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Guardrail on Short Bridge

Question
State IA
Description Text

We've got a situation where we will be updating the guardrail at a very short bridge (34 feet " see attached pics) and we'd like to carry the thrie-beam across it.  Do you guys have a preferred method of attaching guardrail to a vertical parapet like this, and what would be your recommended blockout depth?  Note that we will not be modifying the parapet ends in any way.

 

In the alternative, do you have any other suggestions on how to treat this bridge?  Potential candidate for the MGS bridge rail?  What about using guardrail with base-plated posts?

Keywords
  • Bridge Rails
  • Guardrail
Other Keywords none
Date July 27, 2011
Attachment 20110726092232211.pdf


Response
Response

There have been a few retrofit bridge rails that have included the placement of blocked-out thrie beam on the front face of decorative concrete parapets with some type of curbing. In your situation and depending on test level, you may be able to utilize blocked-out W-beam or thrie beam across the bridge. I assume that you are upgrading the approach guardrail transitions as well. If the parapet stays, there is no reason to use posts with base plates on the deck surface nor the MGS bridge rail. At this time, we have not retrofitted existing deck edges with the new system. However, I would not be too concerned with post placement sufficiently away from deck edge by using cored hole within interior deck region.

 

If the parapet is sufficiently strong, then it may be most cost-effective to retrofit rail across front face.

 

If you desire options for this method, , I am enclosing a small pdf file which contains the cover pages for the noted reports as well as general design details for the retrofits bridge rails. Please let us know if you need any other information. The references are listed below.

 

Crash Tests of R4 Retrofit and Open Parapet Bridge Rails - Final Report, Report No. FHWA-MI-RD-92-01, ENSCO, Inc., Springfield, VA, February, 1992.

 

Gripne, D.J., "Washington State Department of Transportation Development of a Bridgerail Retrofit Program", Transportation Research Record 1198, Transportation Research Board, Washington, D.C., 1988, pp. 45-54.

 

Buth, C.E., and Menges, W.L., Crash Testing and Evaluation of Retrofit Bridge Railings and Transition, Report No. FHWA-RD-96-032, Texas Transportation Institute, Texas A&M University, College Station, TX, January, 1997.

Date August 11, 2011


Response
Response

Thanks for the info, Ron.  Our situation is a little different than what the research has covered, so I'd like your opinion on something.  Notice in the photos that our parapet is essentially vertical throughout the length of the bridge, with a 1- to 2-inch lip curb near the bottom.  We would like to install a blocked-out thrie beam across the bridge, using our standard guardrail transition (http://www.iowadot.gov/design/SRP/IndividualStandards/eba201.pdf) at each end.  My concern with this is the possibility of vehicle snagging at the parapet ends.  What depth of blockout would you recommend using on the bridge in order to minimize the snagging potential?

Date August 23, 2011


Response
Response

Based on the prior approved Iowa AGT attached to a safety-shape parapet (B-47 and B-47A) and a vertical parapet (B-47B), I would utilize a minimum blockout depth of 4 in. on the face of the concrete parapet. This recommendation is based on previously provided a chamfer corner and/or tapered end of 1 to 2 in. behind the parapet face. Thus, if the toe extends up to 2 in. in front of parapet, one would need at least 4 in. of blockout on parapet face. In order to provide some factor of safety, you may consider using a 5-in. blockout depth on the parapet face. Once you traverse past the ends, would you be either (1) anchoring thrie beam to face with end shoe flush or (2) carrying thrie beam across entire bridge?

Date August 23, 2011


Response
Response

We will be carrying 10-gauge thrie-beam across the length of the bridge using 4-inch blockouts. One of the reports you sent showed 10-foot spacing between blockouts, while another showed 6'-3" spacing. Do you see any problem with reducing the spacing to 3'-1½", or should I just stick with 6'-3"?

Date September 1, 2011


Response
Response

I am fine with using a 3'-1½" blockout spacing on the bridge.

Date September 1, 2011


Response
Response

We are actually considering removing the vertical parapet on one of our slab bridges and replacing it with a version of the MGS bridge rail. This particular slab bridge has a deck thickness of approximately 24 inches, so we are looking into the feasibility of side-mounting the tubes that hold the posts. Are you aware of any other states that have tried this yet? If not, do you have any suggestions beyond those that were published in the report?

Date September 1, 2011


Response
Response

At this time, I am not aware of any states which have adapted the MGS bridge rail for use on existing bridge decks with substantial thickness, such as 24 in.


With such a thick deck, one would think that two transverse epoxied rods could be designed to rigidly attach the tube to the outer vertical surface without causing excessive concrete fracture given sufficient deck reinforcement, anchor depth, and upper concrete cover. The lower end of the tube certainly could be side-mounted on the deck edge.


Alternatively, one could likely drill/core concrete holes in the upper deck surface which are sufficiently away from deck edge. Post could be placed cored holes on new steel hardware to receive bridge post ends. New steel hardware may consist of steel pipe with welded lower end plate to fill bottom and an upper welded ring at top to set sleeve depth. The galvanized hardware would be epoxied into thick depth to prevent washer penetration into concrete edges around pipe. Water could sit in closed pipe without concern. If concerned, then fill pipe with grout around post. Grout could be replaced after impacts.

Date September 1, 2011


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