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.
We have information about how to build line posts for MGS when the steep slope begins at the back of post. However, a designer in one of our Districts is dealing with a location where the slope continues in a similar manner all the way to a bridge. Are there any recommendations about adding to the length of posts for the transition from MGS to a bridge parapet?
Our particular transition to the parapet, given adequate support behind the posts is given in our Standard 630001.
At this time, we do not have any available design information for modifying the length of the transition posts when located on a steep slope near the bridge end. However, MwRSF does have a research project with the Wisconsin DOT to provide recommendations for addressing various transition issues. One of the noted issues will be to provide design guidance for situations when steep slopes are found behind the posts. It is expected that this effort will be initiated within 2 months.
For your info I've attached a copy of our proposed railing standard that we discussed on the phone (with the slotted holes in the base plate). The areas in green indicate the proposed changes.
I am working up the bridge rail transition for the MGS Guardrail System. In previous discussions, I recall that you said we could use a transition that was already approved by FHWA, however we would need to incorporate the "transition to the transition".
As you may recall, John Rohde worked with us to obtain approval from FHWA for adapting the Alaska Thrie Beam Transition to connect with WYDOT's TL-3 and TL-4 twin steel tube bridge rails. I have attached a mirror image of the Alaska design along with details for option "K" of the simplified transition recently tested by MwRSF.
I have a few questions to help me understand how to proceed:
I have another MGS question for you. We have a situation where a sign truss foundation is located 13" from the back of guardrail posts. MGS with standard post spacing was installed which would deflect into the concrete foundation. The minimum deflection distance we are using for the 1/4-post spacing installation is 14" measured from the back of post to the near edge of the hazard. The designer is proposing to add posts and to stiffen the rail by doubling up on the rail element thereby further reducing the deflection.
Do you have any comments/objections to this approach seeing that we only need to reduce the deflection by 1"?
Do you have any data on the anticipated deflection distance for this proposed installation?
See attached drawing of the proposed modifications. Note that the rail is gradually stiffened by using 1/2-post spacing and then the 1/4-post spacing. On the departure end of the 1/4-post spacing is 1/2-post spacing needed before getting back to standard spacing?
I have attached a pdf copy of our prior TRB journal paper for the Midwest Guardrail System (MGS). Within the paper, guardrail placement guidelines are provided for treating hazards. These guidelines pertain to the distance between the front face of the rail to the front face of the hazard. As noted, the minimum recommended distances are 1.25, 1.12, and 0.90 m (49, 44, and 35 in.) for the standard, half-, and quarter-post spacing designs, respectively. It should be noted that the width of the steel-post MGS is 0.54 m (21.25 in.). Using this information, one would need to consider using a clear distance of approximately 0.35 m (13¾ in.) between the back of the steel posts and the front face of the vertical hazard when utilizing the quarter-post spacing system.
Below, you noted that the available clear distance between the back of the steel posts and the front face of the rigid, vertical concrete foundation is 0.33 m (13 in.). Based on the guidelines noted above, your noted solution to use the MGS quarter-post spacing design and noted placement would result in 20 mm (¾ in.) less clear distance than that recommended in the paper (as noted above). However, I do not have significant concerns with using the basic ¼-post spacing MGS configuration to shield the noted hazard nor deem it necessary to use nested W-beam rail to cover the 20 mm (¾ in.) deficit in provided clear distance.
Finally, the use of the ½-post spacing in advance of the ¼-post spacing MGS seems reasonable and an appropriate transition in stiffness. In addition, I see no reason to utilize a ½-post spacing MGS on the departure end if reverse-direction impacts cannot occur.
I would like to know what placement range MwRSF is recommending for MGS behind 6" curb. Is it going to be:
0-6 for TL2
6+ for TL2
We are in the process of documenting the recent test and making recommendations. But our current thinking is that the 37-in. rail height relative to the roadway is valid for TL-2 between 4 and 12 feet behind the curb.
We have no evidence to make any other statements about TL-2. For example, there is no point where the rail makes the change from 31" to 37" because we do not know the valid range for the 31" rail height relative to the roadway.
There are currently no plans, or budget, to determine any other valid scenarios for MGS placement relative to a curb. It is believed that any such determination would require additional full-scale testing.
Previously, you had inquired into the allowance for alternative anchorage hardware within the bridge railing system noted above. According to the MwRSF test report (No. TRP-03-53-96), four anchor rods were used to attach the upper tubular steel rail, tubular steel posts, and welded base plate to the top of the reinforced concrete parapet. As noted, these anchor rods were modified from ASTM A307 grade material to ASTM A325 grade material. The report also noted that ASTM A193 Grade B7 material was used due to the unavailability of A325 hardware for testing. Therefore, the final crash-tested system utilized 7/8-in. diameter anchor rods configured with ASTM A193 Grade 7 alloy material with the expectation that either A325 or A193 B7 material could be used in the future.
A comparison of structural properties for 7/8-in. diameter fasteners using A193 B7 and A325 material is noted below, along with information for ASTM A449 material.
Min. Yield (ksi) Min. Tensile (ksi) Min. longation (%)
ASTM A193 B7 105 125 16
ASTM A325 92 120 14
ASTM A449 92 120 14~ As shown above, the structural strength for ASTM A193 B7 material is only slightly greater than that provided by ASTM A325 material. As such, it would seem reasonable that steel anchor rods or bolts meeting the ASTM A325, A449, or SAE Grade 5 material specifications would also adequately retain the rails, posts, and plates to the concrete parapet.
Based on the successful crash testing program, the MnDOT combination bridge railing system was found to provide acceptable crash performance according to the NCHRP Report No. 350 Test Level 4 guidelines. Since higher strength anchors were utilized within the concrete parapet for attaching the metal railing, it would not be appropriate to utilize ASTM A307 anchors in lieu of the higher strength anchors unless deemed acceptable through the use of full-scale vehicle crash testing.
Although the testing program was successful, there was insufficient data collected in order to determine or estimate the actual impact loads imparted to the entire bridge railing system. As such, it is not possible to determine what peak lateral load was actually distributed to the upper metal railing system, including the individual posts, plates, and vertical anchors. Thus, it is difficult to now substitute the use of epoxied, high-strength (HS) anchors for the cast-in-place, high-strength anchors which were used in combination with an embedded steel anchor plate.
If epoxied, HS anchors are desired, it would seem appropriate to construct a short segment of the RC parapet with the alternative vertical anchor systems spaced 6 ft or more apart along the wall. With the posts attached to the parapet, dynamic component testing could be performed at each post location in order to determine the peak load capacity of the various anchored posts when impacted at the upper rail height. If the alternative anchor options are found to provide equivalent or greater load capacity through dynamic component testing, then it would seem reasonable to allow their use within the bridge railing system.
I have the following questions for your consideration and reply related to recent DOT inquiry on design of
Post and Tube Bridge Rail (see attached typical section). I have also included in attached Figure 1.jpg an excerpt from 2004
AASHTO LRFD Bridge Specifications Section 13, Table A13.2-1: