On October 3, 2015, Professor Stephen Ressler sat down for a live Q&A session with his fans from across the globe. The chat is over, but the transcript is posted below for you to enjoy.
DAVID LÖFQVIST: How are 3D printers and designing software for the masses changing engineering?
RESSLER: In industry, both 3D printing and computer-aided design are revolutionizing engineering–primarily because they allow engineers to try many more design alternatives before building an actual prototype.
FHPRINTMKR: Viewing your courses I was amazed at the detailed planning and exacting execution of many of the systems and structures. It seems to me that significant construction duration projects (often decades long), and that the constructors were held to such stringent standards required a centralized and autocratic government. Of course now with computers, modern concrete, and plastics we don’t have to spend decades building even the largest structures. But do you think that a democracy… given that we all now much prefer living in a democracy… is even capable of such a sustained effort?
RESSLER: Yes, of course. The Interstate Highway System and the Panama Canal are good examples. What’s needed is a combination of both political will and popular support.
BARRY YATES: Romans constructed massive projects all over their world. What kind of formal training were the engineers given for standardization? Thx
RESSLER: We know that the Romans had very well-developed systems for training their military engineers–and that these engineers typically worked in the civil sector as well. But we don’t have a lot of in-depth info about how these educational programs operated. Clearly they were amazingly effective.
GEB BLUM: Are we approaching an engineering limit on the slender and tall buildings constructed around the world? The bases of these buildings must be undergoing tremendous compressive forces due to the height of construction, and some large tension forces with wind loading.
RESSLER: There certainly is a theoretical upper limit, but it’s much more likely to be driven by wind and earthquake loading concerns than by foundation concerns. But recent tall building projects in the Far East suggest that we haven’t reached that theoretical limit yet.
YEARN2LEARN: What is your perspective on America’s infrastructure (roads, bridges, dams, etc); is it as bad as reported in the media? or worse?
RESSLER: The American Society of Civil Engineers does a detailed analysis of our infrastructure every year and publishes it as the “ASCE Infrastructure Report Card.” It is the work of seasoned professionals in this area and thus is very reliable. And, yes–as noted in the Report Card–the situation is pretty bad.
YEARN2LEARN: In your course on the World’s Greatest Structures you discuss how ancient structures were built based on trial and error, institutionalized learning, and the art of engineering. Now that structural engineering is more science based do you feel that some of the “Art” or craftsmanship of the field has been lost?
RESSLER: Yes and no. Certainly some of the fine craftsmanship we see in older works has been lost. But more powerful analysis and computational tools have made it possible for architects and engineers to create astonishingly creative new forms that simply wouldn’t have been possible in earlier eras–because there would have been no way to analyze them reliably.
JOE S.: If you had 2 days in Rome, what would you go see?
RESSLER: My three favorites–Nero’s Domus Aurea, Trajan’s Market, and the Baths of Caracalla. All three are a bit off the “beaten track,” and all three are magnificent engineering achievements.
CARMEL AND JONATHAN: I’ve always wondered what goes on ‘behind the scenes’ at the Great Courses. Do you do all the lectures in one or two days, one after the other, or one a day. How is the editing done? I’m just curious about how it all works. I’ll bet others are, too. Tell us some “secrets”.
RESSLER: For my courses, we schedule three lectures per day–every day for 2+ weeks. (Other courses without demonstrations typically do more lectures per day.) We also do two separate walk-throughs of all demonstrations with the production crew, so we get all the right shots at the right times.
STEVE K.: As an engineer (of the chemical persuasion), I greatly enjoyed your courses. I recall that after some serious boiler explosions in schools and similar buildings, the ASME adopted Codes that had a safety factor of roughly 4 to 1. Is it possible to state a similar safety factor for skyscrapers in events like hurricanes and earthquakes? Have there been modern day failures of such buildings?
RESSLER: The factor of safety for building design is generally lower–around 2 is typical. There have been a few spectacular structural failures in recent decades–though most have been caused by failure to anticipate a certain loading or structural response (e.g., the Tacoma Narrows Bridge) rather than an inadequate factor of safety.
