Inca rope bridge

Transcription

DR. JOSE BARREIRO: Good afternoon everyone. Welcome back. I trust you had a good lunch. Welcome back to Engineering the Inka Empire: A Symposium on Sustainability and Ancient Technologies. We had a wonderful morning, some very detailed work that was reported on, actually research being done on the Inka Roads, Inka construction, and in particular of course the engineering in support of an exhibition that should open here in 2015 on the Qhapaq nan, the great Inka road, that will incorporate a lot of this research. A good afternoon program as well, this afternoon we are fortunate to hear from John Ochsendorf on the topic of Engineering in the Andes: The Indigenous Suspension Bridge Technology. John Ochsendorf has been on the faculty of the Massachusetts Institute of Technology since 2002 and is the class of 1942 Professor of Architecture and Civil and Environmental Engineering. He earned his Bachelor of Science in Structural Engineering and Archeology from Cornell University where he has informed me that he was in one of my classes back then. If I can take a little credit for his vast knowledge, the student certainly has surpassed the teacher on this one. He earned his Bachelor of Science again at Cornell University where his undergraduate thesis carried out the first of—its hard to find a light back in here—technical study of Andean Suspension Bridges. Ochsendorf earned a Master's of Science in Civil and Environmental Engineering from Princeton University and a PhD in Structural Engineering from the University of Cambridge. He is a founding partner of Ochsendorf, DeJong, and Block LLC, a consulting firm specializing in historical structures. He has won numerous awards for research and structure engineering and architecture including a Graduate Research Fellowship from the National Science Foundation, a Fulbright pre-doctoral scholarship from the J. William Fulbright Foundation, a Rome Prize from the American Academy in Rome, and not the least, a MacArthur Fellowship from the John D. and Katherine T. MacArthur Foundation. Welcome John Ochsendorf. DR. JOHN OSCHENDORF: Thank you very much Jose. It's absolutely an honor for me to be here and I'm excited to share my work and some open questions on suspension bridges in the Andes Mountains. I would like to start with a slightly personal note that I grew up a few hours west of here in the mountains in West Virginia and when I was a child and got to come to the Smithsonian it really lit a fire in me and a thirst for knowledge so it's a tremendous privilege to be here. At that time I was fascinated with indigenous cultures so it's a particular pleasure for me to be here at the National Museum of the American Indian. I was lucky enough at 10-years old to have a really exceptional teacher turn to me and say I'd like you to write an extended research paper and I've chosen the topic for you. I said okay. And she said the topic is the Inka Empire. It wasn't until years later that I found that and that I'd written a couple of paragraphs about the Inka Road system and even mentioned the bridges at 10-years old. It was dumb luck but it makes clear that I've had a fascination with Andean culture and technology for most of my 16-years of life. The Andes are a vertical landscape and within this vertical landscape the Inka culture in particular developed a highly sophisticated engineering responses so here we see a 2 kilometers of straightened river at Pisac where you can see the curving terraces on the left, where the historical route of the river and then by straightening it and building a canal it reclaimed more land for agricultures so we're talking about engineering at a grand scale in the Andes Mountains and that's one of the things that we're here to both celebrate today but also to try to understand and learn from. The road system though is perhaps the largest and most lasting and impressive work of Inka engineering so I'm delighted to be working together with the Smithsonian on a public exhibition on this topic. When I was introduced to the Inka road system in 1995 I was very lucky to go and visit some stretches of the original road with Adriana Von Hagen who's a Peruvian journalist and archeologist whose father wrote a book in 1955 Highway of the Sun so for me you can imagine what a thrill it was to be along parts of the Inka road with Adriana von Hagen and Edward Frankamonta [phonetic], an anthropologist who really introduced me to Andean technology. Since that time I've been actively working on the large-scale suspension bridges which I believe are fundamental to the Inka road system, they make the road system possible. The Andes are divided up into regions with very difficult canyons to cross. Several months of the year they're practically impassable because of the water and Kenneth Wright, who spoke earlier, is braver than I am because he was willing to wade out into some of these rivers and it really is a severe obstacle within the landscape and yet we know at least in the Inka empire large-scale suspension bridges stitched together the road system and these were substantial works of engineering. One of the arguments that I make for that is the dark shapes on the left side of the cliff in this image are tunnels through the rock where Inka builders tunneled through the rock in order to reach the approach span in order to build the abutment for the bridge. This is not throwing a river from one tree to the other tree. This bridge was among the longest bridges in the world at the time it was one of the longest continually used bridges ever built in the Americas and the fact that the tunnel through the rock was constructed meant that, I think it's appropriate to use the term engineering, because it was a premeditated