For millennia, China marched to a different drummer
from the rest of the worid.
Often reaching its own solution to a common problem
using chopsticks for eating
and simplified pictures for writing.
Bridge building is yet another example.
The Chinese invented suspension bridges
using iron chains 1400 years before the Europeans.
While the Romans needed multiple semicircular arches
to bridge wide rivers,
the Chinese met the challenge with a single shallow span.
It would take centuries for such a radical design
to be adopted in Europe.
But of all these ancient bridges,
the most amazing is the Rainbow Bridge.
Its graceful form is the focus of a famous painting,
as familiar an image in China as the Mona Lisa is in the west.
Its builders managed to create a design
that transformed straight beams into an elegantly curving arch.
Made from timber,
the last Rainbow Bridge has long since rotted away.
How it was built is now a complete mystery.
Inspired by a 900-year-old image,
one man, Tang Huan Cheng, spent years trying to figure it out.
In an attempt to finally solve the puzzle,
NOVA travels to China to help Professor Tang build his bridge.
But faced with a totally novel design,
the team's first problem is to understand
what sort of bridge they're building.
They're not designed like this.
They're designed to act purely in compression,
you are forcing these things in bending,
and that's what happens.
I don't like this at all, we're off-kilter here.
Until the experts can get inside the heads of the ancient builders,
all that's coming out of theirs is hot air.
Why the hell do we have abutments in there if this is a beam?
Because you don't need it!
Oh, come on! Why did we spend all this money doing this construction
if you don't need it?
In a land laced with waterways,
China's bridges were created out of necessity.
But they grew into a glorious union of engineering and art
with the power to still stir the emotions.
I have never seen anything like this before.
This is just totally gorgeous.
I don't know what to say.
I'm not worthy,
this is just too much.
Oh, my goodness.
This is in the presence of holiness.
Surrounded by mountains, oceans and deserts,
China's vast empire was separated
from the rest of the worid throughout much of her history.
The Italian merchant Marco Polo was the first foreigner
to leave a record of his experiences in China.
He arrived in 1275,
at the end of a dynamic era called the Song dynasty.
China's economy was booming
and the population was growing rapidly.
Marco Polo was flabbergasted by the size and wealth
of Song dynasty cities,
with populations approaching a million people or more.
This far exceeded anything he had experienced back home,
where Europe was just emerging from the Dark Ages.
Try to imagine life without printed books,
without gunpowder,
without compasses for navigation,
without paper money or even restaurants.
Yet life was like this before the Song dynasty in China
a thousand years ago.
These were their inventions and their great gifts to the worid.
China's fortunes have always been linked to its rivers,
the Yangtze and the Yellow,
which bisect the empire.
To connect these great rivers,
an elaborate system of canals was constructed from north to south.
Historian Robin Yates.
Waterways were the arteries of the Song empire,
and an endless stream of boats and barges
carried silks and tea and rice and salt and people
from village to town to city, all over the empire.
Although they improved the flow of goods,
all these waterways were a huge impediment to foot traffic.
To solve this problem,
the Song government needed to find a way
to quickly build hundreds of bridges.
To permit boat traffic to flow freely,
a single span was required.
Arches had always been constructed from stone,
which is durable, but expensive and time consuming.
Most buildings in China were made of timber,
so wood was clearly an alternative for mass producing bridges.
But wooden beams are straight,
and arches are curved.
Chinese carpenters had already solved the puzzle
of using timber beams to form elegantly curved roofs
and Professor Tang believes Song engineers possessed similar ingenuity.
The scroll of the famous Rainbow Bridge was painted
during the Song dynasty,
so perhaps this was when the problem of creating arched bridges
from straight timbers was also solved.
Professor Tang is both an engineer and a bridge historian.
He dismisses any thought
that the bridge was just an artist's fanciful idea.
The artist Zhang Zhe Duan was trained
in the imperial painting academy,
and this academy had the express purpose
of producing realistic portraits of life at that time.
So the Rainbow Bridge painting must represent a real bridge,
and I can use it to help me calculate its actual size.
