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I am looking for guidance on the use of pipe runners at skewed culverts.  The current IL Tollway standards show pipe runners perpendicular to the roadway when the pipe is perpendicular to the roadway.  Based on a departure angle of 25 degrees, a vehicle leaving the road would hit the pipe runners 25 degrees from perpendicular.  For our skewed pipes and headwalls, the pipe runners are shown parallel to the pipe.  Therefore for a culvert that is on a 30 degree skew (right hand forward) with pipe runners parallel to the pipe, that same vehicle departing at 25 degrees would now hit the pipe runners 55 degrees from perpendicular.  This seems like too much of an angle.  I was under the impression that the pipe runners should ideally be perpendicular to the path of the departing vehicle.  Is there guidance for usage of pipe runners on skewed pipes?

I have discussed your prior emails on the noted subject with my colleagues. Following this discussion, I must report that we are unaware of any design guidance for placing the culvert grates or pipe runners at angles other than at 90 degrees with respect to the traveled way when used with transverse drainage structures.

In recent years, MwRSF successfully performed full-scale crash testing on a culvert safety grate system that was used to protect a large culvert opening on a 3:1 fill slope according to the Test Level 3 (TL-3) safety performance criteria found in NCHRP Report No. 350. This testing involved both small car sedan and full-size pickup truck vehicles leaving the roadway and slope break point at 20 and 25 degrees, respectively, and at a target departure speed of 100 kph. For this test installation, the center-to-center pipe spacing was 30 in. From this testing, MwRSF researchers observed that the test vehicles could safely traverse the culvert grate system at high speeds and when the approach path was not orthogonal to the pipe runners.

Under oblique angles with respect to the pipes, the clear opening distance between pipes is increased from that found when the vehicle path is perpendicular to the pipes. As the approach angle is further increased, there exists a point when the vehicle could no longer traverse the pipes but instead would snag within the pipe system or contact the concrete culvert edge. For vehicles launched off of a fill slope and subsequently landing on the grate system, there would be increased safety risks as the effective clear opening width were increased, such as for higher approach angles or under situations where the pipes were skewed away from traffic.

In your email, you noted that there are situations where the culvert system is skewed with respect to the roadway, thus causing the pipe runners to be installed in the same skewed orientation on the fill slope. As noted above, skewed pipes could increase the potential for vehicles to drop between the pipe runners, thus resulting in front end or wheel snag on the pipes or at the culvert edges. As mentioned previously, we are not aware of any research nor guidance pertaining to the placement of skewed pipe runners. In the absence of testing and/or computer simulation modeling, we offer the following opinions and recommendations based on our best engineering judgment and available information.

1. In general, the culvert grate system was designed with the pipe runners to be installed perpendicular to the roadway. As such, the grate system or pipes should be installed orthogonal to the roadway when placed in combination with skewed culvert systems.
2. When reverse direction impacts cannot occur, pipes may be skewed when the bottom end of the pipes are located upstream from the top end of the pipes.
3. If it is absolutely necessary to skew the bottom end of the pipe runners downstream from the top end of the pipes, then it is our opinion that skew angles ranging between 0 and 10 degrees can be safely accommodated. In addition, there exists the potential that skew angles between 10 and 20 degrees may possibly be accommodated, although the safety risks are believed to be greatly increased.
4. If skewed pipe runners are deemed necessary, it may be possible to utilize a narrower pipe spacing in order to decrease the clear opening width, thus reducing the potential for vehicle snag and instabilities. However, a reduced pipe spacing may also lead to an increased potential for debris to clog the drainage system.
5. Further research and testing is necessary to accurately determine the safety performance of culvert safety grates when installed with skewed pipe runners.