CHAPTER IX._The Anatomy of Bridgework

DECAY AND PAINTING.

The author has collected particulars as to the amount and rate of

rusting in metallic structures which are of some interest. In all such

instances it is very necessary to note the conditions which have

obtained during the process of wasting, as without this, misleading

conclusions may be drawn. The information given relates in all cases to

wrought iron, unless otherwise stated.

A plate-girder bridge, having girders under rails, was found to be badly

rusted. The atmospheric conditions were unusually trying, the air being

damp and impregnated with acid fumes from adjacent steel works. That the

wasting was largely due to this latter cause was indicated by the fact

that the girders nearest to the steel works suffered more than those

farther removed and partly sheltered from the corrosive influence.

The webs were in places eaten right through, having lost a mean amount

of about 1/8 inch full on each surface in twenty-eight years. Painting

had not been well attended to.

In a similar bridge, not a great distance from this, but sufficiently

far away to modify the conditions for the better, considerable wasting

was also observed, but more particularly where the girders had been

built into masonry, which, loosening with the constant movement of the

girder-ends, had allowed moisture to collect, and rust to develop,

without the chance of repainting these surfaces. The amount of waste at

the places indicated was, as in the last case, about 1/8 inch on each

face, and in the same time, other parts of the girders having suffered

less.

[Illustration: FIG. 58.]

A third plate-girder bridge, with outer main girders, cross-girders, and

plated floor, carrying a road over a railway and sidings, and which was

known to have been neglected in the matter of painting, was very badly

rusted, both as to the cross-girders and floor-plates. The atmosphere

was somewhat damp; the chief cause of deterioration was, however, the

smoke and steam from locomotives, which frequently stood for some time,

during shunting operations, directly under the bridge. The webs of the

cross-girders, which were originally 1/4 inch thick, had rusted into

occasional holes during fourteen years--i.e. 1/8 inch from each surface

in that time. When removed a little later the wasting was so complete

that it was possible to knock out with a light hammer the remains of the

web between flanges and stiffeners, so as to leave an open frame only.

One of the cross-girders was so treated by the men engaged upon the

work, when it presented the appearance shown in Fig. 58.

In another case--that of a bridge with lattice girders under rails--the

ends were built into masonry, which had, of course, loosened, with the

usual result. The air of the locality was certainly pure, but somewhat

damp. The general condition of the ironwork was good, but end-bars of

the diagonal bracing, where they had been closed in, had lost 1/8 inch

on each surface in thirty-three years. The top flanges immediately under

the timber floor were in a very fair state, which is of some interest

when it is considered that these were made of steel of the same kind as

that already noticed as being used in the construction of small girders

(see Fig. 46, _ante_), described in the chapter upon “High Stress,” both

cases dating from the year 1861. The painting upon the lattice-girder

bridge had been pretty well attended to; but in the case of the small

steel girders it had been greatly--perhaps altogether--neglected; this,

coupled with adverse atmospheric conditions, had produced the result

that the rate of rusting had for the small girders been much greater

than that of the steel top flange referred to, being fully 1/8 inch on

each surface, as against a negligible amount under the more favourable

circumstances.

Girder-work over sea-water, as in piers, seems to rust at a sensibly

greater rate than inland work under average conditions; but it is hardly

practicable to make any strict comparison, as in either case the rate of

oxidation is so much affected--even controlled--by the care bestowed

upon the structures. This general conclusion is based upon the results

of examination of wrought-iron girder-work over sea-water of ages

varying from fourteen to forty-four years. It should be remarked,

however, that in one case steel girders but five years old, and which

were frequently wetted with sea-spray, were found to be wasting rather

badly--the paint refusing to keep upon the surface.

It may be concluded from the above instances, and from others which have

come under notice, that wrought-iron work, if not properly cared for in

respect to painting, or under conditions otherwise bad, may be expected

to rust at a rate which corresponds to the loss of 1/8 inch on each

surface in from fifteen to thirty years; but with proper care as to

painting, and exclusive of exceptionally bad conditions, it does not

appear to waste at any measurable rate. In some instances, upon scraping

the paint from girders which had been in use for thirty years, the

author has found, beneath the original red lead, the metallic surface

bright and clean, showing no trace of rust.

