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Weak-post guardrail on culverts installation issues

State IL
Description Text

We are dealing with a few installation/compatibility issues with the weak-post guardrail attachment to culvert system described in TRP-03-277-14. Without compromising MASH compatibility with the system, and without introducing a stiffness transition between the MGS and the socketed weak post system:

1. How can the weak post socket system be modified to fit culvert installations where the thickness of the top slab plus the height of the headwall (T+H) is as little as 13 inches? Note the socket would extend past the top of the culvert in these installations.

2. If this is not feasible, how can the required T+H dimension be reduced, and by how much?

3. Our standards dictate that a 6" radius be used of the bottom edge of the top culvert slab on the upstream side of the culvert. How can the weak post socket system be adapted to our rounded top slab detail? For the as-designed T+H value of 17 inches? For some reduced T+H values as noted above.

4. For precast culverts, the headwall is often part of the band or collar joining the wings and barrel of the culvert. What concerns or constraints arise?

5. Are there constraints on how close bolt holes could be placed to the barrel/headwall horizontal joint when the headwall is part of the band or collar joining the wings and precast barrel.

Please see the attached file for details

  • Guardrail
Other Keywords weak-post, culvert, socket
Date January 29, 2016
Attachment Questions Regarding MGS on Culvert.docx


Questions 1 &2:

                This weak post attachment to culvert design was adapted from a bridge rail design that was previously developed here at MwRSF [reference report TRP-03-226-10].  In that design, the bridge deck was only 8” thick, so the socket extended past the bottom of the deck.  To anchor the base of the socket, a steel angle was bolted to the inside face of the socket and to the bottom of the deck (see pages 162 – 164 of the report).  I think utilizing this concept would work perfectly for culverts with short headwalls.  ½” diameter bolts can be used to attach the socket baseplate to one side of the angle, while the 3/4” diameter anchor rods should be used to anchor into the bottom of the culvert slab.  The anchor rods should be increased in size because they now transfer all the load in shear (as opposed to the base plate bearing directly to the culvert headwall. The angle should be at least 3” high (matches base plate), 6” wide (to extend under the culvert), 3/8” thick (to match original bridge rail angle), and 8” long (matches base plate).  Also, the 3/4” anchor rods should remain 6” apart – same as the original base plate attachment. A gusset plat similar to the one utilized in the original bridge rail should be welded in the angle to prevent deformations. Note, the exact height and/or orientation of the angle would change based on the actual height of the headwall (the angle could be flipped around to open toward the culvert for headwall heights near or just exceeding the depth of the socket).

 Question 3:

                We have come up with 2 reasonable options to retrofit these curved edges at the bottom of the culvert slab.  Simple sketches of these options are shown in the attached file.

Option 1 utilizes a bent plate with an inside radius of 6” to match the radius of the culvert edge.   This bent plate could also be cut from a pipe with an inside radius of 6”. The plate would contain two holes that would allow ¾” anchor rods to be inserted through the plate and into the curved face of the culvert, thus attaching the plate to the culvert.  A gusset plate would need to be used to bridge the gap and attach the outside of the plate to the inside face of the socket.  The gusset plate would be welded to both the socket and the plate.

                Option 2 (preferred) utilizes the steel angle concept discussed above.  The only difference would be that the steel angle would need to be wide enough to extend past the curved portion of the culvert and few inches into the flat section of the slab.  The ¾” dimeter anchors should be utilized to attach the angle to the flat portion of the culvert slab. Note, the angle could also be fabricated from a bent plate to obtain the desired dimensions.  The same ½” diameter bolts can be used to connect the angle to the socket base plate, and a gusset should be welded within the angle/bent plate as noted above.  Dimensions and orientation of the angle/bent plate would be dependent upon the culvert dimensions.

 Question 4:

                I believe all dimensional concerns were discussed in the previous questions and answers, so I assume that the sockets have the space needed to be mounted on the headwalls.  As you have stated, if no headwall is present, then another barrier option needs to be utilized.  Additionally, you would want to avoid any rebar/bolts/rods/etc.  critical to joining these components together.  Thus, the holes for the epoxied anchor rods should be drilled with care as to not cut through any critical connection hardware.

