The Maintenance of Iron Bridges, with special reference to the present state of the Brooklyn
Bridge.
Paper read by PROFESSOR KERNOT, Dec. 3, 1902.
There is a widespread tendency to neglect maintenance both of bridges and other engineering works. Designs are pre- pared with great •labour and skill, and carried out under supervision of the most stringent character, and then the work is left to take care of itself. The stitch in time that saves nine is, either through attention being preoccupied by other matters of immediate urgency, or through some petty cheese-paring economy, neglected, and the structure goes from bad to worse, becoming unserviceable or dangerous long before it has attained a fair and reasonable old age. I do not say it is always so. Doubtless there are many instances of care- ful, regular and competent inspection and maintenance; but no one will, I think, dispute my allegation of the reverse of this being all too frequent. For example, a huge timber arch, nearly 200 feet span, situated at the lower termination of a long cutting, the side of the valley being so steep that there was no approach embankment. The original designer had provided a system of drain pipes to intercept the rain water from the cutting, and discharge it clear of the bridge, but the maintenance was so ignorant or careless that the drains were allowed to become choked. The water flowed over the deck, and percolated to the arches below, which speedily rotted and collapsed, and the bridge needed complete renewal years earlier than it should have done.
In another case, I dis- covered huge cavities washed out beneath the footings of a stone arch, and no attempt made to fill them up with concrete or masonry, though to do so would have cost only a few pounds. And each of these structures was on a main road, carrying heavy traffic, and each was within a mile of the town hall of an important and wealthy city. And now news arrives that the great Brooklyn Bridge, which is by fax the largest public road bridge in the world, and cost some mil- lions of money to build, has been shamefully neglected, the most distinct and urgent signs olr distress being ignored, until at last sudden and alarming fracture of important parts has aroused public attention,. and led to the partial closing of the bridge to traffic, pending extensive repairs. In view of such facts, it appeared that a paper on maintenance of bridges would be appropriate and useful.
THE MAINTENANCE OF IRON BRIDGES. 91 The first necessity for proper maintenance is a record plan, showing the structure as built, and accompanied by full numeri- cal particulars as to loads, stresses, results of tests of material, and any accidental peculiarities of construction. It rarely happens that a structure is carried out exactly as designed, and not infrequently the finished work differs most seriously from the original drawings. It was my lot some years ago to be engaged in reporting upon a number of large, important and costly bridges in an adjoining Australian State. I found in more than one case that the drawings supplied were utterly useless, the structures having been altogether altered from the original design. Further, no information could be obtained as to the loads intended to be carried, as to. the stresses calculated, or as to the results of the tests of the material, while locomotives of weights far in excess of those used when the bridges were built were being introduced on the railway of which they formed a part. In one case, where continuous girders, extending over three. spans were used, there was a serious difference of level in the piers; but to the all-important question, from a stress point of view, whether the difference of level was part of the original con- struction or had subsequently occurred, no answer could be obtained.
This want of accurate record drawings is a subject of grave complaint at the Brooklyn Bridge, the official report upon which says :—
"There is no complete set of plans showing fully and
"reliably the present features of the structure. Those which
"do exist are few in number, insufficient and unreliable, and
"in many instances without date. Some of them show the
"structure not only not as it is at present, but as it never
"has been. A fire destroyed part of the plans some years
"ago, but ample time has elapsed since to allow of their being
"replaced by others. showing the actual construction.
"Efficient supervision should, in our opinion, include the
"securing and maintenance up to date of a complete and
"reliable set of plans showing the actual features of the struc-
"ture, plans of sufficient completeness and reliability to allow
" a decision to be made from them with confidence on all
"questions of strength." I agree most thoroughly with this, and would call special attention 'to the words "maintenance up to date." I know of a case in which the timbers of a wooden deck on an iron bridge Were arranged originally in a certain way in order to minimise the stresses on the iron- work. Years elapsed ; the timber deck needed renewal, and
this was accomplished in a way that most seriously increased the stresses on the ironwork. The person making the
renewal—I will
not call him engineer, for he was not worthy92 THE MAINTENANCE O1 IRON $RIDGES.
of that honourable name—in order to save himself a little trouble, either ignorantly or knowingly imposed serious extra stresses on parts already fully loaded up to their proper working strength. Now, any person consulting the original plans of this bridge would be most seriously misled as to its strength and safety.
