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"Paper read at the R.V.I.A. General Meeting, June 26th, 1904 The paper was illustrated by lantern slides prepared by A. Henderson (V.P.), and by a large model of a floor in reinforced concrete."
Monash's diary suggests a date of 28 June, rather than 26th. V.P. = Vice-President.
"The use of metal, in the form of steel or iron, in combination with concrete, has been known for a very long time. Railway rails and rolled joists, covered with concrete, for the formation of heavy floors, bridge decks, and the like; and rolled joists or plate girders, forming the abutments of concrete arches, also in floors, are familiar examples.
Such construction is, however, not of the class that it is proposed to consider; and in current literature and text-books, is not covered by the term "re-inforced concrete".
The essential characteristics of this construction are - Firstly, that the whole of the metal used shall be placed in those positions, and be so proportioned, as to section, that it can take up the greater portion of all the tensile and shearing stresses in the structure, but does not act by itself in cross bending. Secondly, that the concrete is not to be treated as a mere filling or cover, but must be so placed, and so proportioned, that it can take up the greater portion of all compressive stresses. Thirdly, that the whole of the metal shall be entirely embedded within the mass of the concrete, being nowhere exposed to view. Fourthly, that the whole construction shall be as far as possible continuous and monolithic.
The term re-inforced concrete has many synonyms, of which "armoured concrete", "armed concrete", "ferro-concrete", "sidero-concrete", "concrete-steel", are often met with in English journals. These various terms by no means indicate different materials, nor do they imply any difference in their distribution or relative arrangement; precisely the same physical, chemical and statical principles governing the correct consideration of every one of these forms. This difference in nomenclature arises largely from the exigencies of trade competition.
The same observations apply to the names of the several so-called "systems" of re-inforced concrete, it being the habit in Europe to designate various methods of design and practice (though differing often only very slightly in minor and unimportant details), with the name of a "system", coupled with the name of the designer. Very many of these minor variations in method obviously depart from sound and scientific practice, and it can only be concluded that their propounders chiefly hold in view the desire to keep clear of sounder methods where those are already covered by Letters Patent.
Confining our attention only to those "systems" which can claim a prudent adherence to scientific principles, we have, as the principal form the Monier system, which is in general use all over Europe and in the United States, and is the chief constituent of all types, and as modifications or developments of it, the Hennebique system, most largely used in France, which superadds special provisions for the shearing stresses; the Ransome system, which is most popular in America and is characterised by the use of twisted bars instead of round bars of metal; and the Roebling system, also often used in America, where the bars are partly rectangular, instead of round, in section.
It is not my province, nor would time permit, to enter into a discussion of the relative merits of these different systems in points where they conflict. It is sufficient to say that the several systems which I have enumerated can fairly claim to embody the best principles of re-inforced concrete design, as founded upon patient and well-directed scientific research.
Nor can I hope in the short space of a single paper, to dwell at all upon the history of the development of this form of building construction. I prefer to devote the available time to a brief explanation of its principal characteristics, and to illustrate upon the screen some notable examples of its successful application.
Some general data as to the observed phenomena in the behaviour of a combination of steel with concrete will be of interest.
The concrete used in the construction is always cement concrete. Standard practice is to specify a concrete with a rich mortar, about 2 to 2 1/2 parts of sand to 1 part of cement thoroughly mixed with stone or gravelly aggregates, or sometimes coke, of fine gauge, 1 in. and under, in the proportions most usually of 1 part of mortar to 1 or 1 1/2 parts of stone, gravel, etc. Intimate mixture and uniform distribution of the cement are here as important as in ordinary concrete. With good materials, properly manipulated, the following average ultimate strengths in compression may be expected:
Concrete of 1 part cement, 2 1/2 parts sand, and 5 parts screenings-
|At 7 days||2,500 lbs. per square inch|
|At 28 days||3,500 lbs.|
|At 6 months||4,500 lbs.|
|At 2 years||6,500 lbs.|
As materials vary all the world over, an absolute standard for allowable stress cannot be fixed. The building regulations framed by the authorities in the German Empire direct that the safe working stresses to be used in the concrete in compression shall not exceed one-fifth of the ultimate strength at 28 days, to be based upon actual tests of the particular materials employed in each particular case. This allows of a design up to 700 lbs. per square inch, but I may add that the practice usually obtaining is to design for a stress in compression not exceeding 500 lbs. per square inch under a full load. French practice is 420 lbs. per square inch.
The concrete of the composition above referred to may be expected to have the following ultimate strengths in tension, viz.:-
|At 7 days||150 lbs. per square inch|
|At 28 days||220 lbs.|
|At 3 months||250 lbs.|
It is however generally laid down by Building Regulations, and uniformly adopted by designers, that the tensile strength of the concrete in a re-inforced concrete construction shall be wholly disregarded in the stress calculations. This is an assumption which is very appreciably in the direction of safety.
It has been established experimentally that there is a very considerable strength of adhesion between cement concrete and the metal embedded therein, due partly to physical and partly to chemical reasons. These experiments have been carried out exhaustively in many different countries, and during a series of years. The most notable are those by Bauschinger, at Munich. At an age of 28 days, the force necessary to overcome this adhesion has been found to average about 500 lbs. per square inch of surface of metal in contact with the concrete, varying with the percentage of water used. With good design the surfaces of the metal are usually, necessarily, so considerable, that the forces which tend to disturb this adhesion are only a small fraction of this ultimate adhesive strength.
It is also an established fact based upon observations extending over many years, that steel or iron wholly embedded in concrete is entirely protected from corrosion in any form, and this even when the work or specimen are continuously submerged in, or saturated with, water. The reason is that, chemically, neither carbonate nor oxide of iron can be formed in the presence of the limes and silicas in the cement. It will be found that if a piece of metal, such as a bright wire nail, be partly embedded in a pat or block of mortar or concrete, and the specimen submerged in water for an indefinite period, the piece of metal will be found, upon breaking up the specimen, bright and un-corroded where embedded, but considerably rusted at the exposed end. As the lasting qualities of concrete made with sound cement are well established, it follows that in re-inforced concrete we also have a material, which in the matter of mere durability, is unimpeachable.
It is further claimed for re-inforced concrete that it is probably the best fire-resisting form of construction yet known. Modern experiences in large conflagrations seem to fully justify the claim so made; and I hope to show, later, some illustrations bearing upon this point. The reasons for this may be briefly stated. While the specific heat of iron or steel is very high, that of concrete is extremely low. By this is meant that concrete absorbs heat at a very slow rate or is a poor conductor of heat. A high temperature surrounding a mass of concrete will penetrate but very little into the mass - experiments tend to show that the penetration will be less than one-quarter of an inch. It follows that the temperature of the embedded metal cannot differ appreciably from that of the concrete which surrounds it. It remains only to add that the coefficient of expansion under heat of steel or iron, and of cement concrete, agree very closely within wide ranges of temperature. Thus a disruption of the composite materials through unequal expansion of either may be inferred to be improbable. Many experiments and actual conflagrations have shown that such a disruption does not, in fact, occur. As both materials are non-combustible it follows that the constructions are, as nearly as possible, completely fire-resisting.
Having reference again to the fact that the external heat can penetrate only slightly into the mass, it will be seen that a beam, column, or floor of re-inforced concrete is not, and cannot be, subject to those considerable changes of form, which in the case of metal columns, joists and girders directly exposed to the full temperature of the flames, often lead to the complete collapse of the external walls of the building. This deduction has also been amply demonstrated by experience.
Ordinary cement concrete has a very low modulus of elasticity. The modulus of elasticity for iron or steel is about 30,000,000; that of concrete averages only about one-tenth of this. In plain words, concrete will stretch under a given pull, much more than iron or steel would stretch under a pull of the same intensity.
But, on the other hand, the elastic limit of concrete is very low, while that of iron is very high. In other words, concrete, when not re-inforced, although it can be made to begin to stretch more easily than iron, will stretch but very little before entirely rupturing, while iron will stretch very considerably before rupturing.
Without tracing all the steps in the reasoning, which are lengthy, and somewhat abstruse, it follows that if a given tensile stress or pull be applied to a mass of re-inforced concrete then momentarily (while the pull is slight) the concrete will bear a considerable share of the pull, and the iron but little; but as the pull increases, an ever-increasing share of the pull is taken up by the iron by reason of its refusing to stretch so easily as the concrete. If the experiment be continued, this will go on until the stress has become so great that the small share of stress left to be borne by the concrete becomes greater than the concrete by itself can bear; then the concrete will crack, and the specimen, so far as it is a piece of true re-inforced concrete, will have failed; thereafter the metal will bear the whole of the tension until its point of rupture is also reached. Now it may be shown, both analytically and experimentally that this failure of the concrete, or, as it is called, the appearance of the "first cracks", will not occur until the elastic limit of the metal has been passed. Hence to ensure the immunity of a piece of re-inforced concrete from cracking when subjected to tension, it is only necessary to provide a sufficient section of metal to ensure that the stress upon the metal shall not exceed its elastic limit. Now, in mild steel the elastic limit is often as high as 40,000 lbs. per square inch; so high a limit is not relied upon, and it is usual to provide that the greatest possible stress upon the metal used in the tension side of a beam shall not, under the fullest load, exceed from 15,000 to 20,000 lbs. per square inch. German, Swiss, and Austrian building regulations provide for a maximum stress upon the metal used in re-inforced concrete of 750 kilograms per square centimeter for iron, 1,000 kilograms per square centimeter for steel.
In this connection it remains to mention briefly that the presence of the metal re-inforcement in its correct position in the mass of concrete lends to the whole structure a remarkable elasticity, or ability to deform or deflect, and completely recover its original form, without the smallest sign of any cracking of the concrete. I have on several occasions conducted destructive tests of beams which showed deflections easily perceptible to the eye without the aid of instruments, completely elastic in character, and without any signs of cracking of the concrete.
Having now referred to the principal characteristics of the two elementary materials in their combination, I desire , before showing upon the screen examples of actual works, to indicate briefly the elementary methods of application to building construction.
These applications may be classified into its use as arches, beams, and pillars. But inasmuch as an arch partakes of the character either of a pillar, when uniformly loaded, or of a beam, when partially loaded, we may confine ourselves really to the two elementary forms of pillars, or cases of longitudinal compression, and beams, or cases of cross bending.
The consideration of "pillars" covers not merely direct compression, but also long column action subject to lateral buckling; while the case of "beam" (using the term in its mathematical sense) covers floors, plates, girders, joists, cantilevers, and all parts subject to cross bending, including also walls subject to lateral, wind or water, or other pressures.
The typical form of pillar or column may be of any desired section in plan, either square, circular, hexagonal, or octagonal. The re-inforcement consists of two separate systems, viz. - (1) a series of vertical bars disposed generally near the outer surface, and uniformly distributed around the perimeter, and (2) a series of horizontal ties binding together the vertical bars at short intervals in height. (See figures 1 and 2).
Fig. 1. "Column - Sectional Plan."
Fig. 2. "Column - Perspective."
The effect of this arrangement is to greatly postpone, and generally to wholly prevent, the usual method of failure of an un-re-inforced concrete column, by what is familiarly known as the "pyramidal" fracture. The ultimate strength of the column in direct compression is enormously increased, while a great resistance to lateral buckling is provided by reason of the outer surfaces being capable of resisting high tensions, due to their reinforcement. It has been shown by Bauschinger, in Germany, and Considère, in France, that the combined strength of the two materials in combination is several times greater than the sum of the strengths of each tested separately.
The elementary form of a re-inforced concrete beam is shown in figure 3, which is a cross section showing a series of round bars disposed as closely as practicable along the under surface.
Fig.3. Cross-section of a reinforced concrete beam of breadth = b, depth to centroid [or area] of tension steel = d, depth to neutral axis = d', distance of centroid of steel from bottom of beam = c, distance from neutral axis to centroid of steel = ζ.
The exact determination of the position of the neutral axis depends upon the relative elastic moduli of the respective materials, and the proportion or percentage of metal used. As a general rule this percentage is from 1 to 2 per cent. of the total volume. As already mentioned, it is usual to wholly neglect in the calculations the tensile strength of the concrete below the neutral axis, hence the beam is computed as if this concrete were not present at all or only present in sufficient quantity to connect the tensile members with the compressive member, or in other words, to transmit the shearing stresses.
From this, it is only a short step to the next, but very important, development, that of the T beam, of which figure 4 shows a type, also in cross section:-
Fig. 4a shows the cross-section of a T-beam with breadth of flange (or slab) = b, thickness of flange = d, depth of web = h, and distance from bottom of web to centroid [of area] of tension steel = c.
Fig. 4b shows the stress diagram superimposed on a part-elevation of the beam. D is the compressive force in the concrete and Z the tensile force in the reinforcing bars. The maximum compressive stress in the concrete is σb and the tensile stress in the steel is fe. The distance (lever arm) between forces D and Z is (h - c + 2d/3). This implies an assumption that the neutral axis is at the bottom of the flange (as confirmed by the stress diagram) which would occur only with very heavy tensile reinforcement.
Here all the metal in the lower member has been brought together into fewer, larger bars, and most of the concrete below the neutral axis has been omitted. Although such a beam is not quite as strong as the beam shown in figure 3, it is relatively very much stronger per unit of concrete used, and also much lighter.
It will be observed that the upper member of this T beam has a considerable width relatively to that of the rib. Of course, this ratio is not unlimited, but a few trial computations will show that a very moderate width of top member will provide an ample compression area to balance the tensile strength of the rib, with the percentages of re-inforcement usually adopted.
Fig. 5. "Pair of T-beams."
If we now place two of these T beams side by side, a little distance apart, as shown in figure 5, and connect their upper members, we realise a floor supported by two ribs or joists. Now, the floor is formed by, and itself forms the compression members of, these T beams, and for this function requires no re-inforcement, But the floor has another function, which is, to bridge over the space between joist and joist, and must for this purpose be re-inforced according to the method of figure 3, and also by bars near the upper surface, at the points of support. The combination shows in outline the modern and important principle of monolithic construction, which has been developed to a high degree of complexity, floors, joists, and girders being all one continuous mass. It involves two important features, each of which makes greatly for economy and strength. The first is that the effective depth of the girders, joists, or ribs is to be counted from the position of the re-inforcing rods, not merely up to the under side of the floor, but right up to the top surface of the floor; a further consequence of this being a substantial saving in height in every floor. The second feature is that the floors, and in the fuller developments, the girders and joists, also act as continuous beams, whereby the bending moments are greatly reduced, and the stresses to be provided for are lessened.
I apprehend that this general description has now advanced to a sufficient stage to render intelligible several of the views which have been prepared.
A photograph was shown upon the screen taken during the actual destructive testing of such a T beam as is described above. It was conducted in Gorinchem, near Amsterdam, in Holland. The clear span was 23 ft. The horizontal top plate was 4 ft. 6 in. wide, and 3 1/8 in. thick. The rib was 8 in. wide by 12 5/8 in. deep, so that the overall depth of the specimen was 15 3/4 in. The relation of span to depth was therefore about 17 to 1. The principal re-inforcement, at the bottom edge of the rib, consisted of five round bars each 20 mm. dia. - giving a total section of metal of slightly under 2 1/2 sq. in. The photograph showed the specimen when supporting a load of 49,400 lbs. Its age was then only four weeks.
Plate 1 shows the roofing of the underground railway, Vienna, which encircles the city, taken near the main railway station, showing monolithic re-inforced concrete construction in the simple form just described i.e., a repetitive arrangement of T beams with floor deck between. The span is 42 it,, and the spacing of the ribs is 5 ft. 6 in. centre to centre. The length of line covered in this way is 2,200 yards. The upper surface is a public road, carrying heavy public traffic, and was designed for a 40 ton rolling load and a distributed load of 500 lbs. per square foot.
I trust the general principles have now been sufficiently outlined to justify my passing on directly to more complete examples and more complex developments. I have only to add that whenever, in the nature of the design, the direction of the bending moment is reversed (as near the ends of fixed girders), so as to bring the tensions to the upper surface, it is only necessary to reverse the design of the beam, by placing the re-inforcing rods near the upper surface also.
Plate 2 is a typical diagram showing the functional parts of a monolithic floor, comprising columns, main girders, joists or cross girders, and the floor plate. By reason of the continuity of the girders, considerable spans may be so treated, thus involving much less interference with floor space than is usually necessary. The small vertical members shown in the picture are introduced for the purpose of re-inforcing the concrete against shearing stresses. An examination of the location of the remaining rods will show that they are placed wherever the tensions are liable to occur.
[At this stage, a number of actual working drawings were exhibited upon the screen chosen from those prepared for the construction of the Palace of Justice, in Verriers, Belgium.]
Some characteristic dimensions of this building are as follows:- Main girder ribs, 29 ft. 3 in. net span; deck, 3 1/8 in. thick. Columns, 17 ft. 9 in. high, 14 in. x 14 in. Working load for floors, 102 lbs. per square foot, and for columns 43 tons.
[Several pictures were shown of factory and workshop interiors showing completed constructions. Of these Plates 3, 4, and 5 are reproduced.]
Plate 3 shows the interior of one of the floors of a large electrical factory in Stuttgart, Wurttemburg. In this the extent of free floor space may be seen. The work is as left after stripping the moulding boards.
Plate 4 is a view of the ceiling of an electric power station, at Amstetter. In this case, no columns are used, and a somewhat different arrangement of ribs may be seen.
Plate 5 represents one of the floors of the store of the Metallurgical Institute in Aussig, Saxony. A feature of this picture is the trimming out for the lift well, which can be seen in the centre of ceiling, and also in the floor. The heavy loading of the floor, with what is probably mineral ores, may also be noted.
Time will scarcely permit of any detailed description of the methods employed in the actual work of construction. It must suffice to say that it is in the first place necessary to construct a complete mould for the whole of the work in contemplation, of softwood timber, supported upon wedges so placed as to permit of easy demolition after the concrete has set hard. This timber work can, of course, be used repetitively, with a certain small percentage of waste for each re-adjustment of form. It is usual to so arrange this centering or framework that the supports may be removed from under all the floor plates and minor joists, without disturbing the central supports of the more important beams. In this way the supports of the floors generally may be removed in from ten to fourteen days after the placing of the concrete, while those to the main girders may be left for three to four weeks, offering little obstruction to other work proceeding around.
As soon as the timber framework is placed in position the whole of the re-inforcing rods are then brought into place, all requisite cutting and bending is done, and the rods are loosely tied, where they intersect, with fine wire, and the whole skeleton work is temporarily supported in its correct position. When all is ready the placing of the concrete is commenced, and proceeds as a continuous operation, without pause of more than, say, one hour, until the whole area to be dealt with as one monolith is completed. The points to be insisted upon in this work are - (1) a thorough mixing of the materials, both dry and wet; (2) a uniform percentage of water, adjusted to suit the prevailing temperature ; (3) a thorough compacting of the concrete into its place.
