flyf15
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No Stopping On Bridges

Tue Oct 10, 2006 10:34 am

Lots of major airports out there have taxiway bridges over roads.... many of them, I've noticed, have "DO NOT STOP ON BRIDGE" signs (or something similiar). Why is this?
 
VTBDflyer
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RE: No Stopping On Bridges

Tue Oct 10, 2006 11:33 am

Heavy loading on the bridge. I suspect that even though the bridge is very thick, the aircraft putting all that weight on anywhere from 3 points - 5/6 points of contact will put high stress on a relatively small area.

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LimaFoxTango
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RE: No Stopping On Bridges

Tue Oct 10, 2006 11:39 am

I remember watching the United 777 DVD from justplanes.com and the captain said the reason they aren't allowed to stop on brigdes isnt due to weight, but if they had to do an emergency evacuation, the slides would deploy right over the sides of the bridge. Not a pretty sight to say the least.
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Bobster2
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RE: No Stopping On Bridges

Tue Oct 10, 2006 12:22 pm

Taxiway bridges are supposed to be at least as wide as the taxiway. There should be enough room for slides.

I think loading could be an issue. Stopping and starting puts more stress on the roadway, it would shorten the lifetime.

Also, if emergency vehicles need to cross the bridge, a stopped plane might block them.
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2H4
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RE: No Stopping On Bridges

Tue Oct 10, 2006 12:30 pm




Quoting Flyf15 (Thread starter):
many of them, I've noticed, have "DO NOT STOP ON BRIDGE" signs (or something similiar). Why is this?

It's a result of rubbernecking A.netters not paying attention to the road, and causing traffic accidents as a result.  Wink



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Mir
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RE: No Stopping On Bridges

Tue Oct 10, 2006 1:35 pm

Quoting LimaFoxTango (Reply 2):
I remember watching the United 777 DVD from justplanes.com and the captain said the reason they aren't allowed to stop on brigdes isnt due to weight, but if they had to do an emergency evacuation, the slides would deploy right over the sides of the bridge.

This is what I remember as well.

Quoting Bobster2 (Reply 3):
Taxiway bridges are supposed to be at least as wide as the taxiway. There should be enough room for slides.

IIRC, on the larger planes, the slides deploy far enough out that it would be a problem.

-Mir
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EssentialPowr
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RE: No Stopping On Bridges

Wed Oct 11, 2006 3:14 am

Quoting VTBDflyer (Reply 1):
Heavy loading on the bridge. I suspect that even though the bridge is very thick, the aircraft putting all that weight on anywhere from 3 points - 5/6 points of contact will put high stress on a relatively small area.

baloney.

The problem is lack of emergency vehicle access if an a/c stops, as well as congestion to the rest of the airport by blocking a throughfare.
 
Lemurs
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RE: No Stopping On Bridges

Wed Oct 11, 2006 3:20 am

Quoting Bobster2 (Reply 3):
Taxiway bridges are supposed to be at least as wide as the taxiway. There should be enough room for slides.

Taxiways are designed for taxing though, not for evacuating people. When you want to evacuate, you expect people to run off into the grass as far away from the airplane as they can manage. You also want emergency vehicles to be able to get to the airplane from directions other than directly in front and behind. Bridges are just bottlenecks for emergencies at the end of the day...if weight were really an issue, the airplane shouldn't be allowed to roll over top of it to begin with!
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VTBDflyer
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RE: No Stopping On Bridges

Wed Oct 11, 2006 6:30 am

Quoting Essentialpowr (Reply 6):
baloney.

The problem is lack of emergency vehicle access if an a/c stops, as well as congestion to the rest of the airport by blocking a throughfare.

Just throwing my idea out there.

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kaddyuk
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RE: No Stopping On Bridges

Wed Oct 11, 2006 9:10 am

Quoting VTBDflyer (Reply 1):
Heavy loading on the bridge. I suspect that even though the bridge is very thick, the aircraft putting all that weight on anywhere from 3 points - 5/6 points of contact will put high stress on a relatively small area.

On that basis, the bridge wouldnt be able to take the loading of an aircraft at all...

Quoting Essentialpowr (Reply 6):
The problem is lack of emergency vehicle access if an a/c stops, as well as congestion to the rest of the airport by blocking a throughfare

This was my understanding of the situation...
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RichardPrice
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RE: No Stopping On Bridges

Wed Oct 11, 2006 5:16 pm

Quoting Kaddyuk (Reply 9):
On that basis, the bridge wouldnt be able to take the loading of an aircraft at all...