act of construction in order to place the abutment in the most favorable location. These bridges survived up until the late 19th century. I used to think I was born a century too late because chroniclers throughout the ages documented these bridges and really their technology wasn't surpassed for centuries they were so appropriate to the Andean context and frankly the very long spans with the difficult rivers beneath. Looking down one of these bridges in the late 19th century you can almost transport yourself back to the Inka period and envision how impressive these works are to us today and how impressive they must have been in the 15th century and in the height of the Inka Empire. Major bridges like this connected the main roads of the Inka road system leading to Cusco and these bridges played a really important and strategic role not only in the growth of the empire and control of it but also in moving goods and data as we're going to hear next from my distinguished colleague Gary Urton. They also give us insight into construction and maintenance of public works in this period and the give us a sense of really cutting edge technology and bridge construction and I'm going to try to make that argument to you. On their strategic role one of my favorite chapters in any book is this chapter from Book 3 of Garcilaso de la Vega, titled "Many tribes are reduced voluntarily to submission by the fame of the bridge". If that doesn't convince you that these bridges were something special in that the Inka was welcomed as a result of these because of the marvelous new work that seemed only possible from men come down from heaven. We can question the how accurate Garcilaso is on some levels but it's very clear from this that the bridges had an importance and there's much more research that needs to be done on the dating of the Inka road system and therefore the dating of the bridges, but most of the major bridges seem to date to the Inka period and I've estimated that at the height there may have been as many as 200 suspension bridges throughout the Andean region that helped connect the road system. These were used to cross natural barriers to also conquer new regions. You can see the growth of the Inka Empire and during and after conquests the bridges were strategically used. If you read accounts during the contact period the bridges are constantly being used in a very strategic way. One army will cross the bridge and then they'll cut it or burn it and keep going and the other army will be delayed several weeks and aren't able to pursue them as a result. Both the Spanish and the Inka used this technique. This gives us a sense of how insurmountable these barriers were without the bridges. The second thing about the organization its clear from the chronicles at least that the Inka himself is claimed to have played an active role in the planning and construction so according to another chronicle by Betanzos the Inka himself made a painting and a drawing of the bridges and this is, for me, absolutely fascinating the notion that the responsibility for public works went all the way down to the individual level of the Inka. We also find provisions for constant repair and protection were carried out both as part of a tax to the Inka on the part of smaller communities and keeping the major roads in operation with bridge keepers, who we believe for the large bridges, were stationed at the bridges in guardhouses, they repaired them continuously, they controlled who was allowed to cross, and we have some evidence that there were royal roads and royal bridges and there were others that were used by commoners and so there's more to be understood there. One thing we've learned is that there were smaller bridges that were the responsibility of local communities. The other thing I want to emphasize is that this technology was not only among the longest spans at the time anywhere in the world but they're also very durable and long-lasting. They were not surpassed until really the industrial revolution in terms of ability to span such long distances and in particular the whole concept of spanning a long distance intention was really the opposite of European method of bridge construction which was most common building arch bridges in stone. Arch bridges are limited to a much shorter span because you have to build a wooden formwork first, it won't stand up until the last keystone of the arch is in place then you can remove the wooden formwork. That doesn't work very well when you have deep canyons and very fast flowing rivers and therefore the Inka solutions were both appropriate for the terrain and also very unfamiliar to the Spanish when they arrived who in their world a bridge looked like this, stone on stone and compression. We have evidence that some of the brave conquistadors crossed on their hands and knees. They called the bridges the work of the devil and yet they also claimed that the bridges were as solid as the streets of Seville and we know that horses and cannons were able to cross these bridges, so these are substantial works of construction but really fundamentally different world view and in some ways emblematic of this clash of two worlds. Throughout the Andean region today you can find remains of Inka suspension bridges up and down the sacred valley and there's ongoing research needed to document these remains because many of the bridges were in use up until even a hundred years ago you can find things like these two stone towers, the bridge crossing the Ken Wright taught me about, and taught all of us about this morning, may have had stone towers like this that could have been washed away over the centuries. Then abutments in the floor used horizontal stone beams to tie the cables off to