STEVEN KNAPP: Given your background at West Point, have you ever thought about developing a course on the long history and amazing accomplishments of military engineering?
RESSLER: I would love to do such a course. Indeed, I’ve had a few informal discussions with The Great Courses about this subject. Recognize that TGC won’t produce a course unless there is significant customer demand–so be sure to make your voice heard!
LJ NEAL: How do you think building materials will change in the future? Is anything being done to make them more environmentally friendly?
RESSLER: Yes, though materials for large-scale structures tend to change more slowly that they do for high-tech applications like aerospace and electronics. Still, we are seeing some interesting new developments in the use of composites these days. I address the subject of sustainability in construction extensively in my new course (Everyday Engineering). Yes, a lot can be done–especially with the use of engineered materials instead of wood.
RODGER EWY: How did Theodoric’s minions get the monolithic dome (10+metres) up on its structure in Ravenna?
RESSLER: This was a dark era, so there aren’t a lot of good sources about how construction was actually accomplished. I’m pretty sure that this dome was made of brick. The Roman method of casting monolithic concrete domes had been lost by Theodoric’s time. So they would have erected a huge set of temporary wooden centering within that octagonal space, and then set the bricks in mortar across the top of this structure to create the dome.
MARY: When did you begin building models for illustrating various concepts? For your West Point classes or before that? Have you always been a model-builder?
RESSLER: I’ve been a model-builder for as long as I can remember. But I didn’t start building models for instructional use until I arrived at West Point in 1991. Amazingly, my department had a tradition of using models that dates back to the early 19th century!
JMOP1: What can we learn from the ancient engineers and technicians that would be of benefit as we tackle today’s problems relative to the significant impact modern humans have on our environment?
RESSLER: Great question! The ancients had a strong grasp of many “sustainability” concepts that we mistakenly think are modern. Many principles of modern “passive solar design” are beautifully articulated in Vitruvius’s treastise from the 1st century BC.
STAYLOR: Do you feel that the Great Cathedrals of Europe in the 11th to 15th century were the great architectural and engineering achievements of the western world since Roman times?
RESSLER: Yes, absolutely. In many ways, the great Gothic cathedrals surpassed Roman engineering prowess–for the first time in over 1000 years.
KEITH_PORTER: Do you know how the Romans might have used mathematics in their construction? Would their number system have been a hindrance in any way?
RESSLER: The Romans used math in essentially the same way the Greeks did. They used numbers to define the geometric proportions of structures, to enhance their appearance and function. But they had no equivalent of our modern mathematical models that are capable of predicting structural behavior. Lack of decimals and the number zero would certainly have been a hindrance.
MARY: Have you visited or studied any of the structures which are being destroyed in the Middle East? What can be done to preserve their memories?
RESSLER: I have admired the great structures at Palmyra and have mourned their destruction. I do believe that archeologists have documented most of them very well–so we at least have a record for posterity.
DONALD MCLEOD: France is building a 35 story wooden skyscraper. Portland, OR, is building a 12 story wooden high rise. After what you said in the lecture on the materials used in construction and the limitations of wood, why are these designers going back to wood?
RESSLER: I’m not familiar with these specific structures–but the most common reasons for using wood are aesthetics (wood really is inherently beautiful!) and ease of construction. But in large buildings, fire resistance and the potential for insect damage can be huge challenges.
ROBERT BERNAT: Hello Professor. Very much enjoyed your course. It fit in nicely with my volunteer work at the American School of Classical Studies in Athens where I worked with Sherry Fox. You are well-known to many of my colleagues here. I pass the Acropolis daily and continue to be in awe of the structure, the artistic and mechanical achievements. How did they get all that material up there? Thank you.
RESSLER: I devote two lectures to this subject in Understanding Greek and Roman Technology. In Lecture 3, we begin at an ancient quarry and follow a stone block as it is extracted, shaped, transported, finished, and lifted into position in the wall of a Greek temple. In Lecture 4, we examine the construction of the temple itself—stone by stone, using a really cool computer animation.
