Friday, 7 February 2020

What is Prestressed Concrete?

Despite its excellent qualities as a structural material, concrete has some weaknesses, too. One is that it has almost no strength against tension. Concrete can withstand a tremendous amount of compressive stress, but when you try to pull it apart, it gives up quickly. Concrete’s other weakness is that it’s brittle. It doesn’t have any “give” or stretch or ductility. Combine these two weaknesses, and you get cracks. Concrete loves to crack. And if you’re designing or building something made of concrete, understanding how much and where it’s going to crack can be the difference between the success and failure of your structure.

To understand how an engineer’s design reinforced concrete structures, we first have to understand design criteria - or the goals of the structure. The apparent goal that we all understand is that it shouldn’t fall down. When a car drives over a bridge and the bridge doesn’t collapse, the structure is achieving its design criterion of ultimate strength. But, in many cases in structural engineering, avoiding collapse actually isn’t the limiting design criteria. The other important goal is to avoid deflection, or movement under load. Most structural members deflect quite a bit before they actually fail, and this can be bad news. The first reason why is perception. People don’t feel safe on a structure that flexes and bends. We want our bridges and buildings to feel sturdy and immovable. The other reason is that things attached to the structure like plaster or glass might break if it deflects too much.

In the case of reinforced concrete, deflection has another impact: cracks. The reinforcement within concrete is usually made from steel, and steel is much more elastic than concrete. So, to mobilise the strength of the steel, first, it has to stretch a little. But, unlike steel, concrete is brittle - it’s doesn’t stretch, it cracks. So that often means that concrete has to crack before the rebar can take up any of the tensile stress of the member. Those cracks not only look bad, but in an actual structure, they could allow water and contaminants into contact with the reinforcement, eventually causing it to corrode, weaken, and even fail.

One solution to this problem of deflection in concrete members is pre-stressing or putting compressive stress into the structural member before it’s put into service. This is usually accomplished by tensioning the reinforcement within the concrete. This gives the member compressive stress that will balance the tensile stresses imposed in the member once it is put into service. A conventionally reinforced concrete member doesn’t have any compression to start with, so it will deflect too much well before it’s in any danger of not being strong enough to hold the load. So with conventional reinforcement, you don’t even get to take full advantage of the structural strength of the member. When you prestress the reinforcement within concrete, you don’t necessarily increase its strength, but you do reduce its deflection. This balances out the maximum load allowed under each of the structural design criteria, enabling you to take fuller advantage of the strength of each material.

There are two main ways to prestress reinforcement within concrete:

Pre-tensioning: It’s pre-stressed because the steel is stressed before the member is put into service, but pre-tensioned because the steel is stressed before the concrete cures.

Post-tensioning: In post-tensioning, the steel is stressed after the concrete cures, but still before the member is put into service.

It’s important to point out that we didn’t necessarily make these beams stronger. Both the steel and concrete have the same strength as they would without prestressing the steel. But, we did increase the serviceability of the member by reducing the amount of deflection under load. Pre-stressed concrete is used in all kinds of structures from bridges to buildings to silos and tanks. It’s a great way to minimize cracking and take fuller advantage of the incredible strength of reinforced concrete. 

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The unparalleled growth in the construction sector has widened the scope for prestressed concrete. Keeping its growing importance in mind, PHI has brought out a textbook Prestressed Concrete by Muthu et al.

Interested to know more about this textbook: PRESTRESSED CONCRETE

Thursday, 6 February 2020

Should Computer Science Be Required for Every Student?

Every college student needs a computer science course. Educators are beginning to recognize this truth, but we are a long way from meeting the need.

Should we require all college students to take a computer science course? That is, perhaps, debatable. But, without question, we need to make such courses available to all students.

Colleges and universities offer the opportunity for any student to take as many courses as they desire in Mathematics, History, English, Psychology and almost any other discipline, taught by faculty members in that discipline. Students should have the same opportunity as computer science. But at far too many institutions today -- including many of the most prestigious in the country -- students who are not computer science majors encounter severe enrollment caps, watered-down computer science for non-majors courses or courses that just teach programming skills. They deserve better.