and multiple beams arranged in series and what's very clever about this is that the masonry in the roadway serves as a level walking surface but it also serves as a mass to resist the very large tension pull from the cable, so this is really an ingenious design and these have not been well documented. In fact, you can see the stone in the top here which is a stone lintel but then it's carved and rounded and smoothed in order to receive the cable. We need more documentation of these sites before they're lost. We have from Guaman Poma the main elements of an Inka bridge, what you notice is a braided rope serves as the roadway so travelers are walking along the rope that's suspended between two stone abutments. There are vertical ties linking these together. Here we have all the elements from the 1500s of what an Inka bridge looked like and almost incredibly surviving today in a rather remote region of the Andes at one of these bridges has been there since the Inka period. This bridge near a community today of Huinchiri in Peru has been there for at least 500 years and most remarkably perhaps the bridge is built entirely out of grass that you see growing on the hillsides. You ask how you make a grass bridge survive for 500 years. I've had many students at MIT over the years come up with various answers; you dip it in carbon fiber solution with some kind of epoxy resin, but the short answer is that the grass is integrated within the communities on either side and every single year in a 3-day festival the bridge is rebuilt entirely, so not only do we have the bridge as an artifact from the Inka period but we also have the process of construction. On the first day each household produces about 50 meters or 150 feet of cord to make ropes. The second day the old bridge is cut, new ropes are installed, and on the third day the bridge keeper installs the handrails and completes the bridge. I'll just take you briefly through the festival. Collecting the grass, which is a type of icchu [phonetic] grass that grows and is commonly used throughout Peru. You can see a small cord about the size of your index finger emerging from the twisting. Those are laid out in bundles of 24 cords and about 700 community members come together to build this and over two days those cords are twisted and then braided in a traditional ropewalk, so you can see the ropes being passed as the braid is being done. Then the cable that emerges is about the thickness of your thigh and that's the main structural element for the bridge which is then passed across with a small traveler rope to the stone abutments and even though they've been rebuilt and repaired over time we believe they date and the crossing dates from the Inka period. Here you see the main structure of the bridge with four cables for the floor which are then tied off to a stone abutment. The scale is a little misleading. This abutment's quite large because those cables look small in this image, and are then tied off to the stone blocks embedded within the masonry walkway. At the end of day two, in case you're tempted to cross, there's a banner put across that says it's not quite ready. Most of day two is spent pulling on the cables taking slack out of the bridge and you can see that it's almost horizontal, it can't go quite all the way horizontal and then on the last day it comes down to the work of the Chakacamayoc or bridge keeper who today is a man named Victoriano whose family has been responsible for this phase of the construction for more than 300 years. He's a direct descendent of a long, long tradition of master bridge builders and his job is not easy over the rushing river braiding these cables together and while he's doing this there are about 600 inebriated friends on the hillside who are yelling at him and calling for his demise. It's also a very important religious moment, the bridge is you can imagine is extremely important for the communities and many offerings are made to pachamamma [phonetic] throughout the construction of the bridge and I became involved when a NOVA television production filmed this, and this was the producer who was the first to cross that year and he, like the Spanish, was rather terrified and he said you know, I'm afraid if I lean out on the cables maybe it'll just move out into space and if I grab the other one the bridge will flip over and dump me into the canyon. He said does that ever happen and one of the community members said oh yeah, that happens all the time, which of course, is not the case. I'm absolutely thrilled; this is Victoriano that a new documentary filmed The Last Bridge Masters documenting his family and their struggles to pass on this tradition in the face of a lot of other pressures in the period that we live in today and I'm particularly excited about having him explain in his native Quechua what this bridge means to him and his family and his communities. This is really an incredible piece of Inka engineering that we are fortunate that has survived until the present day. As an undergraduate I was lucky enough to tackle this project as a senior thesis which involved a typical engineers question; how strong were the ropes. I did load testing on these ropes and found that a single main cable could support about 4,000 pounds in tension before failing so that's about strong enough to lift up two small cars. Then I said to my advisor nobody's ever measured the bridge; I really need to go to Peru and she was terrific and said yes, you do and I got to go to Peru and measure this bridge and I found that the span is 99 and a half feet which we'll call a hundred feet, and did an analysis that showed what, of course the community knows, that the bridge has plenty of load capacity. This is about as many people