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Can
you comment on using the proposed ½” steel plates on either side of our
existing 42” Type C Single Sloped Median barrier will suffice for TL-3 or TL-4
(NCHRP or MASH) as shown in second drawing above “024 SS 104…”?  
 

Plates
will not be recessed into barrier but ends of plate will be beveled. Bolt heads
that anchor plate into barrier may or may not be countersunk (is countersinking
critical or can bolt heads be exposed?).  Length of overlap of plate at
each barrier end will be 12” as shown. each end.

The
plan is to remove 4 feet of barrier and replace with the steel plates.

Any
concerns? Any crash reports that cover this?

The ½-in. plate strips option does not appear to be close to having sufficient capacity on paper. We may expect up to 70 kips to be imparted to plate when distributed over 4 ft for MASH TL-3. With significant plate bending/deformation and possible fracture, we would likely have high risk for vehicle snag and/or pocketing on downstream end of joint. A 1-in thick plate is getting closer to design requirements. Plate ends would need to be recessed. Anchors would need to be farther from edges and spaced enough apart to not cause capacity reductions.

A prior proposal that remains unfunded in Pooled Fund has been brought up that addresses a similar issue. A CALTRANS system researhc in that proposal is shown but not crash tested to the best of our knowledge. 

Attached is what we came up with where we need to span a 4ft gap in our median barrier curb for drain inlets. I believe our median barrier is NCHRP 350 TL-4, so we are looking at a plate resistance that can match.

We have uncertainty about the type of bolts such that they can be recessed in slotted holes for expansion. The question has come up about slotting and recessing while still maintaining strength. Any help with this would be appreciated. The CALTRAN plate (shown below) in your “Open Joints Proposal” shows some flush bolts. Do you know the type of bolt this is?

The plate will be galvanized with either galvanized or stainless 1” dia steel anchors. The plate thickness is 1” also.

I want to discuss this system with Scott/Bob but have a few initial questions. Here are a few questions to start:

1. Refresh my memory on test level and barrier height;
2. Will top plate be recessed down to eliminate and/or reduce snag risk of engine hood/quarter panel? If not, can it be? I somewhat thought that it would be recessed if the ends were recast.
3. Is there one directional or two directional traffic at locations where this plate will be used?
4. Will downstream end of plate also be anchored with bolts (front and back) using slotted holes to allow for differential movement?

We will need to look up the other items before commenting.

Below

I want to discuss this system with Scott/Bob but have a few initial questions. Here are a few questions to start:

1. Refresh my memory on test level and barrier height;[Gregory E. Sanders]  TL-4, 42”, Safety shape median barrier we call Type C
2. Will top plate be recessed down to eliminate and/or reduce snag risk of engine hood/quarter panel? If not, can it be? I somewhat thought that it would be recessed if the ends were recast.[Gregory E. Sanders]  Yes since ends are recast. We chose to recess 1-1/8” (an extra 1/8”) for 1” thick plate in lieu of recessing only 1” and beveling plate ends.
3. Is there one directional or two directional traffic at locations where this plate will be used?[Gregory E. Sanders]  One directional. Median divides I-70 in St. Louis.
4. Will downstream end of plate also be anchored with bolts (front and back) using slotted holes to allow for differential movement?[Gregory E. Sanders]  Both ends will be secured with bolts. Only one end needs to be slotted, which end may not matter. We need to allow for expansion since the barrier has no open joints. I figure that reducing any compressive force in plates due to expansion is a benefit when impacted.

We will need to look up the other items before commenting.

Here are our thoughts on this topic after additional discussion.

1. We had originally thought about this configuration in line with using a 70-kip TL-3 MASH loading condition. Since TL-4 is to be considered, then a 80-kip TL-4 MASH loading condition is now applicable.
2. For a concrete safety shape barrier with a 4-ft long gap, the dual-sided plate needs to continue downward to a point of 3 in. above the road surface (i.e., stop at point of lower vertical toe of 3 in.). The lower portion of the plate would then need to match the second slope of the “F” safety shape (incline from 3 in. to 10 in. heights). The lower portion of plate prevents passenger vehicles from snagging on the exposed barrier at lower toe at downstream side of gaps. Single slope barriers would utilize a constant slope plate on front and back over full height.
3. For wide gaps, the 1 in. plate should incorporate a lateral stiffener welded or bolts between front and back plates at the midpoint (2-ft) to allow both plates to be engaged together when impacted on one side. A plate stiffener of ½ to ¾ in. thick over the plate height should suffice. Other built-up structural sections could also be used.
4. Two columns of bolts would be used on both sides of the gap. Dome head bolts could be used. Tapered head bolts could be used if tapered holes are drilled to allow for flush mounted heads. Dome head would require hex shapes or star shaped to allow them to be tightened.

Please let us know if you have further questions regarding this matter. Thanks!

Thanks for your feedback from the group. Much appreciated.

We are hung up on maintenance: ease of removal of the barrier plates for cleaning out the median drains. Using plates on each side worries the resident engineer; cumbersome, clunky, difficult to re-attach eve if removed as a unit (2 plates and a stiffener). Whole metal barrier skin (two straight sides and a welded top plate) may be easier to maneuver routinely. Barrier is 42” single sloped.

