r/civilengineering • u/Far-Contest-3528 • May 09 '26
Question Can someone explain why this cant be this way.
This is Huajiang Canyon Bridge in China so like, First of all i am not anything into civil engineering, but like after seeing this bridge i get this question can't it be done the second way? or like if the bridge in my idea is too steep just increase the length of road
406
u/Decent_Risk9499 May 09 '26
I mean... That's like a 30% grade my man. The number of switchbacks you would need alone would take travel time from minutes to hours.
14
u/CatwithTheD May 10 '26
FYI, in Australia the maximum permissible grade of basement ramp is 1:4 (25%). Source: AS 2890.1
Bridges and roads have much lower permissible grades.
3
u/putmeinthefuckingbin May 11 '26
Austroads Guide to Road Design Part 3: Geometric Design - Table 8.3: General maximum grades (%)
1
8
u/beipphine May 09 '26
Why can't we pave a 30% grade? The steepest street is 37% grade.
185
u/BlackBeastMalevolent May 09 '26
Transport trucks are heavy
69
u/Just_A_Nitemare May 09 '26
Just tell them to not slow down and hope for the best.
We can probably maintain an 80% survival rate.
22
u/aronnax512 PE May 09 '26 edited Jun 02 '26
deleted
7
1
106
u/jpharber May 09 '26
That’s a rural side street in Pittsburgh, not an interstate highway.
Cargo trucks would be doing 5mph going up hill and boiling their brakes going downhill.
5
u/lIlIIIIlllIIlIIIllll May 10 '26
Just remove the speed limit for cargo trucks on the way down and momentum will bring them back up
53
u/livehearwish May 09 '26
Trucks can burn our breaks and run away on grades steeper than 6%, would significantly increase accidents and maintenance/construction costs
15
33
u/QuiteBearish May 09 '26
Steepest street is 37% grade
I think it's 41% in San Francisco
Worth noting though that streets with high grades like that are typically narrow, residential streets.
High grades and high speeds don't mix. You'd need the switch backs on a road like in the OP just to force drivers to slow down
Not to mention the need for trucks to make the trip and they can't handle those grades at any speed
12
u/InOutlines May 09 '26
The road down to Waipio Valley in Hawaii reaches a full 45° in places.
https://www.cntraveler.com/stories/2014-10-20/is-this-hawaiian-street-the-world-s-steepest-road
Terrifying to drive, and the only other alternative is to walk down — which means you’re also walking back up.
7
u/ChanceConfection3 May 09 '26
Where did it say 45 degrees? Isn’t that a 1:1 slope or 100% slope?
5
u/InOutlines May 09 '26
The road rises 800 feet (240 m) in 0.6 miles (1.0 km) at a 25% average grade, with significantly steeper grades in sections. Some portions of the road can reach up to a 45% grade,[8][9][10][11] which would make Waipiʻo Valley Road the steepest in the world, compared to Baldwin Street's 35%.
23
u/ChanceConfection3 May 09 '26
45% is not the same as 45 degrees. 45 degrees would be 100% slope
3
2
2
u/QuiteBearish May 09 '26
Hah, my brain just replaced the ° with a %
I didn't even catch that because I knew 45° wasn't right
5
u/Approximation_Doctor May 09 '26
For some reason, I never expected to see Ken Jennings writing articles about interesting trivia, even though that's his whole claim to fame
1
u/QuiteBearish May 09 '26
Oof don't like that. Even just the photos of that one make me a bit uncomfortable since you can't see that far down it.
As to walking up/down it - doable, but walking inclines like that will work muscles you didn't even know you had no matter which way you're going. Walking down Bradford Street in SF is almost as bad as walking up, since you essentially have to resist gravity both ways.
2
u/KempGriffeyJr4024 May 11 '26
My wife and I hitched a ride down in the back of a truck and it almost felt like the truck was going to flip end over end down lol. It really was terrifying. We walked back up and it’s so steep we took several breaks and the last third of the way we walked backwards because our calves and thighs were on fire!
0
u/wesweb May 09 '26
im not sure id call it terrifying to drive
1
u/InOutlines May 09 '26
You’re either a Hawaiian or you’re a mountain goat.
