Reinforcing A Post Or Column

A column or fence post should be so reinforced that it will resist tension on all sides because in a fence post, for example, as it is set in a fence line, pulling strains may come upon it from any one of four directions. Animals might try to break in or out of the enclosure. Strain of the wires may come from either direction along the fence line, and as one cannot tell just how the fence post may be required to act, it is necessary to place reinforcing rods at each of the four corners of the square or rectangular fence post and at positions corresponding to a square in a circular post. Rods should be placed just as near the surface as possible and at the same time permit thoroughly surrounding them with concrete.

Example Of Column Loading

The strains that are brought to bear upon a loaded column may be illustrated in a simple manner. Suppose we make a cylinder of tissue paper and fill it with sand. If handled carefully the tissue paper will withstand the tendency of the sand to burst the paper and spread itself out in a pyramidal pile. If slight pressure from one's hand is applied to the top of the filled paper cylinder, the compacting of the sand will cause it to burst the tissue paper. If the cylinder is made of tin, we can readily see that it would require a great deal more pressure than one could exert by hand to break the tin. Going a step farther and taking a concrete column instead of the cylinder of sand, we can realize that if hoops of steel are properly embedded in the concrete as reinforcement, they will act in a manner similar to the tin cylinder in preventing the sand from bursting it.

Reinforcing Materials

As already mentioned, not every kind of material nor every kind of steel is suited for reinforcing concrete. Generally speaking, some types of round or square twisted steel rods or bars are used more extensively for reinforcing concrete than any other material. Next to these perhaps are some of the mesh fabrics and so-called expanded metals. Various kinds of patented bars are on the market. Most of these involve some alteration of the shape of the bar, such as forming lugs or projections on it while rolling the metal and in this way attempting to prevent any slipping of the steel in the concrete-in other words, increasing the mechanical bond. Tests have shown, however, that if concrete is properly proportioned and of the right consistency and properly placed, few if any of these patented types of bars have anything to recommend them for choice in the ordinary classes of concrete construction.

One of several types of expanded metal used as reinforcement for concrete.

Many of the reinforcing fabrics are similar to certain types of woven wire fencing with the exception, however, that the meshes or openings in them are of uniform size throughout the width and length of the material. Expanded metal is made by slotting or cutting thin sheets of steel and then stretching or drawing the sheets so as to increase their area or "expand" them, thus forming a sort of a fabric. The advantage of many of these metal fabrics and expanded metals is readily apparent in such work as stucco or plastered surfaces. They provide a good bond or key for the cement mortar plaster and are also fireproof and practically permanent. Many of the mesh fabrics or different types of expanded metal are particularly suited to reinforcing some of the concrete work illustrated and described in this book.