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I have noticed that at some airports like FLL and SEA, there are bridges that make up part of the actual runway and specifically the threshold/touchdown area. How many landings are bridges like that actually designed to take over time and are pilots given any sort of guidance or preference to land past it if possible to reduce wear on the bridge over time?

I don’t know the answer but look at Funchal, half the runway is built on pylons out over the sea. That takes runway bridges to a real extreme and they don’t seem to have had it collapse (yet).

At least at FLL the majority of landings are on the other end of the runway.

I would have to assume that they design the tunnels to be able to support the dynamic loads of landings for the planned lifetime of the runway before it needs refurbishment.

Bridge spans are incredibly strong and durable. The surface concrete will wear out well before the bridge will show signs of fatigue.

jetmatt777 wrote:

Bridge spans are incredibly strong and durable. The surface concrete will wear out well before the bridge will show signs of fatigue.

Indeed. Look how some bridges in Europe, especially Italy, that are still in use today and are hundreds of years old.

This is probably more a question for tech ops, because there is actually specific guidance about runway loading limits and specs at every major airport. For example, here's the information about LGA's runway 04-22 from the chart supplement: 63 /F/B/W/T, S-80, D-170, ST-360.

Now, hopefully I don't screw any of this up because I technically haven't gotten this far in my actual flight training, but I've known the basics for a long time. Decoded, those specs mean Flexible pavement/B subgrade strength/High tire pressure category with no pressure limit/data based on a Technical evalulation of the runway, then the following are weight limits for total gross weight of different types of landing gear - 80,000 lbs. for single MLG, 170,000 lbs. for dual, 360,000 lbs. for single tandem. The "63" is the Pavement Classification Number - basically you have to cross-check this number against the Aircraft Classification Number in your airplane's POH under different configurations to determine if you can use the runway. The number for your aircraft can change depending on loading and other factors.

If you compare these runway specs to, say, JFK's runway 13R/31L, you can see the difference: 90 /F/B/W/T, D-210, ST-550, DT-1100.

Much stronger.

If you just browse around different airports on a site like SkyVector, you will see these specifications for every runway at every large (class B or C) airport. Class D and E airports still usually have a subset of the weight limits but usually don't (or probably never) have the PCN rating or other coded specs.

There is definitely no requirement or even suggestion that pilots land beyond the piers if a runway has them. If a runway is marked with a threshold at a certain spot, that runway is fully usable from that point forward. If the piers were not intended to be part of the runway, they would be marked that way. If a runway is a runway on top of piers, that means the piers were built to withstand takeoffs and landings to the published weight and load specs.

Dominion301 wrote:

jetmatt777 wrote:

Bridge spans are incredibly strong and durable. The surface concrete will wear out well before the bridge will show signs of fatigue.

Indeed. Look how some bridges in Europe, especially Italy, that are still in use today and are hundreds of years old.

Reminds me of this old commercial:
https://www.youtube.com/watch?v=aK70mATVGhs

a320fan wrote:

I don’t know the answer but look at Funchal, half the runway is built on pylons out over the sea. That takes runway bridges to a real extreme and they don’t seem to have had it collapse (yet).

That runway is built on pillars fully over land on a hillside, not over water.There is now a park underneath it.

Not sure if this belongs here. I remember LAX until the the late 80’s, aircraft of a certain weight could not depart on the south runways, due to the bridge. Landing was okay though. That is midfieldish, not approach end though.

SeaTac doesn’t have any runway made partially of a bridge does it? If so I never realized that.

Haneda’s runway D does IIRC, so that the Tamagawa river. can empty into Tokyo Bay without impediment.

Those same style of pylons are used in high-rise building construction and hold the wait of
enormous buildings above them. Have enough of them and thats gonna weigh a lot more
than any A320 ever will. Funchal used to have DC-10s and everything regularly serve it.

Civil engineers do know how to design things. Even back in the slide rule days there were runways over roadways. In Denver, Stapleton had a bridge on I-70 that every plane using the N-S runways or taxiing out to the north crossed the interstate. It is sets of moving loads for the 747, 777, 737, 380, 350, 330, 320, as well as all heavy military transports. Those loads will have fatigue values too, for NB's it is one per 5 min for 20 hours/day x 365 x 50 years in terms of cycles. The A380 loads will go into the fatigue categories at 20 per day over the same 50 years.

