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Alternative F-shape Barrier Connection Pin Detail

Question
State MN
Description Text

Per our discussion last week, I am sending you a request for your consideration and approval of two Portable Concrete Barrier (PCB) Connecting Pin designs.

 

Minnesota uses an F shaped, 12.5' long, pin and loop, portable concrete barrier system.  The design was developed by Midwest Roadside Safety Facility.  The supporting FHWA acceptance letters are, B-41 for the original design, and B-122 for the current design.  Our design matches the current design, as proposed for the Barrier and Hardware Guide (SWC09) through task force 13.  See attached (SWC09 10-29-08.pdf). 

 

The current connector pin is located at

http://aashtotf13.tamu.edu/Guide/Hardware/Components/FMW02.pdf

 

We have been told by our construction office that the current connector pin design is difficult to work with when installed.  Especially when there is tension in the barrier system, thus having the effect of locking the pins into the loops.  Construction personnel often use hammers to tap the pins loose, which in turn causes damage to the upper plate of the connecting pin design (FMW02).

 

Our two proposed options are a "T" shaped pin and a "Cane" shaped pin.  See the attached drawing (pin_11_22_10.pdf).  Both proposed designs provide the same 1.25" diameter and 25" long vertical pin design as FMW02.  The proposed changes are to the top configurations of the bars only.  The "T" shaped top is the preferred design, however the "Cane" shaped top is less expensive to make, and still provides the necessary durability in the field.

 

Also attached is our proposed standard 8337C plate (StandardPlateReviewForm_8337C_Draft.pdf).  Our intention is to allow all three connecting pin types within our standards provided you approve.  Our Proposed 8337C plate 3 of 3, will be revised to include all three options.

Keywords
  • Permanent Concrete Barriers
Other Keywords none
Date November 22, 2010
Attachment StandardPlateReviewForm_8337C_Draft.pdf
Attachment pin_11_22_10.pdf
Attachment SWC09_10-29-2008.pdf


Response
Response

We have looked through your proposed pin designs and we have a couple of comments/concerns.

 

1.       We believe that the T-handle design would work acceptably, but we are concerned with the weld between the main pin and the T-handle. The current pin design has a ¼" fillet weld on the top and bottom of the plate. This is a weld length of approximately 7 7/8" and a weld area of 1.39 in2. The top of the pin can be loaded with significant vertical loads as the barriers rotate adjacent to one another, especially in a tie-down or anchored configuration. Thus, we are concerned that the T-handle pin does not have sufficient weld area to handle vertical loading similar to the tested pin and plate design. Our experience in welding round sections perpendicular to one another has found it very difficult to develop load capacity.

2.       We also have concerns with the cane type pin. The concern here is that under high loads, the short extension on the cane pin could be pulled into the loops and compromise the joint. The bent end of the pin would be free to rotate when installed and could be in a position that allows it to be pulled into the loops when loaded, or large barrier and joint deflections could pull the relatively short bent end into the loops.

3.       If the issue at hand is damage to the plates at the top of the pin, increasing the plate thickness should address that.

Date November 23, 2010


Response
Response

For furthered consideration is the attached proposed detail combining both request of MNDOT and MwRSF concerns on satisfying weld length & area requirements.

Date November 24, 2010
Attachment Proposed Details_112410.pdf


Response
Response

We would like to pursue some type of "T" bar option. I would like to propose that you consider taking out the 2 ½ " of bar between the 4"x2.5"x0.5" plate and the top 6" horizontal bar. The 4"x2.5"x0.5" plate could be welded on both sides, but on the top of a 2' 1-1/2" bar, and then the 6" long horizontal "T" top could be welded to the plate, extending 1" beyond either side of it. The 6" top could be round or square stock.


There seems to be a discrepancy with the drawings. The AASHTO link and B-122 (2003) show only a one sided weld for the plate to the pin. B-41 (1997) shows welding on both sides of the plate. Do you know which one is correct since one gives twice the weld area as the other?


If only one side needs to be welded, the plate could be brought up to the T handle and welded on the bottom without worrying about welding the handle for retrofit use if the pin length is acceptable.


Minnesota uses an F shaped, 12.5' long, pin and loop, portable concrete barrier system. The design was developed by Midwest Roadside Safety Facility. The supporting FHWA acceptance letters are, B-41 for the original design, and B-122 for the current design. Our design matches the current design, as proposed for the Barrier and Hardware Guide (SWC09) through task force 13. See attached (SWC09 10-29-08.pdf).
Date November 24, 2010
Attachment SWC09_10-29-2008.pdf


Response
Response

I have given some additional thought to the T-top connection pin for the F-shape barrier. I have included some additional comments below.