Of ordinary steelwork the same cannot be said, the common experience

being that mild steel is very liable to be attacked by rust. With

passable care in the bridge-yard during manufacture, such that with

wrought iron no after-trouble would be noticeable, steel is very liable

to show, within a year of being built up, numerous little blisters on

the painted surface; any one of these being broken away discloses a

small rust-pit. This is more often seen on the flange surfaces

(horizontal) than on web surfaces (vertical), but it is probable the

position has little to do with the matter, and that it is rather due to

the fact that rust has been earlier started on the flange-plates, upon

being put through the drilling-machines and inundated with slurry, which

occurs only to a more limited extent with webs having fewer holes. The

heads of steel rivets do not show this tendency to “pit,” or to early

development of rust. The riveting is about the last operation in making

a girder, each rivet being freed of all rust by heating, and quickly

coming under the protection of oil or paint. It may happen in this way

that the heads of rivets on a girder may be exposed without protection

for as many hours only as the rest of the work for weeks, which fully

accounts for the difference in behaviour.

The essential point to be observed in all steelwork is to prevent, if

possible, the first development of rust, for once begun it is much more

difficult to arrest than in iron; for this reason, oiling of all

material for a steel bridge, at a very early stage of its existence,

cannot be too strongly insisted upon. This practice, however, makes the

work so objectionable, and even dangerous when being lifted--because of

the liability to slip--to the men engaged upon it, that it is commonly

very difficult to ensure it being done sufficiently soon to satisfy a

careful inspector. If the work is carried out under cover, the

requirement is less urgent. Strictly, all material should be oiled so

soon as rolled, but the author does not remember to have seen this done

at any of the mills he has visited, though it is common enough to find

it specified.

Ironwork does not need the extreme care which should be bestowed upon

steelwork, but it is desirable that it should be painted as soon as

possible, the surfaces being first thoroughly cleaned.

There is, probably, for painting girder work nothing to beat good red

lead as a protective coating; but there is considerable difficulty in

getting it reasonably pure, without which quality its utility will be

greatly reduced. The question of purity will, however, be found to be

largely a question of price. It may be stated broadly that, whether for

steel or for iron, the first protective covering is, perhaps, the most

important of any it will ever receive.

In repainting old work, care should be taken to remove all traces of

rust previous to laying on the new coat. It is not an altogether

uncommon practice to repaint old structures by dealing only with the

parts readily accessible, which, being less liable to rust, probably but

little need it; leaving those parts which are difficult of access, and

where rust is developing, untouched; treating the whole business as a

matter of appearance simply. This, it need hardly be said, is

indefensible. It is better rather to neglect the surfaces freely exposed

and ventilated, and devote the whole care upon those other parts,

confined and difficult to get at; taking the trouble necessary to remove

ballast, timber, or whatever may obstruct the operation, in order that

the bad places may be thoroughly scraped, and then painted. Those parts

which most need attention may cost, perhaps, to reach--and deal with

when exposed--ten times as much per yard of surface as the rest of the

superfices, which needs little, and is always accessible; but the cost

should not deter the proper carrying out of the work, as it will prove

the very worst sort of economy to deal with painting in a perfunctory

manner.

It should be noted that girder work, whether of wrought or cast iron,

when embedded in lime or cement concrete, or mortar, generally proves to

be very well preserved, provided that close contact has obtained.

Cast-iron girders, when carrying jack arches resting upon the bottom

flanges, are found after long use to be in remarkably good order, when

finally taken out, having, indeed, the surface appearance of new

girders. Much the same remarks apply to girders of wrought iron carrying

jack arches, where protected by the brickwork; provided that the girders

are sufficiently stiff to minimise deflection, and allow the masonry or

brickwork to adhere to the surfaces.

Such girders are in a very different condition to those previously

referred to, in which the ends of the girders, carrying a light floor

structure, are built into masonry where the deflection slope is

greatest; though, apart from the few cases where adherence can be relied

upon, building-in is an undesirable practice, and has the disadvantage

that after-examination is only possible by removing portions of the

masonry, which it is evident would very seldom be resorted to.

Cast iron has ordinarily--unlike wrought iron or steel--great capacity

for resisting rust, and will, after many years of absolute neglect,

appear but little the worse; an advantage which is the more pronounced

when considered relatively to the greater thickness of the thinnest

parts in cast-iron girders, the percentage of waste being

proportionately lessened.

Cast iron does, however, behave somewhat badly in sea-water, the metal

sometimes losing its original character, and becoming in time quite

soft; though, if not worn away, as by the attrition of shingle,

maintaining its original bulk.