In the US, concrete structures are designed with a 1.4 Dead load factor and a 1.7 Live Load factor as well as a phi factor to address quality. Forces from cross winds, breaking, thermal etc. are reviewed with specified load combinations.

Pavements, either asphalt or concrete, tend to wear out. Say the concrete runway is a reinforced 18 inches thick. This runway would continue over the bridge and there would be an added 12" of gravel below it, then there would be like a 3 foot thick bridge deck that is heavily reinforced. This 3 feet of 'cover' on the bridge spreads out the wheel load by a factor of 10 onto the deck. It also allows for conduits, ductbanks, water lines, etc to be excavated or installed , and or runway repairs/replacement without getting into the bridge deck.

I've designed short 'bridges' for shipyard portal cranes on rails. I recall it was a series of 8 50 ton loads spaced 3 feet apart for all the wheel bogies, with a 2nd set of 8 for the next legs with a 20 foot gap. Depending on the part being checked, the moving load would be parked at the worst load case for review. The crane with its load was in the 1,500 ton range, about 1.5 747's.

Jet-lagged wrote:

SeaTac doesn’t have any runway made partially of a bridge does it? If so I never realized that.

Haneda’s runway D does IIRC, so that the Tamagawa river. can empty into Tokyo Bay without impediment.

Seatac 34R south end has 188th street below it. It is past the threshold so lots of rubber all over above the bridge.

https://www.google.com/maps/@47.4345833 ... 384!8i8192

Dominion301 wrote:

jetmatt777 wrote:

Bridge spans are incredibly strong and durable. The surface concrete will wear out well before the bridge will show signs of fatigue.

Indeed. Look how some bridges in Europe, especially Italy, that are still in use today and are hundreds of years old.

Italy may not be the best example... You probably want to go back prior to the existence of the Italian state. Since then, engineering hasn't been exactly amazing.

https://www.youtube.com/watch?v=Asts66_hrdI

XRadar98 wrote:

Not sure if this belongs here. I remember LAX until the the late 80’s, aircraft of a certain weight could not depart on the south runways, due to the bridge. Landing was okay though. That is midfieldish, not approach end though.

Yes, you never used to see 747s land or takeoff on the 25's until they strengthened the tunnel under Sepulveda Blvd, which goes under those two runways.

Funchal (FNC) bridge span is over 180 pylons. The aiming point of runway 23 is located in the bridge part of the runway and landings occur frequently there (although not as frequently as on runway 05).

Since year 2000, about 300'000 landings occurred on both runways, empirically I would say that 100'000 of those were on the bridge span.

The bridge was designed to sustain 400 tons impacts (based on the B747-400 at that time). Found an article from 2011 where the engineering laboratory said the conservative life of the concrete part of the runway was 75 years. I assume that is based on the frequent landings of the larger aircraft, not that frequent in FNC, so the real life should be quite longer than that.

Although the pylons are built over land, that piece of land was actually an enbankment at sea.

Quite an old video, but gives a very good perspective of how the bridge was built:

https://www.youtube.com/watch?v=ipzluQR3Wpw

horpdorp wrote:

Dominion301 wrote:

jetmatt777 wrote:

Bridge spans are incredibly strong and durable. The surface concrete will wear out well before the bridge will show signs of fatigue.

Indeed. Look how some bridges in Europe, especially Italy, that are still in use today and are hundreds of years old.

Italy may not be the best example... You probably want to go back prior to the existence of the Italian state. Since then, engineering hasn't been exactly amazing.

https://www.youtube.com/watch?v=Asts66_hrdI

That bridge was an unfortunate case, but in general Italy's post-war engineering has been incredible. The Autostrade are truly a wonder of the world.

LGA 4/22 also has part of the runway built overwater.

YYZLGA wrote:

horpdorp wrote:

Dominion301 wrote:

Indeed. Look how some bridges in Europe, especially Italy, that are still in use today and are hundreds of years old.

Italy may not be the best example... You probably want to go back prior to the existence of the Italian state. Since then, engineering hasn't been exactly amazing.

https://www.youtube.com/watch?v=Asts66_hrdI

That bridge was an unfortunate case, but in general Italy's post-war engineering has been incredible. The Autostrade are truly a wonder of the world.