I would, therefore, urge that in the case of any large or important bridge a complete set of record plans be kept.
These, of course, may be the original plans or lithographed or blue printed copies of the same, but with every alteration and deviation from the original design, whether made at first or subsequently, shown most clearly and fully, say in red or some other distinctive colour. Attached to these should be a full set of calculations of sizes and strengths, with all the data upon which they are based, including tests of material used;
also a clear description of every 'departure from the original design, with the history and reason of the same. These docu- ments should be all signed and dated by some competent and responsible person, and deposited in a fire-proof safe or strong- room, so as to be available to meet any question arising at a future time. Had this been done in the case of the Brooklyn Bridge, or the other bridges referred to earlier in this paper, much Iabour and expense would have been saved, and unpleasant doubts as to the state of stress in vital portions of the structure would have been removed.
Again, it will be well known to those conversant with bridge design that, in order to secure simplicity of construc- tion, many parts are made 10, 20, 50 and even sometimes 100 per cent, larger than they need be, while others are made the exact calculated size. For example, the diagonals of an ordinary lattice girder, if . constructed strictly in accordance with theory, will require so many different sizes of metal as to lead to inconvenience and confusion in manufacture. To obviate this and reduce the number of sizes, probably at least half of these parts are made stronger than necessary, so as to be duplicates of their more heavily stressed neighbours. Thus the various portions may be divided into two classes—first, those that are accurately proportioned by calculation of strength; second, those that for practical reasons are some- what larger than considerations of strength demand. Now, it is obvious that to secure the best result the material and workmanship of parts of the first class should be faultless;
while in those of the second class, considerable deficiencies in these directions are innocuous. Therefore, I would recom- mend that all parts not more than 10 per cent. in excess of their theoretical size be tinted or shaded in the drawings, so as to distinguish them from the rest, and that special care be taken in construction that they are of the full size shown, and of unexceptionable material and workmanship; while in
THE MAINTENANCE OF IRON BRIDGES.
the remainder of the structure rather more latitude be per- mitted in these respects. I have known, for example, the most heavily stressed bar of a bridge to be subjected to severe extra punishment through grossly careless workmanship, while the contractor is often worried unnecessarily in order to secure the most perfect material and workmanship in a part that has a factor of safety 50 or 100 per cent. greater than is needed. This same distinction should be observed in the subsequent maintenance, the most rigorous inspection, and the greatest precautions against corrosion and other deterio- rating influences being employed in the case of those por- tions which possess little or no margin above the dimensions required by theoretical computation. Blixt as things have been in the past, it has been no uncommon thing for much anxiety to exist about, and much money be spent in rectifying small defects in parts possessing an enormous excess of strength, while others that were heavily stressed were allowed to suffer extra punishment unheeded. With such a set of plans as I recommend, anyone without special mathematical knowledge would at once be able to tell whether a few defec- tive rivets or a badly fitting joint were worth troubling about
or not.
Next, as to the actual periodical work of inspection and maintenance. Firstly, every part of the structure should have a definite designation such as a name, number, or letter.
A book should be provided with these names, numbers; or letters in a column on the leftrhand side of the page and the inspector should carefully go over the structure, say, once a year, and enter in the book any defect or peculiarity he may notice. Thus by comparing the records of several successive inspections it would at once be seen whether any given pecu- liarity or defect was increasing or not. The necessity of fill- ing up the various lines in the book would compel the inspec- tor really to look at each part, and not shirk the inspection, as I have known to be done. Further, the defects noted in former inspections would be a guide to show what parts of the structure should have the most immediate or closest scru-
tiny. This book, when not in use, should be kept with the plans in the fire-proof safe or strong room.
In ordinary iron or steel girders the lower parts are those most likely to deteriorate rapidly. The bottom member is often of trough sections ; this holds water, dust, and, in road bridges, horse-dung. In these parts of the Brooklyn Bridge water 4 to 8 inches ..deep was found, also large quantities of horse-manure. To prevent this, it is well in future structures to avoid this trough section, and. substitute one that will not hold water and rubbish. In existing bridges all that can be done is to make numerous and large holes for the exit
of
water, putting them, of course, in places where calculation 93
94 THE MAINTENANCE OF IRON BRIDGES.
shows there is an excess of strength, and to frequently brush out dust and dung. Further, all small corners where water and mud is likely to cling may be filled with • Portland cement, asphalt, or other suitable material, care being taken to have the metal clear and bright before applying the cement. I have known a case of heavy, dangerous, and unsuspected cor-
rosion where cement was applied to a surface of painted iron.
