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In determining the comparative economics of continuous and non-continuous trusses for any proposed bridge, the application of the preceding
findings would have to be somewhat modified in case the structure has to be
erected by semi-cantilevering. Under such a condition the continuous
trusses have an advantage over the non-continuous ones, at least to the
extent of the extra metal required by the toggles for the latter over the
center pier. Again, it is probable that some of the lighter truss members in
either type will need reinforcing for erection stresses; and this consideration
is likely to affect the non-continuous trusses more adversely than it does
the continuous ones.
Summary of Conclusions
Summarizing the results of the entire investigation, the following conclusions are reached:
First. For long spans the divided-triangular trussing is decidedly
superior to the Petit trussing for bridges with continuous-truss spans, but
not much so, if at all, for those of simple-truss spans.
Second. For long spans there is an important saving of metal by the
adoption of continuous trusses, and the said saving is nearly twice as great
for standard highway bridges as for modern, double-track railway-bridges.
Third. For long-span bridges the method of treating the matter of
stress reversal has practically no effect upon the comparative economics of
continuous and non-continuous trusses.
Fourth. For comparatively-short-span, steam-railway bridges, the
continuous truss has a small advantage over the simple truss only when the
divided-triangular trussing is used and stress reversals are ignored. In
all other cases the comparison is either a stand-off or in favor of the simple
truss.
Fifth. For comparatively-short-span, steam-railway bridges, the
divided-triangular trussing is generally more economic of metal than the
Pratt trussing.
Sixth. In no case should either the Pratt or the Petit truss be employed
for continuous spans, because in these the divided-triangular truss is more
economic.
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