Atbara Bridge
1899 'As a result of a question asked in the House of Commons
by Sir Alfred Hickman, the documents relating to the contract for the Atbara Bridge have been issued as a Parliamentary panper- Egypt, No. 6, 1899. The purchase of the
bridge superstructure was in the hands of Lieutenant-Colonel
W. S. Gordon, R.E., and the interest of the papers lies
almost entirely in his report. From this we find that it was
evident that if the bridge was to be completed before the
floods came down it would have to be of a type that could be
launched or built up without false work in the river. The
whole question, therefore, became one of quick delivery:
price was practically not considered. Four English firms
and two American were asked to quote in accordance with
a rough specification. Only one British firm - the Patent
Shaft and Axle Company - tendered, and apparently the least
time in which they could deliver the bridge was about
twelve mouths, and they felt confident that no other British
firm could complete it in a shorter time, as they had
special facilities. Under these circumstances every effort to
get q ick quotations elsewhere were made. The Pencoyd
Company, Philadelphia, replied to an invitation to quote
that they offered eight 150ft. through spans at 28,000 dols.,
delivered in six weeks, the load to be two consolidation
90-ton locomotives, and the train load 26 cwt. per linear foot.
This quotation slightly modified was finally accepted. Tbe
report concludes as follows
"The whole of the material for the bridge has been inspected by a representative sent from England by Colonel
Western, and a copy of his report is attached to this memorandum, from which it will be seen how thoroughly the work
has been carried out.
"I have not referred to price, since that point was not
considered by us of such great importance as the time for
delivery; but, having in my former memorandum stated that
the price quoted by the nearest English firm was £10,400, I
may mention that this was calculated on the weight of the
English bridge being the same as the American; but I
now gather that it would have been about 200 tons more,
and at £15 15s. per ton would mean an increase of £3150.
"Finally, I wonder that comment has not been made on
the fact that the more expensive portion of the bridge was
contracted for by an Italian fir ; the answer to this being
that there is only this single firm in Egypt possessing the
required plant for sinking cylinders, &c.; and, until a British
firm set up a plant in this country, I conclude that all
bridges whose superstructures built as those of the
Atbara and others will continue to be contracted for by this
Italian firm."
The final report of inspection and tests of the Atbara Bridge, as built by the Pencoyd Ironworks, Philadelphia,
which is also among the papers, states that work in the shop
actually began on February 8th, but little progress could be
made on account of difficulty in procuring materiaJ, especially
the plates, in sufficient quantity to allow the shop to make
a fair start on the work. It was further delayed by the
great snowstorm on February 13th until the 19th, when
the work fairly began. The first shipment was made on
February 25th, 1899. The second shipment was made on
March 15th, 1899. The character of the workmanship was very good, and was in accordance with American practice.
Where finish was required for the transmission of stress or
for the strength of the piece it was done ; where it would
serve no good purpose it was not done, which was in accordance with American practice in all class of structural and
constructional work. The character of the work was quite
satisfactory.'[1]
1899 Technical data and brief description of the bridge in The Engineer, 2 June 1899[2]
See here for a recent American articleon the bridge.
1910 Reconstruction
'Probably no bridge contract ever gave rise to so much
controversy as did that placed in the early part of
1899 with the Pencoyd Ironworks (A. and P. Roberts),
of Philadelphia, for the steel superstructure of the
bridge required to carry the Sudan Military Railway over the river Atbara, thus rendering possible the extension of
the railway to Halfaya - Khartum North. Full detailed
particulars of this works were given in THE ENGINEER of
June 2nd, 1899, and subsequently in our issue of
August 11th of the same year - the circumstances under
which the contract was given to an American firm in
preference to British engineers were set forth at some
length. It may, however, be useful to recall that the
Atbara bridge is of the ordinary standard American single-track through pattern, of seven spans, each 147ft. centre
to centre of end pins of trusses, and of a total length of
105ft. between end piers. The trusses have their
bearings on cylinders, 8ft. 6in. in external diameter, and
built up of 1/2in. plates, finished at the top with cast iron
caps. Each span consists of two trusses, placed 16ft. 2in. apart from centre to centre, connected at the top by a
system of horizontal bracing and at the bottom by bracing
and floor beams carrying the railway track. The trusses
are parallel, divided into five completely braced panels and
two end struts. The length of each of the five panels and
the two end bays is 21ft. The total weight of the seven
spans, including cast iron cap plates, is 1,391,899 lb. The
assumed dead load of the structure was:-
[Table of weights]
The assumed live load was two locomotives, one weighing 46 tons, the other 34 tons, followed by a train of one
ton per foot run. The limiting stresses were nine tons
per square inch of dead load and live load multiplied by
1.5, provided that the dead load assumed was never less
than half the live load.
