raised suspended bridge design

7/23/2019 Raised Suspended Bridge Design

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What is Tres Unidos und why we decided in favour of it?

Coincidentally we came across the American charity “Bridges to Prosperity”. In atechnical magazine for bridge design and engineering we read about the donation of acompetition’s award to this organisation.

The organisation helped us to identify Tres Unidos, a Peruvian village which has about2.000 inhabitants, as a community urgently in need of a bridge. Tres Unidos lies deep inthe Amazon region. A river divides the village from its farmlands and makes it difficult forthe people to reach their fields.

To ease the hard life of the village people and to ensure the children, working on thefields, can daily reach their school in time, we decided to help them building a simple butdurable suspension bridge.

Furthermore we wanted to see the money and our engagement going directly to the

villagers and not into bureaucracy of institutions.

The challenge of Tres Unidos

aims



philosophy

Our visions und dreams

The vision was to build a bridge by sharing our knowledge with the Peruvians. Wewanted to teach the villagers on how to erect a bridge by just using local tools andmaterials. They should make use of simple solutions, do almost without bolts and use

alternative ways of connecting elements instead.

Based on these facts the residents should understand the bridge as their own.With our assistance from time to time they should erect the bridge themselves.A big challenge!

Also, we wanted to hold the costs on a low level. All in all we faced the challenge tocreate a footbridge that can be rebuilt elsewhere – basic in design, out of localresources, cheap to erect but nonetheless durable and safe.

aims



time schedule

We had an ambitious time schedule, already taking into account, that two very differentmentalities will collide.

Jan. 06 1 engineer, 1 week on place for evaluation

May 06 1 engineer, 1 week purchasing material, teaching residents

May – Sep. 06 people of Tres Unidos excavation, concrete

Sep. 06 1 engineer, 1 week foundations, teaching, material

Sep. – Jan. 07 people of Tres Unidos preparation wood and site

Jan. 07 4 - 6 colleagues, 2 weeks erection with help of 50 villagers

schedule



highlights of the bridge

self-made non-electrical winch

special hangers an old bottle

a bottle with an importantfunction in connection withstability of the bridge

no bolts, no tools and

efficient functionality,easy replacement

our Swiss example to reach the

needed preload-forces

highlights



shoe saddle anchor rods

support and hinge, it was a

significant detail to createa bridge protectedagainst rotting

an easy solution to transmit

movements and forcesinto the pylons, acting aspendulum support

simple reinforcement barsact as an underground-connection between

foundation and cables

highlights

highlights of the bridge



materials and tools

directly fromTres Unidos

- sand

- water- welder

- bending tool

- blades for saw- paint for anchor rods

- wood for pylons and

bridge deck

from the vicinityof Tres Unidos(Tarapoto)

- reinforcement steel

25, 16, 10, 6 mm- cement- tools, welding equipment

- ropes

- plates and bars for fixationof the pylons

from thecapital Lima

- cable 460m, 19mm AA

(2 x 110m, 2 x 120m),- 56 clamps

We tried to support the locals by buying as much material as possible in Tres Unidos.The resources which we could not get in Tres Unidos were found in the surrounding

villages. Equipment we could not get in the vicinity of the village we bought in Lima.

local



bridge construction

bridge deck foundation pylons

1:18 4m sag

5 ,

5

3 ,

0

1 , 5

North

South

1 , 1

4 ,

3

16,3

4 , 2

3 , 0

15,2

72,3

3,5

16 ,2

13 ,6

3 , 0 10,3

9,1

1 2, 1

1 0, 0

design



bridge types we talked about

After certain evaluation we decided for the bridge with a sag, because of different reasons:

The valley is flat, the water level will not reach more than 3m. Nevertheless it is possible that extremelyrarely floating trees touch the deck.

We had to go for design details which allow an optimal combination of locally available conditions,durability to resist collisions, safety and the ability for repair works by the residents. Therefore we

chose a heavy bridge deck, four instead of only two load bearing cables and tension/pressurependulum pylons.

Further, a slim construction now avoids the use of “motocarros” and the crossing oftoo many people at once.

design



Bridges with two counteracting

cables are more stable and

resistant to wind.

However in our point of view thisdid not countervail thedisadvantages of the erection withtwo abutments on each side,

higher pylons and pre-stressedcables.

Moreover the building site in TresUnidos is not so exposed to

strong wind loads anyway.

design



wood – Ana Caspi

One of our first decisions was the type

of the wood we wanted to use for ourproject.

According to our philosophy we had toget informed about the typical wood

types available in this area and find outwhich one was the best for us.

Finally we opted for Ana Caspibecause of its long fibres, highbending resistance and high density

(0.9).

The decision was based on therecommendations of the residents:a good co-operation!

wood



anchorblock

We needed two kinds of foundations: one for the anchorblock and two for the pylons oneach side.

We found out CAD drawings are completely useless since the unskilled residents did notunderstand these very technical information.

After many explanations, convincing and with help of the wonderful sketches of Roland

Weber, one of our supporters, the villagers did the whole excavation themselves.

foundation



The problem was not to calculate the excavation but to find a way to handle thesedimensions by manpower. We developed the cavity step by step and safely for thepeople who did the digging.

Smart details like long bended ends of the anchor rods ensured that the villagers castthe cage with the right amount of concrete. We told them to pour concrete in until theends are covered. A concrete cover of 20cm (!) excuses even the biggest castingmistake!

long end of the anchor rod

foundation



Our Peruvian friend Damaso from Tarapoto whospoke perfectly English used checklists weprepared for the execution of all works. This waywe did not need to be on place all the time.