Vertical or sloping parts generaly suffer most at their lower ends, the water running down and collecting there. These lower ends should therefore be fully open to inspection, and should be kept well painted.
The upper parts of girders as a rule do not suffer much, and being generally plainly visible, are not likely to escape in- spection. The flat, horizontal top, however, may be an excep- tion to this statement, as under the intense rays of an almost vertical sun the paint tends to peel off and the horizontality of the surface causes water to remain and corrode the metal instead of running away. Hence, then, top surfaces should be looked after, and well and frequently painted. If wooden beams or planks rest directly on the top of the girders prevent- ing inspection and painting, special care should be taken to interpose some desirable protective coating before laying the woodwork. Further, periodically (say, every 5 years) the woodwork should be lifted and the ironwork cleaned, inspected and re-coated with paint or other suitable preservative, other- wise, in course of years, serious corrosion may go on unsus- pected. This lifting of the woodwork need not be a costly process. The removal of a few bolts and the jacking up of the timber three or four inches at a time when the deck is not loaded would permit inspection, cleaning, and painting.
When plates or bars of metal are in contact, but not so close as to exclude water, heavy corrosion will sometimes take place, and great accumulations of rust bulge this metal apart.
I witnessed notable instances of this at the great Charing Cross railway bridge, London, near which. I resided two years ago. Metal half an inch thick had been so affected, the mass of rust between being f to 3-8ths inch thick. This was being removed at the time and additional rivets inserted to keep the plates close. In view of this experience, I would sug- gest that plates in contact be held by rivets spaced not more than ten times the thickness of the plate apart, that plates be thoroughly painted before being rivetted together, and that any gaps where water might enter be kept thoroughly filled with paint or other preservative material.
The ends of girders are sometimes badly placed for inspec- tion purposes being built solidly into masonary or brickwork, placed in dark and inaccessible positions, or exposed to damp arising from below or to contact with earth ballast or cinders
—under these circumstances heavy local corrosion often oc- li
THE MAINTENANCE OF IRON BRIDGES.
curs. I have kicked the toe of my boot through the wet plate of a girder of an important railway bridge so affected. It cannot be too strongly insisted that all parts of bridge girders be visible, accessible, well ventilated, dry and free from con- tact with earth, and if not so designed originally they should be altered. A method of ensuring these advantages is dis- cussed on page 9 of my work on Common Errors in Iron Bridge Design.
Some bridges, the Brooklyn amongst the number, possess screw or wedge adjustments in certain parts. These should be most carefully treated, and unless intelligent maintenance is certain, their presence is a distinct source of danger. The previously quoted report on the Brooklyn: Bridge animadverts upon the scandalous ill-treatment of these parts. Bolts that should have divided the load equally were so ill-adjusted that one of a pair would have to carry the full load, accompanied by most improper and dangerous bending actions, while the other had nothing to do, or the load was brought on one edge of a nut instead of fair on its face. (See sketches.)
I
,I.
.1
Fm. 1. FIG. 2.
Adjustable parts should be carefully adjusted by a com- petent person, who understands stresses, and who has a suf- ficiently musical ear to be able to tell approximately the tension on a bolt by the note it gives out when struck lightly 95
•
96 TIM MAINTENANCE OF IRON BRIDGES.
with a hammer ; and no other should on any account be al- lowed to interfere with it. When so adjusted; the exposed thread should be well painted to check corrosion.
Most bridges are provided with slides, rollers or other mech- anism to permit of expansion and contraction with variations of temperature. These parts are often neglected, and be- come fixed by rust and dirt. It was so at Brooklyn to a most serious degree. The gigantic saddles or carriages supporting the cables on the summits of the towers had become absolute fixtures, and the load on the masonry was seriously increased thereby, while many of the hinged joints in the vertical rods near mid-span, and which were subject to an angular motion of several degrees, were set fast by rust and dirt, over which a thick coat of paint had been spread.
These important details should be so placed as to be visible and accessible, and should he kept perfectly free from dirt and rust, and periodically lubricated.
It would be a departure from present practice, but I fail to see any good reason why, when convenient, they should not be placed in an oil bath with a closely fitting, but easily removable, cover of sheet metal to exclude dust. In this way their free working would be assured acid corrosion prevented.