By examining the number of people standing next to the railing,
I can estimate the length of the bridge.
And when I compared this number with historical records describing
the width of the river,
it confirmed my calculation
that the bridge had a span of about 60 feet.
After years of trying to figure out
how it was designed and constructed,
finally, with NOVA's help,
Professor Tang will have the opportunity
to bring the Rainbow Bridge back to life
in the town of Jinze, just outside of Shanghai.
Jinze already has a Song dynasty bridge
it's made of stone.
Professor Tang thinks perhaps a Rainbow Bridge once stood
in its place.
But like the hundreds of wooden bridges built by the Song engineers,
it was eventually replaced by arches of stone.
Halfway around the globe,
Professor Tang's Rainbow Bridge
is beginning to intrigue another bridge builder.
Bashar Altabba is a structural engineer
working on Boston's Big Dig.
NOVA has asked Bashar to help build the Rainbow Bridge in China.
I'd like to start today's lecture
with a very interesting course of events that happened -
In his course on bridge structures at MIT,
he now includes analysis of the Rainbow Bridge.
And I've got to tell you,
I've never seen anything like this before.
In fact, I'm not really sure whether it's a beam
or whether it's an arch.
Is it a beam structure or is it an arch?
I don't know of any similar type that exists like it in the west.
The problem of how to categorize
the Rainbow Bridge is very confusing.
And the odd thing, also, apparently there is no similar structures
like it in existence in China.
I've been in this business for 15 years,
and when I first saw the scroll,
I couldn't quite figure out what to make of it.
The good thing about the scroll is it gave us an underside
of the bridge,
so we could tell - we could reconstruct this model
and tell what we think this bridge looked like.
And at first look it seems like it's made out of straight pieces
woven over these cross girders,
and the question is: Does this behave like a beam or like an arch?
This is not just an academic question.
The foundations needed to support an arched bridge
are quite different from those of a beam bridge.
The job of a bridge is to transmit the weight of its load,
be it pedestrians or vehicles,
to the foundations on either bank.
Beams bend under tension,
and arches are compressed.
While compression is a more efficient method for transmitting a load,
arch bridges require very substantial foundations.
The puzzle facing Bashar is that the Rainbow Bridge
is made of beams,
but has the form of an arch,
so what kind of foundations will it need?
With Bashar about to leave for China,
a more pressing problem is coming up with a method
to erect the bridge.
Now, the little ones are centimeters and the big ones are inches.
Okay.
Also traveling to China is Marcus Brandt,
a timber framer from Pennsylvania.
Marcus is an expert in using traditional techniques
for building large wooden structures.
My feeling is, to get at least the skeleton perfectly set,
and then you can feed the pieces -
Marcus and Bashar are meeting today
in order to finalize a construction plan
that meets the requirements of the bridge site in China.
that will keep those two teams tied down to this girder.
Because what happens when this thing starts locking
This bridge will have to be built over a 40 foot wide canal
with heavy boat traffic,
and what we learned from trying to build this model together
is, it is not a simple operation.
You're going to have to interlace those beams right under
and over these girders,
and these beams are quite heavy,
they're about 200 pounds each,
and then have them just the right geometry
and just the right length.
You're going to have to get these things
to butt against each other
so that the two beams butt
right over the girder with very little seating,
and then while you're doing that,
trying to lash it
over a river with very heavy traffic.
It's a very, very difficult
and precarious operation.
You can see, the whole thing is very unstable at this stage.
That's okay, that's okay -
The solution is to preassemble the bridge in two halves
on the riverbank.
If I'm going to do this kind of connection,
you need to be on dry land.
And the way to do that is to prefabricate this into two halves,
one on either side of the river,
and then launch it into place
and then just do one set of connections over the water.
Leonardo da Vinci once said
that an arch consists of two weaknesses
which, leaning on each other, become a strength.
It's going to be very different.
I mean, here is a very comfortable environment.
Once you get there it's the real worid
and you never know what you're going to come across.
The town of Jinze is enjoying a major Buddhist festival
hosted by the local monastery.
The first member of the bridge building team
to arrive here is Robin Yates.
