Excerpt from Practical Hints for Concrete Constructors

The elastic theory as applied to concrete arches is very elegant and every concrete designer should be familiar with it. But in all cases except in that or very aat reinforced arches the common theory, intelligently applied, is all that is neces sary. In the design of the Piney Branch bridge;now under construction in Washington, both theories were applied and the maximum pressures diirered under the two analyses by only 5 lbs. Per sq. Ih. Don't spend much time worrying about the exact position of the neutral axis or a beam. Anter you have inspected work under construc tion for a time you will be convinced that the way concrete goes into the work does not permit 01 any hair-splitting application of theory. Don't stress your concrete in compression higher than 300 lbs. Per sq. Ln. (mixture in beams, columns or arch-work. Unless you are going to have flrst-class inspection; 400 lbs. Per sq. Ih. Is high enough when the work is going to receive the best class of inspection. It is true that the French use very much higher stresses in their work, but they do a better class or work than we are at present doing in this country. Select a design which gives little tension or shear where possible. Remember that concrete is strong in compression and weak otherwise. The parabola catenary and circle are the best curves for a concrete arch. Arches designed on these lines require little steel reinforcement. Ellip tical arches and basket handles and other flat arches must be heavily reinforced with steel and should not be used unless the waterway or grade demand it. Very flat arches act more like beams than arches, in spite of the elastic theory. Variety OF mixtures. - In your plans and specifications for small work. Don't call for two or three different mixtures of concrete where it is not necessary. You are figuring pretty close when you differentiate between. Say. 1-2-4. 1 255-5 and 1-3-6 mixtures. It is axiomatic that a 1-2-4 is stronger than a 1-3 - 6. Other things being equal. But is the practical diflerence sogreat that you cannot safely get along with a 1-3-6. Or is the added cost or the material of the 1-2 - 4 really a matter of monetary import? If you study the ultimate crushing strength of the several classes of concrete, you will probably conclude that the practical difference as to com pression is not great. I do not mean to say that you should not use a 1-2-4 for reintorced-con cret'e work or other high - stress design; but on plain. Every-day work you will often find a 1 - 2 - 5 or 1-3 - 6 is amply strong for an entire job. In foundations you can frequently use a 1-3-7, 1-4-8 or 1-5-10 in perfect safety it you have good materials and good inspection. If you can in safety do so, specify one lean concrete for your foundations. Say 1 - 4-8, and use a 1 - 2-5 (or depending on your aggregate a 1-255-5) in your superstructure.

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