Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
Transcripts

"Secrets of Lost Empires: China Bridge"

PBS Airdate: February 29, 2000
Go to the companion Web site

Sponsor: During the following program, look for NOVA's Web markers which lead you to more information at our Website.

NARRATOR: For millennia, China marched to a different drummer from the rest of the world. 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.

BASHAR ALTABBA: 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.

MARCUS BRANDT: I don't like this at all, we're off-kilter here.

NARRATOR: Until the experts can get inside the heads of the ancient builders, all that's coming out of theirs is hot air.

BASHAR ALTABBA: Why the hell do we have abutments in there if this is a beam?

MARCUS BRANDT: Because you don't need it!

BASHAR ALTABBA: Oh, come on! Why did we spend all this money doing this construction if you don't need it?

NARRATOR: 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.

BASHAR ALTABBA: 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.

Sponsor: Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.

Sponsor: CNET.com, helping you choose the right technology product.

Sponsor: This program is funded in part by Northwestern Mutual Life, which has been protecting families and businesses for generations. Have you heard from The Quiet Company? Northwestern Mutual Life.

Sponsor: Additional funding for this program is also provided by the David H. Koch Charitable Foundation. And by the Corporation for Public Broadcasting, and by contributions to your PBS station from viewers like you. Thank you.

NARRATOR: Surrounded by mountains, oceans and deserts, China's vast empire was separated from the rest of the world 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 in1275, 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.

ROBIN YATES: 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 world.

NARRATOR: 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.

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.

NARRATOR: 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.

PROFESSOR TANG [voice over translation]: 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.

NARRATOR: 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.

BASHAR ALTABBA: I'd like to start today's lecture with a very interesting course of events that happened -

NARRATOR: In his course on bridge structures at MIT, he now includes analysis of the Rainbow Bridge.

BASHAR ALTABBA: 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. I don't know of any similar type that exists like it in the west. And the odd thing, also, apparently there is no similar structures like it in existence in China.

NARRATOR: Is it a beam structure or is it an arch? The problem of how to categorize the Rainbow Bridge is very confusing.

BASHAR ALTABBA: 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?

NARRATOR: 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.

MARCUS BRANDT: Now, the little ones are centimeters and the big ones are inches.

BASHAR ALTABBA: Okay.

NARRATOR: 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.

MARCUS BRANDT: My feeling is, to get at least the skeleton perfectly set, and then you can feed the pieces -

NARRATOR: Marcus and Bashar are meeting today in order to finalize a construction plan that meets the requirements of the bridge site in China.

BASHAR ALTABBA: - 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 -

NARRATOR: The solution is to preassemble the bridge in two halves on the riverbank.

BASHAR ALTABBA: 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.

NARRATOR: Leonardo da Vinci once said that an arch consists of two weaknesses which, leaning on each other, become a strength.

BASHAR ALTABBA: It's going to be very different. I mean, here is a very comfortable environment. Once you get there it's the real world and you never know what you're going to come across.

NARRATOR: 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.

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.

NARRATOR: The festival also celebrates the restoration of the oldest bridge in the town.

MARCUS BRANDT: Hey Robin!

ROBIN YATES: Oh hi, Marcus, hi! Welcome to China.

MARCUS BRANDT: You know Bashar.

ROBIN YATES: Oh yes, Bashar.

BASHAR ALTABBA: Robin, nice to meet you again.

ROBIN YATES: Let me introduce you to Professor Tang.

BASHAR ALTABBA: Professor Tang.

NARRATOR: 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.

BASHAR ALTABBA: Then erecting them will be fairly straightforward, by just basically elevating -

NARRATOR: 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.

BASHAR ALTABBA: 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.

NARRATOR: 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.

ROBIN YATES: 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.

NARRATOR: 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.

BASHAR ALTABBA: They're not building it according to this drawing.

ANDREW LI: No, he said in order to save time, they're using two pieces, and that they'll fix it -

BASHAR ALTABBA: Oh, but that's no good, see because if they're using two pieces here, you're losing all your interlock -

NARRATOR: 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.

BASHAR ALTABBA: 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.

ANDREW LI: He's saying we could cut this one here, and then we could excavate a little bit -

BASHAR ALTABBA: 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 -

NARRATOR: 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.

BASHAR ALTABBA: There is no question this is acting as an arch, no question about it.

NARRATOR: If this is correct, strong abutments are needed to counteract the outward push of the arch when it's loaded.

BASHAR ALTABBA: 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 -

NARRATOR: 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.

ROBIN YATES: Scaffolding on the boat -

____: On the boat.

NARRATOR: Scaffolding and an ancient lifting device known as an A-frame, or sheer legs, are put in position.

MARCUS BRANDT: 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 -

NARRATOR: Marcus, an expert rigger, is extremely concerned about the security of the stakes and rope that will anchor the A-frames.

MARCUS BRANDT: 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.

NARRATOR: 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.

ROBIN YATES: From a historical point of view, it's quite clear that the Song had the most advanced iron and steel technology in the world. And so it certainly would have been possible for them to have iron nails.