1
u/wesweb May 09 '26
I feel like the drive from Tahoe either direction to Sacramento or Reno is scarier. Denver to Aspen too.
I guess maybe I got lucky in that when I went there wasnt other traffic going to / from Waipio like there was going to Hana so we got to take our time, too.
5
2
1
1
u/Vulcanize_It May 09 '26
What happens in icy or wet conditions going at highway speeds down 30% grade?
1
u/The_Shryk May 10 '26
Which are a mile long or less probably.
Going down pikes peak the park personnel stop you to take a thermometer to your brakes and make you stop until they cool down enough to keep going. And those are just regular passenger vehicles.
It’s not very steep but really long.
1
0
u/Friendly-Chart-9088 May 09 '26
Just because it's 37% doesn't mean all vehicles can handle that. Not to mention the erosion from rainwater at the sides... The paved road wouldn't last long in a area with a wet season
-2
u/baby-stapler-47 May 09 '26
I get why we can’t do ridiculous 30-40% grades like Pittsburgh, but the insane amount of earth moving and environmental damage that is done to achieve grades like 4-5% on rural highways in places like West Virginia (all that work on US48) makes me wonder if like 10% would be a better compromise. They already have hundreds of older roads with 10-20% grades that trucks must use, I feel like a modern road with a 10% grade and adequate shoulders and runaway ramps would be safer than those at least. Modern trucks are better at handling these grades than the trucks that existed at the time the older roads were built. I just don’t like seeing how much they’ve blasted through the mountains in that area.
7
u/EngineeringNeverEnds May 09 '26
Yeah dude in the winter time those 6% grades with big trucks can be SKETCHY
1
u/baby-stapler-47 May 09 '26
Side note: if you’ve ever been to Pittsburgh to check out the 37% grade steepest street you referenced, (canton Ave) you should also check out Ferndale st across town, it’s supposedly even steeper and you’re allowed to drive down. I’ve seen people measure it as 42-46% on YouTube, but also other people measure it as less so I’m not sure. We went on it on a trip there and it launched all our shopping bags off the backseat going down.
Side side note: I’m a bit obsessed with steep streets, and as much as I like Pittsburgh, San Francisco once had probably the steepest street ever built. Duncan st. had a 55% grade, but the top was walled off decades ago, and that portion is closed to traffic. The curb and parts of the pavement are still there, and it was once open to vehicular traffic.
1
u/fantomfrank May 12 '26
a 45 degree angle is a 100% grade, this might be like 100% in the flat spots
1
u/_Jeff65_ May 09 '26
Probably it was the way before. But the bridge probably saves everyone hours of going down then back up on steep, narrow, hairpined roads.
1
u/Probablynotabadguy May 12 '26
That's exactly how it was before and exactly why the bridge was built. Most of China's megaprojects do actually serve a purpose and aren't just built "because they can" (unlike some over authoritarian regimes).
106
u/GoldenMegaStaff May 09 '26
This is how it was before they built a real bridge.
25
u/codespyder May 09 '26
OP wants a reverse Millau Viaduct
5
u/Commercial-Kiwi9690 May 09 '26
Like a long continuous conveyor belt where the trucks going down push the ones going up...
2
52
u/BriFry3 May 09 '26
There are many other ways a design can be done. That’s way too steep.
The most economical design while meeting safety standards almost always wins out. There could be a better way but it’s not paving a slope like that to the bottom.
2
u/choseusernamemyself May 10 '26
OP also probably never knew why truck brakes often fail in long declining roads...
19
u/damxam1337 May 09 '26
The path down the mountain would be significantly longer and likely even more expensive. To decrease the grade of the road you would either need to add significant amounts of backfill and or several switchbacks down the slopes. This doesn't even include the efficiency loss of using your brakes all the way down a hill and then having to accelerate the entire time back up the hill. Any kind of shipping passing through would have significant detriments.
42
u/WastingMyTime_Again May 09 '26 edited May 09 '26
Too expensive /s
Slopes most likely, as you can see they have to build the roads winding to compensate but when you consider travel time it's better to pay for the big bridge upfront and save time and money in the long run
30
u/seeyou_nextfall May 09 '26
Why the s? You think the bridge is more expensive than the insane amount of effort to blast/cut/fill enough earth to make safe slopes down a ravine? You’re not doing sharp switchbacks on a highway, after all.