In ancient China, the Buddhists,
in order to build up good karma for the community,
they helped build bridges and to maintain them.
So here we have a ceremony,
particularly of all the old women
making offerings of all different sorts.
So this is a very - what the Chinese call "renao," lots of action.
The festival also celebrates the restoration of the oldest bridge
in the town.
Hey Robin!
Oh hi, Marcus, hi! Welcome to China.
You know Bashar.
Oh yes, Bashar.
Robin, nice to meet you again.
Let me introduce you to Professor Tang.
Professor Tang.
Marcus and Bashar arrive in Jinze
with the town in a party mood.
But Bashar is anxious to get started,
and he quickly puts together a model
to demonstrate the plan for deploying the bridge from the banks.
Then erecting them will be fairly straightforward,
by just basically elevating -
Chen Fu Xiang is in charge of the construction crew,
and neither he nor Professor Tang believes
there's enough room on the banks
to prefabricate the bridge in two halves.
This is disappointing news to Marcus and Bashar,
who want their role to be more than back seat driving.
Well, it's quite disappointing, actually.
When we came to the site,
we found everything is so tight in here.
The villagers build their houses almost all the way
to the edge of the water,
and the paths are so narrow leading to the bridge.
You just cannot bring any large pieces to the site.
So the whole idea of prefabricating in large pieces
I mean, it's just not going to work here.
And it's really been a disappointment,
because now it means we don't have an alternate method.
Robin is also disappointed with the site of the Rainbow Bridge,
because he thinks it has not been aligned
in accordance with the ancient Chinese principles of fengshui.
To check if his suspicions are correct,
Robin employs another Song dynasty invention.
Before the Chinese started any building project,
they had to decide what the most auspicious time to do it was,
and they also had to align the structure in relationship
to all the others round about it.
And they asked a fengshui master to come in
and make those determinations.
And he used an instrument like this, a luopan, a compass.
Here our river runs north south,
and our bridge runs east to west.
To the east of us, a Buddhist temple.
But something which is not very appropriate is,
just to the south of us, a lavatory.
And that is very unfortunate,
because lavatories are yin
and they should be to the north.
In addition to having bad fengshui,
Bashar is worried that the bridge may also have bad foundations
because the workers are not building them according to plan.
They're not building it according to this drawing.
No, he said in order to save time,
they're using two pieces, and that they'll fix it -
Oh, but that's no good,
see because if they're using two pieces here,
you're losing all your interlock -
Work on the foundation begins
when wooden pilings are driven into the subsoil.
Granite blocks are then placed on the pilings.
The abutments require several layers of stone,
each tied into the layer above
by vertical blocks to make one interlocking mass of masonry.
But in order to speed up work,
the stone masons simply laid the top layer on the one below,
omitting a critical step:
cutting a row of L-shaped blocks
to lock together the top two layers.
Without L-shaped stones locking the top layer to the masonry below,
Bashar worries that the bridge could do the splits.
This is worse than I thought.
The actual drawings had it right.
But to save time, they've actually cut -
I know exactly what they -
they wanted to do this as one piece.
This - it's supposed to be one piece,
and it was supposed to be locked completely in the whole abutment.
But to save time, they've actually cut it into two pieces,
so now the top part is completely floating independent.
This is going to just push right back.
There is no way. This has to be fixed.
He's saying we could cut this one here,
and then we could excavate a little bit -
No, no, wait, wait.
I'm assuming all these were done in place.
And that's why - I mean, it's crucial to have this L shape.
If you have a vertical piece, you are locking the horizontal -
Bashar is so agitated
because computer modeling of the Rainbow Bridge
performed just before he left Boston convinced him
that the structure has all the characteristics of an arch.
There is no question this is acting as an arch,
no question about it.
If this is correct,
strong abutments are needed
to counteract the outward push of the arch when it's loaded.
There is one simple trick, though.
If the abutments move, the arch will collapse.
I'm beginning to think about possible remedies here.
OK, what do you -
While Bashar frets over the problem of fixing the abutments,
Professor Tang has come up with a new idea
for erecting the bridge.