MARCUS BRANDT: 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 -

BASHAR ALTABBA: Careful, because we're going to have to dig it through.

MARCUS BRANDT: - 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.

NARRATOR: Everyone agrees that for safety, the iron clinch nails are essential to stabilize the emerging structure.

BASHAR ALTABBA: I think this is fine. I don't see any problem with using these for erection purposes.

PROFESSORT TANG: Yes, yes. That's all right.

BASHAR ALTABBA: Okay, so we agree? Okay!

NARRATOR: 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.

MARCUS BRANDT: They're discussing now how to lash the various pieces together and make it work using the materials they've got at hand.

NARRATOR: A thousand years ago, the lashing materials could have been hemp rope, or something that's very widely used in China: bamboo.

MARCUS BRANDT: 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 -

PROFESSOR TANG: This is wetted -

MARCUS BRANDT: It's what? It's wetted, yeah, yeah.

PROFESSOR TANG: When it is dry, more tight.

MARCUS BRANDT: Right. Okay, so it pulls tight when she dries. Well, that makes for a wonderful lashing material, then.

NARRATOR: The problem with the strips of bamboo on hand is that they are too short for making a good lashing.

ROBIN YATES: Here, all we've got is actually three meters, and the Chinese don't actually know how to link them together.

MARCUS BRANDT: To get a longer rope.

ROBIN YATES: So we're going to try and figure out whether we can make a rope about 15 meters long altogether.

MARCUS BRANDT: Right. It's awfully strong, but it's the length that's hurting us.

ROBIN YATES: So we need at least -

NARRATOR: 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.

.

MARCUS BRANDT: I think it would take me a month of Sundays to get that down to a science.

NARRATOR: 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.

MARCUS BRANDT: Well, obviously they -

ANDREW LI: They still do it this way.

NARRATOR: 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.

MARCUS BRANDT: It's like plaiting my daughter's hair before she goes to bed.

NARRATOR: 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.

MARCUS BRANDT: 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.

BASHAR ALTABBA: 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.

MARCUS BRANDT: In layman's terms, we're going to pre-fab it.

BASHAR ALTABBA: That's right.

NARRATOR: 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.

BASHAR ALTABBA: Fine, we can flip it again, but we need to untangle the ropes first.

ANDREW LI: Okay, calm down, calm down.

NARRATOR: In the confusion, Professor Tang keels over. But fortunately nothing is damaged, except Bashar's pride.

BASHAR ALTABBA: Please ask them to hold it in place. I will solve this problem!

ANDREW LI: Oh wonderful, superman.

BASHAR ALTABBA: Will you please ask them to hold them -

ANDREW LI: Oh, God!

NARRATOR: As tempers fray, Bashar suspects that some of his instructions are getting lost in translation.

BASHAR ALTABBA: Come with me -

ANDREW LI: You just told him not to move!

BASHAR ALTABBA: Could we have another - Miss Zhou, please - would you please ask them -

NARRATOR: So he insists that Andrew be replaced by a local translator.

BASHAR ALTABBA: - and just hold them, stay in place.

MARCUS BRANDT: 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.

BASHAR ALTABBA: It's just, we need to think about this, just hold it - okay, this -

MARCUS BRANDT: Oh boy.

BASHAR ALTABBA: Marcus, look, the problem starts from here. This one is going across fine, but this one is wrapping around all these and coming here.

PROFESSOR TANG: The problem is wrongly rolled and wrapped, roll - if you, with same steps, like this way you have no problem, you see?

NARRATOR: Three hours later the faulty rolling and wrapping problem is finally untangled. But before work can proceed, Marcus calls for time out.

MARCUS BRANDT: 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.

NARRATOR: With the suspension cables anchored at both ends, Marcus uses a device called a Spanish windlass to start raising them.

BASHAR ALTABBA: Why don't you let it go? We're going to bring it up one more time.

MARCUS BRANDT: All right. If you look across the river, Bashar, we're launching the bridge.

NARRATOR: Wooden piers hold the main suspension cables high above the river, enabling Marcus to get a sense of how strong bamboo rope can be.

MARCUS BRANDT: 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.

NARRATOR: 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.

MARCUS BRANDT: 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.

NARRATOR: But then an unexpected problem arises.

BASHAR ALTABBA: Okay, it seems that the boat is sinking, it's filling up with water. Oh man!

NARRATOR: 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.

MARCUS BRANDT: 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.

ROBIN YATES: 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.

NARRATOR: 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.

ROBIN YATES: For the first time you're seeing a real Song bridge being put up.

PROFESSOR TANG: Yes.

NARRATOR: To celebrate, the team does something the Song builders might have done a millennium ago.

ROBIN YATES: 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!

NARRATOR: What was fueling the growth of lively restaurant-filled cities during the Song dynasty? Robin believes it had a lot to do with rice.

ROBIN YATES: 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.

NARRATOR: 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.

BASHAR ALTABBA: 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.

NARRATOR: 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.