JACK SWING: What are your thoughts on a cost effective method for rebuilding our country’s aging infrastructure. And, are excessive regulations the problem?
RESSLER: There is nothing technically difficult about rebuilding our infrastructure. We have the technology and the know-how. All we lack is the political will to commit adequate resources to what just about everyone knows is the right thing to do. Regulations are generally not a problem. Indeed the engineering codes and specifications that guide this process are triumphs of human ingenuity. They make our lives better every day.
GERRIE: What is the long term outlook for the Tower of Pisa and the city of Venice?
RESSLER: The long-term outlook is excellent, thanks to some truly brilliant geotechnical engineering work to shore up the foundations from below. The building is reasonably well stabilized now.
TOM DOE: In engineering we would hope to anticipate failures and prevent them rather than learn from failures after the fact. Your courses provide excellent examples of mistakes and how we learned from them. Without taking on any liability, are there any areas of current civil engineering thinking that make you nervous?
RESSLER: The thing that occasionally makes me nervous is the great potential for over-reliance on computers for structural engineering analysis. Computers are extraordinarily powerful design tools–but their use must always be informed by the engineer’s deep understanding of fundamental principles. In this era, our engineering schools sometimes emphasize computational skills over fundamental understanding–and that’s not good.
TRAFFICAL: If you had to pick one historical example of an engineering work that embraced the breadth of ancient civil engineering knowledge, what would that be?
RESSLER: Definitely the Baths of Caracalla, addressed in Lecture 15 of “Greek and Roman Technology.” Here in one amazing infrastructure system, we have monumental structure, an integrated water supply and drainage system, under-floor heating, roads, urban planning, and even water power. It’s all there under one vaulted roof!
DAVID: Dr. Ressler, ever since I took an art course years ago, I’ve wondered about the fluting on ancient Roman and Greek columns. I know that they were intended to give the appearance of strength, but, structurally, do they?
RESSLER: No, they are purely aesthetic. They serve no structural purpose at all.
DANIEL: What is the most exciting innovation in civil engineering in the past 10 years that you consider to have the greatest potential to re-shape the world as we know it?
RESSLER: I personally am inspired by the Green Building movement–the holistic integration of sustainability concepts into the design and construction of structures. It’s a fascinating subject–and a lot of smart people are doing some very thoughtful and innovative work in this area.
MARIE STERLING: I’m a civil engineer who watches your courses with my 13yo son. He did not enjoy STEM in middle school but you have turned him back on to the subject. Thank you.
RESSLER: Thanks for sharing this! Engineering outreach to kids is my greatest professional passion.
LISTENER: Your lecture on Mortise and Tenon Joints is posted below. But isn’t all the stress focused on the tenons further weakened by the dowels? Why don’t these structures just snap at the seams?
RESSLER: The tenons were made of hardwood, which is much stronger than the pine planks. Yes, the holes weaken the tenons, but this loss of strength is more than offset by the huge number of tenons that share the applied loads.
AJBRAMLETT: Next Tuesday, I will be doing a presentation on the temple of Artemis at Ephesus — any pointers?
RESSLER: This immense building represents the upper limit of stone post-and-lintel construction. Any larger, and the stone architraves would have been incapable of safely carrying their own weight and the superimposed loading from above. It’s too bad that more of this great structure has not survived for us to appreciate.
AJBRAMLETT: In the ancient world, the builders had limited materials, could they have built bigger structures?
RESSLER: It depends what you mean by “bigger.” They certainly could have built larger pyramids, for example, because the pyramid is such a stable structural form–and the compressive strength of stone is much greater than needed to carry the weight of the stone that constitutes the structure. On the other hand, it’s doubtful that stone arches or un-reinforced concrete domes could be built with significantly larger spans than, say, the Pont St. Martin or the Pantheon.
TOM DOE: One of the really great themes in your structures course is the contrast of British empiricism and French mathematical analysis. What is the current balance in current engineering practice and does it tip one way or the other?