  1. First, I have reviewed the T-pin design that you have proposed which includes a 2.5"x4"x1/2" plate welded to the top of the 1.25" diameter connection pin. The T-pin is then welded to the top of this plate. I don't see any issues with this design. The F-shape barrier was originally tested to NCHRP 350 with a top plate mounted exactly as you propose. I have attached details. If the restraining plate at the top of the pin is attached with lower capacity welding than the original design that was tested, there concern that the top cap could disengage from the pin and allow the pin to exit the connection loops. This in turn would eliminate the integrity of the connection. However, because you are welding the top plate with the same weld used in the tested design, there should be no strength issues and attachment of the T-pin should be acceptable. Thus, I believe that your proposed design should function acceptably.
  2. We also discussed the T-pin design that has currently been made by your barrier fabricators. This design consists of a 1.25" diameter T-pin welded directly to the top of the 1.25" diameter connection pin. Again, the concern here is that the T-pin may not be connected to the connection pin with sufficient weld to have similar capacity to the tested design and ensure that the T-pin does not disengage from the connection pin during an impact. We cannot determine exactly what the loads were on the top plate during testing of the original pin. Thus, we must require that any modification of the connection pin must have similar or greater capacity.


I do not believe that it is possible to get sufficient weld area (and corresponding weld capacity) in the fabricators design to match the tested pin. The strength and capacity of a given weld is determined by the throat area of the weld. Weld throat area can be determined by the formula At= .707hl. In this formula, At is the throat area, h is the height of the weld, and l is the weld length. The tested pin cap was attached to the connection pin with a throat area of 1.39 in2. Thus, we would require that the attachment of the T-pin to the connection pin have similar throat area and weld capacity.

  1. It may be possible to retrofit the existing T-pins that have been fabricated. I have attached a detail for a proposed retrofit. This retrofit would attach the tested pin plate to the pin using the standard ¼" fillet weld on the bottom. The plate could be slid up the pin from the bottom. Then the plate would be welded to the T-pin on top with a flare bevel weld along the length of the plate. This would require checking to make sure the retrofitted pin still extended into the barrier loops (had the same effective length) as the tested design. Let me know what you think.


With respect to the weld details, there are different weld details floating around out there. There are currently three details.


  1. The original pin cap was welded with the cap flush with the top of the 24.5" long pin. The cap was welded to the pin with a ¼" fillet weld on the bottom of the cap and the top of the cap was welded to the pin with a flare bevel weld. This pin design was used when the free-standing barrier was originally tested to NCHRP Report 350.
  2. The pin cap weld configuration was used when the steel strap tie-down was developed for the F-shape PCB. At that time, we used a 27 ¾" pin that mounted the cap plate 1" below the top of the pin. This cap was attached with ¼" fillet welds on both the top and bottom of the plate.
  3. The remaining F-shape PCB testing was conducted with a 28" long pin with the pin cap mounted 2.5" below the top of the pin. The pin cap for this design was welded with a ¼" fillet weld on the top of the pin cap only. This pin was a design originally submitted directly to us by KsDOT when we switched from the two loop to three loop connection design. It was used in both the MASH testing and the other tie-down and transition testing conducted at MwRSF.


Based on the different configurations above, we have typically recommended that the second configuration with top and bottom fillet welds be used. However, the single fillet weld design has passed the free-standing barrier MASH test, and it was used in all of the tie-down and transition designs excluding the steel strap tie-down. Thus, it would be okay to use the third pin configuration as long as you did not plan to use the steel strap tie-down. The steel strap tie-down would still require the second pin design.


My previous weld areas were calculated based on the second pin design. If you went with the third option, then your revised T-pin design would require ½ the weld area. This would be a throat area of 0.694 in2. I don't believe that you can get that much weld area with the welding of the T handle directly to the pin. Thus, some form of retrofit would still be needed. However, the retrofit I proposed could be simplified by only using the fillet weld on the underside of the pin cap and then welding the T-handle to the top of the pin cap plate. No filler weld needed on the top of the pin cap to attach it to the pin.
Date February 3, 2011
Attachment T-pin_retrofit.PDF


Response
Response

We have put together a design which is similar to what Bob had suggested below. Please see the attached PDF.


We are proposing that the plate be attached with the ¼" fillet weld on the underside of the pin plate. We are not proposing any additional welding on the top side of the plate. The proposed modified pin design does state that this design is not to be used with the steel strap tie down.

Date February 14, 2011
Attachment T-pin_retrofit.PDF


Response
Response

The detail looks consistent with our discussions, and I have no issues using this pin.


Date February 14, 2011


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