In some bridges loose rivets have been a source of com- plaint. They may be usually detected by stains of rust pro- ceeding from them. They vary in looseness from those that can be shaken and turned round by the finger and thumb, to those whose slackness can just he detected when struck on one side by a light hammer, the finger being placed opposite. In some cases they are due to careless construction, rivets being used too short to permit of their being closed up properly.
In others they are due to insufficient rivet area to carry the stress. When loose rivets occur, the first thing to be done is to carefully check the calculations of stress, and if the result is unfavourable, the rivets should be removed a few at a time, when the structure is free from external load; the holes should then be carefully enlarged with a suitable rhymer or cutting tool and large rivets inserted ; or, if there is room, new holes may be bored and additional rivets employed. If, on the other hand, the rivets are not overstressed, it woul suffice to replace them by new ones of the same size.
I do not think that much is to be learnt by testing the bridges periodically with loads much more severe than the ordinary working load. I think, however, it would he well to have a bench mark or permanent reference point over each support, and at the centre of each span„ and to take the rela- tive levels of these before the bridge has carried any load, after it has carried the test load usually applied before opening it for traffic, and at intervals of, say, five years afterwards,
THE MAINTENANCE OF IRON BRIDGES. 97 both with and without its ordinary load. Any progressive yielding or increase in deflection would thus be detected.
The frequency and stringency of inspection should vary with circumstances. Structures near the sea or any large body of salt water, or much exposed to coal smoke or the fumes of chemical works, are likely to deteriorate much more rapidly than those at a distance from these prejudicial agencies., and should be treated accordingly. Further, structures with a low factor of safety, or of which this failure would involve specially serious risk to life, may fairly claim the more careful and constant maintenance.
Discussion (December 3rd, 1902).
The chairman (Mr. G. Higgins) said that certainly, like all that Prof. Kernot read or said at the Insititute, this paper was a very practical and important one. He thought it would give rise to a very interesting discussion. There was present Mr. W. R. Rennick, who had a good deal to do with railway bridges of Victoria. He understood that the existing bridges on the North-Eastern line were about to• be strengthened to permit or the new and heavier locomotives running on them.
There was also present at the meeting Mr. Mountain who would perhaps introduce what machinery is used by munici- palities in connection with the testing of bridges. Iron and steel bridges required very careful attention. It was not only important that the members of the Institute should have their attention drawn to this subject, but the attention of the public should be called to the need of care for the safety of bridges; it was a matter that he would sug-
gest for the consideration of the meeting. The paper might also lead to the question of the maintenance of machinery.
Mr. Mountain concurred with Mr.. Higgins that the members invariably listened with considerable interest to anything that the Professor was good enough to supply. He thought it was one of the most practical papers that had been read before the Institute for a long time, and that it would be often referred to when in print. The Professor had put together in such a systematic manner what was required that it would be an aid to memory of many engineers. He agreed that this paper extended far beyond bridge construction. The neglect of maintaining bridges and constructions of all kinds was com- mon in Australia.. The engineer was not to blame for this in all cases. First of all in the case of this bridge, the designer had undoubtedly great genius,, but he assumed that after the completion of the bridge, as he, designed other bridges, that one ceased to interest him any more. Another man would
98
THE MAINTENANCE OF IRON BRIDGES.come on and he might not have sufficient knowledge of the critical parts of the structure through lack of proper data and information being left him. In AustraliaL it seemed that all public bodies had the impression when a building or structure was finished that no further expenditure would be required, and metaphorically used the words of Macawber when he signed the promissory note, "Thank God that job is settled." In connection with this matter he had no doubt that if the Institute would present a copy of the paper, with its compli- ments, to the Hon. Thomas Bent, the present Minister of Rail- ways, it might do some good in the Railway Department. In
his own case, he had tried repeatedly to get funds to keep works in first class condition, but the exigencies of the situa- tion prevented him obtaining them ; "Want cf funds," was always the answer. He (Mr. Mountain) thought that a depre- ciation fund should always' be provided. In this respect the body with which he was connected had become fully alive to this fact, and had wisely provided a depreciation fund. If this was done in all cases it went a long way to systematise the idea that Professor Kernot had stated.