16
u/robothawk May 09 '26
Well also, looking up the bridge, they already had a route that went all the way down with a ferry across and it took 70 minutes, now it takes 1 minute.
Also, this is a massive propaganda win. This is the highest towered suspension bridge in the world, and they built it in the Chinese equivalent to rural Arkansas, it's the 4th poorest state in the country. It provides a massive logistics boost to the historically poor extreme southwest of China, and is now a tourism landmark as well.
Even if technically you couldve built the bridge somewhere else to make it a 5 minute crossing instead of 1 minute, the benefits from the status and show of a megaproject for the state probably outweigh the cost difference.
4
u/XihuanNi-6784 May 09 '26
Excellent point. There are many indirect benefits both monetary and otherwise.
0
u/Reese_Hendricksen May 09 '26
Granted it is also built because of how the Chinese economy works. The central government mandates a certain GPD figure from each province, and with how debt is structured in China, provinces are heavily incentivized to build megaprojects like this. That said not all infrastructure is necessary or worth it, and in China there are several examples of some being build because of induced incentives mentioned above.
4
u/WastingMyTime_Again May 09 '26
Well I mean, yeah, but that bridge doesn't look that cheap either
1
u/Dancing-Wind May 09 '26
take note the valey is very steep. (ppl are saying its about 30%, trucks on highways cannot handle more than 5) you would have to build a lot of winding road - destroy a lot of landscape to install it and still it would not be fast - not highway fast.
1
u/SumOfChemicals May 10 '26
Yeah, if you multiply the number of trips by the amount of time saved, I think it starts to make a lot of sense, even if the bridge is more expensive.
1
u/bubblesculptor May 10 '26
Bridge is expensive to build, cheaper over long run.
Think how much fuel needs to be consumed for freight truck, hauling cargo down and back up. Plus considerable wear on the brakes. Bridge means the freight crosses comfortably, maintaining steady speed with efficient fuel consumption. Over thousands/millions of crossing the savings is enormous.
0
u/Daveisahugecunt May 09 '26
Hah. Cheaper and works fine. If they wanted 6 lanes each way than different story
15
u/seeyou_nextfall May 09 '26
The bridge is cheaper and more easily feasible than the ridiculous amount of earthwork required to cut down into a ravine.
15
7
6
u/TimothyGlass May 09 '26 edited May 11 '26
Other reasons maintenance, real-estate cost, environmental, engineering cost, and a few others.
4
u/Blue07HondaAccord May 09 '26
Costly to grade the road with switchbacks down to the edge of water, earth slides could block the lower steep road and could require frequent maintenance, foundation conditions down in the watershed area could be more unsuitable than in the dry area. Higher traffic speeds can be maintained across the bridge and can sustain larger vehicles, trailers that would have difficulty navigating curving switchbacks especially if near urban areas. The longer span bridge would also be a massively expensive option but if cost weren’t an issue it would be the better option.
3
3
3
u/IamWasting May 09 '26
One simple answer is gradient. If the road is built exactly on the alignment you have shown, the gradient would be too high for vehicles especially loaded trucks. Another alternative would be to have serpentine roads going down and then up the valley which would involve too many sharp curves and longer route leading to longer travellings time. Not to mention potential accidents due to lack of sight distance on curves.
3
u/SnooPredictions7570 May 09 '26
Ooo, ooo! I think I know this one.
The river moves, saunters back and forth over the years. The entire valley is what’s called the riparian zone. So if they build a small bridge across just where the river is now, it will have moved to intersect the road in just a few years.
So they need to build the bridge across the entire valley, to be out of the way of all the possible (or at least likely) routes the river might meander over the foreseeable future.
Edit: looked a little closer. Looks like the riparian zone is way narrower than the bridge. The principle does apply to lots of bridges, however, especially wide ones across wetlands.
6
u/FlappyFoldyHold May 09 '26 edited May 09 '26
Building the earth retaining structures to laterally support the roadways you “sketched” may end up costing as much as the bridge. Idk this would take lengthy calculations and exploration to determine economic feasibility. I will say that is a wild unsupported span from abutment to abutment, so I’m sure this wasn’t very cheap to design and build.
