Delaware gazette and state journal. (Wilmington, Del.) 1883-1902, 1893-01-05

Enicientys
4. Graham Gribblc, in his department
of civil engineering in the current
number of The Engineering. Magazine,
thus summarizes the several types OF
movable bridges hitherto constructed.
They are:
First, swing-bridges. These are three
fold: (1) Those having centre pier IN
the waterway about which the whole
bridge rotates, so as to form two open
ings; (2) those having their centre Oh
rotation on one of the river banks, the
further end spanning the whole water
way and the shore end counterbalanceu;
(3) those having centres OF rotation at
both banks, each shore end being coun-
terbalanced and the outer ends meeting
at the centre of the stream.
Second, traversing or draw bridges.
In these bridges the river Is spanned by
the protrusion of the bridge either en-
tirely from one side or by two portions,
one from either bank SO as TO meet in
the center.
Third, lifting-bridges. In this type
the entire bridge is suspended from
towers ut either side of the stream and
countcrbalanced like window in its
sash frame, so that the lifting required
is only that due to tho resistance OF
| of friction.
Fourth, bascules. These are bridges
which might be termed also draw
| bridges, because they are built on the
same principles as those guarding the
| ancient castle meats which were termed
| draw-bridges Either one or two leaves
rotate about a horizontal axis like the
lid Of box. All other rotating bridges
turn about vertical axis. The Hartman
folding bridge is bascule having two
leaves, each of which has two indie
| pendent axes of rotation.
Fifth, pontoon swing bridges. In this
type one shore end is on a pivot having
vertical axis. The other end is fur
nished with Il pontoon which permits
the end to rest on the bank, but, when
the bridge is to be opened, water is
pumped into the pontoon to lift the end
off its bearing. The pontoon is fur
nishcd with propeller and ferries tho
end round to the other bank, the bridge
lying lengthwse of the stream and tho
pontoon running into recess.
There is space here for only few
general remarks upon the relative
efficiency of these various types.
The swing-bridge is adapted TO the
widest waterways and is most effective
where there is plenty of room for the
centre pier. Its advantage is that it
leaves the banks of the river entirely
free to ships at their moorings, and that
it requires no counter balancing. Its
disadvantage is that the centre pier oc-
cupies that part of the river which is
best adapted tonayigation, and in a mar
row stream leaves less room for vessels
l 4 -
wharyes. In the single form with a
shore end counterbalanced the advant-
) ages previously stated are lost at the one
end, while the waterway is, on the Other
hand, left more free.
, The traversing bridge has the advan.
tage of leaving a clear waterway, but il
, requires a considerable space for the
overlapping ends, and it offers small re-
sistancc to coliding vessel. 1t also re-
Qures considerable material for the
Counterbalance
The lifting bridge occupies no more
: room on the shore than that occupied
by tho towers; but these are expensive
. structures, first, because they have to b.
i Of great height to accommodate maste,
a vessels; second, because they must sup
/ port double the weight of bridge,
being counterbalanced.
v The pontoon bridge is Simple AM
cheap movable bridge. 1t gives a clea
4 waterway with the exception of tn
q space occupied by the bridge along th
=. shore when open. Its disadvantage
y that where there is a swift current tn
). pontoon. unless provided with powerful
:. engines, is slow in ferrying the free en-,
round. 1t also requires a good deal o
II clcarway for its sweep, and, general,
II speaking, is more suitable to waterway
II with moderate traffic.
The bascule as hitherto constructed
is adapted Only to small spans. Tn
largest yet At Copenhagen. 1t wa
opened in 1861 and covers a passage Q
:, So feet inches. A pair of leaves coun
J" terbalanced at the shore ends flap uT
wards. The tail ends are UNUSUAL
: short--13 feet inches in length.