RESSLER: It definitely tips strongly in favor of science- and math-based design–sometimes excessively so–though empiricism will always have a vital (if under-appreciated) role in analysis and design.
FRITZ USMA ’66: Given the current water resources problems on the west coast, do you think there are practical civil engineering solutions to getting water where it is required and is simply moving water around going to be enough? The Romans seemed to have solutions but they did not have the magnitude issues we have here.
RESSLER: Although I’m not an expert in the subject, it certainly seems that desalination provides a viable (if expensive) solution to this challenge.
DONALD MCLEOD: I would love to see you do a course on forensic civil engineering. I am always amazed looking at the twisted wreckage of a failure in a structure that anyone can come in and figure out the cause.
RESSLER: I have actually proposed such a course to The Great Courses. Thus far, it hasn’t gotten much traction–I think because many would consider the subject to be too morbid.
MERLIN DORFMAN: I’m sure you’ve read “Brunelleschi’s Dome” by Ross King and perhaps seen the PBS “Nova” program on the same subject.
RESSLER: Yes, I read “Brunelleschi’s Dome” as part of my research for “World’s Greatest Structures.” King does a very nice job of covering the human dimension of the story; however, I was somewhat disappointed with his coverage of the technological dimension. How did the “herringbone brick pattern” actually work? You won’t find the answer in this book.
STEVE K.: Do civil engineers and architects sometimes have … “serious discussions” about a proposed design? B-}
RESSLER: Yes! In general, the architect has the lead role on a building design team. The engineers are subordinate. The architect is primarily concerned with form and function; the engineer with structure. Since form, function, and structure are not necessarily in harmony, the architect’s and engineer’s relationship is also sometimes not in harmony.
JEC: I enjoyed your course on structures particularly because of the emphasis on concepts and qualitative relationships. It reminded me a lot of the teaching style led by Prof. Harry Meiners at Rensselaer Polytechnic in the late 1960s. Conversely, the curriculum in engineering mechanics at the time was heavily influenced by tensors and matrix math. That was typically thorough and informative but much more difficult to grasp. Have you been able to successfully collaborate to bring your teaching methods to campuses other than at USMA?
RESSLER: Perhaps to some extent. For the past 17 years, I have been extensively involved in a teacher training program (called Project ExCEEd) sponsored by the American Society of Civil Engineers. Through this program I’ve had the opportunity to advocate certain aspects of my teaching style (most importantly, the use of physical models) with workshop participants from academic institutions around the world.
BECKY KARO: Question: how did the Romans actually build the Pantheon dome? With centering? Love your courses and use them in my teaching – as does my daughter with her high school Latin classes.
RESSLER: There is no surviving record of the construction method, so we can only speculate. But in my view, the builders must have used centering of some sort. There simply would not have been any other practical method of supporting all that wet concrete.
TIM MCCOLLUM: We thoroughly enjoyed the two courses. We just returned from Italy, where we visited, and in some cases revisited, a number of the structures you discussed. We even got to the bridge at Pont St. Martin. Is there any book simple enough for a non-engineer to continue studies from?
RESSLER: Thanks! I have not been to the Pont St. Martin myself–so I am quite envious! As I noted above, my favorite resource for ancient engineering is The Oxford Handbook of Engineering and Technology in the Classical World, by J. P. Oleson. For coverage of building structures through history, try Building: 3000 Years of Design Engineering and Construction by Bill Addis.
SALLYANDMILT: I live near Hoover Dam and have been amazed at the engineering and construction ingenuity that went into that project, and that it was finished two years ahead of schedule. I would love to see that and other engineering feats (such as the moving of the Abu Simbel Temples in Egypt) featured in a future lecture series.
RESSLER: I would too! I have proposed a course called “Masterworks of Infrastructure,” which would address the Hoover Dam (as well as other great civil works like the Panama Canal and the Interstate Highway System) in great detail, though this proposal hasn’t gained much traction so far.
PAUL FRANCO: Q: During a volunteer mission trip to Nicaragua this past winter, I heard talk of the Chinese, for one, having a financial investment interest in building a second transoceanic accessway as a competitor to, or a sister to the Panama Canal. With so much inland water in Nicaragua it seems very feasible.
