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|Description Text||It was mentioned during the meeting that the new recommended minimum barrier height for TL-4 barriers is 36”. Below is shown a table found in the AASHTO LRFD Bridge Design Specification. Table A13.2-1 - Design Forces for Traffic Railings Is there a similar table which shows equivalent values for MASH? If not, is there a corresponding equivalent load for the 36” height that is different from the current 54 kip?|
|Date||March 1, 2013|
The short answer to your question is… no, there is not a complete bridge rail design table for MASH similar to the one found in AASHTO’s Bridge Guide. However, there are multiple pieces of this table to be found in various testing reports and journal papers. For example, TTI published a journal paper through the Transportation Research Board that summarizes recent MASH TL-4 testing into 32” and 36” tall parapets. Within this paper, they recommend a minimum height of 36” for TL-4 rails and a lateral force capacity of 75 kips. A reference to this paper is below:
Sheikh, N. M., Bligh, R. P., and Holt, J. M., Minimum Rail Height and Design Impact Load for Longitudinal Barriers that Meet Test Level 4 of MASH, Transportation Research Record, Journal of the Transportation Research Board, No. 2309, January 2012.
MwRSF agrees with the 36” minimum rail height as our own crash simulations indicated the minimum height to be 35”-36”. Additionally, both MwRSF and TTI have conducted MASH TL-4 impacts on 32” tall F-shape / Jersey shaped barriers that resulted in the 10000S vehicle rolling over the barrier. However, MwRSF feels that the 75 kips design load may be on the load end of the range. We have been designing MASH TL-4 barriers to a capacity of 90-100 kips based on numerical analysis, simulation, and crash testing results. Now, clearly, TTI’s barrier with an 80 kip capacity successfully redirected the vehicle, but the calculated 80 kips value may vary depending on who is running the analysis and the methods applied. As we discussed over the phone, there are multiple ways/assumptions to calculate concrete bending strength and barrier strength. For example, including the compression steel in the bending strength calculations (TTI has historically only included the tensile steel, so I would assume this to be true of their 80 barrier design). Thus, their 80 kip design strength may calculate out to 85-90 kips if both layers of reinforcement were included in the design. Additional varying ways/assumptions made during calculations may include reduction factors, concrete strength, and barrier depth for irregular / non rectangular barrier cross sections. So, to sum up this paragraph, we are not saying that 75 kips is incorrect, but rather that it is on the low end of the MASH TL-4 design strength range and MwRSF is more comfortable with a more conservative 90-100 kip design strength… unless of course the barrier has been crash tested and shown adequate.
Similar to the above discussion, MwRSF has long felt that the design capacities listed in by AASHTO are on the low side. A complete analysis of TL-5 loads was previously conducted and documented in the attached report. From this analysis, the TL-5 design load was determined to be 210 – 225 kips. Note, there was no change between NCHRP report 350 and MASH concerning TL-5 testing criteria. Of course, it would follow that AASHTO’s TL-6 design load 175 kips was also low if the TL-5 design load is over 200 kips already.
MwRSF also feels the AASHTO TL-3 design load was on the low end of the range. For NCHRP Report 350 criteria, we had regularly witnessed TL-3 impact loads between 55 and 70 kips. Under MASH, the pickup truck increased in mass and the impact angle was also increased from 20 to 25 degrees. Thus, TL-3 impacts have an increased Impact Severity and would be expect to impart higher loads to the bridge rail. Subsequently, MwRSF has used a 70-80 kip range when designing MASH TL-3 bridge rails.
Hope this helps. Let me know if you have any questions.
|Date||March 2, 2013|
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