Engineering Question

[PeteC]

A lot of engineers on here so thought I'd ask this rather than researching it. I caught a bit of a show about the Royal Canal. I assume the one in Ireland and not the Royal Military Canal in England. A guy has a fully restored motorized canal boat about 15 meters long and almost as wide as the canal itself. He's trying to navigate the entire length and talk about the history as he goes.

At one point the barge actually goes over a bridge about 30 meters in length, over a road beneath. The bridge is Roman arch style and looks in rather rundown condition. It looks very similar to a Roman aqueduct, and in fact is like one as water flows through it deep enough to allow vessels to pass through it.

To the question......as the vessel passes over this water filled bridge what is taking the weight of the boat, the water or the bridge itself? In other words is the displacement of the water in direct proportion to the weight of the boat....or is the bridge structure taking the weight....or both the water and the bridge structure?

Perhaps an easy answer/concept, but a head scratcher for me. Pete

[BaaBaa.]

I'd say both, more so the water.

They're all over England Pete, I've got 3 such Canal bridges in my town.

Nice little pub on the Canal lock up the road too.

[kendo]

Hi pete I would say both but an object ie a boat is 40% lighter in water than a dry weight.
Please Google an amazing engineering man and some what of a legend in my book
Isambard Kingdom Brunel.

Kendo

[Terry]

Pete - you're having a Eureka moment.................

To get the full explanation check out

http://en.wikipedia.org/wiki/Displacement_%28ship%29
and
http://en.wikipedia.org/wiki/Archimedes

which describes amongst other things 'Archimedes' Principle'

This principle states that a body immersed in a fluid experiences a buoyant force equal to the weight of the fluid it displaces.

The key word in your question is 'displacement' and an understanding of 'buoyancy' also helps. Both the above are good articles.

So - to answer your question.

As long as there is sufficient water in the canal to provide the buoyancy and allow the boat to float and displace it's total load then there is no difference to the load on the bridge, if there is a boat passing or not.
The effect on the bridge is the same - with or without the boat.
The displacement of the water IS in direct proportion to the weight of the boat.


Kendo

Isambard Kingdom Brunel - my all time engineering hero who's 53 short years left an amazing legacy of structures and engineering concepts still in use today
Check out http://en.wikipedia.org/wiki/Isambard_Kingdom_Brunel
There is also a great biography of this genius L. T. C. Rolt (1957; reprinted with new introduction 1989, Penguin). Isambard Kingdom Brunel. ISBN 0-582-10744-X, ISBN 0-14-007986-6, ISBN 0-14-011752-0..Rolt's highly acclaimed biography of Brunel is still the best and most complete

[PeteC]

Thanks everyone. That's what my gut was telling me Terry. In other words, back a few centuries ago when the bridge was designed, all he had to figure out was the cubic water capacity of the span over the bridge, then calculate how much that water would weigh....I imagine with about 10% + for safety. Pete

[Dannie Boy]

Thanks everyone. That's what my gut was telling me Terry. In other words, back a few centuries ago when the bridge was designed, all he had to figure out was the cubic water capacity of the span over the bridge, then calculate how much that water would weigh....I imagine with about 10% + for safety. Pete

Although with the recent rains back in the UK more like here in LOS, maybe 20% would be a safer margin

[STEVE G]

When I was young we used to go on holidays on canal boats in the UK and I remember going over the Pontcysyllte Aqueduct in Wales which is absolutely huge. It's a bizarre feeling being so high up in a small boat.

[Dannie Boy]

When I was young we used to go on holidays on canal boats in the UK and I remember going over the Pontcysyllte Aqueduct in Wales which is absolutely huge. It's a bizarre feeling being so high up in a small boat.

I only did a narrow boat once on the Kennet and Avon canal and thoroughly enjoyed it, but you'd need a head for heights for the one in your photo - oh to be so grateful for our Engineers!!

[PeteC]

I look at that photo and my first reaction is that the bridge is over engineered? Maybe an optical illusion and there is more water in that trough than it appears, but look at the massive supports and the quantity of them. Maybe needed due to the water sloshing around, and the momentum/sway that would build up if it was less rigidly supported. I wonder if they would build it the same way today? Pete

[BaaBaa.]

I look at that photo and my first reaction is that the bridge is over engineered? Maybe an optical illusion and there is more water in that trough than it appears, but look at the massive supports and the quantity of them. Maybe needed due to the water sloshing around, and the momentum/sway that would build up if it was less rigidly supported. I wonder if they would build it the same way today? Pete

What did the Romans ever do for us?

[PeteC]

^ They never made a pizza as good as the Neapolitans! Pete

[pharvey]

When I was young we used to go on holidays on canal boats in the UK and I remember going over the Pontcysyllte Aqueduct in Wales which is absolutely huge. It's a bizarre feeling being so high up in a small boat.

Which brings us to Thomas Telford - along with Brunel, a great engineering hero of mine.....

[Nereus]

When I was young we used to go on holidays on canal boats in the UK and I remember going over the Pontcysyllte Aqueduct in Wales which is absolutely huge. It's a bizarre feeling being so high up in a small boat.

Which brings us to Thomas Telford - along with Brunel, a great engineering hero of mine.....

And the engineer that built the above bridge.
The dimensions are given, so it is not difficult to calculate the weight of the water alone. There are a multitude of other considerations on the design of any structure, and the weight of the water alone is probably only a small percentage of the total static and dynamic load. The picture on Wiki gives a better perspective.

http://en.wikipedia.org/wiki/Pontcysyllte_Aqueduct

[Terry]

A lot of people do not realise the 'power' of water.

Remember - it's quite heavy
1 cu mtr of water weighs 1 tonne.

So with in excess of 10,000 cu mtrs of water at my place that's over 10,000 tonnes.

Just look at the damage that a Tsunami does. The Kinetic energy held by moving water is not something to be underestimated.

So - when you look at water retaining structures like canal bridges (Aqueducts) and most notably Dams, you can well understand that they are built to relatively massive proportions.

[PeteC]

I wonder how long it would take 20 of us to drink 1 cu mtr of beer. , and would we each gain 112 pounds in the process. Ow, forgot to calculate discharge frequency and volume. Pete