He'll assemble it piece by piece over the river.
Two boats will act as platforms for workers
connecting the beams over water.
Scaffolding on the boat -
On the boat.
Scaffolding and an ancient lifting device known as an A-frame,
or sheer legs, are put in position.
I'm not exactly sure what the procedure is here,
but we've got sheer legs set, ready to go up.
It's kind of precarious, and from my personal point of view,
I'm - when somebody yells get the hell out of the way,
I won't understand what they're saying.
That really worries me -
Marcus, an expert rigger,
is extremely concerned about the security of the stakes and rope
that will anchor the A-frames.
These ropes are loaded in such a weak way.
I really worry about that.
This is a really good way to get people killed.
The anchor that they're anchoring back to
is just a piece of angle iron,
using a frayed rope,
and they're not tying it all that well.
I'm really quite scared.
I've spoken with the powers
that be on the construction crew and they seem to be quite happy with it.
Once in position,
the A-frame will support the beams
cantilevering out from either bank.
The first arch of the Rainbow Bridge
consists of three sets of beams.
The horizontal beams in the middle lock the arch in place.
Interwoven with the blue three-sided arch
is the red four-sided arch.
Combined, the two arches should make a very strong bridge.
At least that's Professor Tang's theory.
As part of his plan to build the bridge one beam at a time,
Professor Tang has come up with an ingenious idea.
Like a tailor pinning his cloth before he sews the seam,
Professor Tang will use iron nails for holding the beams in place.
From a historical point of view,
it's quite clear
that the Song had the most advanced iron and steel technology
in the worid.
And so it certainly would have been possible
for them to have iron nails.
This is a clinch nail.
Instead of using a bolt with a nut on the other end of it,
you can see how this is split here.
Can you see how that's split? And one side will go -
Careful, because we're going to have to dig it through.
yeah - one side will go this way,
the other side will go that way.
It will get bent back
and clinched into the timber and that will hold it in place.
Everyone agrees that for safety,
the iron clinch nails are essential
to stabilize the emerging structure.
I think this is fine.
I don't see any problem with using these for erection purposes.
Yes, yes. That's all right.
Okay, so we agree? Okay!
But with the first beams in place,
Professor Tang points out a detail in the scroll
which appears to show that the timbers were also lashed together.
They're discussing now how to lash the various pieces together
and make it work using the materials they've got at hand.
A thousand years ago,
the lashing materials could have been hemp rope,
or something that's very widely used in China: bamboo.
It's tremendously strong. I mean, look at this
everything I can do, it's going to cut my hands if I pull on that
yeah, you're right. It will - it is exceeding -
This is wetted -
It's what? It's wetted, yeah, yeah.
When it is dry, more tight.
Right. Okay, so it pulls tight when she dries.
Well, that makes for a wonderful lashing material, then.
The problem with the strips of bamboo on hand
is that they are too short for making a good lashing.
Here, all we've got is actually three meters,
and the Chinese don't actually know how to link them together.
To get a longer rope.
So we're going to try and figure out
whether we can make a rope about 15 meters long altogether.
Right. It's awfully strong, but it's the length that's hurting us.
So we need at least -
In order to learn more about how to make bamboo rope,
Marcus and Robin travel to the Yellow Mountains, 200 miles away.
Bamboo is actually a grass,
and no other living plant grows as fast.
It can reach its full height in just 60 to 90 days.
And the Chinese use bamboo for everything,
from musical instruments to scaffolding to food.
The first step to making rope is to split the bamboo.
Next, it is halved.
Then halved again,
and again.
I think it would take me a month of Sundays
to get that down to a science.
In a neighboring village,
these strips are turned into samples of thick bamboo rope.
Joining Marcus and Bashar is Andrew Li,
an expert in Song dynasty building techniques
from The Chinese University of Hong Kong.
Well, obviously they -
They still do it this way.
The cable consists of two parts.
The core is made of strips
from the tougher outer part of the bamboo plant,
while the more pliable strips from the inner layers
are woven into a sheath which surrounds the core.
