One for all you Civil engineers

[Aftermarth]

ok, I am doing a project called EWB (Engineers without Borders) and our group of 5 consists of the following:
2 Mech Eng students
1 Flexi first year engg student
1 biomed engineering student and
1 aeronautical engineering student

(if u want to see the EWB website its here: www.ewb.org.au/ewbchallenge/[/URL])

we have to design projects that will benefit the people of cambodia, and our team decided a low cost bridge would be a good thing to do.
and we are hitting major problems.

we decided to use a pontoon bridge so the cambodians can simply build as many sections as they want and span the rivers, and we plan to anchor each end to the shore by concreting some poles. Now, the lecturer has asked us what we plan to do when the floods come in.

we came up with a few ideas...
1. pull the bridge in and wait until the worse of the flood is gone then respan the river
2. place large wooden poles in the river on either side of the bridge at various intervals to stop it floating away
3. same as #2, but this time the poles are in the centre of the bridge
4. Fix the pontoons well enough so that they won't go anywhere

any other ideas or ways we can implement any of the four above mentioned ideas?

 

[turbo]

One thing might be to implement a shallow gradient on the upstream sides of the pontoons so that faster current would raise the pontoons instead of allowing them to be pushed downstream. Another idea might be to mold in a downward curve into the bottom surface of the downstream pontoon's bottoms so that the pontoons might be lifted a bit and tilted into the flow in high flows, making the angle of the upstream pontoons even more shallow and increasing their lift. Play with this a bit, and consider making the upstream/downstream pontoon molds asymmetrical to maximize the effect If you are going to use longitudinal pairs of pontoons on each bridge section. I assume that you will want to use pontoons aligned in the direction of the river flow, however, since these will result on the lowest lateral stress on the bridges.

Edit: Keep the vertical profile of any such bridge as low and as permeable as possible, since strong winds can play hell with these flexible bridges.

 

[Aftermarth]

ok we will look into it
but the only problem seems to be the materials
we are only using wood, concrete, bamboo, rope and small amounts of metal (no more than you would find lying around in any third world country)

its 'appropriate technology' we have been told

 

[turbo]

Disclaimer: I am NOT a civil engineer, but studied general engineering, and have spend a lot of days running white-water in kayaks and canoes in class 4-5 water. Water (even at low velocities in rather flat deltas) is powerful and engineering stuff to withstand its vagaries is not trivial.

 

[Aftermarth]

thats ok you don't have to be a civil engg. none of us are either remember?

thanks for ur ideas thou :)

 

[Nick M]

I'm also not a Civil Engineer, but a sophomore in Engineering.

Since the pontoons float, you could attach them to the anchored shafts on the shore with a sliding bearing. When the waters rise, the pontoons will ride up the poles.

 

[Mech_Engineer]

Since the pontoons float, you could attach them to the anchored shafts on the shore with a sliding bearing. When the waters rise, the pontoons will ride up the poles.

This is what I thought of too, just have anchored poles on the shore at the ends of the bridge that the attachment points can slide up and down on. That way, the bridge will stay on top of the water, even if the water level rises.

 

[Nick M]

With strong enough anchors, you could design platforms that were mounted (with teh same bearings) at three/four points and have end platforms. That way when the ground is underwater, you would have a couple stable platforms that people could dock at - with a bridge that spans the gap.

EDIT:

This would really come down to the current - the force applied to these pontoons by the current would be magnified by the length of the poles to the anchoring point, and you would need to be careful that the torque wouldn't be too much for the poles.

 

[Aftermarth]

ok, so we anchor the bridge to the shore with the sliding bearings as you guys suggest...

a few things
1. what materials do we need and how much in $US do they cost?

2. would someone need professional skills to repair them, as remember the bridge is for cambodia, and once it is built they are going to be left on their own to maintain the thing. We have looked into educational programs but we can only teach them so much before it becomes really technical so they won't understand. We are trying to go by the concept that 'lets engineer it so that even normal (non engineers) can look after it without assistance'. All about appropriate technology again.

3. Is this design suited to being in water for most of the year? i believe Cambodia has a wet season for around 3-4 months (correct me if i am wrong) and it floods quite often during this time, so whatever we use to support the bridge must be waterproof

4. the problem we have is not so much to do with floating the bridge, but more so to do with SECURING the bridge so that it doesn't float away

 

[faux]

I wouldn't trust the poontoon solution for this. Rivers are quite dynamic and for areas with abundant rain, rivers can be dangerous. I would rather go for a hanging bridge made of indigenous hemp rope as the suspensions with planks as the treads. The suspension system will be tied to timber poles acting as pylons sufficiently anchored to the ground. To stabilise the poles, utilise wire ropes pre-tensioned and anchored to concrete blocks (dead man) that are buried.

 

[Pyrrhus]

Hey there, a civil eng here. As we know pontoon bridges are never permanent structures unless the river has a reasonable stable water level, reasonable lack of strong currents and/or extreme depth, ... The hydrological conditions of the zone are a key factor needed to be taken in consideration like How frequently and by how much in average the water rises? does the river always gets flooded?, are the currents always strong?, how much time usually the conditions are suitable for the bridge?. Also other factors like is this bridge critical, does it connect two towns? helps to transport large amounts of goods?, what are the reasons for building this bridge?...

In my opinion if we have to build a pontoon bridge, i will go for the first option depending on the hydrological study, if not i would consider other bridges types depending on the projected traffic and main use of the bridge.

Also, maybe you should provide dimensions and some hydrological data, so we can asess the situation better...

 

[turbo]

Here's something else to consider, since the OP mentioned withdrawing the bridge during floods. While some floods are predictable (rainy season, moonsoon) some are decidedly not. Runoff from heavy rains in the hills and mountains far from the bridge location can result in some unexpected flooding, and withdrawing the bridge might not be feasible or safe under such circumstances.

 

[Aftermarth]

Ok from our data flood waters rise up to 6 meters almost every wet season. Banks have a shallow slope so there isn't much to worry for major drops in banks.
also, flood waters do flow, but how fast? Well again - it is Cambodia, and it is awfully hard to find anything concrete on data of their flood waters. So we have taken the assumption yes it flows, but no, not at fast rates.

and turbo? u don't think withdrawing the bridge will help?

 

[faux]

^^assuming against conservatism is a no-no in engineering. it's better to err in the safeside (meaning designed the structure with a factor of safety or sometimes called factor of 'ignorance' ), than to be daring with the design when you don't have much data to support with.

Withdrawing a bridge is a logistical nightmare since this is a semi-permanent structure and when this is located in a quite isolated location, nobody would even dare to go and withdraw it come rainy season due to dangers from wild animals such as snakes, besides the typical debris that is being carried by the river itself which accumulates on the poontoons and will eventually destroy it. I suggest a permanent structure such as a hanging bridge since this is quite 'easy' to construct and does not require much of an 'engineering'.

Btw, I'm a practicing civil eng with varying experience from tall bldgs to bridges, to wind engineering to earthquake engineering.

 

[Aftermarth]

ok thank you faux. much appreciated.
our design was only a suggestion which we decided on because we liked the idea of being ontop of the water. Unfortunately, we are too far beyond the point of changing our design now, so we will have to design around what we have.

maybe the fact none of us are civil engineers hasnt helped haha :)