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Analysis of Concrete Median Barrier without Transverse Steel

Question
State	WI
Description Text	We had a project were the contractor did not insert 1 bar longitudinal reinforcement bar in our barrier design. Initially the project staff rejected the barrier. The contractor then had a structural engineer do some LRFD calculations. If memory serves me correctly, I don’t think LRFD calculations will work because the middle section of the barrier has no vertical steel. Am I remembering this correctly? Could you take a look at these calculations?

Question

State

Description Text

We had a project were the contractor did not insert 1 bar longitudinal reinforcement bar in our barrier design.

Initially the project staff rejected the barrier. The contractor then had a structural engineer do some LRFD calculations.

If memory serves me correctly, I don’t think LRFD calculations will work because the middle section of the barrier has no vertical steel.
Am I remembering this correctly?

Could you take a look at these calculations?

System Performance Evaluation

System Types
Permanent Concrete Barriers

Applications

System Features

Date October 11, 2019
Previous Views (133) Favorites (0)

Response
Response (active)	You are correct that you can’t conduct Yield Line analysis on a barrier that doesn’t have transverse steel. Yield Line methods assume that a plastic hinges form in the structure and absorb energy as the hinge rotates through an angular displacement. Unreinforced concrete is brittle and does not form plastic hinges in bending (steel reinforcement required for plasticity). The calculations on the barrier utilize an equation to estimate the capacity of an unreinforced structure, but due to the lack of plasticity, the concrete would fail and not absorb the same amount of energy assumed by Yield Line theory. Further, without the barrier being anchored with transverse steel, the moment capacity at the base of the barrier would be zero. Because of these two issues, I do not believe that the yield line equations can be used to calculate capacity. Also of note, the calculated capacity is compared to a design load of 54 kips. This load has been superseded by the design loads in NCHRP 22-20(2), which specify a TL-3 load of 70 kips and a TL-4 load of 80 kips. That all being said, I do not believe that the omission of the single longitudinal bar would be detrimental to the barriers performance based on comparisons to other tested systems. Recall that an unreinforced, single-slope, concrete median barrier was successfully tested to MASH TL-3 for Ohio DOT. This barrier was 42” tall, 12” wide at the top, 28” wide at the base, and used a 1” asphalt keyway. The TL-3 test resulted in minimal damage to the barrier. Although the Ohio barrier is wider and a little shorter, your 56” tall barrier has nearly the same cross section area and contains longitudinal reinforcement. As such, I believe your barrier would also perform adequately for MASH TL-3.

Response

Response
(active)

You are correct that you can’t conduct Yield Line analysis on a barrier that doesn’t have transverse steel. Yield Line methods assume that a plastic hinges form in the structure and absorb energy as the hinge rotates through an angular displacement. Unreinforced concrete is brittle and does not form plastic hinges in bending (steel reinforcement required for plasticity). The calculations on the barrier utilize an equation to estimate the capacity of an unreinforced structure, but due to the lack of plasticity, the concrete would fail and not absorb the same amount of energy assumed by Yield Line theory. Further, without the barrier being anchored with transverse steel, the moment capacity at the base of the barrier would be zero. Because of these two issues, I do not believe that the yield line equations can be used to calculate capacity.

Also of note, the calculated capacity is compared to a design load of 54 kips. This load has been superseded by the design loads in NCHRP 22-20(2), which specify a TL-3 load of 70 kips and a TL-4 load of 80 kips.

That all being said, I do not believe that the omission of the single longitudinal bar would be detrimental to the barriers performance based on comparisons to other tested systems. Recall that an unreinforced, single-slope, concrete median barrier was successfully tested to MASH TL-3 for Ohio DOT. This barrier was 42” tall, 12” wide at the top, 28” wide at the base, and used a 1” asphalt keyway. The TL-3 test resulted in minimal damage to the barrier. Although the Ohio barrier is wider and a little shorter, your 56” tall barrier has nearly the same cross section area and contains longitudinal reinforcement. As such, I believe your barrier would also perform adequately for MASH TL-3.

Date October 11, 2019
Previous Views (133) Favorites (0)