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RE: airborne vehicles

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A good reference on the issues surrounding the design of vertical curves is the AASHTO "Policy on Geometric Design of Highways and Streets". I do not know if that is available online from AASHTO or elsewhere. 

My somewhat older copy at hand Figure III-38 gives the crest and sag vertical curve parameters and it's all laid out in terms of the constant rate of curvature per unit length (parabolic) for a given design speed. In addition to the pure physics that Christopher notes, AASHTO also addresses acceptable sight-stopping distances. I imagine this is not a very long bridge crossing a river, but sight distances would have to be considered so that the following vehicles can safely stop if one of the leading vehicles needs to slow down or stop as it exits the opposite side of the bridge.

For example, from Table III-32, "Design controls for crest vertical curves based on stopping sight distances", a design speed of 20 mph requires 125 feet. A design speed of 35mph requires 250 feet. If the length of the vertical curve (the bridge) is less than the sight distance required, sight distance will govern, and the maximum curve elevation (the vertex of the parabolic path) will be limited to what enables sufficient sight distance to be maintained.

Any good highway design text should address the above in detail.

Best regards,

Thomas Honles, SE, PE
Los Angeles, CA


-----Original Message-----
From: Christopher Wright [mailto:chrisw(--nospam--at)skypoint.com]
Sent: Tuesday, February 15, 2005 4:43 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: airborne vehicles



On Feb 15, 2005, at 3:51 PM, Candi Anderson wrote:

> ’m working on the design of an off-road vehicle river crossing 
> (bridge) and am looking for information regarding speeds and 
> inclinations when ATV and 4WD vehicles would go airborne.
Check your physics book. If the road way lies below the parabolic path 
that the truck would take through the air, given its initial speed and 
direction corresponding to the slope of the road, the vehicle will 
leave the ground, at least momentarily. You can probably take the 
inclination a horizontal, assuming that the truck is at the crest of 
the bridge, and figure the parabola on that basis.
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 
1864)
http://www.skypoint.com/~chrisw/


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