the installation described above in place?
Keywords
  • Approach Guardrail Transitions
  • Bridge Rails
  • Guardrail
Other Keywords Downstream, pocketing, rupture
Date April 22, 2014


Response
Response
We have had similar inquiries in the past regarding guardrail on the trailing end of a bridge. As noted, the concern for pocketing associated with approach transitions is low. However, the answer to the amount of load in the rail and the anchorage of the w-beam to the bridge rail has not been fully defined. See below.

http://mwrsf-qa.unl.edu/view.php?id=338 

We have not previously addressed the potential for rupture of the W-beam rail due to increased tensile loading and/or bending of the rail around the end of the bridge, but the concern may be valid. This concern could be mitigated somewhat by the location of the first post downstream of the bridge rail. By placing the first downstream post closer to the end of the bridge rail, the propensity for the rail to be bent around the end of the bridge would be lowered. Thus it may be worth considering placement of the first post 3.125 ft (quarter post spacing) or less from the end of the downstream bridge end. However, rail tensile loads may still be relatively high.

The some state DOT's have observed W-beam guardrail tears in vehicle accidents downstream from a stiff point in the barrier system (e.g., a larger post or a post installed in asphalt/concrete). These situations can be similar to the departing ends of concrete bridge rails on one-way roadways where the W-beam is directly attached to the parapet. In this scenario, the rigid concrete barrier will not deflect, thus potentially producing high tensile and/or shear forces in the rail at the edge of the rigid parapet that may result in tearing or rupture. Thus, full-scale crash testing may be necessary to fully evaluate this concern.

Illinois proposed this as a problem statement in Year 22 of the Midwest Pooled Fund, but it was not funded. 
Date May 6, 2014


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