Q1: Any news on the likelihood of such a project?
Q2: Although Central America is wider in Nicaragua, the extensive inland waterways lessen that problem. I suppose it was probably one of many likely alternative locations in Panama Canal site selection, perhaps it was engineeringly less feasible or was it politically or economically scratched?
RESSLER: I’m not too well-versed in the history of the Panama Canal–though you are definitely correct that a route through Nicaragua was seriously considered before Panama was ultimately selected as the site. I believe a major advantage of the proposed Nicaragua route was much flatter terrain, which would have facilitated a sea-level canal. The final decision was highly politicized.
VINCENTKEITH02: Each age has its own advances in structural engineering. What would you say is the most innovative example of the current age?
RESSLER: I think the most interesting (and surprising) is the increasing use of high-strength concrete as the principal structural material for extremely tall buildings, like the Burj Khalifa.
DONDZ: With the present environment will we see improvements in the infrastructure or will it take some major breakdown to initiate action?
RESSLER: I hope not; but if the infrastructure doesn’t receive some serious short-term attention, such failures are practically inevitable.
GARY: Regarding the Roman architectural designs, did Buckminster Fuller incorporate and even improve upon Roman engineering postulates in his geodesic designs? It seems that there’s a common, recurring theme in hemispherical advantages around load balancing, structural integrity, and simplicity that seems to be intuitively sound and wondering if these earlier Roman engineering principals have not changed significantly in the modern era?
RESSLER: I don’t know about Buckminster Fuller’s motivations. I think we can safely say that every long-span dome built after the Pantheon owes something (if only inspiration!) to the Pantheon. But there were many evolutionary and revolutionary developments in domed structures between the Romans and Fuller’s era–and all of these developments must have influenced him in some way as well. Lectures 13 and 22 of “World’s Greatest Structures” address many of these developments.
CLYDE PORTER: Empirical Structural Design Methods vs Scientifically Based Structural Design Methods for URM Structures — what are some of your comments?
RESSLER: I have no personal experience with the design of modern un-reinforced masonry, and I’ve never taught the subject–so I can’t really comment without doing some research. I expect that empirical methods would play a significantly larger role than in the design of steel framed structures, for example.
JOE T: Having worked as a Civil Engineer for nearly 30 years, it seems like Civil Engineering is in kind of a rut. Considering that 99% of public works engineering is the everyday stuff of roads and utilities, there seems to be little applied advancement (trenchless pipe technologies seem to be the exception). California, for example, is still using the Hveem method for asphalt (developed in the 1930s when Model A Fords were on the roads), and the only published reason why they are finally switching slowly to Superpave, is that they cannot find anyone to manufacture, maintain, and calibrate the Hveem equipment. There’s a lot of pilot studies on various methods and materials, but they seem to not go much past that stage, a few lectures are given, and there is no compendium comparing the benefits and limitations of new technologies. So new tech seems to just die, and there is less incentive to innovate. Considering the deteriorating state of public works in this country, and the increasing costs, is there any movement to correct this and move things forward?
RESSLER: The only effort I know of is ASCE’s “Raise the Bar” initiative, which is attempting to establish the master’s degree as the minimum academic prerequisite for professional licensure. And I do think that this initiative, if successful, will address the issue you’ve raised, at least in part. I suggest that a new generation of engineers with higher levels of academic preparation will be more receptive to the use of new technologies.
CONTESSA: How many hours have you spent on your models? Where do you store them?! They really make your lectures interesting and understandable for the average non-engineering student.
RESSLER: For my recently completed course, “Everyday Engineering,” I spent over two months, working 12 hours every day, to design, build, and test the models. Yes, it’s a lot of work–but worth it, because there’s simply no more convincing way to illustrate how engineered systems actually work. (You can manipulate a computer model or animation to show anything you want, but physical models must obey the same laws of physics as the full-scale systems they represent.)