When they were built maybe (and even that I take exception with when you compare it to the Autobahnen or the Eisenhower Interstates). They've been horribly maintained by a corrupt corporation since then and Genoa is only a small piece of the infrastructure problems Italy is going to be dealing with in the next decade or so.

But I'm happy to agree to disagree here, I may be a little jaded as a Southern Italian

horpdorp wrote:

YYZLGA wrote:

horpdorp wrote:

Italy may not be the best example... You probably want to go back prior to the existence of the Italian state. Since then, engineering hasn't been exactly amazing.

https://www.youtube.com/watch?v=Asts66_hrdI

That bridge was an unfortunate case, but in general Italy's post-war engineering has been incredible. The Autostrade are truly a wonder of the world.

When they were built maybe (and even that I take exception with when you compare it to the Autobahnen or the Eisenhower Interstates). They've been horribly maintained by a corrupt corporation since then and Genoa is only a small piece of the infrastructure problems Italy is going to be dealing with in the next decade or so.

But I'm happy to agree to disagree here, I may be a little jaded as a Southern Italian

Obviously Italy is facing serious deterioration, but you should see the bad state of many American Interstates. Minneapolis also had a deadly bridge collapse a few years back. The U.S. also doesn't have anywhere near as many amazing highways as Italy, where virtually the whole length is either on viaduct or in tunnel. The U.S. (other than the Big Dig in Boston), doesn't have a single expressway tunnel over 5km, while Italy has at least a half dozen, if not more. That's not to mention high-speed rail, which Italy has been far more successful at building (cheaper and more extensive) than Germany or the Netherlands, let alone the US. Italy has its issues for sure, but you should be proud of your country's incredible engineering achievements.

JayinKitsap wrote:

Civil engineers do know how to design things. Even back in the slide rule days there were runways over roadways. In Denver, Stapleton had a bridge on I-70 that every plane using the N-S runways or taxiing out to the north crossed the interstate. It is sets of moving loads for the 747, 777, 737, 380, 350, 330, 320, as well as all heavy military transports. Those loads will have fatigue values too, for NB's it is one per 5 min for 20 hours/day x 365 x 50 years in terms of cycles. The A380 loads will go into the fatigue categories at 20 per day over the same 50 years.

In the US, concrete structures are designed with a 1.4 Dead load factor and a 1.7 Live Load factor as well as a phi factor to address quality. Forces from cross winds, breaking, thermal etc. are reviewed with specified load combinations.

Pavements, either asphalt or concrete, tend to wear out. Say the concrete runway is a reinforced 18 inches thick. This runway would continue over the bridge and there would be an added 12" of gravel below it, then there would be like a 3 foot thick bridge deck that is heavily reinforced. This 3 feet of 'cover' on the bridge spreads out the wheel load by a factor of 10 onto the deck. It also allows for conduits, ductbanks, water lines, etc to be excavated or installed , and or runway repairs/replacement without getting into the bridge deck.

I've designed short 'bridges' for shipyard portal cranes on rails. I recall it was a series of 8 50 ton loads spaced 3 feet apart for all the wheel bogies, with a 2nd set of 8 for the next legs with a 20 foot gap. Depending on the part being checked, the moving load would be parked at the worst load case for review. The crane with its load was in the 1,500 ton range, about 1.5 747's.

Live loads, dead loads and allowed deflection. That’s most of what we do.

Deflection is often the limiting factor. Too much deflection causes bounce, decay and deterioration.

Routine Inspections of bridges are mandatory and part of the design documents.

PS: I remember designing those kind of cranes in school. One of the most important parts of the industrial revolution and most people don’t even think about them.

YYZLGA wrote:

horpdorp wrote:

YYZLGA wrote:

That bridge was an unfortunate case, but in general Italy's post-war engineering has been incredible. The Autostrade are truly a wonder of the world.

When they were built maybe (and even that I take exception with when you compare it to the Autobahnen or the Eisenhower Interstates). They've been horribly maintained by a corrupt corporation since then and Genoa is only a small piece of the infrastructure problems Italy is going to be dealing with in the next decade or so.

