sign, the two
tables agree very well. In bridge work this occurs only in some of the
bracing bars.

It is a matter of discussion whether, if fatigue is allowed for by the
Weyrauch method, an additional allowance should be made for impact. There
was no impact in Woehler's experiments, and therefore it would seem rational
to add the impact allowance to that for fatigue; but in that case the
bridge sections become larger than experience shows to be necessary. Some
engineers escape this difficulty by asserting that Woehler's results are not
applicable to bridge work. They reject the allowance for fatigue (that is,
the effect of repetition) and design bridge members for the total dead and
live load, plus a large allowance for impact varied according to some
purely empirical rule. (See Waddell, _De Pontibus_, p.7.) Now in applying
Woehler's law, f_{max.} for any bridge member is found for the maximum
possible live load, a live load which though it may sometimes come on the
bridge and must therefore be provided for, is not the usual live load to
which the bridge is subjected. Hence the range of stress,
f_{max.}-f_{min.}, from which the working stress is deduced, is not the
ordinary range of stress which is repeated a practically infinite number of
times, but is a range of stress to which the bridge is subjected only at
comparatively long intervals. Hence practically it appears probable that
the allowance for fatigue made in either of the tables above is sufficient
to cover the ordinary effects of impact also.

English bridge-builders are somewhat hampered in adopting rational limits
of working stress by the rules of the Board of Trade. Nor do they all
accept the guidance of Woehler's law. The following are some examples of
limits adopted. For the Dufferin bridge (steel) the working stress was
taken at 6.5 tons per sq. in. in bottom booms and diagonals, 6.0 tons in
top booms, 5.0 tons in verticals and long compression members. For the
Stanley bridge at Brisbane the limits were 6.5 tons per sq. in. in
compression boom, 7.0 tons in tension boom, 5.0 tons in vertical struts,
6.5 tons in diagonal ties, 8.0 tons in wind bracing, and 6.5 tons in cross
and rail girders. In the new Tay bridge the limit of stress is generally 5
tons per sq. in., but in members in which the stress changes sign 4 tons
per sq. in. In the Forth bridge for members in which the stress varied from
0 to a maximum frequently, the limit was 5.0 tons per sq. in.,