We are trying to implement the KsDOT Barrier in the next week.
I question how to raise the bottom of the barrier 2" (preferred).
Bar 4a1 is a stirrup which usually comes within 2" of the bottom of the barrier/ ground.
The rise on the left half of the barrier is preferred, the right is an odd option I think will lose the 3" base piece.
Can the stirrups labeled 4a1 be cut to keep the steel from being closer than 2" to the bottom when we raise it like the right half?
Or can 4a1 be bent slightly different to meet our 2" distance to the outside of the barrier?
I have reviewed your email questions below and have a few comments. See below!
**I am not sure that I understand the concept of raising the bottom of the barrier by 2 in. However, I assume that you are referring to the need to add increased hydraulic drainage flow to specific locations on the bottom of the barrier.
**In the original TCBs, the vertical stirrups and lower longitudinal rebars were placed close to the barrier's base with an acceptable concrete cover for the steel bars. Later, the Pooled Fund member states met in St. Joseph, MO to discuss the standardization of the TCB as well as the modifications/addition of some other features, including fork lifting slots. When the fork lifting slots were incorporated, the shape and placement of the vertical stirrups remained the same but the lower longitudinal rebars had slightly moved upward to provide cover above the fork lifting holes. The affect of these changes can be garnered by viewing the original details and comparing them to those details listed for the KS/FL (Midwest) TCB free-standing and tied-down systems.
**The modifications on the left side of the barrier depict a distinct fork lifting slot and a separate drain slot. However, on the right side, there exists a fork lifting slot that it integrated into the drain slot, thus requiring a modification to the vertical stirrup and reduced barrier contact with the ground. Personally, I like the detail on the left side more than that on the right side. If provision for drainage is really needed, I would almost rather see four fork lifting slots centered between the vertical stirrups such that the vertical stirrups do not require modification and barrier support is better distributed. With this change, there would not be narrow, 3-in. wide segments. On another note, are you counting on the 3-in. wide drainage slots between the barrier ends? If not, would the two fork lifting slots be sufficient?
**If the bottom side is raised under vertical stirrups, the bottom of the bars would be bent inward at a higher elevation to fit under the lower longitudinal bars. However, it is not recommended to have this change occur near the tie-down locations. Thus, the proximity of the drainage slots near the outer tie-down locations could result in increased concrete fracture when the barriers are anchored to a paved surface or bridge deck.
**As noted above, it would be possible to bend the lower stirrups ends inward at a slightly higher elevation to meet concrete cover. However, the drain slots should be integrated such that they do not pose concerns for increased concrete fracture at the tie-down locations.
The Nebraska Department of Roads (NDOR) has recently contracted with a contractor to provide 25,000 Lin. Ft. of new concrete barrier and FHWA-NE is requesting the NDOR to update its design to something similar to the Kansas Portable Concrete Barrier (PCB) which has incorporated a pin and six connection loop system and some of these low-cost improvements to which Mr. Horne alluded. The Kansas PCB has been crash tested and is accepted by FHWA in letter HSA-10/B-122. As we have previously discussed, NDOR has determined that instead of directly adopting the Kansas design, we prefer to adopt a modified design that incorporates features that Nebraska has found to be beneficial.
NDOR is requesting that MwRSF review the following changes to the Kansas design and advise if the barrier will continue to perform satisfactorily with the desired changes.
Modifications to:
Loop Steel:
The Kansas plan calls for "1.25 times the yield strength but a minimum of 80 KSI" The ASTM standards for A706 steel include this. Both plans call for yield strength of 60 KSI. The minimum bending diameter for ASTM A706 steel is 4 x (3/4" dia.) = 3" our plan shows this in the bending diagrams.
Six Loop System " Connection Pin/ Retaining Bolt:
The six loop system does not require a retaining bolt at the bottom of the connection pin used to connect adjacent barriers; NDOR's barrier has this detail and we have elected to keep the detail only requiring it to be used when using the strap near a drop-off.
Anchor Bolt Block Out:
The Kansas plan shows a standard detail for an anchor bolt block out to allow the barrier to be bolted to the substrate; NDOR elected to make the anchor bolt block out optional and to be built at the discretion of NDOR since it is not required for all projects. The U-shaped steel bars labeled 6A2 required for the anchor bolts to transfer load are also omitted when the anchor bolt blockout is absent.
Tie-Down Strap:
The Kansas barrier plan does not have an alternate of using a tie-down strap to mount the barriers to the substrate. The tie-down strap was tested with the NCHRP 350 testing procedures and is an accepted detail for the 12.5' barrier. NDOR desires to retain the option of using the tie-down strap.
Foot Print - bottom of barrier in contact with the ground:
The Kansas plan has a foot print of 12.9 square feet the proposed Nebraska design has 14.4 square feet. NDOR prefers that there be additional lifting slots for drainage conveyance under the barriers to reduce ponding on the roadway and allow movement by larger forklifts.