Professor Kernot said that he had to thank the members for their appreciation of his little paper. H'e had had the material on hand, and it was simply the matter of writing it out. If it did any good he would be only too well satisfied. It surely was a small matter to keep a record, only, as had been pointed out, it was one of those things which in the hurry of the com-
pletion of a large contract was likely to he overlooked. The point about the Brooklyn bridge was that there was less excuse there than anywhere else. There was a staff of engineers to look after the bridge. Now, if the work had been one or two thousand miles long, there would have been an excuse. . Con- sidering that the parts that broke were those that one could have touched while walking along the road, it seemed extraor- dinary that things were so utterly neglected, and that nothing was done until. the breakage occurred.
Mr. Mountain thought that the facts being as Professor Kernot stated, if there was a sta
ffof engineers, the engineer was personally to blame. Evidently the money was supplied for the maintenance of the bridge ;; it was a case of the man shirking. his duty.
Professor Kernot said there was evidently no lack of money, for the bridge charged a toll on the ordinary road ; then there were the cable-cars, etc. ; altogether the revenue must have been enormous.
It was decided, in view of the importance of the matter,
and considering the reluctance of public bodies to maintain
important structures, that copies of the paper be sent to the
daily papers.
THE MAINTENANCE OF IRON BRIDGES.
99
Discussion, zst A~Yil, 1903:
Mr. W. R. Rennick said that in the Railway Department they sometimes experienced ; the difficulties which the Professor had encountered in finding accurate records of structures as they stood. To keep the records properly a very efficient system was required, and such a system was introduced into the Department a few years ago, and is found to answer very satisfactorily.
Mr. Menz said it might be of interest to the members of the Institute to know that in the Prussian Government Railways, when a contractor finished the construction of a bridge about three sets of complete drawings were prepared. One set went to the Permanent Way Department, one was kept in the office and another was hung up in the station nearest the bridge, for the information and instruction of any men connected with the maintenance, etc., of the bridge. Speaking of the preparations used for the preservation of girders, etc., in Prussia, they used cement to a great extent, and in the case of lock gates they had used cement made up in skimmed milk— very cheap and very effective in making it waterproof. Another important thing to be dealt with was the deteriorating effect of soot, especially when in conjunction with water.
Mr. Geo. Turner concurred with Mr. Menz that the action of smoke on roofs or other iron-work was very marked. Any roof near a chimney suffered exceedingly, and unless frequently attended to, its deterioration would be excessive.
Mr. W. R. Rennick said that in the Railway Department they were faced with the same problem. Where the engine stands under a bridge at a platform, in spite of the girders having been repeatedly scraped and tarred, the ironwork was corroded. A lot of bridges in the suburbs were`being investi- gated to see how they stood. The blast of the engine being at its highest, any paint that was put on might be blown off.
Professor Kernot was pleased that his little paper had in- voked so much discussion, and had brought out such valuable points. The maintenance of iron bridges was a very important matter, and the treatment of them had been in a great many instances very far from what was desirable. The "stitch in time " that " saves nine " had been most conspicuous by its absence in most cases. The difficulty of keeping a large number of papers in order so as to get them when wanted was very great.
Unless at the outset a rigid system was arranged and adhered to
most rigorously, and heavy penalties imposed for the slightest
departure from it, there was bound to be trouble. With
reference to the record plans of bridges, one question arose as to
I00 THE MAINTENANCE OF IRON BRIDGES.
the size of the plans and the form in which they should be kept.
A rolled plan occupied a great deal of unnecessary space because of the hollow inside. In the Public Works Department of New South Wales they had a method which was worth mentioning.
For each important structure they had the plans photo-litho- graphed down to a comparatively small size They were not so small as to prevent all the particulars and dimensions being clearly read if a good magnifying glass were used. Having brought the plans down to this small size they made books of foolscap size, and in it they had the complete specification and all the plans of the bridge. The plans were folded in the book.
He had in his possession about half a dozen of these books which the Public Works Department of N.S.W. had sent him. These foolscap books were bound in stiff covers, and in the small book that occupied very little space, were all the particulars of a large bridge. Thirty of them could be placed in every foot of shelf.
Of course it might be necessary to add to them particulars of alterations, departures from original design, etc., but still that could be done without making the book very much thicker.
When one considered the value of bridges and the frightful results of a bridge accident, it became apparent that great care should be taken to keep them in order. Keeping a plan of the bridge at the Station near the structure as they did in Prussia was a thing that the Victorian Railways had not yet thought of.
The system of introducing skimmed milk with cement was a revelation to him, and he would very much like to see experi- ments conducted in this direction.
1
