When contrasting the vertical lifts and the heel-trunnion bascules, it was assumed that, for low vertical clearances, the lengths of moving spans would be the same in the two types, the fenders being placed as close to the piers as possible. Since the bascule can rarely be rotated through more than 83 or 84 degrees, a vertical line through the face of the fender

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near the trunnion will generally intersect the bottom chord of the fully-raised span at some height above the water. This height will depend upon the horizontal distance from the trunnion to the said vertical line, the height of the trunnion above the bottom chord, and the angle of inclination of the said chord to the vertical; but it will generally be about 50 or 60