CHAPTER XXIV. Explanation of Tables. GENERAL REMARKS. The following tables have been added to this volume in order to form a ready reference for the inspectors and builders while in the field. Some of these tables are entirely indispensable for the inspector in checking up the sizes of various steel beams or columns. Other tables may be used advantageously in investigating floor framings in cases of overloading, and under many other conditions. The material in some of these tables has been compiled by the author from various tables in common use at the present time. Many good tables are found in the mill books issued by the various rolling mills, like the Carnegie Steel Company, the Cambria Steel Company, the Bethlehem Steel Company, and so on. The author believes, however, that the tables contained in this volume will be found sufficient for all field purposes. Of course, the various tables in existence have been carefully compared, and the doubtful figures recomputed. All the tables have been rearranged, most of them have been extended in order to make them more useful for field uses, and several of the tables are entirely original. In considering them in order, wew shall point out a few of the many ways in which these tables can be used in inspecting iron and steel constructions. Table I. Wire and Sheet Metal Gauges. A "wire gauge" is a method of indicating the diameter of wires or the thickness of sheets of metal by referring to the numbers of a table arranged on a certain fixed and arbitrary basis. There are at present at least ten different wire gauges, resulting in great confusion. The most important gauges in use in this country are as follows: The United States Standard Steel Plate Gauge, which is the only legal gauge in this country. It is given in the fourth column of the table, and is mostly used by the manufacturers of sheet iron, steel and tin-plate. The Brown & Sharpe guage is given in the second column of the table, and is commonly used for copper wires, sheet copper, brass, and sheet iron or steel, i. e., by special order of the Bureau of Buildings all outside metal smoke flues in Manhattan must be made of galvanized sheet steel, not less than No. 8 B. & S. gauge in thickness. According to the table, this would mean a thickness equal to .128490, or a little over one-eighth of an inch. Similarly, the metal used in making iron treads and risers for interior stairways is often specified to be of No. 12 gauge. With poor supervision this would be replaced by sheet metal No. 16 gauge. To determine gauges in the field, either one of these methods may be used: (a) By means of a gauge ring. This consists of a circular metal plate with indentations all around the outer edge. The indentations are made to correspond with the diameters of the various wire gauge numbers. (b) By weighing any convenient portion of the sheet metal, say, several square feet. The weight per square foot for steel plates corresponding to the various gauge numbers will be found in the third column of the table. (c) The gauge numbers are generally painted on the original plates by the manufacturers. Table II. Shearing and Bearing Value of Rivets. The values given in this table are safe values, in accordance with the New York Building Code. Single shear is figured at 10,000 pounds per square inch. As the ultimate shearing strength for steel is about 50,000 pounds per square inch, it follows that good rivets will stand in shear before failing about five times as much as given in the table. The shearing resistance of a rivet in double shear is just twice the value for single shear given in the fourth column. Table III. Length of Rivets for Various Grips. In this table the length of the rivet is taken to mean the distance from under the head of the cold rivet to the free end of the shank. This is the common meaning of the length of a rivet or bolt and is illustrated on top of the table. Table IV. Properties of American Standard and Special I-Beams. This table gives the depth, weight and area of American sections. The thickness of the web is also given. Column five gives the width across the top of the beam or the width of the flange. Checking Sizes. The figures in determining the weight of a beam. column five are used in For instance, a 24 in. I weighs 80 pounds per foot when it measures 7 in. across the flange; or it weighs 100 pounds per foot when the width across the flange is 7.25 in. As an additional check compare also the thicknesses of the webs, as given in column four. Moment of Inertion. Consider an I-beam section as shown at the bottom of Table V. Draw this section for any one I-beam in full size. Then divide the section into, say, twenty equal parts. Then draw line AA. Now multiply the area of each part by the square of the distance between the centre of such part and the line AA. You will get twenty products. Their sum is an approximate value of the Moment of Inertia of that particular section and will not be far from the values given for each section in column six. The line AA is called the Neutral Axis at right angles to the web. The line BB is another neutral axis, but parallel to the web. The Moment of Inertia of a section is the algebraic sum of all the products obtained by multiplying each small particle of the area of the section by the square of its distance from the neutral axis. The moments of inertia for various sections with reference to axes AA and BB are given in columns 6 and 7 under I and I' respectively. Radius of Gyration. Divide the moment of inertia by the area in square inches. The square root of the number thus found is called the radius of gyration. The radius of gyration for various sections are given in columns 8 and 9. Section Modulus. Divide the moment of inertia by onehalf the depth of the section in inches. The result is the section modulus. Section moduli for various beams are given in column 10. This is the more used section modulus, or the one about the axis AA. Table V. Properties of American Standard and Special Channels. The arrangement of this table is similar to that of Table IV., and no further explanation is deemed necessary. Table VI. Properties of Standard and Special Angles. This table gives the size, weight and area as well as other properties of angles with equal legs. Column 4 gives the distance from the centre of gravity of the angle to the back of the flange. The moments of inertia, section moduli and radii of gyration about two axes are given. These axes are shown in the diagrams as AA and BB. For a list of common angles with unequal legs see Table XIV. |