NCHRP report 350

[Soldierson]

I am designing an extended curb that will anchor a test level 3 traffic rail per the NCHRP 350 spec and need an impact force. I know how to calculate impact forces, but the report only designates weight, initial speed, and impact angle. I need change in velocity (lateral) for the calculation. The report does give limiting occupant speed and decel (g's) and I am thiking of using those. Any thoughts or sugguestions?

 

[Ron]

Check the AASHTO specs for guardrails...it gives you what you're looking for.

 

[jdonville]

Gerald,

AASHTO requires a design (unfactored) impact load of 400 kips for structures adjacent to a roadway that are not protected by an independent barrier.

Or you could assume a final velocity of zero and use F*deltaT = m*deltaV (impulse relation from high school physics). The smaller the deltaT, the higher the F.

Jeff

 

[Soldierson]

Thanks for the input. I havent found the reference in my google searches on AASHTO guard rail specs. 400 kips for a TL3 design seems very high. The TL3 is a 2000kg pick-up traveling 100 km/hr with a 25 degree impact angle. For a complete stop, yes the loads are very high, but for a glancing blow, veloctiy change is much smaller, but what is it? The 12 m/sec occupant limitation yields 106 kips, which seems more reasonable.

 

[jdonville]

Gerald01,

If I recall correctly, the 400 kips was for a tractor-trailer impact or a train impact.

But when the contract doesn't specify the requirements (as a recent letting project did), I bid it high to cover my butt. Didn't win the job, but I'm not eating the cost of missing it from the design-build item.

Jeff

 

[Soldierson]

OK, that makes sense, this will be a secondary road with lower speed limits. I just found a detail on the TxDOT bridge standards web site that I believe is useable Rail Anchorage Curb for box culverts that I believe I can use without undue concern:
ftp://ftp.dot.state.tx.us/pub/txdot-info/cmd/cserve/standard/bridge/racste01.pdf
Thanks for your input