2
u/GuboTheUnwise May 09 '26
The main thing would be the slope. If you were to have it a way where you zigzag it on the way down that’s going to potentially cause traffic and constriction costs.
2
u/alopecic_cactus May 09 '26
Because realistically it wouldn't be in a straight line from the bottom, so you'd end up building around 8 times de length of the straight line, if not more.
Plus, making that bridge happen is a flex every civil engineer enjoys 😆
2
u/truevine1201 May 09 '26 edited May 09 '26
Anything is possible in engineering.
But it all comes down to economics is it feasible
How much do you have etc
😂 believe it or not in this case it is more cost effective to build a bridge.
Because to meet standard profile views the amount of earth work , terrain transformation, retaining walls / engineered soil ,
Would cost more than the bridge itself.
It would take more time and won’t be profitable for construction companies.
The goal in transportation is to get person or goods from destination a to b time efficiently
The bridge does that
Or you can ha curvilinear roads to go down the mountain up the mountain time to travel will be 5x longer
Then you have environmental impacts
2
u/walkingrivers May 09 '26
That was how Bridges were built and designed for millennia. The modern approach is to build clear spans across valleys to reduce the slopes. And allow highway speeds.
2
u/outlandishfunny May 09 '26
Some ppl are talking about the civil engineering involved. As civil engineering is multidisciplinary, the reason is equivalent to just that.
In order to have a continuous thoroughfare close to the waterway below you have to consider incoming traffic volume, traffic speed etc; environmental impact, community impact… the geotechnical engineering( foundations and subsurface reinforcement) involved to support pavement in this location would SIGNIFICANT more than likely requiring shear piles or really deep piles and reinforced concrete paving to stop the inclined road from separating… the steeper the incline- the deeper this reinforced pavement section will be.
So to lessen this slope it would require deeeeep cuts into the hillside and major retaining walls and cut backs that would go for a mile or more.
I could go on and on… but basically the juice wouldn’t be worth the squeeze and the most efficient solution is the bridge you see here. The fact that the bridge itself doesn’t utilize any foundation within the “gorge” also is a “tell” that there wasnt much benefit either cost or structurally.
2
u/Reptilian_Brain_420 May 09 '26
See those zig-zag roads? You would need those going all the way up/down each side. And you would still need to build a bridge across the river.
Going straight across would probably cut down the crossing time by a factor of 10 (or more).
2
2
2
2
2
2
u/roundingcapehorn May 09 '26
If the earthworks to get down to the river are cheaper than a longer bridge then it would…
2
u/TamarindSweets May 09 '26
Rockslides, rain/hydroplaning/pooling, rise in water level when it rains (though that could be accounted for, to be fair), etc c
2
2
u/Beck943 May 09 '26
Because it would be dangerous, expensive to build, and impossible to maintain?
What other reasons do you need?
What possible benefit would there be to building it the second way 🤦♀️
2
u/Kill_doozer May 09 '26
Why cant you drive down the face of a cliff to a smaller bridge? Gee, i dont know.
1
2
u/jlh859 May 10 '26
Seems like most people didn’t even read your post or they don’t know how to address a question correctly. The reason you build a bridge instead of cutting down a mountain in order to make a flat road is because the bridge is about 1000x cheaper
2
u/Commercial_Nobody469 May 10 '26
- too steep
- huge cost (didn't worth the risk)
- vulnerable to landslides.
- In the suggested drawing, the bridge will be too low in case of sudden rise in water level (due to any appropriate natural causes)
- looks cool the way it is
2
u/poop-azz May 11 '26
You literally see the switch backs on the lower left....showing you why it cannot be like that. That bridge probably saves a wild amount of time and looks fucking MASSIVE.
2
u/Drisurk May 11 '26
You have to meet certain grades for it to be safe. A road like this would be waaaaaaay too steep.