The modification is shown on the elevation view as being an additional 1' of barrier on each half elevated 3" from the ground for the width of the barrier and results in a reduction of 3.75 square feet of foot print. To mitigate this decrease in the area of the barrier in contact with the ground the NDOR plan removed the 7" wide x 1" high inverted V-shape on the bottom of the Kansas plan, shown on Section B-B of the Kansas plan, this had held 6.12 square feet from contact with the ground.
NDOR requests that MwRSF review this information along with the attached plans and advise if the Kansas barrier, modified as proposed will continue to function as tested and accepted or include further suggested modifications to perform satisfactorily.
NDOR further requests an opinion on whether the Nebraska PCB designs (both 4-loop & 6-loop) and the modified Kansas PCB can be pin connected together and be considered to perform satisfactorily to NCHRP Report 350 or MASH Test Level 3 evaluation criteria.
I have reviewed the enclosed NDOR materials and have the following comments.
(1) The reinforcing steel for loop bars are shown to conform to ASTM A706 Grade 60, which infers a minimum yield strength of 60 ksi, a minimum tensile strength of 80 ksi, and a minimum % elongation in 8 in. equal to 14% for no. 6 or ¾-in. diameter bars. A footnote also reads that the tensile strength shall not be less than 1.25 times the actual yield strength. It is acceptable for NDOR to denote that the loop bars conform to ASTM A706 Grade 60.
(2) Historically, the loop bars and reinforced concrete barriers have been fabricated and crash tested using a 2¾-in. pin diameter to achieve the specified loop geometry. Florida, Iowa, Missouri, and Kansas all utilize a 2¾-in. pin diameter. NDOR has depicted a 3-in. pin diameter. In order to maintain the same drop pin and rebar loop clearances, it would be recommended that NDOR utilize the 2¾-in. pin diameter.
On another issue, Iowa and Kansas specify that the steel rebar for loop bars pass the 180-degree bend test using a 3½-in. pin diameter, while Florida specifies that a 2¾-in. diameter pin be used for the 180-degree bend test. Missouri does not identify a bend-test requirement. NDOR does not currently identify a 180-degree bend test requirement. ASTM A706 denotes the bend test to demonstrate that the bar can be bent around the pin without cracking on the outside radius of the bent portion. Thus, if a bend test were to be performed, it would seem appropriate to run the 180-degree bend test using the same diameter that would be used in the final loop configuration.
(3) A six-loop rebar connection system with drop pin is shown in the NDOR CAD details. At the base of the drop pin, a horizontal retainer bolt was originally configured for use with the four-loop rebar connection system as well as for the tie-down strap anchor system. However, the retainer bolt is not required in free-standing TCB configurations that utilize the six-loop rebar connection.
(4) An alternative tie-down system was originally developed for the Midwest F-shape temporary concrete barrier which consisted of vertical bolts or rods penetrating the barrier's toe. At these anchor locations, horizontal rebar loops were incorporated to strengthen the TCB at the attachment locations. The NDOR temporary concrete barrier does not include these additional rebar loops in all sections, unless the barrier section will later be used in tied-down applications. It is acceptable to leave out these 6A2 bars if the TCB will only be used in free-standing applications or anchored using the tie-down strap.
(5) NDOR noted that Kansas does not utilize the alternate tie-down strap with the F-shape TCBs. However, I reviewed the Kansas standard plans and found detail RD622B which depicts the tie-down strap anchor method.
(6) NDOR has proposed to increase the length of the lateral openings on the underside of the TCB to allow for improved water drainage flow from the roadway to travel under the barrier, thus reducing concerns for water ponding near the travel lanes. The detail with four separate drainage slots is acceptable. After considering alternatives, it would also be acceptable to combine the two slots on each half of the barrier into one slot measuring 2 ft - 3 in. long and shifting the outer edge inward slightly to provide additional concrete cover near the outer tie-down holes. For this second alternative, the 4A1 bars above the slot would need to be modified slightly. The 2-ft long middle support section would be maintained.
Based on the features identified in Item Nos. (1) through (6), it is our opinion the modified NDOR F-shape TCB will provide an acceptable safety performance when used in similar applications to those approved for the Iowa, Kansas, Florida, and "Midwest" TCBs. The aforementioned barrier versions have been previously crash tested in free-standing and tied-down applications according to either the NCHRP Report No. 350 or MASH impact safety standards.
Lastly, NDOR requested that MwRSF provide comment regarding the safety performance of a free-standing, TCB system which utilizes one end of a four-loop connection to attach to another end of a six-loop connection. As noted previously, the four-loop, Iowa TCB system was successfully crash tested under the NCHRP Report No. 350 impact safety standards. Later, the six-loop, Midwest/Kansas/Florida TCB system was successfully crash tested under the MASH impact safety standards. When the six-loop connection was integrated into the F-shape TCB section, the geometry of both loop connections was considered to ensure that the two designs could be attached to one another. Therefore, it is our opinion that a TCB barrier system which contains joints where both loop connections attach to one another would be considered crashworthy and capable of meeting the Test Level 3 impact conditions.
For informational purposes, I have attached PDF copies for the TCB CAD details for Kansas, Iowa, Florida, and Missouri.
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