As the use of counter-braces is to counteract the effects of a variable load, the greatest strain upon them is equivalent to the greatest strain produced by the same load on a brace, and will be equal to that upon the braces of the middle panel. The sectional area will therefore be uniform throughout.
Take the strain on the vertical rod at the abutment as equal to one-half the load on the bridge, and on that next the centre as equal to one-half the load on the middle panel. These rods follow the same rule as the braces under a uniform load in diminishing from the abutment to the centre of the bridge.
The floor-braces are to stiffen the bridge against the effects of high winds, etc, which may be treated as a load uniformly-distributed. Half the force will be borne by the braces next the abutments, increased in proportion of the length of the brace to the width between the girders, and diminishing in each brace towards the middle of the span in the same manner as the braces of the girders, Art. 368.