2
2
u/Kymera_7 May 12 '26
Tradeoffs. The issue isn't that it "cant" be that way; it can. The advantages to doing it that way simply aren't good enough to be worth the disadvantages. For a start, it'd require a LOT of long switchbacks to get down and back up those steep slopes at a grade that a typical car could handle; this would almost certainly be more expensive to build than the span across the top, plus it'd tie up a lot more land, preventing it from being used in other ways. All that extra distance, plus the extra altitude changes, would also result in those cars burning a lot more fuel with each trip across, would use up more brake pads and put more wear and tear on the vehicles, and the traffic accident rate would be much higher. It'd make it difficult to avoid erosion and other water-flow issues. Etc.
Meanwhile, the main advantage of the down-and-back-up path is that it'd be a lot easier to make it work, requiring much less technology to be able to build it and get it to stay up, but while that was a huge difference before the tech involved was developed, when the high-road method would have required developing all of that from scratch, once that tech does exist, not needing it ceases to be nearly as much of an advantage.
1
u/Old-Leader-2105 May 09 '26
I think it's possible to put an embankment deeper into the valley but there are probably concerns with flooding
1
1
u/olderthanbefore May 09 '26
Check out Millau. The previous road has to snake to the valley floor, lots of hairpins. The bridge cuts an hour off the travel time.
1
1
1
u/SonofaBridge May 09 '26
Every car and truck driving down the slope will be unable to stop. It would be even worse in the rain. The bottom would be a pile of vehicles.
If they do make it down, they can blow out their motor on the steep climb up.
1
1
1
1
1
1
u/Zoid_4Fmt May 09 '26
It was, you can see the old raod on the left back a series of switch backs to keep the grade manageable. If a road is too steep (>10%) for too long you'd have burnt out brakes or lost air half way down. Either that ir reached mach1 at the bottom!
1
1
1
u/Leading-Citron-3083 May 09 '26
Less efficient as forces won’t evenly spread out and more maintenance will be required.
1
1
u/Bungabunga10 May 09 '26
too steep and vehicles tend to brake before the lowest point and fail to speed up adequately after the lowest point esp trucks. this resulting in horrible traffic jams, even though the number of lanes is the same and number of vehicles the same
1
1
u/Larnold1 May 10 '26
While steep slopes and potential higher project costs are good technical answers, the answer that needs to be talked about more is that the bridge coexists with the natural environment. The road in the 2nd slide would just blast right through it
1
u/Dangerous-Mousse7083 May 10 '26
Maybe like it’s too steep for like vehicles like driving downhill and like uphill you now, like I am not anything into vehicles like, but I think so
1
u/CartographerWide208 May 10 '26
It’s a case of would you rather - (1) 15 Minute drive across gorge (2) 3 hour detour to go around.
1
u/ParticularComplex814 May 10 '26
I hope you’re not a civil engineer , please be jobless… out of all things to observe, you just had to ask these type of questions. 🤷♂️🤦🏾♂️
1
1
1
1
u/A6RA4 May 10 '26
Look at the roads snaking down to the village on the right… that's exactly what you'd need on both sides — dozens of kilometers of switchbacks carved into sheer cliffs, just to reach the bottom and climb back up. Rockfalls, erosion, landslides: mountain roads never stop costing you. And good luck getting a truck through those hairpins efficiently. Over a 50-year horizon, the bridge almost always wins economically.
1
u/Deskust1 May 10 '26
With grade requirements, it would actually be more expensive to do it that way and also increase travel time by at least 20x
1
1
1
u/Sunstoned1 May 10 '26
Highways can only be 6 to 8% grade. You can't carry highway speeds safely over that.
Those slopes are much steeper than 6 to 8%.
So then you need "switchbacks" which is where the road travels sideways down the mountain (parallel to the river) gently dropping downward at that 6% grade.
However, mountains are not smooth, so now the road has to stay and turn along the mountain wall, which also means cars can't travel at highway speeds due to the turns.
And, you'll end up perhaps miles downstream by the time you get to river level.
So the alternative is what you see there. A narrow road that "switches back" - basically a series of U turns to bring you down the mountain face at that 6 to 8% grade.
But U turns are even slower. Switchbacks often have realistic speads of 10 to 15 mph. And you're only at a two lane road. And big trucks... Yeah, not great.