 Question 5:

                Anchor bolts/rods do lose capacity when placed adjacent to the edge of a concrete member.  However, none of the anchors should be loaded in shear in a direction toward a horizontal joint.  Thus, there would be minimal effect.  Anchors could be placed within a few inches of a joint – as long as the concrete isn’t damaged/split/spalled when the holes are drilled.
Date January 29, 2016
Attachment socket to culvert attachment sketchs.pdf


We are working on updating Illinois Department of Transportation Highway Standards related to guardrail, and have a few questions to resolve in order to finalize the draft highway standards to forward to Design and Environment.

 MGS Attached to Culverts

 Q1:         For the MGS attached to culverts (weak post), in Option 2, provided by Scott, (see attached “socket to culvert attachment sketchs.pdf”) there is a leg of the bent angle that goes under the culvert top slab radius.  Because the radius at the inlet end of the top slab can be about 6 inches, the leg of the angle into the culvert could be 10 inches or so.  In the original design referenced by the MGS attached to a bridge deck, the corresponding leg of the angle was just 7 inches.  The thickness of the original angle with a 7 inch leg was 3/8”.  For a 10 inch leg should the material be thicker?

 Q2:         What is the minimum bend radius we should use for this bent angle?  The radius used here slightly affects the bottom plate gusset dimension.  See the following related question.

Q3:         Please review the attached weld type and dimension we’ve shown for the bottom plate gusset. 

 For questions 2 and 3, please see the “MGS attach culvert bottom bracket assembly.pdf” and “MGS attach culvert bottom bracket parts.pdf” attachments.

 Q4.         Please review the IDOT spec for “Chemical Adhesive” and the related materials testing procedures (attached “TestingChemicalAdhesives.pdf”), and advise us if this satisfies the intent and requirements of the adhesive used in testing the MGS attached to culverts.  Our current list of approved materials (“chemicaladhesives.pdf”) is also attached.  We note that the AC 100+ Gold by Powers Fasteners is listed, but the AC 50 Silver, apparently a lower grade, is also listed.  (Or we might refer this to our Bureau of Bridges and Structures, and/or to our Bureau of Materials, but we need to understand the “bond strength” definition and requirements.) 

 Note:  We do need to look at threaded rod embedment, because IDOT culverts use 3500 psi concrete, while the MwRSF testing used 4000 psi pcc.  Also, we need to investigate if any special spec is required for the overhead use of 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 ITP for Chemical Adhesives and be on the

Department’s qualified product list.


Date April 5, 2016
Attachment MGS attach culvert bottom bracket parts.pdf
Attachment MGS attach culvert bottom bracket assembly.pdf
Attachment socket to culvert attachment sketchs.pdf
Attachment TestingChemicalAdhesives.pdf
Attachment chemicaladhesives.pdf


Q1:        The thickness of the bottom angle can remain at 3/8”.  The compression force the socket transfers to the angle should be equivalent to the bridge rail system, so the same material thickness and cross section seems reasonable.


Q2:         Typically, the minimum bend radius for a steel plate is considered to be equal to the plate thickness.  3/8” is getting pretty difficult to bend, so I understand if a fabricator wanted a larger radius.  The gusset can handle a radius as large as ¾” without needing modifications, so I would place the maximum bending radius at ¾”, or 2 times the plate thickness.  

 Another option would be to turn this bent plate into a welded assembly of two flat plates (eliminates the bend if you are having difficulty with it).  A welded angle assembly should also work just fine.  


Q3:    The weld and gusset appear to have the same specifications and dimensions as the original bridge rail attachment.  I see no issues here


Q4.       The strength of an epoxied anchor can be calculated through procedures provided in ACI-138 (Appendix D in 2011 version, or Chapter 17 in 2014 version).  Using the equations provided in these sections along with the anchor size and strength, concrete strength, embedment depth, and adhesive bond strength, you can calculate the tensile and shear strengths of a particular anchor.  The strength of the epoxy will only be applicable anchors loaded in tension. The anchors on the under-side of the culvert slab (overhead installation) would be loaded in shear and therefore only critical for concrete breakout and steel failure.  The top mounted socket designs for culvert attachment also load the headwall anchors in shear, so again the adhesive is not critical.

 Only the epoxy anchored version of the side-mounted sockets (originally design concept D2 in report TRP-03-277-14) for culvert attachment are loaded directly in tension.  So, if you are utilizing that particular attachment design, you should check your anchorage strength (embedment and bond strength) against those of the tested design.

 Since your culverts were constructed with a 3500 psi concrete, you will want to utilize the noted ACI-318 sections to ensure that the embedment of the anchors will develop sufficient concrete breakout strength and concrete pry out strength (tension and shear loading) for all anchors.

Date April 5, 2016

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