While its unlikely a bridge that is likely to see large amounts of traffic would be built in such a way, its possible for a structure to take less static weight than dynamic weight and thus would be fine for an aircraft crossing but not to hold its weight for an extended period.

But it is unlikely that such a design would have been approved...
 
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HAWK21M
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RE: No Stopping On Bridges

Wed Oct 11, 2006 10:29 pm

Stress loads on the Bridge for a longer duration.
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Bobster2
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RE: No Stopping On Bridges

Thu Oct 12, 2006 1:31 am

Quoting HAWK21M (Reply 11):
Stress loads on the Bridge for a longer duration.

I assume this is a maintenance cost issue. The bridge needs periodic repairs, and perhaps the interval can be extended by avoiding unnecessary stress. It's not a safety issue because the bridge is rated to carry the weight, but near the load limits there will be an increasing in cracking and other problems.
"I tell you this, no eternal reward will forgive us now for wasting the dawn." Jim Morrison
 
US AIRWAYS
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RE: No Stopping On Bridges

Thu Oct 12, 2006 2:44 am

I'm pretty sure in PHX I've actually stopped on the taxiway bridges the are over the departures road near Terminal 4. The bridge is rather wide though so that may be why.
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A/c train
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RE: No Stopping On Bridges

Thu Oct 12, 2006 6:54 am

Theres been some fantastic amounts of BS spoken on this topic !!! quite entertaining !
 
777236ER
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RE: No Stopping On Bridges

Thu Oct 12, 2006 7:15 am

There is some nonsense written in this thead. It's got nothing to do with static vs. dynamic loads.
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RichardPrice
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RE: No Stopping On Bridges

Thu Oct 12, 2006 8:26 am

Quoting 777236ER (Reply 15):
There is some nonsense written in this thead. It's got nothing to do with static vs. dynamic loads.

I agree, which is why I said its unlikely such a specific design would have been approved with such a limitation, but static verses dynamic loads is far from nonesense in any materials discussion and is likely to play a part in any bridge design.
 
lredlefsen
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RE: No Stopping On Bridges

Thu Oct 12, 2006 1:19 pm

Quoting RichardPrice (Reply 10):
its possible for a structure to take less static weight than dynamic weight

Funny, I've always thought gravity pulls *down*, no matter how fast you're going (ignoring aerodynamic effects, which shouldn't matter on a taxiway -- if they do, you've got other problems).

If the pilot nailed the brakes while they're directly on the bridge, I guess the load would have a horizontal component to it...
 
EssentialPowr
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RE: No Stopping On Bridges

Thu Oct 12, 2006 1:49 pm

Quoting 777236ER (Reply 15):
There is some nonsense written in this thead. It's got nothing to do with static vs. dynamic loads.

I tried "baloney" long ago like a smart button, but we still have people wanting to design airport bridges with unsafe working loads. Good thing they're just on a forum...
 
Fly2HMO
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RE: No Stopping On Bridges

Thu Oct 12, 2006 2:22 pm

Quoting US AIRWAYS (Reply 13):
I'm pretty sure in PHX I've actually stopped on the taxiway bridges the are over the departures road near Terminal 4. The bridge is rather wide though so that may be why.

I was spotting there this past weekend, and at the 3pm departure push, they had up to 2 A320s sitting on the bridge for a pretty decent time, all of them headed for runway 25R and originating from gates from the north side of the terminals. But, as you can see from my shameless plug, the bridges themselves aren't that very long:


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RichardPrice
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RE: No Stopping On Bridges

Thu Oct 12, 2006 3:49 pm

Quoting Lredlefsen (Reply 17):
Funny, I've always thought gravity pulls *down*, no matter how fast you're going (ignoring aerodynamic effects, which shouldn't matter on a taxiway -- if they do, you've got other problems).

Theres an element of how much compression a material can take before fracturing, and the less time the weight is on the structure, the less compression takes place.

Its not all instantaneous.
 
CM767
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RE: No Stopping On Bridges

Thu Oct 12, 2006 11:45 pm

Quoting LimaFoxTango (Reply 2):
if they had to do an emergency evacuation, the slides would deploy right over the sides of the bridge.