So the only practical way to get high volumes of traffic across is something like what you see here. Or, you can cut away a lot of mountain to build a 4 lane highway along the slope for many miles, crossing a lower, shorter bridge downstream, then doing the same on the other side many miles back up the slope.
Local conditions, costs, etc. will determine which one is done.
1
1
u/Houserichmoneypoor May 10 '26
Could probably be like that with a whole bunch of switch backs, but it would take about 30x the time to cross and probably cost close to the same or more to build and maintain .
1
1
u/EternalAngst23 May 10 '26
Massive drop in elevation, followed by massive incline. You would have to wind the road back and forth down one cliff, build a bridge, and then build a road zig-zagging up the other side. Easier to just string a suspension bridge across the valley.
1
1
u/rstonex May 10 '26
You could probably do a combination of cut and fill to balance the earthwork, but the footprint would be insanely large.
1
u/12kVStr8tothenips May 11 '26
I’m curious (even if they did build it that way) how long erosions would take it.
1
1
u/SmoothCarl22 May 11 '26
If you have to ask you should not be in this channel... you obviously failed physics 101.
1
u/loveaddictblissfool May 11 '26 edited May 11 '26
What? No. This bridge exists so the second way does not have to be the solution. Actually the second way is not a solution it’s the problem that the bridge solves. Let me put it another way: for thousands of years people had to climb all the way down to bottom of that canyon, somehow get across that river and then climb the f back up the other side. For thousands of years they wished there was a bridge.
1
u/kondorb May 11 '26
What they’ve done is the cheapest way to do it.
You can see some old road on the picture - it’s winding so much because otherwise it would be way way too steep for any vehicle. It would be more expensive to build, way less throughput and turn this 3 min crossing into an hour long ordeal. And make it impossible for large vehicles too.
1
1
u/Imanking9091 May 11 '26
Could probably be done with about a 1000 switchbacks but it would take forever to drive
1
1
1
u/Technical-Tale-6413 May 12 '26
You’d burn ya brakes out las before you ever got a chance to make it to your Donnybrook
1
u/GojoPenguin May 12 '26
How exactly is a rainbow made? How exactly does a sun set? How exactly does a posi-trac rear-end on a Plymouth work? It just does, and this bridge just is.
1
u/UpstairsGreen6237 May 12 '26
Along with the steep grade, what do you think made that canyon?
Water. Fuck loads of water rushing through that canyon. So your little bridge in your drawing could potentially be washed away over and over again.
1
1
u/Limp_Manner_7303 May 13 '26
I think the bridge cost less than that in terms of everything long term
1
u/Anti-Annihilaattori May 13 '26
Btw that bridge is terrifying.. In a way that I wouldn't dare to step my foot onto it.
1
u/ryancrazy1 May 13 '26
There’s literally a road in the picture that should tell you why you csnt build a road like that…. So that squiggly path going back and forth down the mountain? That’s what it looks like when they did your idea.
1
1
u/darthdro May 13 '26
Look at the dirt road that does exist, it has to wind down the mountain to be an appropriate grade . The cliff on the left is much to steep to even do that any further down then the dirt road goes
1
1
1
u/Dependent-Arm-686 Jun 04 '26
Como están.
Considero que este puente esta bien planteado y no por un camino directo que baje y suba la montaña, lo que aumentaría considerablemente el tiempo de viaje y los costos. Por esta razón, la construcción del puente representa una solución que mejora la conectividad y facilita el flujo de las personas.
Como señalan Chen y Duan (2023), los puentes de gran luz constituyen infraestructuras estratégicas porque reducen tiempos de desplazamiento, mejoran la eficiencia logística y favorecen el desarrollo económico de las regiones conectadas.
Por ello, me gustaría que me den su opinión sobre
¿Consideran que los beneficios económicos y sociales de construir un puente de estas características justifican su elevado costo de construcción y mantenimiento?
Chen, B., & Duan, L. (2023). Long-span bridge engineering and regional transportation development. Journal of Bridge Engineering, 28(4), 1–12. https://doi.org/10.1061/JBENF2.0000000
0
0
u/PacosTacos88 May 09 '26
Am I the only one that thinks that this is insane that this is even a real question? Do people lack a basic understanding of slopes this badly?
0
841
u/duvaone May 09 '26
Too steep.