Rules Specifications

SPECIFICATIONS, RULES, AND TABLES FOR REINFORCED CONCRETE BRIDGE CONSTRUCTION Reinforced concrete construction has been tried and has been proved satisfactory in many thousands of structures, and still it must be admitted that it is in its infancy in the matter of time. For this reason, "good practice" in such construction is not so definitely defined as is "good practice" in the use of older materials in construction. The men most experienced in the use of concrete, however, are pretty well agreed as to the essential requirements for first class work, and there have been a number of rules and specifications adopted by various so cieties, commissions, contracting companies, and railroads, which cover the main considerations, and which together furnish a safe guide to all for construction work. In addition to such spe cifications, there have been numerous articles written by specialists in concrete work, which give an additional source of reliable information for those who wish to accumulate a knowledge of the subject.

The following extracts* are from an article by Mr. W. J. Douglas, Bridge Engineer of the District of Columbia: "Some Points in Detail Design. The first thing that a concrete engineer has to do, after his general lines are *Reproduced by permission from a copyrighted article in "En gineering News," December 20, 1906, subsequently issued as Reprint No. 12.

laid out, is to select the general details of design. . . .

"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 of very flat reinforced arches, the common theory, intelligently applied, is all that is necessary. In the design of the Piney Branch bridge, in Washington, both theories were applied, and the maximum pressures differed under the two analyses by only 5 lbs. per sq. in.

"Don't spend much time worrying about the exact position of the neutral axis of a beam. After you have inspected work under construction for a time, you will be convinced that the way concrete goes into the work does not permit of any hair-splitting application of theory.

"Don't stress your concrete in compression higher than 300 lbs. per sq. in. (mixture 1:2:3) in beams, col umns, or arch-work, unless you are going to have first class inspection ; 400 lbs. per sq. in. is high enough when the work is going to receive the best class of inspec tion. . . .

"Select a design which gives little tension or shear, where possible. Remember that concrete is strong in compression, and weak otherwise. The parabola, cat enary, and circle are the best curves for a concrete arch.

Arches designed on these lines require little steel rein forcement. Elliptical arches and basket-handle and other flat arches must be heavily reinforced with steel, and should not be used unless the waterway or grade demands it. Very flat arches act more like beams than arches, in spite of the elastic theory.

"Steel Reinforcement. It is easier to design concrete work by using steel reinforcement than otherwise. The designer does not have to study his work so carefully in order to make sure that the concrete will not be subjected to tension or shear. When in doubt, reinforce with steel, as it is an axiomatic fact that in all cases it strengthens the work. Steel is particularly useful for beams, flat arches, and structures whose foundations are poor (in order to allow for deformation), and for reinforcing the foundations.

"Concrete cannot be safely used as a beam without reinforcement, except by using absurd depths. Fre quently it will be found, in figuring out the cost of de signs, that unreinforced concrete will be cheaper than reinforced concrete on the basis of the same factors of safety, because of the additional expense required in reinforced work for forming steel and the additional cost of placing concrete in narrow forms. . . .

"Bars having a mechanical bond are stronger than plain rods, particularly in work where the inspection is not the best. Bars having a mechanical bond are particularly desirable in vertical and approximately ver tical reinforcement, because in these positions they are frequently struck by the laborers and mechanics han dling material on the work, and not infrequently by the concrete buckets. When bars are so struck, they lose their adhesion to the abutting concrete, if it is partially set up. This loss of adhesion, in bars of an inch thick ness, will extend down as far as 18 inches from the top surface of the concrete. This is one of the many reasons why light reinforcement (small bars in larger number) is better than heavy reinforcement. When the adhesion is broken, you are forced to rely on the mechanical bond. Rods should not be placed closer to corners or other face work than three diameters. It is not uncommon, where the reinforcement is closer, that the concrete on the outside of the rods (particularly where the reinforce ment is vertical) will spall off. Small mesh reinforce ment must not be used where the aggregate is large, as it is difficult to force the concrete through the mesh so as to completely encase it. It is well in design to keep in mind in a general way the practicability of holding reinforcement in place during the concreting.