As Professor Tang remembers,
before being replaced by steel 25 years ago,
all the cables in the An Lan suspension bridge
were made from bamboo.
It's like plaiting my daughter's hair before she goes to bed.
More primitive suspension bridges also employed bamboo rope.
From the braided main cables hung a thinner variety of rope
for carrying the walkway.
And it is this small diameter rope
that interests Marcus and Bashar,
because it looks suitable for lashing together
the beams of the Rainbow Bridge.
It is made by spinning fine strips of bamboo
and then plying together two strands of it.
To test out the strength of this bamboo rope,
Marcus and Bashar will build a primitive suspension bridge.
Its design is based on a bridge Professor Tang saw
in the Himalayas as a young man.
Because the bridge must span a 60-foot wide river,
Bashar wants the thin ropes for the walkway
to be attached to the main suspension cables on dry land.
Well, it's Bashar's suggestion,
I think it's a really good one,
to lay out the whole bridge here on the gravel bar.
So what we're trying to do here,
instead of working over water and building the roadway,
we're going to lay it all out
and configure it on dry land
and then just hoist it in place.
In layman's terms, we're going to pre-fab it.
That's right.
Although prefabricating the bridge seemed like a good idea,
somewhere in the process of manhandling the two main cables,
the multitude of smaller connecting ropes
have gotten hopelessly entangled.
Fine, we can flip it again,
but we need to untangle the ropes first.
Okay, calm down, calm down.
In the confusion, Professor Tang keels over.
But fortunately nothing is damaged,
except Bashar's pride.
Please ask them to hold it in place.
I will solve this problem!
Oh wonderful, superman.
Will you please ask them to hold them -
Oh, God!
As tempers fray,
Bashar suspects that
some of his instructions are getting lost in translation.
Come with me -
You just told him not to move!
Could we have another - Miss Zhou, please
would you please ask them -
So he insists that Andrew be replaced by a local translator.
and just hold them, stay in place.
So I came back over in the boat and
just to see what the problem was,
and we have spaghetti junction going on over here.
It's just, we need to think about this,
just hold it - okay, this -
Oh boy.
Marcus, look, the problem starts from here.
This one is going across fine,
but this one is wrapping around all these and coming here.
The problem is wrongly rolled and wrapped, roll
if you, with same steps,
like this way you have no problem, you see?
Three hours later the faulty rolling and wrapping problem
is finally untangled.
But before work can proceed, Marcus calls for time out.
Let's work safe - we've had -
Professor Tang fell over, I fell over,
Bashar here has almost fallen over.
We're at the point now where we can get hurt real easily,
so let's put our thinking heads on.
With the suspension cables anchored at both ends,
Marcus uses a device called a Spanish windlass
to start raising them.
Why don't you let it go?
We're going to bring it up one more time.
All right.
If you look across the river, Bashar,
we're launching the bridge.
Wooden piers hold the main suspension cables high above the river,
enabling Marcus to get a sense of how strong bamboo rope can be.
I can't get over how strong this rope is.
It's only about a quarter of an inch in diameter.
It's taking a full load.
I would not trust just a regular Manila
or sisal rope that diameter with my full body weight.
Pretty amazing.
The suspension bridge managed to support 12 people,
convincing Marcus that bamboo rope was strong enough
for lashing together the heavy beams of the Rainbow Bridge.
This is where it gets really interesting.
This is where the lashing's tightened up.
We're going from the wraps here,
these are called fraps over here.
This is real Boy Scout stuff,
but I'll tell you,
it works really well.
But then an unexpected problem arises.
Okay, it seems that the boat is sinking,
it's filling up with water. Oh man!
A leak in one of the boats that supports the scaffolding
threatens to bring production to a halt.
They bale furiously,
and despite having to manhandle 200 pound beams
on top of a listing platform,
the builders push forward with erecting the arch.
This is a lot hairier than anything I like to see,
it's up there.
They're driving the nails through.
This one's really scary,
they need some big hammers here.
You'll notice I'm standing here where if something falls,
it falls around me and not on top of me.
