for any span length, provided the live load per lineal foot remain unchanged.

C. To find for any span length the truss weight  T '  per lineal foot, for a total load  p'  per lineal foot from the corresponding known weight  T  for a load  p,  the following approximate empirical formula may be used:

This is quite accurate for all ordinary spans, but for very long ones it gives too great a variation between  T'  and  T.

After finding the value of  T',  the value used for  p'  should he checked; and if there be any serious dlsagreemeent between the value assumed and that found, the substitution in the formula should be made anew, and so on until a satisfactory agreement between the said values of  p'  be obtained.

Fourth. General Layout of Structure.— The general layout should consist of a profile, a plan, and enough cross-sections to illustrate properly the entire substructure, superstructure, and approaches, all being made to exact scale. For long crossings, a scale of one fortieth of an inch to the foot is the most satisfactory, but for short crossings the scale should be made larger.

The proportioning of the skeletons of the trusses should be done in accordance with the suggestions given in several of the preceding chapters, and the dimensions of the piers should be determined by the principles established in Chapter XXII.

Each general layout should give the following information:

Elevations of bed rock, low water, standard high water, extreme high water, lowest part of structure, grade-lines and tops of piers, lengths of all spans between centres of end-pins or centres of bearings, distances between centres of piers, all leading dimensions of,piers, heights of trusses, and lengths and kinds of approaches.

As soon as the general layout is completed and finally adopted, the computations of stresses and sizes of members of spans may be begun.