YEARN2LEARN: In your course you talk about how the ancients who preceded Newton and the understanding of classical physics depended on trial and error, passed down learning, and the “art” of engineering. Yet many of their structures (e.g. Roman bridges, Egyptian and Mayan Pyramids, Japanese Pagodas, etc) have lasted for centuries. Now that engineering is more science based, do you think that the “art” and/or craftsmanship of classical engineering is being lost?
RESSLER: Yes and no. Certainly some of the fine craftsmanship we see in older works has been lost. But more powerful analysis and computational tools have made it possible for architects and engineers to create astonishingly creative new forms that simply wouldn’t have been possible in earlier eras–because there would have been no way to analyze them with sufficient accuracy.
HENRY STAUB: I learned in other courses and a visit to Egypt that Egyptian architecture did not change much in the almost 2000 years of its prominence. Have any reasons for this stability but little development been brought forward?
RESSLER: I suspect the two principal reasons were (1) the lack of widely available building materials beyond stone, brick, and wood;and (2) the lack of science-based design methods. The revolution that ultimately produced modern architecture stemmed, first and foremost, from the widespread availability of iron and the adoption of science-based design methods.
HENRY STAUB: Has it ever been determined what ingredients made the Roman concrete used in many structures that are 2,000 years old last so long?
RESSLER: This issue has been studied extensively by archeologists, with a lot of support from chemists and materials scientists. There’s a fairly good summary in the “Material Properties” section of this article: https://en.wikipedia.org/wiki/Roman_concrete#Material_properties.
IRVING NAZARIO: Can you give us a brief overview of how modern concrete came to be developed?
RESSLER: There’s a good summary at http://www.auburn.edu/academic/architecture/bsc/classes/bsc314/timeline/timeline.htm.
DALE LANDIS: Can you give a bit of info on how modern math and computers have affected your field during your time teaching it? I can certainly guess there have been major changes – what do you think are the most significant?
RESSLER: Computer-based methods of structural analysis have revolutionized structural engineering–by facilitating highly accurate analyses of structural forms that would have been impossible to analyze by traditional methods and by allowing engineers to consider many more design alternatives before selecting an optimum.
PAUL: Hello, Today we build great structures using more engineering and lighter materials. However, many of these structures require more ongoing maintenance than structures of the past. Do you think that the savings in materials justifies the ongoing cost (frequently unbudgeted) and risk for the long term survival of these structures, particularly bridges?
RESSLER: I think that some modern construction does over-emphasize minimizing the up-front cost, rather than designing for long-term serviceability and sustainability. Most engineers understand the trade-off, but owners need to be willing to increase their up-front investment–and this doesn’t always happen.
JOHN: How do you think the pyramids were built? I saw an interesting theory using flotation bags and a clever canal system within the pyramid structure.
RESSLER: I’m aware of many interesting theories–including a very compelling one that ramped tunnels inside the structure were used as the pathway for hauling stones to the upper levels. But I’ve never studied this subject, so I can’t comment on it with any authority.
MATT: How often do engineers fail on a project?
RESSLER: I haven’t seen any stats, but it’s certainly clear that engineering failures do happen more often than we’d like. And if recent disasters like the BP Deepwater Horizon, the GM ignition switch failures are any indication, a significant problem is the tendency of individual engineers to surrender their professional autonomy and responsibility to their corporations’ economic interests.
ALAN: Without wandering into “Ancient Aliens” nonsense, is it possible that some ancient societies employed technologies that we’ve only recently rediscovered? (The Victorian worldview of history as smooth progress is challenged by the idea of falling back and forgetting in Dark Ages.)
RESSLER: Certainly. The Antikythera Device is an amazing example of an extraordinarily sophisticated ancient machine that no one anticipated before it was discovered.
VIN: I recently visited Grant’s tomb and I’m not sure what the difference is between squince and pendentive. Why one and not the other and what are the merits of each?
RESSLER: They both perform the same structural function, but the pendentive is both more aesthetically pleasing and allows for smoother flow of forces from the base of the dome to the supporting structure below.
DERYL SHIELDS: How did the Romans build the multi-level aqueducts? Were they actually able to build scaffolding up THAT high?