The fireworks are to drive away evil spirits,
and the red color on the top beam and on the guy ropes
is for good fortune.
So this is absolutely essential
when you're building a bridge.
The firecrackers appear to be doing their job,
because Professor Tang and the bridge builders
successfully conclude the first phase of construction
erecting the skeleton of the three-sided arch.
For the first time you're seeing a real Song bridge being put up.
Yes.
To celebrate, the team does something
the Song builders might have done a millennium ago.
The Song actually invented the idea of the restaurant.
And the reason for this was that
they had a very vibrant urban and commercial culture,
and so going out to restaurants was one of the ways
in which they enjoyed themselves best of all.
Fortunately, the basic idea of a Chinese meal
is to have all different sorts of colors.
The most delectable parts go to the most honored guest,
which is Tang Laoshi.
Gan bei!
What was fueling the growth of lively restaurant-filled cities
during the Song dynasty?
Robin believes it had a lot to do with rice.
During the time of the Song empire,
there was a massive population explosion
and this caused two related revolutions:
a commercial one and an agricultural one.
In order to feed the population,
the government imported a new strain
of early ripening rice
and this allowed the farmers to grow two crops a year,
giving them a surplus which they could sell on the open market
in the cities.
Their customers used paper money which the government printed,
and they used that to tap the wealth
that these two revolutions were producing.
The thriving economy enabled the Song government
to embark on ambitious improvements
to the transportation system.
Bashar visits one of the best surviving examples:
the almost mile and a half long An Ping bridge.
It was built from massive granite beams
to permit travel across a tidal estuary in Fujian province.
The interesting part with this kind of construction,
which is a pier beam construction,
is that it's so flexible,
that in case of an earthquake
and you can see, this thing is so skewed,
probably from the action of earthquakes
moving those piers around,
but it does not necessarily cause a failure of the system
because of the flexibility built into it.
This is the longest bridge in China right now.
And this here is the longest beam of the longest bridge.
I've just measured it,
and it measures just about two and a half feet deep.
It's 35 foot long,
and two and half foot wide.
It probably weighs about 20 tons,
and you can really here see
how the Chinese pushed the limit of granite
to its maximum strength.
Any longer and this thing would collapse.
As in the piers of the bridge,
granite works best in compression,
so Song engineers were really pushing the envelope
when they used it in tension for the beams.
To demonstrate the risk of using stone beams in bridges,
Bashar compares how wood and stone behave
when put under tension.
While wooden beams bend when overloaded,
stone gives no warning when it's about to fail.
So overall it's an amazing accomplishment
for the time period it was done,
but structurally it's quite primitive.
It also limited river traffic.
In order to create longer spans
and allow larger boats to pass through,
Chinese engineers had to master the art
of a more sophisticated structure:
the arch.
The trouble with semicircular arch bridges is the hump,
which makes it awkward for all but pedestrians to cross.
As you can see,
the semicircular arch has a very high rise to span ratio.
Okay, that caused two problems.
One, you needed a lot of false work
to be able to create the arch form.
Secondly, the high rise meant
that you needed a very high approach
coming to it to level the roadway.
In 600 AD, Chinese engineers solved this problem
by flattening out the hump
and coming up with a radical new concept for an arch.
When constructed,
the Anji bridge had the longest span of any arch in the worid.
Oh, wow. It's gorgeous. I don't know what to say.
I'm not worthy.
This is just too much.
Oh my goodness!
And it would take another eight centuries
before such a daring design was attempted in the west.
At around 600, the Chinese made this incredible leap forward
in engineering arches.
What they did,
they sunk the arch down
and only used a segment of it.
A better word for it is a shallow circular arch.
Okay, the price you pay for that
of course, first of all,
the advantage is that you have less form work
and you have a lower rise,
so you don't have to build as much.
But the price you pay is that the lower the rise,
the higher the thrusts on the abutments.
In fact, it's a direct relationship.
As you halve your rise,
you double your force on the abutments,
so you have to have a larger abutment,
and clearly the Chinese were comfortable with that
because they built them this way.