But I'm happy to agree to disagree here, I may be a little jaded as a Southern Italian

Obviously Italy is facing serious deterioration, but you should see the bad state of many American Interstates. Minneapolis also had a deadly bridge collapse a few years back. The U.S. also doesn't have anywhere near as many amazing highways as Italy, where virtually the whole length is either on viaduct or in tunnel. The U.S. (other than the Big Dig in Boston), doesn't have a single expressway tunnel over 5km, while Italy has at least a half dozen, if not more. That's not to mention high-speed rail, which Italy has been far more successful at building (cheaper and more extensive) than Germany or the Netherlands, let alone the US. Italy has its issues for sure, but you should be proud of your country's incredible engineering achievements.

Well I live in the US so I've seen a few highways here.

You're conveniently ignoring scale. I 90 alone is as long as 2/3rds of all of the Autostrade in Italy. The US system is way more impressive, even if they don't have as many bridges and tunnels, as if that's somehow a metric of how "amazing" a highway is.

As for high speed rail, Acela is again 2/3rds as long as the entire HSR network of Italy. The scale that's required to do things in the US is just so enormous. But also comparing Italy's HSR to Germany or Netherlands, countries that don't have all their cities in basically a straight line like Italy, feels a bit misleading.


American infrastructure is second to none, but I know how en vogue it is right now to talk about how horrible America is.

ikramerica wrote:

JayinKitsap wrote:

Civil engineers do know how to design things. Even back in the slide rule days there were runways over roadways. In Denver, Stapleton had a bridge on I-70 that every plane using the N-S runways or taxiing out to the north crossed the interstate. It is sets of moving loads for the 747, 777, 737, 380, 350, 330, 320, as well as all heavy military transports. Those loads will have fatigue values too, for NB's it is one per 5 min for 20 hours/day x 365 x 50 years in terms of cycles. The A380 loads will go into the fatigue categories at 20 per day over the same 50 years.

In the US, concrete structures are designed with a 1.4 Dead load factor and a 1.7 Live Load factor as well as a phi factor to address quality. Forces from cross winds, breaking, thermal etc. are reviewed with specified load combinations.

Pavements, either asphalt or concrete, tend to wear out. Say the concrete runway is a reinforced 18 inches thick. This runway would continue over the bridge and there would be an added 12" of gravel below it, then there would be like a 3 foot thick bridge deck that is heavily reinforced. This 3 feet of 'cover' on the bridge spreads out the wheel load by a factor of 10 onto the deck. It also allows for conduits, ductbanks, water lines, etc to be excavated or installed , and or runway repairs/replacement without getting into the bridge deck.

I've designed short 'bridges' for shipyard portal cranes on rails. I recall it was a series of 8 50 ton loads spaced 3 feet apart for all the wheel bogies, with a 2nd set of 8 for the next legs with a 20 foot gap. Depending on the part being checked, the moving load would be parked at the worst load case for review. The crane with its load was in the 1,500 ton range, about 1.5 747's.

Live loads, dead loads and allowed deflection. That’s most of what we do.

Deflection is often the limiting factor. Too much deflection causes bounce, decay and deterioration.

Routine Inspections of bridges are mandatory and part of the design documents.

PS: I remember designing those kind of cranes in school. One of the most important parts of the industrial revolution and most people don’t even think about them.

I'm BSCE 77 from U of Michigan in Structural, did 4 years designing, fabricating, and installing cranes and crane runways in Detroit. Got to drive a couple of the Zug Island Ore Bridges, seen lots of foundries, stamping plants, and steel mills. However, the Portal Cranes at PSNS dwarf all of that. In the following image the 250 Ton Hammerhead is inactive, used to lift Battleship turrets back then. But the 2 dozen portal cranes there keep things busy.

https://www.artanderson.com/wp-content/ ... ap-i14.jpg

The biggest risk is likely crack induced corrosion in marine conditions; the dynamic loading would tend to increase cracking, and water intrusion causes rust and degradation of the reinforcing steel. This too though can be planned for...

Sorry, but why runway or taxiway bridges shall be not durable. Frankfurt Airport A3, there are two bridges for two taxiways over the A3 highway, the high-speed ICE railway and a street. I have used the highway dozens of time.
https://www.youtube.com/watch?v=17EKx1KvbY8

https://de.wikipedia.org/wiki/Rollbahnbr%C3%BCcke_(Flughafen_Frankfurt_am_Main)#/media/Datei:RollbahnbrueckeWest.jpg

And regarding collapsing bridges; outside of construction and breaking down and as long as not a highspeed train crashes into it, with 250 km/h, they have not to collapse. Check them regular, maintain them accordingly and replace them, when they have to be replaced.

