I was recently “informed” by an old friend that orginally the height for highway overpasses/bridges was determined by the height required of military vehicles carrying missles, so that they could be transported via interstate roadways. Theory then that more modern trucks/semis are then based off of this maximum height.
I find this unlikely, although it does seem to have a bit of logic to it, and assume that it is an urban legend. Unforunately, I have not found any references in my cursory Google searches supporting either side. I want to disprove this with some feedback or evidence, as my friend is a very stubborn engineer who tends to deny truth even when it is obviouly staring him in the face.
WAG, but this may be a story misappropriated from Nazi Germany - Hitler built the Autobahn network to far surpass anything in Europe at the time. And it certainly had a military role, so I doubt there wasn’t input as to the ‘requirements’ from military top brass. (He also had plans for a broad-gauge trans-Europe rail network, to deliver tanks and other military hardware as needed.)
I haven’t heard that one, but it is entirely possible. I have read that at least a one mile stretch out of every five miles must be straight and not have any overpasses. This is to allow war planes to land on the interstate if needed in a time of war.
So maybe the FIRST part is probably correct, the US (Eisenhower) saw the benefits of the German Autobahn in WWII, and basically the design for the Federal Highway System went from two lane roads to the larger and wider roads we have now, with land purchased on each side wherever possible. The Federal Highway Act of 1956 really got the system started, and we were fresh out of the Korean Conflict at the time.
:smack: I read that one a “Road Geeks” web site, where they seem to know everything about the interstate system. And then I couldn’t find the link so I went ahead and posted anyway. My apologies… I wonder what other wrong paths they have led me down.
The Interstate standard clearance is 15’2" or 15’6" (I forget which.) Does anyone know how tall a missile-laden military truck is? Those suckers must be pretty tall – if an ICBM is more than 15 feet wide at the base, that would put the kibbosh on the legend right there.
Quite a few urban highways aren’t build to Interstate standards, mostly freeways which were built before the system was established. Bridges that don’t meet the height clearance have to be marked. (Presumably, the military has a list of these somewhere…or else they’d be hitting the brakes REEEEALL fast!!) They probably don’t typically drive missiles right through downtown L.A. very often, though.
I always feel a bit awkward hijacking a roadway or airway thread but…
I’d once heard that the architect of the Merit Parkway (Route 15) in Connecticut purposely designed the overpasses to be too low to allow busses to fit, thereby keeping the poor stuck in the City and away from the rich folk’s country homes. There was also the insinuation that they (the overpasses) were artfully designed so as to discourage future remodeling.
I never took it too seriously, but from what I’ve heard from reputable sources about other projects in and around New York City, it wouldn’t surprise me if there was some truth to it.
Back in college, one of my engineering profs told us that the overpasses on Long Island freeways were deliberately made too low for New York City busses.
Yes they do, it’s called a truckers road atlas. They list bridge heights along with a lot of other info that is useful for truckers. You can pick them up at truck stops if you’re willing to pay for it. They are nice but probably not much use for anyone except truckers.
I had always heard that Robert Moses built the parkways such that the overpasses between NYC and Jones Beach couldn’t accommodate the height of the busses, and that Moses was a racist who didn’t want blacks coming to the beach. Never really found a reputable cite, though. This article refutes the old story.
A lot of Long Islanders accept this story as fact, but I’m not so sure.
The 4.9 m figure is correct as per the AASHTO Green Book. 4.9 m clearance over the entire traveled way is required over freeways, except that in highly developed urban areas 4.3 m may be used if there is an alternate freeway facility with a minimum of 4.9 m clearance. Also, sign trusses and overpasses have a clearance requirement of 5.1 m. And an allowance of 150 mm is recommended to all clearances to accomodate future resurfacing.
I’ve been told that in the event that the military was to use the freeways to move equipment and if a bridge prevented such passage, that they are quite able to clear the bridge quickly if need be.
Well, here’s the relevant quote from the AASHTO (American Assoc. of State Highway and Trans. Officials) Standard Specifications for High Bridges, 17th Ed., 2002:
(any typos are mine)
I had never heard the military angle before (I’m a bridge engineer) but I guess it’s possible. The TexasDOT Bridge Design manual, specifies 14’-6" as the absolute minimum for clearance with 16-6" being preferred if possible. And actually, nowdays, the districts have been wanting 17’-6" if it can be done. (what clearance is possible depends on a lot of things like allowable steepness, length of approach, number of levels to an interchange, superelevation, etc.
You blew me away at first. I had never heard of Standard Specs for High Bridges and I’ve been in the business for 26 years. Oh yeah, the Standard Specs for Highway bridges. Yep, 2.2.3 does read that way.
The military does affect bridge design in another way- 3.7.4 specifies alternate load of two axles 4 feet apart weigh 12T each. Never controls with us but it’s in the specs.
:o Yeah, my typing skills are pretty crappy (I really need to get my nails cut down). Dang it, I did preview tho’…
Yeah, we almost never even check the military loading anymore since it doesn’t usually control in our cases. (here in Texas, the vast majority of our bridges are prestressed concrete simple spans) Are y’all using LRFD yet? Every bridge engineer in the state is freaking out over the prospect fo switching…