RESSLER: Yes! Building scaffolding that high doesn’t seem like a huge challenge to me, because the first-level arcade would provide ample support for the second-level scaffolding–and so on, as the structure progressed upward.
RICHARD PECK: Please discuss how the Gothic master builders got such incredible production from their workers.
RESSLER: I can only guess–but the medieval era was one of very strong religious fervor, so it makes sense that entire communities would be strongly motivated to participate in constructing their places of worship.
JMOP1: Are there any plans to offer an additional course on civil or structural engineering in the future?
RESSLER: Time will tell. TGC has a very rigorous process of developing course proposals and polling their audiences to gauge whether a proposed new course has a sufficiently large audience. If there’s sufficient demand, they will certainly support development of such courses.
JIM: With the salt-sea environment of Tenerife, what is being done to preserve the Calatrava thin shell structure of the Concert Hall?
RESSLER: I don’t know–but you are correct that salt can severely accelerate the deterioration of both concrete and its steel reinforcement. I know there are some innovative new coatings that are being used to mitigate this problem, though I don’t know if any of them were used at Tenerife.
HUGH REPLOGLE: In lesson (22 ? – development of the catapults) you mention that the “composite bow was developed in the second millium BC. Really that early? Who developed that technology? That’s too early for the Greeks. Stepp nomads? Assyrians? Persians?
RESSLER: According to https://en.wikipedia.org/wiki/Composite_bow, the inventors were Asiatic nomads.
PETER TEITELBAUM: 1. Is the Raftsundet bridge thickest at the supports because the supports cause a rather sharp convex-up bend at that point? Why thinnest at the mid-spans where there would be convex-down flexure stress? 2. I presume a beam is weakest at the midpoint between supports? How is that explained?
RESSLER: The answer to this question is quite complex—but it’s covered in some detail in Lecture 6 of Understanding the World’s Greatest Structures. The key is to focus on the concept of “bending moment” and use of the Moment Diagram as a tool for understanding flexure. In short, Raftsundet is a “continuous beam bridge”—which experiences maximum negative moments over the supports and significantly smaller positive moments at the mid-span regions.
AVECON: We were in Athens recently. How was the Parthenon built?
RESSLER: I devote two lectures to this subject in Understanding Greek and Roman Technology. In Lecture 3, we begin at an ancient quarry and follow a stone block as it is extracted, shaped, transported, finished, and lifted into position in the wall of a Greek temple. In Lecture 4, we examine the construction of a typical Greek temple—stone by stone, using a really cool computer animation.
H. CARLSON: Are construction resources and materials in sufficient supply?
RESSLER: Generally yes. Iron is one of the most abundant minerals in the earth’s crust. Moreover, in the U.S. today, nearly all structural steel is made from recycled scrap. Concrete is also made from widely available natural materials–limestone and clay. Wood is more problematic, though most wood used in modern construction is grown in managed forests that are, to some extent, sustainable.
DAVID BUSHMAN: Professor, what are the most durable (long lasting) occupied structures in the ancient world and how were they built? What are the longest occupied communities?
RESSLER: I don’t have a definitive answer. But I will note that, if you travel in Italy today, you’ll find many examples of 2000-year-old Roman buildings that are still serving as residences or businesses. The key to this success is the Roman’s use of an integrated system of brick and concrete for foundations, walls, arches, and vaulted roofs.
ANY INSIGHTS: State Transportation Department officials told the San Francisco Chronicle that tiny cracks found in two rods – which are about 26 feet long and up to 4 inches in diameter – might also be present in many other rods, placing them at risk for sudden fracture. The cracks appear to be related to corrosion caused by water leaking into the spaces around the rods. How concerned should I be driving over this bridge?
RESSLER: Don’t be concerned. If the DOT knows about the issue, then professional engineers have certainly inspected the structure and are continuing to monitor it. They are technically educated and professionally obligated to protect public health and safety. I believe that we can and should trust them to fulfill their responsibilities–particularly in such a high-visibility situation.