We bought a digital camera for him and he madeimages which he sent us from time to time.This way we were updated frequently.

checklist

foundation



placeholders

Another example for the simplicity we introduced:

The water filled bottles inside the reinforcement cagemade it easy to assemble and adjust the “shoes” for thepylons.

After the concrete dried out our Peruvian friends removedthe bottles and we could assemble and adjust the shoes

together within a week. To do this, an engineer fromour side was on place.

first sample shown by one

of our colleagues

foundation



During the run-up of the construction phase wediscussed all possible conditions the bridgecould be exposed to.

It was a prerequisite to use wood and as lessbolts as possible.Due to the slim bridge design and the vicinity to

the water we designed tension/pressurependulum pylons. Easy to build and durable.

Each H-shaped pylon is clamped by twoadditional wooden inclined logs on each sidewhich were connected to the foundation too.

pylons



foundation for the pylons

The pylon’s foundations are simple reinforced concrete blocksof 1.5 x 1.0 x 0,8 metres.

pylons



The so-called "shoe" is the

connective part between concretefoundation and wooden pylon. On ahorizontal plate which acts as abearing, four reinforcements rodswere welded inclined. These are

cast into the concrete foundation.On the upper side a vertical plateprotrudes into the notch of thepylon and is connected to it withthe another reinforcement rod.A distance of 15 cm to the soilprovides a good protectionotherwise sooner or later even AnaCaspi would start rotting.

pylons

shoes



saddles

The saddles form the upper end of the pylons. Here, a bent steel sheet was welded on a

flat steel plate. All cables are guided over them, but they are not supposed to move.We designed a commuting pylon to avoid tension forces in the wood.

pylons



anchor rods

To connect the anchor block and the cables under ground we used reinforcement bars,25mm.

In Picota, the neighbouring village about an hour away, a blacksmith could handle therods. He bended the ends on one side and forged eyes at the other.Finally they were welded in Tres Unidos and painted five times to avoid corrosion.

eyes

cables and winch



As aforementioned we deprecated the use of winches, heavy tools etc.A simple bar with a fork at one end inserted into the eyes was used to coil a rope to

tighten the main cables.

Not only there but also for erecting the pylons we used this technology.

winch

textile rope

anchor rod with an eye

steel cable

“winch” with a fork

cables and winch



“Swiss style” vs. “Peruvian style”

cables and winch



One of our most challengingproblems was the surface coating

of the anchor rods.

We did not expect that painting inthe understanding of the

residents has only a visualmeaning. We could not explain

them, that the main reason iscorrosion protection.

So, only under strict andpermanent supervision theycarried it out in an acceptable

way.

Amazing - here we reached alimit!

corrosion protection



requirements for the bridge deck

- little lateral buckling

- simple installation- easy production- high durability- no bolts- exchangeability- slip-proof surface

- heavy weight (approx. 8t)

bridge deck



The bridge deck was the last constructive element we had to plan for.It was the most difficult design part since we wanted to work with as less connectingelements as possible, still providing a stable construction but flexible in terms of motion.

The bridge deck foremost had to be a safe path to cross the river even when it rains. Sincethe bridge is long (free span altogether 74 meters) its deck had to be heavy (mass inertia)and non-slippery.

It's trapezoidal shape providesadditional resistance to movement.

bridge deck



boards

All boards are notched for a safe fit on the lower cable.

We did not use any other connectors than the hangers, all floorboards are pressedtogether by their own weight.The thickness of every second board differs by one centimetre from its neighbourto ensure a non-slippery surface.

bridge deck



The solution for connecting the upper and lower main cablewas discussed intensively. We performed tension tests in

Germany and came to the conclusion to be seen here.

The hangers are made of 6mm reinforcement.This will not get a design award but it is the cheapestalternative with material easy to get and very easy toreplace.In distances of about 1m, they are twisted around the upperand lower cable and around an extra 10mm reinforcementrod laying on top of the boards to avoid their lift-off . Thespace between is secured with fibre ropes.

hangers 6mm

10mm rod

hangers

bridge deck



Puente Machingero

Puente Machingero



transfer of knowledge and know-how

It was very important for us see that the resident of Tres Unidos understood the designand methods in order to reconstruct this type of bridge on a other place.

We tried to explain and show that just by using simple tools also for surveying, men canbuild things like a suspension bridge.

This is not a privilege of the “first” world!

demonstrationlaying out excavation

conclusion



wood Ana Caspicables, galvanized 19mm AA 460mtotal length 110mfree span 73m

sag 4,50mmax. pylons height 5,20mtotal weight 10twidth boardwalk 90cm

boards, 130 x15 x 7/8cm 660 pcs.pylons, 20 x 20cm 13 pcs.

concrete 16m³reinforcement 500kgmaterial costs/labour 9.800 EURtravel costs 12.200 EURerection time 1 year

people involved in Tres Unidos approx. 120people involved in Germany 17 (incl. donors)

tools: shovels, two hammers and chisels, one pincer, one wrench,a hand-driven drilling machine, a welding machine, chain saw, metal saw, truck, twopieces of a metal pipe, 8 bolts 60cm M12 and - not to forget - few meters of textile rope

from the local market

facts Puente Machingerofacts



Most important was to have a village and its residents who really wanted to build abridge without getting paid for it.

Same important was our English–spoken contact person Damaso Martinez.

Without him the project would be unimaginable too!

Thanks to all who worked with us and provided a huge support base!

Alpin Technik und Ingenieurservice GmbH

Plautstrasse 80 www.alpintechnik.deD-04179 Leipzig [email protected]

facts

raised suspended bridge design

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