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.
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.
For furthered consideration is the attached proposed detail combining both request of MNDOT and MwRSF concerns on satisfying weld length & area requirements.
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.
I have given some additional thought to the T-top connection pin for the F-shape barrier. I have included some additional comments below.
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.
With respect to the weld details, there are different weld details floating around out there. There are currently three details.
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.
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.
The detail looks consistent with our discussions, and I have no issues using this pin.
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