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Expansion and contraction of the guardrail connection to a bridge parapet

Question
State	MN
Description Text	Two questions; 1. How concerned should we be with expansion and contraction of the guardrail connection to a bridge parapet. As you know from our recent questions, this has become a hot discussion issue, but I would think that each post connection must allow some movement with the bolt and slot design, otherwise we would be seeing problems on long runs. 2. If we do move the parapet/end post off the integral approach panel abutment, then how much of a gap can we allow before we have to start considering a cover plate. Attached is a consultant concept that illustrates the concept. Note that I told the consultant that they would not be able to use this concept without an acceptance letter from the FHWA.

Question

State

Description Text

Two questions;

1. How concerned should we be with expansion and contraction of the guardrail connection to a bridge parapet.
As you know from our recent questions, this has become a hot discussion issue, but I would think that each post connection must allow some movement with the bolt and slot design, otherwise we would be seeing problems on long runs.

2. If we do move the parapet/end post off the integral approach panel abutment, then how much of a gap can we allow before we have to start considering a cover plate. Attached is a consultant concept that illustrates the concept. Note that I told the consultant that they would not be able to use this concept without an acceptance letter from the FHWA.

System Performance Evaluation

System Types
Approach Guardrail Transitions (AGTs)

Applications

System Features

Date September 24, 2015
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Attachment End Post.pdf

Response
Response (active)	I will try to respond to your questions noted below. First, I will begin with question No. 2. We re-examined the issue of a critical gap size or length. When considering 25-degree approach angles for passenger vehicles, we believe that excessive gap length could lead to increased vehicle snag at open joints of rigid parapets. Historically, we have used a 2 in. limit for allowable wheel overlap on the upstream end of rigid buttresses when associated with acceptable snag under thrie beam attached to approach guardrail transitions. The affiliated gap length would be around 4.3 in. Thus, I might suggest holding the gap length to 4 in. maximum. I recall that this issue was raised many years ago following our testing of an open concrete bridge railing for the State of Nebraska. I looked through our old Pooled Fund Consulting Q&A site and found the following inquiry and response. Years ago, we also recommended a maximum gap length of 4 in. and the use of chamfered corners/edges. http://mwrsf-qa.unl.edu/view.php?id=386 With regard to question No. 1, we are somewhat concerned with large longitudinal movements in the steel guardrail near the bridge end. If bolts are located at the ends of rail slots, would it be possible for the rail to pull over posts in the same direction of weak-axis bending. Of more concern, we believe that excessive slot length at the buttress and end shoe location could allow for increased vehicle pocketing/snag on the buttress end as well as greater risk for vehicle instabilities during close impacts near the bridge but in AGT. Please let me know if you have any further questions regarding the information noted above.

Response

Response
(active)

I will try to respond to your questions noted below.

First, I will begin with question No. 2. We re-examined the issue of a critical gap size or length. When considering 25-degree approach angles for passenger vehicles, we believe that excessive gap length could lead to increased vehicle snag at open joints of rigid parapets. Historically, we have used a 2 in. limit for allowable wheel overlap on the upstream end of rigid buttresses when associated with acceptable snag under thrie beam attached to approach guardrail transitions. The affiliated gap length would be around 4.3 in. Thus, I might suggest holding the gap length to 4 in. maximum.

I recall that this issue was raised many years ago following our testing of an open concrete bridge railing for the State of Nebraska. I looked through our old Pooled Fund Consulting Q&A site and found the following inquiry and response. Years ago, we also recommended a maximum gap length of 4 in. and the use of chamfered corners/edges.

http://mwrsf-qa.unl.edu/view.php?id=386

With regard to question No. 1, we are somewhat concerned with large longitudinal movements in the steel guardrail near the bridge end. If bolts are located at the ends of rail slots, would it be possible for the rail to pull over posts in the same direction of weak-axis bending. Of more concern, we believe that excessive slot length at the buttress and end shoe location could allow for increased vehicle pocketing/snag on the buttress end as well as greater risk for vehicle instabilities during close impacts near the bridge but in AGT.

Please let me know if you have any further questions regarding the information noted above.

Date October 1, 2015
Previous Views (139) Favorites (0)