In selecting a chord section the radius of gyration should be kept as large as possible, the member at the same time satisfying the following requirements: (1) The thickness of the top cover plate should not be less than 1/40 the distance between the centers of the rivets connecting the plate to the angles or channels. (2) The thickness of the side plates should not be less than 1/30 the distance between the centers of the rivets connecting it to the angles. (3) The angles should not be thinner than three-fourths the thickness of the thickest plate attached to them. (4) The radius of gyration of the member about both axes should be approximately the same. Areas, moments of inertia, radii of gyration, eccentricities and other data for built chord sections are given in the tables in Appendix III. DESIGN OF COMPRESSION MEMBERS.—The allowable stresses in compression members are given in the standard specifications in this chapter. For the details of the calculations of the moments of inertia, radii of gyration and allowable stresses in compression members, see Chapter VI. Lacing.-Lacing bars are used to join the parts of the member together, and make it act as a solid member to resist the shear due to bending and the diagonal shear in the member. Lacing bars are commonly made with a thickness of not less than 1/40 the distance between end rivets for single lacing, or 1/60 of the distance between rivets for double lacing riveted in the middle. The spacing should be such that the part of the column between the rivets is stronger than the column as a whole. Specifications require that the lacing bars make an angle with the axis of the member of from 60 to 45 degrees. The American Bridge Company's standard lacing bars are given in Fig. 8. Design of Lacing Bars.-The lacing bars in a column hold the parts of the column in line, carry part of the diagonal shear, and transfer part of the stress in columns with an eccentric loading. The stresses in the bars required to hold the parts of the column in line are small for stresses in the column within the elastic limit of the material. The maximum diagonal shear, S, in a solid member is S P, where P = the total direct axial load on the member. In columns composed = = of channels, angles, etc., the flange area is ordinarily sufficient to carry this shear without producing large stresses in the lacing bars. The moment, M', due to the eccentric loading is M' P.e, where P the total direct load on the column and e = the eccentricity of the loading. The lacing bars will take the shear due to this bending moment, if the flanges are light. It will be seen from the foregoing that the stresses in lacing bars depend (1) upon the make-up of the column, (2) upon the care used in building the column, and (3) upon the eccentricity of the loading. For a column with a concentric loading, experiments show that the allowable unit stress may be represented by the straight line formula P 16,000 - 70l/r lb. per sq. in., where P = allowable unit stress in the member, = length of the member, c to c of end connections, and r = radius of gyration of the column, both in inches. Now the allowable unit stress on a short block is 16,000 lb., and the 70l/r represents the increase in the fiber stress in the column. Now if we assume that this fiber stress is caused by a horizontal load, W, applied at the center of the height = |