To demonstrate the need for strong abutments
at either end of the arch,
Bashar and Marcus have set up an experiment.
All right, there it goes.
A car jack is placed between the arch and the abutment.
For demonstration purposes, we have a jack at one end.
Okay, we are demonstrating here this huge force,
the thrust we call it,
that is going to push on the abutment
and the effect that it's going to have on the arch
if you fail to resist it properly.
As the jack is released,
the powerful outward thrust produced by the two sides of the arch
is clearly demonstrated.
These marks slide past each other,
and then the stones start creeping -
Slowly,
it's flattening out,
and notice these joints now start opening.
One,
two,
three all right.
Despite the strong forces to self-destruct
generated by this type of arch,
the Anji is China's oldest standing bridge.
Back on the construction site,
the Rainbow Bridge is beginning to take shape.
In order to continue its metamorphosis from beam to arch,
the poles of the four-sided arch
must be interlaced with those of the three-sided arch.
So now they are about to do the next most difficult thing,
which is weaving the four-sided red arch
into the three-sided blue arch,
as I'm about to do now.
It's going to have go right over,
under, and then over,
and it's a little tricky.
No easy trick.
There it goes
and then the next piece on the other side.
Tom Peters is head of the department of architecture
at The Chinese University of Hong Kong,
and an old friend of Professor Tang.
He's never seen this kind of construction before.
This is a really special kind of construction.
Every construction in the worid,
every tradition in the worid,
layers construction,
one thing on the other thing.
You start with the foundations,
you put something on top,
whether it's stone or brick or wood,
and then you build up from there,
layer by layer,
as you go higher and higher
until you get to the top.
And this is a completely different kind of construction.
This is interwoven,
and the reason it's interwoven is that once it's together,
it will jam tight
and be a much more efficient type of structure.
But you can see from the problems they're having,
what the problems of interweaving are.
They're a real problem to get things to fit together.
Yeah, I mean, look at the size of the gap
that you have between the cross piece and the piece
they've just inserted now.
These two pieces should be touching.
One should be sitting on the other.
We've got so many pieces
we have to weave and get exactly together
at the right time and the right place,
and very little tolerance.
And if it's going to work as a bridge,
they all have to kiss at the same time.
Professor Tang's technique to get the beams kissing
is to force them together with a steel bolt.
A modern shortcut,
it upsets Bashar,
because he feels that bending the beam in that way
could weaken the wood.
And what they've done,
they actually went in and drilled bolts through
and forced this gap close,
like so.
And this is just exactly what you don't want to do
with this kind of structure.
Now, to my mind,
I would rather have seen it wedged, and just -
and not worry about it.
Of course, of course.
But -
Marcus, with his hands-on knowledge of timber framing,
has always argued that some of the beams in the arch
still behave like beams:
that is, they bend.
Marcus, the problem with these things
is that they're not designed like this.
They are designed to act purely in compression.
You are forcing these things in bending,
and that's what happens.
There is - but -
You don't have -
The material itself has certain tolerances.
First off, this is a piece of green wood.
It's going to take that shape,
if you crank it up, in a few years.
Once you start interweaving a construction,
where you have all sorts of forces doing all sorts of things
that you don't expect,
you don't really know how to understand these things
and deal with them.
We've got so much redundancy in there,
perfectly done,
two of these things could take the whole load.
Redundancy is there for a different purpose.
Redundancy is there in case one member fails accidentally.
Redundancy is there to provide factor of safety.
You're not supposed to be using that up during construction.
Who says you're not supposed to? This is a -
You're not supposed to, this is -
This is a very traditional kind of structure.
In the days when these structures were conceived,
they didn't know about arches behaving only in compression,
beams behaving only in bending.
In fact, these things behave in many different ways together.
There is only one way that this system behaves, it's a pure arch!
And that's Bashar's way.
It's a pure arch.
For example, if you take this member here,
let's get it to touch,
and you load this,
you are getting compression down this member,
you're also getting a little bit of bending in this member.
You get a little bit of bending in a pure arch from a point load!
Ah, ah, that's exactly what I said.
Ah, bingo.
