President's Memo
August 2003
Let's Keep Connected to the Design Process
by José M. Izquierdo-Encarnación
I am sure many young engineers today are totally unfamiliar with the engineering slide rule, but I was part of the last generation who used this instrument. Then, in the summer of 1975, my father bought me a Texas Instruments (TI) electronic calculator. I was impressed. I could now solve sines and cosines accurately without referring to a table and, even better, I could determine square roots more exactly! It was a time when we would compare the latest TI models with the latest Hewlett-Packard (HP) calculators. (I did tell you in a previous column I was a nerd.)
These were definitely the "coolest" calculators until programmable models with mini-printers came on the scene. The most venerable programmable model in the early '80s was the HP 41C. It had 2K of memory! Later, Casio produced calculators capable of handling subsets of Basic Language (now almost extinct). It also included a color plotter with 2-in.-wide paper. I remember analyzing hundreds of beam designs and strips of multiple spans with such a device.
Back then, Luis Mora, my concrete design professor, taught the virtues of using the coefficient of resistance nomograms and what a great tool the ACI Design Handbook was. The handbook illustrated the different procedures for determining member sizes, required reinforcement, and development lengths; configuring footings and pile cap foundations; and solving tough column designs using the tables provided. Mora, however, also taught us that with the new calculators it was better to ignore coefficient of resistance nomograms and the iterations involved in using them. The new devices now permitted a more direct approach to designing concrete beams and slabs.
Whitney's rectangular solution for the compression area simplified tremendously the design for flexure. (Refer to PCA Notes on 318-02, p. 6-3, for the development of the compression area.) This stress block approach is based on the equilibrium of the tension and compression (Eq. 1) and the forces that make up the flexural capacity of the beam (Eq. 2). An equation is developed by using the definition of the compression force in the flexural formula. A quadratic equation is then formed (Eq. 3)-the solution of which yields a straightforward definition of a based on a particular applied moment (Eq. 4). By knowing a, the area of steel reinforcement is then determined (Eq. 5).
and solving the quadratic formula, we get:
This solution is for beams where tension controls. Because ours is an earthquake- and wind-prone area, we design according to this condition. Programmable calculators, electronic spreadsheets, and greater computerization everywhere make it a more direct design approach. This approach applies to other concrete components as well, not just beams.
My interest is not to transform this President's Memo into a demonstration of concrete design basics. For that, I refer you to the ACI Design Handbook or the PCA Notes on 318-02. (Both can be found in ACI's catalog or online bookstore.)
My purpose, instead, is to emphasize how our engineering methods and thinking must continually evolve to remain useful as time and technology change. The design of concrete structures is no exception. Yet, we still tend to carry the mental baggage and associated tools from when slide rules, charts, and nomograms were God's gift to designers. Textbooks and manuals change in response to code changes, but the design process itself sometimes remains unchanged because, though we abandoned the Stress Design Method, we essentially have been following the same thinking process it implies.
When ACI develops codes or design information, let's make sure we are conscious of the current art and practice of structural design. By understanding today's available tools and office environment, ACI should be able to provide relevant information that more readily yields economical, safe, and durable concrete structures.
José M. Izquierdo-Encarnación, President
American Concrete Institute
pepe@porticus-ingenieria.com
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