After having been in use for less than eleven years, the
main girders of the Atbara bridge have become too weak for the present rolling loads, which have been very greatly
increased during recent years. The cross girders and rail
bearers are, however, of sufficient strength, and it has,
therefore, been decided to renew the main girders and
use the old cross girders and stringers. During the past few days a contract has been awarded by the Sudan Government to the
Cleveland Bridge and Engineering Company, Limited, of Darlington. The new structure will comply with the following
special conditions :-(1) The new main girders to be
accommodated to the existing bearings, which rest on
cast iron cylinders and cannot be removed; (2) the clear
width throughout the structure to be of a minimum of
15ft.; (3) the main girders to be built so as to suit the
existing cross-girders, and to have provision for the
attachment at some future time of sidewalks on both
sides of the bridge, supported on brackets. The existing
cross-girders have a length overall of 15ft. 3 1/2in. The
web members of the new main girders will have a width
of 1ft., in order to provide sufficient lateral stability, and
assuming the connection plates of the main girders to be
1/2in. thickness, the total width of the new spans from
centre to centre of main girders will be 15ft. 4 1/2in. The
width of the existing bridge is 16ft. 2in. The extra width
of 2 1/2in. will be provided by an alteration in the upper
portion or "saddle" of the present bearings. This saddle
is built up of plates and angles, and rests on rollers at the
expansion end and on a bearing plate at the fixed end of
the span, and carries the main girder itself by means of a
pin 6in. in diameter. When the main girders are renewed
it will be necessary to remove the saddle and to detach
the vertical webs from the base plate. A new base plate
will be provided and drilled at the site of the bridge, with
holes for the connection of the vertical webs to suit the
main girders. The bearings will then be riveted up.
Apart from this alteration it is proposed to use the existing
bearings. The pin and rollers are of sufficient strength,
and any alteration of the rollers will necessitate raising
the rail level of the bridge. The direct connection of the
existing cross-girders to the new main girdera will
make an excellent joint at this point and will obviate any
necessity for special packing. The only difficulty involved
is in the provision of sufficient clearance at the end posts
of the bridge. The usual design of a girder of this
character would provide for end posts having a construction similar to that of the top boom. If this course were
followed, however, the clear width would be only
14ft. 6 1/2in. The end posts will therefore be built on the
same basis as the intermediate posts of the girder. They
will be of exceptionally heavy construction, and the posts
throughout have been designed to provide for the bending
stress produced by the wind on the upper portion of
the bridge. It is proposed to connect the cantilever
brackets by means of turned bolts, and, as all the
drilling will be done at the Cleveland Bridge Company's
works, it will not be necessary to carry out any work
on the steel work beyond the fastening of the bolts, when
it is decided to equip the bridge with sidewalks. These
bolts have been designed for a live load of 1 cwt. per foot
over the full area of the sidewalk, together with an ample
allowance for dead load, and 66 per cent. in excess of the
number required has been assumed in order to give an ample margin of safety. Only those bolts which are
symmetrically disposed about the centre are assumed to
take stress, and the plate and angles to which the bolts
will be connected are to be made specially stiff to give an
equal distribution of stress over all the bolts. The tension
of the bolts will not produce any excessive stress on the
posts, and the stress will again be transferred to the cross
girders by the stiff gussets with which they will be
provided. The main girders have been increased to a depth of 26ft., which will provide for a clear headway of
17ft., and stiff post and portal bracing will serve to transmit the wind stress on the top chord to the lower
system of bracing. The panels are to be similar to those
on the present bridge; the top boom will be o£ "box" section, open on the underside and provided with lattice
bracing. The lower boom will be made of two 15in.
channels with extra web plates to make up the necessary
section. This part of the new bndge will be similar to
that of the existing bridge, in order to accommodate the
cross-girders. The two channels will be tied together
by the posts and by a lateral plate on the underside at
every post.
The contract includes the building of a temporary
bridge to carry the deviation of the railway across the
river Atbara, this structure to be of such construction as
to carry the main line traffic of the Sudan Government
Railway system. The work is to be commenced not later
than December 1st next, and the new bridge to be completed by the Cleveland Bridge and Engineering Company, Limited, not later than May 31st, 1911, or in time for use before the rising of the waters of the Atbara River.[3]