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panel points were in close agreement. The results of this fourth set of calculations were as follows:
The divided-triangular-truss figures indicated a gain of seven per cent
for the continuous-truss layout over the non-continuous one when reversing
stresses were ignored and no gain at all but simply a stand-off when they were
properly provided for. But, when the Pratt-truss was employed, the non-continuous-truss layout showed a gain of two per cent over the continuous-truss one when reversing stresses were ignored and five per cent when they
were properly provided for.
The said results also indicated for short, simple-truss spans that, when
reversals are ignored, there is no difference in weight of metal between
steam-railroad bridges of the Pratt-truss and the divided-triangular-truss
types; but that, when reversals are properly provided for, the latter has
an advantage of five per cent.
It was not deemed worth while to compute the economics for highway
bridges of short spans; but it might be inferred by a priori reasoning, based
on the preceding results, that, in the case of the divided-triangular trussing,
the continuous spans would have an advantage of thirteen per cent when
reversals are ignored and twelve per cent when they are properly cared for—also that in the case of the Pratt trussing there would be no material advantage in continuity.
Without making another set of computations, the author would not
care to employ a priori reasoning for the determination of the comparative
economics of continuous and non-continuous trusses in long-span, highway
bridges of the Petit-truss type, excepting that it seems pretty safe to assume
that the Petit trussing would have no advantage over the divided-triangular trussing, and that the continuous trusses would probably show a small
advantage over the non-continuous ones.
All the actual results of the calculations made for this study are collected in the two following tables, and are expressed in ratios, unity standing for weights of continuous divided-triangular trusses when the effect of
reversals is ignored.
TABLE 11a
SUMMARY OF WEIGHT RATIOS
Divided-Triangular Trussing
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