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modifying TTI TL-2 31" guardrail

Question
State	WI
Description Text	I noticed TTI has a TL-2 beam guard transition to rigid barrier. I was wondering if it is possible to modify the transition to use nested rails verse 10 gauge thrie beam and using 12 inch deep blocks? I assume that the transition is the area in the square. Do you think that there is a minimum length of beam guard prior to the transition like the MGS TL-3 transition? report number FHWA/TX-12/9-1002-8

Question

State

Description Text

I noticed TTI has a TL-2 beam guard transition to rigid barrier. I was wondering if it is possible to modify the transition to use nested rails verse 10 gauge thrie beam and using 12 inch deep blocks? I assume that the transition is the area in the square. Do you think that there is a minimum length of beam guard prior to the transition like the MGS TL-3 transition? report number FHWA/TX-12/9-1002-8

System Performance Evaluation

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Approach Guardrail Transitions (AGTs)

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Date February 8, 2013
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Attachment Pages from 9-1002-8 tl2 transition tti.pdf

Response
Response (active)	I do not forsee any issues with extending the blockout to 12" from the tested 8" blocks. This would only help minimize vehicle snaging potential. Going from a 10 ga. thrie beam section to a nested 12 ga. section has been considered allowable in previous designs. Nested 12 ga. thrie beam has about a 50% increase in the cross section area, elastic section modlus, and plastic section modulus in comparison to 10 ga. thrie beam. Thus, it is a stronger section. The only issues that I can see arise from this switch would be increased pocketing at the connection between the w-to-thrie segement and the nested thrie. However, due to the short length of the thrie beam (only connects the bridge parapet to the 1st post) and the fact that this is a TL-2 system, I do not have much concern for an increase in pocketing. To answer how much w-beam is necessary upstream of the transition, I will refer to and amend the 3 criteria listed on page 154 of the MwRSF TL-3 transition (TRP-03-210-10). 1. the first required a 12.5 ft seperation from the assemetrical w-to-thire transition segment and the downstream stroke of the energy absorbing terminal. this requirement will stay the same. However, the stroke of a TL-2 energy absorbing terminal will be less than that of a TL-3 terminal. Thus the total distance upstream of the transition segment will be less. 2. the second requirement called for a minimum of 46'-10.5" upstream of the assemetrical w-to-thrie transition segment. This distance was to ensure proper anchorage of the system. With the drop down to TL-2, this distance can be conservatively reduced by 12.5' to a distance of 34'-4.5". 3. the third requirement called for a 25 ft distance between the assemetrical w-to-thrie transition segment and the start of a flared guardrail terminal. Due to the reduction in impact energy with a TL-2 system (compared to TL-3), this distance can be conservatively reduced by 6'-3". Thus, a distance of 18'-9" should be seperating the transition segment and the start of a flare.

Response

Response
(active)

I do not forsee any issues with extending the blockout to 12" from the tested 8" blocks. This would only help minimize vehicle snaging potential.

Going from a 10 ga. thrie beam section to a nested 12 ga. section has been considered allowable in previous designs. Nested 12 ga. thrie beam has about a 50% increase in the cross section area, elastic section modlus, and plastic section modulus in comparison to 10 ga. thrie beam. Thus, it is a stronger section. The only issues that I can see arise from this switch would be increased pocketing at the connection between the w-to-thrie segement and the nested thrie. However, due to the short length of the thrie beam (only connects the bridge parapet to the 1st post) and the fact that this is a TL-2 system, I do not have much concern for an increase in pocketing.

To answer how much w-beam is necessary upstream of the transition, I will refer to and amend the 3 criteria listed on page 154 of the MwRSF TL-3 transition (TRP-03-210-10).

1. the first required a 12.5 ft seperation from the assemetrical w-to-thire transition segment and the downstream stroke of the energy absorbing terminal. this requirement will stay the same. However, the stroke of a TL-2 energy absorbing terminal will be less than that of a TL-3 terminal. Thus the total distance upstream of the transition segment will be less.

2. the second requirement called for a minimum of 46'-10.5" upstream of the assemetrical w-to-thrie transition segment. This distance was to ensure proper anchorage of the system. With the drop down to TL-2, this distance can be conservatively reduced by 12.5' to a distance of 34'-4.5".

3. the third requirement called for a 25 ft distance between the assemetrical w-to-thrie transition segment and the start of a flared guardrail terminal. Due to the reduction in impact energy with a TL-2 system (compared to TL-3), this distance can be conservatively reduced by 6'-3". Thus, a distance of 18'-9" should be seperating the transition segment and the start of a flare.

Date February 11, 2013
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