Of some forty-five cast-iron piles belonging to various structures,

examined whilst engaged upon sea-pier work for Mr. St. George-Moore,

though the author found somewhat diverse results, in no case did there

appear to be any general softening of the whole thickness, but a

distinct change for some definite distance inwards, generally to be

decided without difficulty, beyond which the metal appeared to retain

its original character. In all cases any material depth of softening was

found close to the ground, this depth rapidly decreasing higher up,

till, at a height of 5 feet, but little if any softening could be

detected. At 2 feet above ground the softening was frequently but

one-quarter of that at ground level. There was, too, often a

considerable difference in the behaviour of different piles in the same

structure under similar conditions; one pile being found to have only

one-fourth part of the softening noticed in others, or possibly none at

all. For six different structures the amount of softening near ground

level, of about twenty-five piles examined, was as given in the table on

the next page.

The greatest depth of softening found (see No. 2) was 9/16 inch, 1 foot

above ground, in a pile thirty-six years old. The decayed material when

removed was of a soft, greasy consistency, perfectly black, which a few

hours later was found to have changed to a dry yellow powder, by the

rapid absorption, it may be supposed, of atmospheric oxygen. It is

apparent, therefore, from this example that deterioration may proceed to

a considerable depth; but it should be observed that other piles of the

set showed softening at ground level of 1/8 inch only.

SOFTENING OF CAST-IRON PILES IN SEA-WATER.

---+--------+----------+-----------+----------+---------+---------

---+--------+----------+-----------+----------+---------+---------

2 |36 years|9/16 „ |1/8 in. in |No result | „ |Rubble

---+--------+----------+-----------+----------+---------+----------

The least rate of softening noticed, apart from those structures of a

more recent date, in two of which it was very slight, occurred in a

pier thirty-eight years old (No. 4), where, of three piles tested, two

were quite hard, and the third softened 1/10 inch only.

Whatever may be the precise cause of the change, it does not appear to

be affected by the period or percentage of immersion during the rise and

fall of tides.

[Illustration: FIG. 59.]

This will be clear from the diagram, Fig. 59, which refers to four piles

(No. 3 of table), all of the same age, in the same structure. On each

pile the depth of softening is given at points in strict relation to

each other, and to the tidal range. The percentages of immersion for the

various heights are also given, from a study of which it will be

apparent that these have no relation to the amount of softening; this,

indeed, is always greatest near the ground, at whatever actual height it

may be. For instance, pile A was at ground-level softened 1/4 inch,

that point being 60 per cent. of its life under water; but on pile B, at

a point 74 per cent. of the time submerged, and 4 feet above a lower

ground-level, no softening was apparent; further, at ground-level of

this pile, the percentage being there 87, the softening was no greater

than at ground-level at pile A.

It is probable that while the percentage of submersion in moving water

hardly appears to affect the result, yet prolonged contact with wet

sand, sea-weed, or clinging shell-fish may do so. This seems to suggest

that the process of change, as between the sea-water and the iron, is

slow, and to be effective must be continuous; so that it is only found

to any considerable extent where the water in contact with the surface

is still. In the two worst cases, Nos. 1 and 2 of the table, at points 1

foot and 6 inches above ground-level, the surface was in one pile

shrouded in a thick mantle of heavy sea-weed, and in the other covered

by molluscs; in both instances the surfaces being thus kept moist and

undisturbed. The piles of the fourth case were in hard rock, were clean,

and, where accessible, always either in moving water or quite dry.

However this may be, the power to resist softening certainly appears to

vary largely with the quality of the iron. The piles, referred to above,

in which deterioration proceeded at the most rapid rate were certainly

of a soft metal, the first being markedly so. On the other hand, certain

piles (No. 4) of hard, close-grained iron suffered very little.

It may be mentioned with respect to the last named, as a matter of

interest, that the caps of the lower lengths (just above ground-level)

had been cast with short pieces of wrought iron projecting--possibly for

lifting purposes--which during thirty-eight years had altered in

character to something very like softened cast iron, but laminated, and

harder. Of about 1-1/4 inch original thickness, only 3/16 inch remained

having the semblance of wrought iron. The percentage of submersion was

about 60.

A number of piles, not included in the table, varying from fifteen to

forty-four years old, and of the same structure to which set No. 2

belonged, were all found to be hard, with the exception of one showing

3/16 inch of softening. These are omitted, because the mud surrounding

them was at the time of examination unusually high, so that the more

normal ground-level could not be reached, at which points testing might

have disclosed different results. It is probable that for any piles

standing in soft material examination below the surface would reveal

more pronounced softening than where occasionally exposed.

To meet the effects of sea-water on cast-iron piles, and for other

reasons, it is a common and good practice to make the lower lengths of

greater thickness--say, 3/8 inch more--than that sufficient for the

upper. Occasionally, also, the bottom lengths are filled with concrete,

which no doubt adds to the length of time during which they may be

relied upon.