Runway 10-28 at ATL and parallel taxiway Sierra Golf both span I-285 on the southside. Driving under those bridges--especially the one for the runway--you can tell they are engineered totally differently than a typical bridge intended for auto/truck traffic of the same span. They are intense.

JayInKitsap raised an interesting point about near-continuous moderate loads (freight train traffic or a heavily used automobile bridge) as compared to less frequent but extremely high loads (aircraft touchdown). I was initially assuming the much greater impact of aircraft touchdown would be the engineering goal/limit that was targeted. But it's really many factors, and some like long-term wear from use can be a lot bigger than you might realize.

Found this:
https://en.wikipedia.org/wiki/Aircraft_bridge

Which led to this:
https://www.faa.gov/documentLibrary/med ... 00_13a.pdf

The Wiki article has a few references you can follow. I'm sure somewhere in the AC, or those references, there will be a link to the actual requirements for the strength of a runway bridge.

As already stated the bridge structure will outlast the bridge decking material.

fr8mech wrote:

Found this:
https://en.wikipedia.org/wiki/Aircraft_bridge

Which led to this:
https://www.faa.gov/documentLibrary/med ... 00_13a.pdf

The Wiki article has a few references you can follow. I'm sure somewhere in the AC, or those references, there will be a link to the actual requirements for the strength of a runway bridge.

As already stated the bridge structure will outlast the bridge decking material.

Indeed, while it doesn’t provide hard figures it does point engineers in the Right direction.

Design runway and taxiway bridges to support both static and dynamic loads imposed by the heaviest aircraft expected to use the structures, as well as any concentrated loads due to the main gear configurations. Airport operators should evaluate the future need to accommodate heavier aircraft when designing bridge structures. Overdesign is preferable to the cost and/or operational penalties of replacing or strengthening an under-designed structure at a later date. The use of a 20% - 25% increase in aircraft loading to account for fleet growth is not an unreasonable value to consider during design. Design Load considerations somewhat unique to airfield bridges can include runway load factors due to dynamic loading, longitudinal loads due to braking forces, and transverse loads caused by wind on large aircraft. Braking loads as high as 0.7G (for no-slip brakes) must be anticipated on bridge decks subject to direct wheel loads.

When airlines wanted to use their DC-10's and L1011's at LGA manufactures had to consider the airports over water runway extension specifications in final design.

Josh32121 wrote:

Runway 10-28 at ATL and parallel taxiway Sierra Golf both span I-285 on the southside. Driving under those bridges--especially the one for the runway--you can tell they are engineered totally differently than a typical bridge intended for auto/truck traffic of the same span. They are intense.

JayInKitsap raised an interesting point about near-continuous moderate loads (freight train traffic or a heavily used automobile bridge) as compared to less frequent but extremely high loads (aircraft touchdown). I was initially assuming the much greater impact of aircraft touchdown would be the engineering goal/limit that was targeted. But it's really many factors, and some like long-term wear from use can be a lot bigger than you might realize.

Josh-
Impact is a big issue. The US Bridge Code (AASHTO) applies an impact factor to the basic load. For trucks it is 30%, other types it varies from 10 to even 100%. How often the load is applied plays a big part of the design. A bridge that is OK for 10 20-Ton trucks per day would not be OK for 100 trucks, the allowable stresses are reduced based on the design cycles. In steel fatigue there are stress range limits based on cycles and condition type, the worst may only allow 6 ksi variation in a 50 ksi at yield steel, really limiting. But that only happens around a shaker table weldments. AASHTO lays out the basic loads, load combinations, material properties, and checks. It has standardized passenger, car, train, and truck loads and dimensions. In turn, this standard truck is basically the heaviest legal license weight. Wind and Earthquakes have a simplified design event, as we do not know what the real earthquake will be.

Corrosion was mentioned above, in seawater it is just brutal. At a large pier nearby steel 48" diameter x 1" pipe piles were driven. The piling was epoxy coated and the portion above the mudline was reinforced concrete, then it also had a cathodic protection system. All to get a 70 year design life. The pier being replaced barely made 60 years.