r rivets always, and sometimes for the whole superstructure of a
bridge, but medium steel more generally for the plates, angle bars, &c.,
the weight of the bridge being then reduced by about 7% for a given factor
of safety. Moderately hard steel has been used for the larger members of
long-span bridges. Hard steel, if used at all, is used only for compression
members, in which there is less risk of flaws extending than in tension
members. With medium or moderately hard steel all rivet holes should be
drilled, or punched 1/8 in. less in diameter than the rivet and reamed out,
so as to remove the ring of material strained by the punch.

In the specification for bridge material, drawn up by the British
Engineering Standards Committee, it is provided that the steel shall be
acid or basic open-hearth steel, containing not more than 0.06% of sulphur
or phosphorus. Plates, angles and bars, other than rivet bars, must have a
tensile strength of 28 to 32 tons per sq. in., with an elevation of 20% in
8 in. Rivet bars tested on a gauge length eight times the diameter must
have a tensile strength of 26 to 30 tons per sq. in. and an elongation of
25%.

18. _Straining Actions._--The external forces acting on a bridge may be
classified as follows:--

(1) The _live_ or _temporary load_, for road bridges the weight of a dense
crowd uniformly distributed, or the weight of a heavy wagon or traction
engine; for railway bridges the weight of the heaviest train likely to come
on the bridge. (2) An allowance is sometimes made for _impact_, that is the
dynamical action of the live load due to want of vertical balance in the
moving parts of locomotives, to irregularities of the permanent way, or to
yielding of the structure. (3) The _dead load_ comprises the weight of the
main girders, flooring and wind bracing, or the total weight of the
superstructure exclusive of any part directly carried by the piers. This is
usually treated as uniformly distributed over the span. (4) The _horizontal
pressure_ due to a wind blowing transversely to the span, which becomes of
importance in long and high bridges. (5) The _longitudinal drag_ due to the
friction of a train when braked, about one-seventh of the weight of the
train. (6) On a curved bridge the _centrifugal load_ due to the radical
acceleration of the train. If w is the weight of a locomotive in tons, r
the radius of curvature of the track, v the velocity in feet per sec.; then
the horizontal force