So, now we are thinking about using a whole barrier skin rather than plates. Since the skin has to be removable in order to clean the median drains, the resident engineer came up with an idea of how we can construct the new concrete barrier ends more easily by using the skin as formwork. He calls this formwork a “barrier saddle sole” plate” (attached) where this is used to form new barrier ends and then act as connector between barrier ends and removable whole metal skin. It is hoped it will allow for easier routine removal.

His idea prompts me to ask: what is wrong with using a whole barrier skin that can be just dropped in place over the existing barrier ends (after removing 4’ of median barrier) with minimal resin anchorage to the concrete. It would eliminate the need to pour new barrier ends; it would overlap 3’ at each cut end of barrier and would act to stiffen the cut barrier ends. It weighs over a ton. It would not however be flush with barrier face which is a concern but also presents us with a major problem in trying to overcome. Can we bevel the plate ends to minimize concerns or move to thinner skin say ¾” or ½” since it is whole skin (and we can add more stiffeners)? How can we mitigate this without going to next step of pouring new barrier ends?

We have thirteen of these removable plates/skins to put in place at one site on this rehab job to fix the median barrier drainage openings.

Gregory Sanders, P.E.

Structural Development and Support Engineer

Missouri Dept. of Transportation

See my comments below.

Thanks for your feedback from the group. Much appreciated.

We are hung up on maintenance: ease of removal of the barrier plates for cleaning out the median drains. Using plates on each side worries the resident engineer; cumbersome, clunky, difficult to re-attach eve if removed as a unit (2 plates and a stiffener). Whole metal barrier skin (two straight sides and a welded top plate) may be easier to maneuver routinely. Barrier is 42” single sloped.

**When considering a 4 ft span, I think that you would still want to have an intermediate stiffener regardless as to whether the top horizontal welded plate/cap exists or not. We think that this system would be conservative but necessary in the absence of testing to prove out a more aggressive, cost-effective system.

So, now we are thinking about using a whole barrier skin rather than plates. Since the skin has to be removable in order to clean the median drains, the resident engineer came up with an idea of how we can construct the new concrete barrier ends more easily by using the skin as formwork. He calls this formwork a “barrier saddle sole” plate” (attached) where this is used to form new barrier ends and then act as connector between barrier ends and removable whole metal skin. It is hoped it will allow for easier routine removal.

**We thought that you may have already been considering the cap variation when you referenced the photo below in the earlier correspondence. I agree that the cap variation is heavier than front and back plates only. However, they both seem very heavy to me and painful to remove for cleaning the drain.

His idea prompts me to ask: what is wrong with using a whole barrier skin that can be just dropped in place over the existing barrier ends (after removing 4’ of median barrier) with minimal resin anchorage to the concrete. It would eliminate the need to pour new barrier ends; it would overlap 3’ at each cut end of barrier and would act to stiffen the cut barrier ends. It weighs over a ton. It would not however be flush with barrier face which is a concern but also presents us with a major problem in trying to overcome. Can we bevel the plate ends to minimize concerns or move to thinner skin say ¾” or ½” since it is whole skin (and we can add more stiffeners)? How can we mitigate this without going to next step of pouring new barrier ends?

**Vehicle snag on plate edges is another big concern. We have rolled a pickup truck in the past when it snagged on a vertical plate that was welded to a steel barrier (i.e., exposed sharp edge). One may need to have an exposed edge (sides and top) less than 1/4 in. thick to resolve this concern. Further, vehicle components can gouge into concrete in advance of the steel plate, which may increase effective plate thickness that could negatively affect vehicle behavior.

**The concept in general may work if more stiffeners or support posts were added in the large gap when considering thinner side plates. We presented a similar concept a few weeks ago in the Pooled Fund meeting, although it was applicable to free-standing PCBs versus permanent barrier. The States moved toward using a thrie beam system on the sides along with a lower plate on the toes. With rigid barrier ends, the concerns for vehicle pocketing upstream from a barrier end increase, which could result in vehicle snag and/or instability.

**There has been a Pooled Fund proposal to address this issue, but it was not prioritized last year and remained unfunded. Do you have any space for support posts on or near the drain system?

Ron

Thanks for taking the time to discuss this project. After our phone conversation, I was reminded to bring up two more items.

We must have a drain opening. Can we use up to 6” opening between bottom of plate and ground for our 42” single sloped barrier to account for future overlays and of course to reduce clogging issues?

And, do we need to chamfer the lower part of the exposed barrier if the plate is not full height to reduce snagging? (I highlighted your response in item 2 below)

Gregory Sanders, P.E.

Structural Development and Support Engineer

I have enclosed links to our Pooled Fund Consulting website. See the two topics below.

http://mwrsf-qa.unl.edu/view.php?id=228

http://mwrsf-qa.unl.edu/view.php?id=439

I think that a lower plate height is always better. If you are recasting the ends, it may be advantageous to use a tapered toe to reduce any small car wheel snag concerns on the downstream end of the gap.