Excerpts from John Wolfe Barry's The Tower Bridge. A Lecture (1894), composed in 1893. As Barry explains in a brief preface, the account was prepared partly for people "well acquainted with engineering matters," but "more largely" for those who are simply interested, and who may "wish to form a general conception of its design and of the various considerations which led up to its inception and determined its mode of execution." — excerpts selected, most of the headings added, and photographs (when not otherwise noted) by Jacqueline Banerjee [Click on the images to enlarge them.]
The West Side of Tower Bridge. Photograph (2013) by Ruth M. Landow. [East side of the bridge by the same photographer.]
The work of the foundations was troublesome and tedious, owing to the isolation of the piers, and still more to the great amount of river traffic, rendering the berthing of barges difficult. The substructure thus occupied a considerably longer time than was anticipated. (36)
The Fixed Superstructure
Two photographs showing details of the bridge's construction: Left: The massive joint that connects two sections of the bridge. Right: The laminated steel plates. Photographs by Ruth M. Landow.
The fixed parts of the superstructure of the Tower Bridge consist of two shore spans, each of 270 feet, and of a central high level span of 230 feet. The fixed bridge is of the suspension form of construction, and the chains are carried on lofty towers on each pier and on lower towers on each abutment…. (45)
The north tower with its pier
The piers of the Tower Bridge are essentially different from the piers of an ordinary bridge, inasmuch as they have to contain the counterpoise and machinery of the opening span, as well as to support the towers which carry the suspension chains of the fixed spans and the overhead girders above the opening span. They are thus very complex structures... Their total depth from the roadway level to the London clay, on which they rest, is 102 feet. (23, 25)
The mode adopted for spanning the landward openings is by suspension chains, which in this case are stiffened. The chains are anchored in the ground at each end of the bridge, and united by horizontal ties across the central opening at a high level .... These ties are carried by two narrow bridges 10 feet in width, which are available as foot bridges when the bascule span is open for the passage of vessels. The foot bridges are 140 feet above Trinity high water, and, as their supports stand back 15 feet from the face of the piers, their clear span is 230 feet. Access is given to them by hydraulic lifts and by commodious staircases in the towers. (26-27)
[The landward crossing] is divided into 36 feet for the vehicular traffic and into two pathways each 1 2 feet wide. I may mention in passing that London Bridge is 54 feet wide between the parapets. (53)
The Span for River Traffic
The stipulated dimensions of the opening span [provide], when the bridge is open for ships, a clear waterway of 200 feet in width, with a clear height throughout the 200 feet of 135 feet (which has been increased in construction to 140 feet) from Trinity high water mark. I may mention in passing that I think these dimensions constitute the largest opening span in the world. The next largest opening is, I believe, at the Newcastle bridge, where there are two separate spans of 100 feet each. (36-37)
Opening the Bridge
An early-twentieth-century photograph of Tower Bridge by Feist and Co., publishers of postcards. [the postcard version].
I should mention that when the two leaves of the opening span are brought together, there will be long wedge-shaped bolts, actuated by hydraulic machinery, fixed on one leaf and shooting into the other leaf, to complete the union of the two. All the machinery of the opening span will be worked from cabins on the piers, in which there will be levers like those in a railway signal box, so interlocked one with the other that all the proper movements must follow in the arranged order. (44)
The time required for the actual movement of the opening span from a position of rest horizontally to a position of rest vertically is estimated at about 1½ minutes. To this must be added the time necessary for stopping the road traffic and clearing the bridge, and withdrawing the bolts. This may take, perhaps, some 1½ minutes more, and we then have to add the time for the passage of a ship and the lowering of the bridge. The time of I½ minutes for opening or shutting the bridge gives a mean circumferential speed at the extremity of each leaf of 2 feet per second, which is a moderate speed for an opening bridge. (44)
Left: Heraldry on the bridge. Right: The iron span.
Signals will be provided by semaphores by day and signal lamps by night, to show ships whether the bridge is open or shut. By night when the bridge is open for ships, four green lights will be shown in both directions, and when it is shut against ships four red lights will be similarly exhibited, and then lights will be interlocked with the machinery, so that wrong signals cannot be shown. By day similar intimation will be afforded by semaphore arms on the same posts as those which carry the signal lamps. During foggy weather, a gong will be used in specified ways. (44-45)
One other part of the machinery remains to be mentioned. This is that of the passenger lifts between the roadway level and the high level foot bridge. There are two lifts in each tower, consisting of a cage, 13 feet by 6 feet, and 9 feet high, raised and lowered by an ordinary hydraulic ram with chain gearing, and capable of lifting 20 to 25 passengers in about 1½ minutes, including the delays of opening and shutting the doors. As the lift will have to descend carrying a cargo of passengers before it can take a second load of ascending passengers, we may assume three minutes from one start to the next; or, as there are two lifts on each tower, 1½ minutes. In addition to the lifts, there are ample flights of stairs in the towers. (45)
The Towers (General Appearance and Materials)
Views of one of the towers — Left: Roof, pinnacles, and windows (photograph by R. M. Landow; view in a different light). Middle: View from the roadway approach. Right: Gothic window tracery on tower.
