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W-Beam Guardrail Over Culvert

State KS
Description Text In 2002, MwRSF published the test results on a guardrail system for use on concrete box culverts. For the design, 1" diameter, ASTM A307 through-bolts were used to anchor the post with welded base plate. However, there may be situations where the bolts cannot be placed through the culvert slab when they fall within the location of the culvert walls. What anchorage alternative would be used in lieu of the through bolts?

What are your thoughts for alternative anchor systems when through-bolting is not applicable?
  • Guardrail
Other Keywords none
Date June 18, 2009
Attachment Table.jpg


I have reviewed the archived material for the original R&D  W-beam guardrail-culvert program. From this review, I was unable to find documentation on the embedment length for the threaded rods that were used in the bogie testing program on the concrete tarmac. I do believe that our policy has always been to use sufficient length to allow for the epoxy to develop the capacity of the rod material. Practically speaking, I can only assume from past experience that these 1-in. diameter rods would have been at least 10 to 12 in. long. As shown below, these rods would develop an axial capacity of more than 36 kips. Now, it may be possible that the rods did not experience this 36-kip load in each of the two tensile rods as the base plate was designed to yield and control the peak load imparted to the deck. However, it is difficult to determine the actual load imparted to the rods. We do know that the peak lateral post load was 14.68 kips.


If one were to assume that the post rotated about the back edge of the plate and the outer bolt row carried all of the uplift, then the outer tension row would be expected to carry a tension load equal to (14.68 k x 30.025 in.) / 9 in. = 48.97 kips. Thus, each bolt would see approximately 24.5 kips (bolts are closely spaced in the lateral direction too). In addition, if the base plate actually yields, then one would expect the load imparted to the bolts to be reduced somewhat. In any event, one would try to come up with alternatives to carry this axial load at a minimum to ensure that bolt fracture or release does not occur.


For thin concrete culvert slabs, it may be difficult to find anchors than carry this load for 1" diameter rods, especially when two rods must be located close to one another. As such, one solution may be to design a larger plate that can be anchored to the slab with through bolts on the sides of the culvert walls. Then, the post/base plate would need to be anchored to the large, thick plate. Now, it may be necessary to use structural beams and a plate to make the larger mounting system for which to anchor the post/base plate system on its upper surface. This type of system would also allow for bolt heads/nuts on the underside of the surface mounting system. This system would also require that some minimum thickness of soil be available on top of the culvert slab.

Date June 24, 2009

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