Wendel Bollman
From The Engineer, 6 March 1857:-
'DESCRIPTION OF THE IRON SUSPENSION AND TRUSSED BRIDGE AT HARPER'S FERRY
'Extracted from an account of it by Wendel Bollman, Master of Track
Repairs on the Baltimore and Ohio Railway, by whom it was designed.
The span of the iron suspension and trussed bridge erected at Harper's Ferry, is 124 feet between abutments. The length of cast iron
in stretcher, 128 feet. The weight of cast iron in the R. R. truss
65,137 lb.; of wrought iron, 33,527 lb.; making a total weight of
cast and wrought iron, 98,664 lb.
The wrought iron requires little workmanship; the rods from the
centre to abutments having an eye at one and a screw at the other
end, with a weld or two between, according to length. The long
counter rods have two knuckles and one swivel for adjustment of
strain, and convenience in welding as well as in raising the whole.
The cast iron stretcher is octagonal without, circular within, and
averages one inch of metal. It is cast in lengths according to the
length of panel, and jointed in the simplest manner. At one end of
each length is a tenon, at the other a socket. The latter is bored out,
and the tenon and its shoulder turned off in a lathe to fit the socket;
thus, when thoroughly joined, to form one continuous pipe between
abutments. The ends of the sections of cylinders, inserted to those
contiguous are slightly rounded, to allow a small angular movement
without risk of joint fracture.
A cast iron plate or washer sets on a bracket cast with each abutment end of stretcher, and at right angles to the centre acting rods.
The tension bars are passed through this washer to receive a screw nut
for the erection and adjustment of the system.
The stretcher or straining beam, the vertical posts and suspension bars, compose the essential features of the bridge: each post being
bung by two bars from both ends of the stretcher independently of
all the others; and each post and pair of tension bars forming with
the stretcher a separate truss.
This system, perfect in itself, is additionally connected by diagonal
rods in each panel; also, by light hollow castings, acting as struts.
The diagonal side rods might be safely dispensed with; for the
peculiar merit of the truss is its perfect independence of such provision. They are therefore used as a safeguard only in case of the fracture of any of the principal suspension rods.
By this combination of cast and wrought iron, the former is in a
state of compression, the latter in that of tension - the proper condition of the two metals. It unites the principles of the suspension and
of the truss bridges. Each bar performs its own part in supporting
the load in proportion to its distance from the abutment; so that the
entire series of suspension rods transmits the same tension to the
points of support as would be equally transmitted from thence to the
centre of bridge.
The tensile resistance of the best American bar iron tables at
80,000 lb. per square inch. Its practical value is generally rated at
about one-fourth the nominal value. In this diagram the highest given value of iron is 16,000 ; being reduced below any probable rate
of fibral separation in any previous data.
Now to proceed for proportion of one rib..... [line diagrams and the results of calculations follow] ....
'This bridge, it will be seen, is composed of seven independent
trusses, which transfer the weight concentrated on each floor-beam
directly to the abutments without aid from any other connexion ; and
not from panel to panel, as in general use.
The strain on cast and wrought iron is wholly in direct line; and
the result, the least quantity of metal is required to carry a given
weight. The weight of bridge and load has a vertical pressure on
the piers, towers, &c., the only horizontal thrust being from the expansion of iron, which is accommodated by rollers, sliding of abutment bracket over its pedestal, or by other means. The necessary
dimensions of masonry may therefore be most moderate.
It is evident, from an inspection of the drawing, that no chord is
requisite at the bottom of the truss to resist tension; the only advantage of that employed is to regulate the movement produced by expansion, in the performance of which agency the resistance is one to compression.
Although the abutment bracket casting and its pedestal were so
constructed as to admit of accommodation to expansion by rollers,
yet such contrivance was omitted with the view of fully testing the
effect of greatest expansion throughout the system.
lt is now ten months since th1s bridge was erected at Harper's
Ferry, during which time it has been exposed to extremes of heat
and cold, and to an average run of twenty trains daily.
From the closest inspection we find that the extreme expansion measures as near as possible, five-sixteenths of an inch on each tower, or
five-eighths in the entire length, 128 feet of stretcher; and without the
slightest perceptible derangement of masonry; the dimensions which are 4 feet square of base, 12 feet high, and 2 feet 9 inches at
top.
While on the subject of expansion it may be well to notice the
effect from difference in expansion of the rods. At the first point of
suspension, or where the longest and shortest rods meet, the counter-rod is about four and a half times longer than the acting rod; and
the expansion of the counter is four and a-half times that of the
acting rod. But there is also a proportionate difference in the lengths
of stretcher from the point directly over the centre of connexion to
the extremities of these rods. This has been practically proved in
this bridge.
The suspender bolt, when the expansion is extreme or five-eighths
of an inch in the length of stretcher, exhibits a motive difference of
three sixteenths towards the short or acting rod; which difference is
provided for, as seen by slot-dotted in elevation, where the vertical
suspender bolt moves to accommodate any such difference, and to give
that proportion of we1ght to each rod according to the angle.
It affords easy access for repairs; for instance, should a new floor
beam be required, it is but needed to slacken the horizontal rod and
the keys in longitudnal strut, remove the washer under point of suspension, and let down the beam to be replaced, which can be done
without trestling up any part of the bridge.
In case of fire, the floor may be entirely consumed without any injury to the side truss.
'The permanent principle in bridge building, sustained throughout this mode of structure, and in which there is such gain in competition with every other -viz., the direct transfer of weight to the abutments, renders the calculation simple, the expense certain, and facilitates the erection of secure, economical, and durable structures.'