When an opening bridge was first proposed there was some outcry by aesthetical people on the score of its ruining the picturesqueness of the Tower of London by hideous girder erections, and it seemed to be the universal wish that this bridge should be in harmony architecturally with the Tower.... it was originally intended that the towers should be of brickwork in a feudal style of architecture, and the bridge somewhat like the drawbridge of a Crusader's castle..... Sir Horace Jones unfortunately died in 1887, when the foundations had not made much progress.... Since the death of my coadjutor I have preserved the general architectural features of the Parliamentary sketch designs, but it will be seen that the structure as erected differs largely therefrom, both in treatment and material..... [I]t became apparent that it would not be possible to support the weight of the bridge on towers wholly of masonry, as in the first designs, unless they were made of great size and unnecessary weight. It was, consequently, necessary that the main supports should be of iron or steel, which could, however, be surrounded by masonry, so as to retain the architectural character of the whole structure. (48)
Interior of the Towers
The skeleton of each tower consists of four wrought steel pillars, octagonal in plan, built up of rivetted plates. The pillars start from wide spreading bases, and extend upwards to the suspension chains, which they support. They are united by horizontal girders and many diagonal bracings.... The chains are carried on the abutments by similar but lower pillars.... Between the pillars are spaces for the public stairs and the passenger lifts, and for the quadrants of the opening span when in their upward position. (48, 50)
The main chains, which are 60 feet 6 inches apart from centre to centre, extend from the rollers on the piers to other rollers on each abutment, and support the platform of the bridge by suspension rods, extending from the bottom of the chains to the cross girders of the platform..... It may be asked why are these structures, which look like girders, called chains? They are, in fact, chains, stiffened to prevent deflection, and the object of the form is to distribute the local loads due to passing traffic, which, in the case of an ordinary suspension chain, distort the chain, continually depressing each part as the load passes, and consequently distorting the platform of the bridge. By making the chain, as it were, double, and bracing it with iron triangulations, these local deflections of the chain are avoided. (50-51)
The total weight of steel and iron in the Tower Bridge will amount to nearly 12,000 tons. (53)
The time of construction, some 7 years to the present time, has seemed long, but it may be some comfort to those who are impatient, to remember that old London Bridge was 33 years in building, old Westminster Bridge 11¾ years, and new London Bridge 7½ years, and I think my hearers will have seen that the Tower Bridge is no ordinary bridge, and in no ordinary position. The structure and its machinery are full of the most elaborate and complicated work of all kinds. (64)
The cost of the bridge, with its approaches and including the cost of the property purchased, will be about a million sterling, and the whole of the expense will be defrayed out of the funds carefully husbanded and administered by the Bridge House Estates Committee. Londoners will thus be presented, without the charge of one penny on the rates, with a free bridge. The expense of working the bridge, which will be very considerable from the quantity of machinery comprised within it, will also be paid by the Corporation. (63)
Barry's tributes to Colleagues
First and most important of all, my acknowledgments are due to my partner, Mr. H. M. Brunel, who has supervised the whole of the complicated calculations and details of the structure, and has taken a very active share in the carrying out of the work from first to last. Afterwards follow the resident engineer, Mr. Cruttwell, who has been in control of the works from their commencement; Mr. Fyson, who has had the duty of the preparation of most of the detailed working drawings and calculations of engineering matters, and Mr. Stevenson, who has acted as my architectural assistant. In connection with this subject, I cannot but express my great regret that the work was so soon after its commencement deprived of the architectural knowledge and experience of Sir Horace Jones, and that he has not lived to see the mode in which his conception of a large bascule bridge across the Thames has been realised.... In another branch of duty I have to express my thanks to the various contractors ... lastly, and in a very important degree, to the firm of Sir W. G. Armstrong, Mitchell and Co., to whom is entrusted the hydraulic machinery, which, I believe, is without rival in size and power. (63-64)
The seagoing ships which pass the site of the Tower Bridge, and for which the central span would have to be opened, number on the average, about 17 daily. They pass by chiefly at or near the time of high water, and it may well be arranged that several may pass one behind the other. The number of seagoing ships proceeding above the site of the bridge does not show any tendency to growth, but, on the contrary, the volume of such traffic will rather, I think, gravitate to the docks down stream as time goes on. I am afraid that some disappointment will occasionally be felt when vehicular traffic is stopped by the opening of the bridge, but it may be hoped that no serious delays will occur either to seagoing ships or to vehicular traffic, as the periods during which the opening span will be raised, though sufficient for the accommodation of the river traffic, will not be of frequent occurrence or of long duration. The Tower Bridge will, it is thought, fairly meet all the difficulties of the case, but if the road traffic becomes of greater importance, and the sea-going river traffic grows less, I suppose the fate of the bridge will be to become a fixed bridge. How soon this may happen no one can tell. It is able to fulfil its duties either as an opening or as a fixed bridge. (62-63)
In drawing this description of the works to a conclusion, I may be allowed to express a hope that the Tower Bridge, when finished, will be considered to be not unworthy of the Corporation of the greatest city of ancient or modern times. (64)
Barry, John Wolfe. Tower Bridge. A Lecture. London: Boot, Son and Carpenter, 1894. Internet Archive. A copy signed "from the Author" (though mistakenly dated 1884) and uploaded by the University of California Libraries. Web. 26 February 2012.
Last modified 28 April 2016