This is the real reason.
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EssentialPowr
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RE: No Stopping On Bridges

Fri Oct 13, 2006 7:37 am

People:

Bridges constrain emergency operations from an a/c, and impede traffic flow.
 
deltamike172
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RE: No Stopping On Bridges

Fri Oct 13, 2006 10:46 am

While I can see how stopping on a bridge would effect an emergency, I can't see why the airport authority would put up a sign telling planes not to stop on thier bridge for this reason. I always figured those signs where in place to decrease the time spent on the bridge, so less stress was put on the bridge, and the airport could go a longer time without replacing the bridge.

While this might not seem valid, consider all the airports that have concrete (can take heavier weight for a longer period of time before it fails verses asphalt) strategically placed in places where aircraft tend to remain motionless for long periods of time (departure ends of taxiways and runways, hold pads, and the gate areas). Very often, the concrete will only be installed at the gate itself, not the taxiway that runs along side. Heck, even the drive through at Wendy's has concrete in the drive through line where cars sit and wait.

DM
 
bobbidooley
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RE: No Stopping On Bridges

Fri Oct 13, 2006 11:00 am

Im MEM there is a bridge that is a part of runway 18/C. I have seen heavy AC land on the bridge. Look at google earth, you will see that the bridge over Winchester Rd, is about 50f from the prime landing zone.

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EssentialPowr
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RE: No Stopping On Bridges

Sun Oct 15, 2006 3:58 am

Quoting Deltamike172 (Reply 23):
While I can see how stopping on a bridge would effect an emergency, I can't see why the airport authority would put up a sign telling planes not to stop on thier bridge for this reason. I always figured those signs where in place to decrease the time spent on the bridge, so less stress was put on the bridge, and the airport could go a longer time without replacing the bridge.

While this might not seem valid, consider all the airports that have concrete (can take heavier weight for a longer period of time before it fails verses asphalt) strategically placed in places where aircraft tend to remain motionless for long periods of time (departure ends of taxiways and runways, hold pads, and the gate areas). Very often, the concrete will only be installed at the gate itself, not the taxiway that runs along side. Heck, even the drive through at Wendy's has concrete in the drive through line where cars sit and wait.

Negative...Why or who would design a bridge, or approve such a design, that had a working load that wasn't several times greater than the typical load? It DOES NOT happen in concrete bridge design, highway or airport. The signs are there due to the access issues as mentioned. Bridges are typical extremely substantial, to give them the longest working life. Concrete over time and weather eventually "spalls"; cracks and begins an irreversible process, but even this is considered in bridge design.

Concrete has considerably more strength than asphalt. Asphalt is typically used as an overlay, but certainly has less strength. Asphalt goes down faster, and is cheaper, as as many notice on interstate highway, develops grooves under loads. It is also softens in extreme heat. Asphalt can be milled off, re laid and rolled in 1 continuous process, which is why it is popular in highway applications but is inferior to concrete. Asphalt also cannot be grooved, like concrete.
 
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HAWK21M
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RE: No Stopping On Bridges

Wed Oct 18, 2006 11:32 pm

Quoting Bobbidooley (Reply 24):


Under construction time.
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MEL
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Aer Lingus
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RE: No Stopping On Bridges

Sat Oct 21, 2006 11:29 pm

Quoting RichardPrice (Reply 20):
Theres an element of how much compression a material can take before fracturing, and the less time the weight is on the structure, the less compression takes place.

just to clear things up a little. metals under a compressive stress will always fail through eventual TENSILE stresses. a steel cloumn in a structure is little use if it fails under a compressive load. a metal under loading (leading to failure) will go through its elastic phase, followed by the plastic phase (leading to permanent deformation) and finally fracture.

just as an aside, concrete is useless in tension and is only good for compression, hence the need to reinforce concrete where it is undergoing tensile stresses. also everything in civil engineering is built with a generous factor of safety for that unforseen moment. the notion that the bridge, upon stopping on it, would fail is laughable. it is conceivable, however, that prolonged static loading due to a large mass on any structure designed for mainly dynamic loading would do damage.
 