What do you mean, that's exactly - this is ridiculous!
Oh, please.
As always, Professor Tang has a surprise up his sleeve.
He takes Robin deep in the mountains
southwest of Shanghai to see a bridge.
From a distance, it bears no resemblance
to the Rainbow Arch in the scroll.
But from underneath,
it has an uncanny similarity to the structure
that is taking shape in Jinze.
The slanting beams -
This is the improvement,
just for the - Qingming Shang He Tu Rainbow Bridge.
Because -
Professor Tang tells Robin
that although many details of the design are different,
he's convinced that this isolated bridge is a later,
improved version of the Rainbow Arch
portrayed in the Song dynasty painting.
In fact, up until the 20th century,
Chinese architecture changed very little.
This is because the Song government produced a building code
that standardized the design of all official buildings
a subject of special interest to Andrew Li.
They produced this book called the Yinzao fashi.
It describes a system, a system of parts, building parts,
and rules for putting them together.
And this system works a little like the way you buy a car.
You decide on the model of car that you want,
but you can decide whether it has two doors or four doors,
or an automatic or manual transmission.
Well, in this system,
it works pretty much the same way with buildings.
You can order a temple hall,
you could order a store room,
you could order a park pavilion,
and it all came from the same system.
A system that also solved the problem of creating curved structures
from straight beams.
While a triangular truss dictates
the straight pitch of roofs in the west,
Chinese roofs are curved
because of the arrangement of columns and beams
which support it.
Chinese carpenters could produce a roof of any size or curvature
simply by adjusting the lengths and widths
of these posts and crossbeams.
So once again you see that
the Chinese have found a completely different kind
of solution to a basically similar problem
that we had in the west: how to cover a space.
And if you ask me,
this is really much the more interesting solution.
Back in Jinze,
the rainbow curve is beginning to emerge
from interweaving the beams of the three- and four-sided arches.
Well, that's the last of the pieces for the four-sided arch.
Looks like it's going to fit pretty well, too.
Next the decking will be nailed to the curved timbers
that reduce the steep incline at either end of the arch.
The approach ramps create what is called a reverse curve,
an architectural form that is particularly pleasing
to Professor Tang.
Why is the esthetic of the reverse curve
considered to be the most beautiful?
Because it's found on the human body,
especially the body of a woman.
So I consider it the most beautiful form for a bridge.
Before the bridge is open to the public,
Professor Tang organizes a demonstration
to show his confidence in the strength of the design.
The way this bridge was designed,
it was never really checked for a point load,
a concentrated load at one spot.
So what we are doing now is
we're bringing two kind of large-sized bulls here
and we are going to have them walk from both sides all the way
to the center.
No problem.
Oh, this is definitely an arch.
There is no question this is acting as an arch,
no question about it.
Four tons of water buffalo have no perceptible effect,
and the bridge is declared ready for the masses.
This is - probably for me,
the most interesting part about this bridge
is that it actually doesn't exist anymore.
So recreating it is almost like finding an old fossilized DNA
and trying to regenerate a new dinosaur out of it.
Well, this bridge has given me a tremendous respect
for the Song dynasty builders
that developed this kind of bridge,
it's a very strong bridge.
And I've also learned tremendous respect
for my Chinese colleagues, I mean,
look at the joinery in this hand rail,
absolutely exquisite work.
The esthetics part is particularly interesting
when you compare them to the masonry bridges.
This is so much lighter,
so much more slender and much more elegant.
And when you typically combine those together,
you end up with the structural arts.
You can't get any better than this.
Bridges have always helped bring people together.
You can see it today.
People from both sides of the river
are now more closely connected.
But the Rainbow Arch is also a bridge across time,
from the present to the past.
By bringing this arch back to life,
after so many hundreds of years,
and by using ancient methods to do it,
I feel much closer to the Song bridge builder
who first came up with this elegant design.
China's Song dynasty gave a lot more to the worid
than the Rainbow Bridge.
Find out about other Song inventions,
from printing to porcelain,
the compass to gunpowder.
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Frightening power,
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Brave a close encounter.
"Crocodile."