Form Construction

Causes of Failure.

Sanford E. Thompson, on "Forms for Concrete Construction," in a Na tional Cement Users' Association bulletin, says: "Recent failures in reinforced concrete construction cannot be cast to one side and forgotten with the passing comment so frequently heard that the accident was due merely to poor construction or too early removal of forms. The reasons for every failure should be thor oughly investigated by experts to prevent recurrence of similar accidents.

"Forms, although frequently guilty, are by no means the only culprits. In fact they are frequently blamed when the designer is at fault. Just so long as men who know nothing of the first principles of mechanics are permitted to design concrete structures, and just so long as irresponsible contractors are engaged to erect them, the list of accidents will increase in startling numbers. In every case it is the men, not the inanimate lumber and materials, who are to blame. However, granting its danger in ignorant hands, reinforced concrete as a whole must not be condemned for failures due to im proper conditions, any more than brick should be rejected as a building material for apartment houses, because of the collapse of several unfinished buildings in New York City a short time ago through disregard of frost action upon the mortar." Failures in concrete buildings may be at tributed to: (1) Imperfect design, especially through neglect of essential details in locating the reinforcing metal, and through the adoption of too low a factor of safety.

(2) Poor materials, such as cement which does not properly set up, or sand which is too fine or which has an excess of clay, loam, or other impurities.

(3) Faulty construction, from improper propor tioning, mixing, or placing, or too early removal of forms.

(4) Weak forms.

"A disregard of such important principles," says Mr. Thompson, "is frequently criminal negligence; and yet, in at least one case under my observation, an examination of the structure and the materials, after a collapse in which a number of lives were lost, showed the design, materials, and construction all faulty, so that it was impossible to decide positively which of the four causes named above was the primary reason for the failure.

"Certain general rules are applicable to all kinds of forms. Strength, simplicity, and sym metry are three fundamental principles of de sign. The necessity for strength is obvious, while economy in concrete construction consists in quickly erecting and moving the forms and in using them over and over again.

"The design of the concrete members should recognize the forms. A slight excess of concrete sometimes may be contributed to save carpenter work. Frequently beams may be designed of such widths as to use dimension widths of lum ber without splitting.

"Columns may be of dimensions to avoid frequent re-making. Parcel recesses in walls may be made the thickness of a board or plank. To permit ready cleaning of dirt and chips from the column forms before laying the concrete, at least one prominent contractor provides a door at the bottom of each of them.

"In building construction, the forms must be designed so that the column moulds and also the bottom of beam moulds are all independent of the slabs. The forms may thus be left a longer time upon members subjected to the greater stress.

"The sides of the beam moulds should be held tightly together by wedges or clamps, to prevent the pressure of the concrete springing them away from the bottom boards. At top or bottom of each strut, hardwood wedges are useful when setting and removing it, and also permit testing, to make sure that there is no deflection of the beam or slab. For this purpose, some con tractors loosen the wedges twenty-four hours in advance of the struts. In general, it is prefer able to use comparatively light joists, such as 2 by 8-inch or 2 by 10 inch, with frequent shores, rather than to use lumber which is heavier to handle.

"If forms are to be used but once or must be taken apart when removed, it is sometimes prac ticable to use only a few partially driven nails so that they can be withdrawn without injury to the lumber. It is very difficult to convince house carpenters that the pressure of the concrete will hold temporary panel boards in place with scarcely any nailing."