BASHAR ALTABBA: So overall it's an amazing accomplishment for the time period it was done, but structurally it's quite primitive.

NARRATOR: 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.

BASHAR ALTABBA: 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.

NARRATOR: 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 world.

BASHAR ALTABBA: Oh, wow. It's gorgeous. I don't know what to say. I'm not worthy. This is just too much. Oh my goodness!

NARRATOR: And it would take another eight centuries before such a daring design was attempted in the west.

BASHAR ALTABBA: 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.

NARRATOR: To demonstrate the need for strong abutments at either end of the arch, Bashar and Marcus have set up an experiment.

BASHAR ALTABBA: All right, there it goes.

NARRATOR: A car jack is placed between the arch and the abutment.

BASHAR ALTABBA: 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.

NARRATOR: As the jack is released, the powerful outward thrust produced by the two sides of the arch is clearly demonstrated.

MARCUS BRANDT: These marks slide past each other, and then the stones start creeping -

BASHAR ALTABBA: Slowly, it's flattening out, and notice these joints now start opening.

MARCUS BRANDT: One, two, three - all right.

NARRATOR: 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.

BASHAR ALTABBA: 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.

MARCUS BRANDT: No easy trick.

BASHAR ALTABBA: There it goes - and then the next piece on the other side.

NARRATOR: 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.

TOM PETERS: This is a really special kind of construction. Every construction in the world, every tradition in the world, 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.

BASHAR ALTABBA: 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.

MARCUS BRANDT: 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.

NARRATOR: 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.

BASHAR ALTABBA: 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.

MARCUS BRANDT: Now, to my mind, I would rather have seen it wedged, and just - and not worry about it.

BASHAR ALTABBA: Of course, of course.

MARCUS BRANDT: But -

NARRATOR: 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.

BASHAR ALTABBA: 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.

MARCUS BRANDT: There is - but -

BASHAR ALTABBA: You don't have -

MARCUS BRANDT: 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.

TOM PETERS: 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.

MARCUS BRANDT: We've got so much redundancy in there, perfectly done, two of these things could take the whole load.

BASHAR ALTABBA: 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.

TOM PETERS: Who says you're not supposed to? This is a -

BASHAR ALTABBA: You're not supposed to, this is -

TOM PETERS: 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.

BASHAR ALTABBA: There is only one way that this system behaves, it's a pure arch!

MARCUS BRANDT: And that's Bashar's way.

BASHAR ALTABBA: It's a pure arch.

TOM PETERS: 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.

BASHAR ALTABBA: You get a little bit of bending in a pure arch from a point load!

TOM PETERS: Ah, ah, that's exactly what I said.

MARCUS BRANDT: Ah, bingo.

BASHAR ALTABBA: What do you mean, that's exactly - this is ridiculous! Oh, please.

NARRATOR: 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.

ROBIN YATES: The slanting beams -

PROFESSOR TANG: This is the improvement, just for the - Qingming Shang He Tu Rainbow Bridge. Because -

NARRATOR: 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.

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.

NARRATOR: 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.

ANDREW LI: 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.

NARRATOR: Back in Jinze, the rainbow curve is beginning to emerge from interweaving the beams of the three- and four-sided arches.

MARCUS BRANDT: Well, that's the last of the pieces for the four-sided arch. Looks like it's going to fit pretty well, too.

NARRATOR: 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.

PROFESSOR TANG [voice over translation]: 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.

NARRATOR: Before the bridge is open to the public, Professor Tang organizes a demonstration to show his confidence in the strength of the design.

BASHAR ALTABBA: 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.

NARRATOR: Four tons of water buffalo have no perceptible effect, and the bridge is declared ready for the masses.

BASHAR ALTABBA: 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.

MARCUS BRANDT: 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.

BASHAR ALTABBA: 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.

PROFESSOR TANG [voice over translation]: 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.

Sponsor: China's Song dynasty gave a lot more to the world than the Rainbow Bridge. Find out about other Song inventions, from printing to porcelain, the compass to gunpowder. Log on to NOVA's Website at pbs.org, or America Online keyword: PBS.

Sponsor: To order the "Secrets of Lost Empires" five video set with a free family activity book, please call WGBH Boston Video at 1-800-255-9424. The family activity book is also available separately, for shipping and handling only.

Next time on NOVA: Frightening power, lightning speed and amazing drive for survival. Brave a close encounter. "Crocodile."

Sponsor: NOVA is a production of WGBH Boston.

Sponsor: Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.

Sponsor: This program is funded in part by Northwestern Mutual Life, which has been protecting families and businesses for generations. Have you heard from The Quiet Company? Northwestern Mutual Life.

Sponsor: CNET.com, helping you find the right technology products.

Sponsor: Additional funding for this program is also provided by the David H. Koch Charitable Foundation. And by the Corporation for Public Broadcasting, and by contributions to your PBS station from viewers like you. Thank you.

Sponsor: This is PBS.

 

About NOVA | NOVA Homepage | Support NOVA

© | Created September 2006

Support provided by

For new content
visit the redesigned
NOVA site