With these assumptions, investigation was made of walls with various
batters and differently designed backs. This investigation developed the
fact that the reaction from the superstructure was so great that, for
economy, both in first cost and space occupied, the batter must be
sufficient to cause that reaction to fall within or very close to the
middle third. Nothing could have been gained by having that reaction
fall back of the front of the middle third, as the wall was required
to be stable against the full pressure before the superstructure was
erected, and in case it should ever be removed; or, to state the matter
more clearly, the reaction from the superstructure was so great in
comparison to the weight of the wall, that, if it fell in front of the
resultant of all the other forces, the width of base required would be
greatly increased to make the wall stable after the superstructure was
erected; whereas, if the reaction from the superstructure fell back of
the resultant of all the other forces, the width of base could not be
correspondingly decreased without danger of the wall being overturned
before the superstructure was erected. The least batter that would
answer those conditions was found to be 2 in. per ft.

For convenience in designing, and economy in constructing, the
steelwork, the faces of the bridge seat and of the backwall were laid
parallel to the center line of the Terminal, and in elevation on line
parallel to the top of the curb and as near to it as the economical
depth of steel would permit, without bringing the finished construction
above the plane fixed in the ordinance. As there is a variation of 13
ft. in the elevation of the top of the curb of 31st Street above the top
of rail and a variation of 18 ft. in 33d Street, a uniform batter, with
the top parallel to the center line, would produce a toe varying in
distance from it and from the other constructions. It was decided,
therefore, for the sake of appearance, to make the face of the wall