Many students need computer science to prepare for success later on in the curriculum. Archaeologists write programs to piece together fragments of ancient ruins. Economists apply deep learning models to financial data. Linguists write programs to study the statistical properties of literary works. Physicists study computational models of the universe to analyse its origin. Musicians work with synthesized sound. Biologists seek patterns in genomes. Geologists study the evolution of landscapes. Artists work with digital images. The list goes on and on.

Programming is an intellectually satisfying experience, and undoubtedly useful, but computer science is about much more than just programming. The understanding of what we can and cannot do with computation is arguably the most important intellectual achievement of the past century, and it has led directly to the development of the computational infrastructure that surrounds us. The theory and practice are interrelated in fascinating ways. Whether one thinks that the purpose of a college education is to prepare students for the workplace or to develop foundational knowledge with lifetime benefits (or both), computer science, in the 21st century, is fundamental.

Even students who will not need to program at all are likely to have essential encounters with computational thinking later in life. For example, philosophers, politicians, reporters and, well, everyone -- not just software engineers -- must address privacy, security and ethical issues in software.

Computer science is also fertile ground for critical thinking. How might a given program or system be improved? Why might one programming language or system be more effective than another for a given application? Is a given approach a feasible way to attempt solving a given problem? Is it even possible to solve a given problem? A course or two in computer science can prepare any student to grapple effectively with such questions.

Steve Jobs once said on National Public Radio that “computer science is a liberal art.” Whether one believes that or not, the question is undeniably debatable and in the best tradition of the liberal arts! And one cannot begin to address the issue without familiarity with the basics. Computer science is grounded in logic and mathematics and relevant to philosophy, the natural sciences and other liberal arts, so it belongs in the education of any liberal arts student. Just to pick one example, developments over the past century in computer science have taken logic, one of the bedrocks of the ancient liberal arts, to new levels. Computer science is not just useful. It expands the mind.

Courses for Every Student

Whatever major they might eventually choose, students nowadays know that computer science is pervasive and they need to learn as much as they can about it. But unfortunately, opportunities to do so are limited for far too many students. Before seriously considering the idea of requirements, colleges and universities must focus on how to provide access to courses for all their students.

We are far from a national consensus, but an approach that has proven successful and has promise for the future is to invest in an introductory computer science sequence that teaches the important concepts and ideas in the field, as we do for Economics, Physics, Mathematics, Psychology, Biology, Chemistry, and many other disciplines. 

A well-designed computer science course can attract the vast majority of students at any college or university nowadays -- in fact, there’s no need for a requirement. An important reason to develop a single introductory course that everyone takes is that it makes later courses accessible to everyone, too. Students in Genomics, Linguistics, Astrophysics, Philosophy, Geosciences or whatever field who need a more in-depth background in computer science can quickly get it -- as well as easily transition to computer science as a major or minor.

Perhaps the most important benefit of the approach is that it supports diversity. The typical approach of offering an accelerated curriculum to Steve Jobs wannabes and computer science for non-majors courses to everyone else is inherently antidiversity. It sends the message to the non-majors that they are inferior and puts them in a position where they have little chance to catch up -- when, actually, they are not so far behind.

Does this put computer science majors at a disadvantage? No. They can learn their major in-depth later, as do the doctors, chemical engineers, writers, historians and everyone else. Meanwhile, they can benefit from learning something about the big picture, along with everyone else.

By putting everyone in the same course, focusing on what is important, teaching programming in the context of exciting and diverse applications across many disciplines, avoiding esoteric language details that can easily be saved for later, and mixing in historical context, theory, simple abstract machines and other material that is new to everyone, we can get all students on more or less the same playing field in one or two courses -- pretty much in the same way as we do in other disciplines.

Of necessity, faculty members who are teaching computer science courses around the world have had to find ways to get the job done that are more effective and efficient than traditional methods. In recent years, it has been exciting to see scalable approaches to teaching computing on all fronts. We can replace inefficient and ineffective large live lectures with curated online videos, use modern tools to create new and better textbooks and associated online content, and develop web services to streamline assessments. Like textbooks, these materials can be shared among educational institutions, further leveraging their effectiveness. Curated videos and web services developed at one institution can be used to improve the educational experience for students at another, in the same way as textbooks. Such developments have enabled computer science professors to reach vast numbers of students more efficiently and effectively than ever before.

Should computer science be required of all students? Maybe. But the first step for any college or university is to commit to providing access to at least a full year of computer science for each and every student. That is what their students want and need. Modern technology can help give it to them.

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