In this chapter will be treated only the salient economic features of ordinary designing and detailing, because to attempt to do more would lengthen this portion of the book beyond all reason.

In all cases it is economic to choose the most simple types of structures and details, and especially those that lend themselves best to stress analysis.

One should select members which by form and location are best adapted to resist forces economically and to carry stresses by shortest routes. For instance, in a long-span truss with parallel chords, shear is transferred entirely by the web members; and the path of the web stresses is much shorter by the divided-triangular truss than by the Pratt truss, hence the use of the former is economical. But in short-span trusses the verticals are nearly all of minimum section, hence there is but little, if any, difference in the economics of the two types. Similarly, with polygonal top chords, the shear on the webs being comparatively small, there is not much to choose between the said two types.

In riveted tension members the riveting should be arranged so as to reduce to a minimum the number of rivet-holes to be taken out of a section; but if the number assumed be too small, material will be wasted at splices in developing the net section, because extra holes above the number that would normally be employed cannot be used at joints unless extra section is first developed into the splice-plates or gussets. Furthermore, the section will very likely be reduced in detailing, without the fact being noted. This will overstress the metal at such sections.

In compression members the metal of the section should be arranged to secure the largest practicable radii of gyration without involving the use of metal that by its thinness would transgress the rules of standard specifications, viz., minimum limits of one thirty-second of unsupported width for webs, and one fortieth thereof for cover-plates. It is economic to employ fairly heavy angles, as thereby the radii of gyration are increased and, in consequence, also the strength of the strut. If the unbraced length of the piece is greater in one direction than in the other, the section should be so arranged that the values of the ratios of length to radius of gyration are approximately equal. For instance, in the case of intersecting diagonals of lower lateral bracing, the unbraced length in one direction is twice as great as that in the other. Then for 2-angle sections unequal-legged

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