as I've ever seen on it and in fact, historically there's a certain amount of engineering that's gone on to get the right number of cables for the needs. Then a few years later with colleagues Lynn Hobbs, Heather Lechtman, and Dorothy Hostler at MIT, they taught a class on materials and Andean society and we had students make cables using natural fibers, they braded them and then we built a replica bridge on the MIT campus over this raging river and we replicated the same way of getting the first traveler rope across and then pulling the cables across stretching them between abutments and the students really enjoyed this. You can imagine. We called the project Chakastata [phonetic] for the building next door and our students are somewhat sleep deprived. This was a really terrific exercise for all of us and I got a lot of publicity at MIT for this; it was a great experience for the students and the faculty. When the New York Times wrote an article about how the Inka leapt canyons highlighting this bridge and our reconstruction for three days on the New York Times website this was the number one most emailed article which tells us that the public also has a fascination with these bridges and with the act of reconstructing this particular bridge. This bridge has survived even though a truss bridge from the 1960s was built adjacent to it. work, play, religion, and technology aren't separate activities in the Andes and that truss bridge which was really imported and not integrated into the communities is now corroding and falling into the canyon where the original grass bridge for the last few decades hasn't been necessary to get across the river but the festival must go on and the bridge is produced anyway so we're very fortunate for that. Just in closing thinking about what can bridges can teach us. Working on an exhibition here on the Inka road system I would argue that there are lessons we can learn about their technology, about their social integration within the communities and also about their environmental performance. The scale of the spans that they achieved not only evoked wonder at the time but even more so today the strength of fibers and tension. One of our possible ideas for the exhibition is to mount a replica of this bridge here at the National Museum across the rotunda between two openings which, when we went and measured them recently, we learned that the span was 99 and a half feet. We've been looking into the feasibility of that, have worked together on developing techniques to connect it to the existing building, and the concept is to have Victoriano produce a bridge possibly as part of the folk life festival on the mall in Washington or possibly in Peru and then ship it here to be installed but we want to introduce the authentic bridge made by the community and we really think that would draw people not only to this museum but to the Smithsonian at large. We also want to work to document the festival, something that's really been lacking in the past and these remarkable filmmakers who are working on this social sustainability side of the project and documenting the builders speaking in their native language about the process and technology of the construction, and we think that's tremendously important as well not only to understand how this bridge has survived until today but how it can survive for future generations and to continue thinking about the way that the Andean world view treats large-scale works of engineering like this which, I believe, in some ways is fundamentally different from the way we think about large-scale works of engineering and has things to teach us. The fact that the infrastructure is integrated within the community and that everyone is involved in the upkeep in this way and then also thinking about appropriate technology for the Andean context. When we think about building for generations this bridge is truly sustainable from an environmental standpoint. It's a hundred percent biodegradable, it washes away in the river every year, and it re-grows every year. The primary fuel for building it is the Chicha, the corn beer which is also grown sustainably. For me, each of us has our own passions and our calling in life but I feel very fortunate to have met Inka suspension bridges at an early age and they continue to fascinate me. We have so many things that we still have to learn and so I'll leave you with some longer term goals, some of them we may be able to achieve by the exhibition in two years. One is a map documenting the major suspension bridges and even the minor suspension bridges so we can begin to understand where they were working from primary sources and trying to date and understand the roads as well like some of the work we saw this morning. Then surveying and documenting the abutment remains which in the cultural landscape of the Andes Mountains the abutments haven't ranked high on anybody's list and yet I think they're really important for us to understand the history of this technology and the history of the road system and also shelters for bridge keepers. I want to end by thanking Ed Frankamont, who is known as Inka Ed, who was studying anthropology at Harvard and studying Andean society and he realized if he really wanted to understand the Andes and learn how to weave and to think in an Inka way that he needed to go and live there and he lived for 20 years in Peru and spoke Quechua. I'm very grateful that he introduced me to this topic and there's still so many things that we have to learn and I'm absolutely delighted that the Smithsonian and that the National Museum of the American Indian is helping to create new scholarship and to bring this important message to potentially millions of people around the world. Thank you.