JayinKitsap
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RE: No Stopping On Bridges

Mon Oct 30, 2006 4:49 pm

Reality check time. I've designed bridge, gantry, and portal crane runways, straddle lift bridges, rail bridges, and highway bridges. All structures receiving a moving load have design wheel loadings. These are patterns of wheels that are at the spacing for maximum load with individual wheel loads. For standard highway bridges in the US we have the H-20 and HS-20 loads. The H-20 is a 20 ton truck with 80% of the weight on the back wheels with an track of 6 feet and distance between axles of 14' min. The HS-20 has a 2nd axle with 16 tons 14 or more feet behind the first heavy axle. From this there are various patters of loads used depending on the specific bridge. Railways have a very similar pattern, so do cranes. I am sure for various classes of airports the runway is designed for GA planes up to x for the first class, then RJ's and the like, then 707/737/757 size NB's, then a full blown class for the WB's thru the 747. With the intro of the 380 a new weight class was likely introduced.

Taxiways would have one pattern, runways another. On runways both a taxi position and nose wheel up would also be considered.

There is a static load of the object not moving, add to that an impact load of 30% for most vehicles, add lateral turning and breaking forces, side wind forces on the vehicle etc. For a runway, high impact forces probably on the order of 2 or 3 g are used (300% static weight) with the load on the main gear only.

Parking on a bridge is a lower load condition structurally than moving. For fatigue the number of loading cycles is what is important, the duration of the load is generally immaterial (well if the load on a concrete bridge is there for 20 years you would look at creep issues, but not 20 days). Fatigue would look at 20 applications a day of the standard load for 20 years or similar kinds of numbers. Occasional heavy loads (that AN-224 coming in) would be looked at without fatigue numbers and with a lower safety factor. Steel and concrete structures generally are at a factor of safety of 2 to yield and 2.5 to ultimate load assuming the materials are at the minimum allowed strength.

Again this is for the design vehicle. On a highway bridge that would cover the rare case of every truck being the maximum street legal load parked bumper to bumper on the bridge in all lanes happening occasionally. At an airport it would be 747's crossing at full MTOW every 5 minutes for 16 hours per day, every day for 20 or 30 years if it was LAX or JFK. At DAL, it would be 737's doing that all day, but one or two 747's every year if the runway can handle it.

I think the no parking on the bridge is a combination of being a visual distraction to the highway and for emergency vehicle access. If the plane is on the bridge probably the emergency vehicles can't cross unless there are multiple bridges.
 
mohavewolfpup
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RE: No Stopping On Bridges

Mon Oct 30, 2006 6:29 pm

Quoting A/c train (Reply 14):
Theres been some fantastic amounts of BS spoken on this topic !!! quite entertaining !



Quoting 777236ER (Reply 15):
There is some nonsense written in this thead. It's got nothing to do with static vs. dynamic loads.

this is the case with most threads on a.net I find. wait for the next plane crash to come around and the junior a.net armchair investigators go into full swing to provide advice on what caused it to crash.

I found out more information about the gol airlines crash recently getting onto wikipedia and searching there then mucking through flame fests, armchair "experts" and other riff raff that clog the airplane crash threads that are always popping up and on the 4th and 5th version of it.

I'm not mucking through all that bs just to get photos, status of the investigation, etc. Sometimes I ponder if a.net would be good with a new section for "experts" to flame and debate each other day in and out while the rest of us wait for the facts to come in, even if it's slow and takes a while?

I think alot on the site suffer from news reporter egos and say/do anything to get the first press on what happens including if that means twisting stuff to sound good or echoing what the news agencies say to get the prestige of breaking it first, rather then pull back and await more details..

This site truely has alot of people on it that remind me of a fox news special I saw when a plane landing in california a while back had it's front landing gear turned in a weird way, the experts ran amuck, one even pulling out a aerial photo of the area the plane was going to try landing and they said:

"well, here's a map of the runways, and here's a plane I found on the set" (a DC9 or 10 it looked like, hardly a 737? (or was that a airbus that had the problem? was a jetblue flight, all I can remember) and they proceed to tell us, the viewers, that it may be hard to land the plane because of the runway size and the plane size, all that from a aerial photo and a toy model!

way to go! apparently all airport designers failed engineering school because a toy plane model CAN'T LAND ON THE AERIAL PHOTO RUNWAY!! omg...

when do we get more people telling facts and less armchair experts?
 
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Vio
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RE: No Stopping On Bridges

Tue Oct 31, 2006 5:34 am

Quoting LimaFoxTango (Reply 2):
remember watching the United 777 DVD from justplanes.com and the captain said the reason they aren't allowed to stop on brigdes isnt due to weight, but if they had to do an emergency evacuation, the slides would deploy right over the sides of the bridge. Not a pretty sight to say the least.

 checkmark  ... Yeah, I have that video too. That's exactly what he said.
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