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I’m happy to offer any assistance I can. I spoke about this with Ron and Bob here as they have excellent experience with end anchorages and terminals.

General Thoughts

The Wisconsin design denoted in the report is similar to the trailing end anchorage design which MwRSF utilizes standard in crash testing, but a determination of equivalency would be based on some considerations:

Soil foundation tube length and embedment

Soil plate size

Terminal post(s)

When the MGS was adopted and we formalized the trailing end anchorage shown in the report, we used a deeper soil embedment tube for the posts than the prior design adopted for G4(1S) and G4(1W) BCT terminals. When combined with the strut, no soil plate was needed for the end anchorage. We would recommend that the soil foundation tube be at least 6 ft deep.

Also, we use a modified BCT post (MGS BCT post) as described in the report. During the compliance test at TL-3, the trailing end anchorage produced longitudinal forces in the car which were high and approached, but did not exceed, the allowable thresholds. We would therefore strongly encourage that the MGS BCT posts be used for both of the final two posts in the system to ensure similarity with the MGS trailing end anchorage design, and that other untested post shapes not be substituted.

NOTE: We currently have a project ongoing at MwRSF to evaluate a steel post, breakaway option for the trailing end anchorage. This steel post version should be equivalent to the wood post version. That project is still ongoing, thus no formal recommendations or implementation guidance have yet been provided.

Specific Notes for Florida Design

I would be happy to review a standard drawing if you have one, and offer comments.

Please let me know if you have any further thoughts, comments, or questions. I hope all is well in Florida.

In looking at our FDOT drawings, it looks like we can lengthen our tube for the first post based upon your comment. The “BCT MGS” post shown in Figure 47 of the report looks very similar to our “Short Timber Breakaway Post”, but we can modify if you recommend it. Also, we can then add the tube/BCT at the second post and enlarge the soil plate if required.

Thank you for offering to take a look at our drawings (attached). The full source document can be found here… http://www.fdot.gov/design/standardplans/current/IDx/536-001.pdf

One last consideration is that we still support a double-faced guardrail option with a history of success. I’m not sure how a strut design would mix with it, but I’d prefer to keep and update the soil plate design if possible.

Thank you!

Upon review of Florida’s drawings, we came to the following conclusions:

• I think that your “short timber breakaway post” is reasonably similar to the MGS BCT post we use –measurements are within ½ in. of what we use as standard here.
• I observed that you utilize the MGS design setup with 8-in. deep blockouts, which TTI demonstrated was successful at TL-3 impact conditions.
• The foundation tube with soil plate may provide similar soil resistance as the double foundation tube with channel strut. There is some unpublished research which suggests that a single foundation tube / driven pile with a soil plate can provide meaningful resistance, and we have used installations of single foundation tubes in applications such as the bullnose guardrail system and a temporary barrier anchoring system. However, we do not know what the capacity of your setup is, and as such, we cannot verify that it has an equivalent strength to the MGS. If you have any concerns about the strength of that foundation tube and soil plate compared to the double-foundation tube with channel strut, we could discuss a small testing effort to construct and measure forces and deflections sustained by both designs.
• We recommend deepening the steel tube foundation from 5 ft to 6 ft for similarity with the MGS foundation tubes. We are not aware of tested designs right now which use 5-ft deep foundation tubes without a groundline strut.

One item that Bob observed that warrants some consideration is your double-sided guardrail end termination, page 9 of http://www.fdot.gov/design/standardplans/current/IDx/536-001.pdf. In that design, only one side of the W-beam guardrail is attached to a cable anchorage. There are some concerns that only anchoring one side of the W-beam could reduce the capacity of the W-beam to resist impacts when struck on the other side of the system. It may be helpful to install two cable anchorage assemblies, with the second cable attached at the second post and to the opposite-side rail, as shown below. As a possible alternative, you could consider splicing a second cable onto the existing BCT cable and anchoring it to the other rail, which is similar to the originally-designed BCT system developed at SwRI. Although these double-sided, trailing end anchorage modifications have not been full-scale crash tested, they represent the best advice we have now.

Based on your review, we will keep our short timber breakaway post, but we’ll update the foundation design to include the strut anchorage, double tube foundation, and deeper tube foundations (from 5 to 6 feet).  These changes will be made to both single and double-faced designs.

Regarding the double-faced trailing anchorage, it’s assumed that the cable connection will be on the panel side that is within the clear zone. If both sides of the guardrail were within the clear zone for a typical median application, then we’d require the use of an approach crash cushion instead.  That said, we will reorient our drawing to show the cable connection on the panel side likely to be in the trailing clear zone. For the rarer case of guardrail between merging lanes (traffic in same direction), we’re okay with the assumption that redirective capability of the guardrail may be further upstream on the side lacking the cable connection.

This brings me to my final question. It appears that TL-3 design for the pickup truck requires 5 post spacings downstream of the impact point to successfully redirect the vehicle.  This being the case, should design policy require guardrail to extend 5 post spacings downstream of the hazard being shielded (to ensure only redirective guardrail is adjacent to the hazard)?  The other consideration would be departure angle and offset of the guardrail to the hazard, which may shorten the need to extend guardrail downstream of the hazard.  What are your latest thoughts on extending the guardrail downstream of the hazard?

Thank you!

I am providing you with two references that may address your later questions. See the Transportation Research Record (attached pdf file) for a proposed hazard envelope near the downstream end of the system anchored by the noted hardware in combination with MGS. Similarly, this information is provided in MwRSF research report no. TRP-03-279-13, which can be accessed on our website. Unfortunately, our website is down for maintenance. You will find guidance pertaining to the clear areas and acceptable hazard areas for impacts near the downstream end. Guardrail may need to be extended if hazards are too close to back of rail at end. If hazards are even closer, then stiffened guardrail systems may be required. Please let us know if you have any other questions.

Thanks!

I did have access to TRP-03-279-13, and my apologies for not spotting that information in the 490 page report.   I actually just got lucky and landed on page 238, so I believe that’s what we’re going for!  This is perfect. It does seem that some national publications should update their trailing end design recommendations based upon this. The information here is great… Some might even say it’s impactful.

Thank you again for all of your help. We’ll get our trailing end anchorage up to the latest and greatest soon.