Steel Posts over Underground Structures (405160-12)

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Final Report Link: Steel Posts over Underground Structures


TTI Research Supervisor:
William F. Williams, P.E.
Texas Transportation Institute
Texas A&M University System
TAMU 3135
College Station, Texas 77843-3135
(979) 862-2297
[email protected]
Pooled Fund Technical Representative:
Michael Elle, P.E.
Design Standards Engineer
Minnesota Department of Transportation
395 John Ireland Blvd, MS 696
St. Paul, MN 55155-1899
(651) 366-4622
[email protected]


For guardrail installations across low-fill culverts, small structures, or underground obstructions where proper post embedment cannot be achieved, either long span guardrail or steel post over underground structures similar to the details shown in Figure 1 are desirable.

The details shown in Figure 1 would allow for the guardrail post to be anchored to a shallow foundation that is constructed on top of the obstruction. However, physical crash testing of the details shown in this figure according to National Cooperative Highway Research Program (NCHRP) Report 350 Test Level 3 requirements has not been performed. Moment slabs can become an expensive alternative to most local municipalities that might use a detail such as the one shown in Figure 1. Therefore, there is a need for a more cost-effective design for this situation.

Figure 1.  Steel Post over Underground Structures.

As part of this pooled fund program, a w-beam guardrail system using W6x9 steel guardrail post was anchored to a simulated concrete box culvert and crash tested (see TTI Project Box Culvert Guard Rail TTI Project No. 405160-5).  The height of the fill used on top of the box culvert was 9.0 inches.  The crash test was performed in accordance with NCHRP Report 350 TL-3 Specifications (Test No. 3-11).  The results of the crash test were successful with respect to NCHRP Report 350 requirements for Test Level 3 (TL-3).  For this project, design and crash test information obtained from the steel post anchored to the box culvert project was used.  Information from this study as well as engineering design and pendulum testing of a W6x9 steel guardrail post anchored to a shallow moment slab were used to develop a design for a steel post supported by a shallow moment slab for use over underground structures.

The objective of this study is to develop a steel post design anchored to a shallow moment slab for use on a typical strong post w-beam guardrail system.  This shallow moment slab would be utilized whenever underground obstruction(s) are encountered preventing the use of a full depth guardrail post (typically 44 inches below grade).  This post with shallow foundation design would serve as a viable alternative for supporting a guardrail post where shallow underground obstruction(s) exist.

Engineering analyses were performed on several design options.  These options were submitted to the state technical representative for review and approval.  The state technical representative sent these options to the supporting state members in the pooled fund for review and approval.

Several design options were analyzed.  One option utilizing a 5 ft x 5 ft square footing was recommended for full scale pendulum testing.  Details of this design are presented in Figures 2 and 3.  Based on the performance of this design, a 4 ft x 4 ft square footing was also a testing option.

Figure 2.  5 ft x 5 ft Footing Cross-Section.

Figure 3.  Steel Post Details.

Full scale pendulum testing was performed on two footing designs.  Photos for the 4-ftx4-ft design are shown as follows in Figure 4:

(a) Test Set-up.
(b) After Test.
Figure 4.  Test Photos 4 ft x 4  ft Foundation with Post.

Full-scale pendulum tests were performed on the two footing designs.  The posts were loaded in bending about the strong axis.  The 5 ft x 5 ft footing rotated approximately 6 degrees and the dynamic rotation of the post was approximately 30 degrees. The 4 ft x 4 ft footing rotated approximately 15 degrees from the pendulum impact.  The dynamic rotation of the post for the 4 ft x 4 ft footing was in excess of 30 degrees.  Plastic failure occurred in the W6x8.5 steel posts for both designs tested.  No distress was observed in the anchor bolts or concrete for both tests.   Both tests compared favorably to a standard embedded W6x8.5 guardrail post. posts embedded 43.4 inches in strong soil.

Videos of Pendulum Testing

Pendulum Test on 60-inch Slab

Pendulum Test on 48-inch Slab