THE KANSAS CITY BRIDGE. 109

The draw was built open upon false-works extending up and down the
river ; on the removal of the temporary supports it became a beam balanced
upon its centre, and the same circumstance is repeated whenever the draw is
open. The top chord is then in tension, and the
bottom chord in compression, the strains tending lia
to distort the beam in the manner shown in Fig. a
1. The strains are greatest at the centre, where
the moment of flexure is :—

364" 960 __

Mf = f

~ 15,899,520 pounds.

and the moments throughout the beam will be proportional to the external ordi-
nates of a parabola.* The chord strains will be equal to the moments divided
by the depth of truss, the central strain being

— M- 34.3 — 463,500 pounds.

When the draw is closed, the wedge-plates are driven home under the end
posts, giving the truss a bearing on the piers at each end. The power applied
through these wedges is not sufficient to lift the ends of the draw, but merely to
bring them in contact with the end supports, and prevent them from settling
when a moving load passes. The whole weight continues, as before, to be car-
ried by the pivot pier, and no change takes place in the strains.

When a train enters the draw, the variable load thrown upon one arm, will
be borne in part by the wedges and in part by the turn-table. It will cause
the loaded arm to deflect, and at the same
time lift the end of the unloaded arm from
its bearing on the wedges; the distortions
caused by these deflections will be similar to
those shown in Fig 2, and will increase from the instant the engine enters the
draw until the whole arm is covered by the load. |

The strains at the centre and in the unloaded arms remain unchanged ; those
in the loaded arm become equal to the sum of the strains already existing, plus

 

 

* A diagram of the curves indicating the Moments in the Draw, is given on Plate XII; the curve of

Moments in the Open Draw is drawn in a plain black line.
+ The passage of a heavy freight train has caused the further end of the draw to rise | of an inch.