I select for illustration two typical views of timber framework and metal armatures prepared in readiness for the reception of the concrete. [These views are not reproduced.] They were taken during the erection of the Singer's Club House, in Strassburg, a very pretentious building, the working drawings of which I have here for inspection. The whole of the floors, galleries, etc., in this fine building are of re-inforced concrete.
The first shows the moulds for floor of one of the anterooms, covering an area of 26 ft. x 40 ft. 6 in., without intermediate supports.
The other view is a portion of the gallery or balcony of the large concert hall - in the style of a curved amphitheatre. While for such a case, the preparation of the timber framework is more complicated and expensive yet the finished construction is obviously cheaper than can be realised either in metal work, masonry, or any other incombustible material. This concert hall has sitting accommodation for 1,476 persons.
I have here also numerous photographs of the various portions of this building after construction; together with the full text of the report of the City of Strassburg Building Inspector upon the loading and deflection tests to which various parts of this building were subjected after completion.
Up to this point I may have created the impression that this form of construction is applicable only in the interior of the building, but this is far from being the case. It should be clear that these principles are equally correct for the complete erection of a self-contained structure. The ends of the main girders may rest upon columns equally as well as upon brick or stone walls. Indeed, the more completely and universally in any given building the monolithic principle is applied, the more perfectly are the advantages claimed for the system realised. I propose to show several examples of this complete construction, the first being on a modest scale only.
Plate 6 shows a grain stage or magazine, the proportions of which may be judged from the horse and carriage, and also the portable engine standing beneath. The load-carrying capacity may also be inferred from the stack of bags on the first floor. This structure is in Vienna, and is designed for a loading of 6 cwt. per square foot.
The next example is on a much more imposing scale being a view (plate 7) of a large warehouse in Alsace during its construction. As may be seen, every portion of the building which performs stress-resisting functions is built of re-inforced concrete. It is to be observed, as a broad principle, that the advantages and economies in the use of the system rise in proportion to the severity of the load-carrying functions to be performed, and are therefore more striking for heavily loaded warehouses than for residential buildings. In the present example the points of junction between the girders and the columns may be stiffened to any desired degree by iron insertions so as to form a joint, offering whatever degree of resistance to distortion is required. Thereby the stability of the whole building is ensured in a far higher measure than when reliance has to be placed merely upon the individual stability of the external walls. In this way the external walls cease to serve any purpose in carrying the building proper, or its contents, and are relegated to the minor function of forming a clothing or curtain to the building, and giving to it its conventional external appearance. It will be observed also that by this means the largest light openings may be provided without the result of unduly weakening the outer walls. The building illustrated, which has eight floors, including basement and attics, was completely erected in four months. The floors are designed for loads of 3 to 4 cwt. per square foot. It has been stated that the cost of this building was 30 per cent less than if carried out in the orthodox fashion with heavy external walls and steel pillars and girders.
I am also able to show you a view (plate 8) of this building after its completion and occupation.
I now introduce to your notice another example (plate 9) equally imposing, but of a totally different character. This is Ingall's Building in Chicago, designed and carried out by Messrs. Elzner & Anderson, of Cincinnati; and was completed in September of last year. It has a height of sixteen stories. Footings, columns, girders, walls, stairs, floors, and roof were constructed entirely of re-inforced concrete, without any exposed structural steel whatever. The outside walls of the building are a mere veneer of marble, brick and enamelled terra-cotta. It is 100 ft. x 50 ft. 6 in. in plan, and 210 ft. high above street level, and was completely erected in 198 days. In each floor, the erection of framework and moulds took five days, and the concrete work was erected complete in two days; each story was allowed twenty-one days to harden, and operations thus proceeded on three stories simultaneously. The principal factors in the design were:
|Resistance to wind-pressure||30 lbs per square foot|
|Live Loading:-||1st floor||200|
|all other floors||60|
Each main girder supports a floor area of 32 ft. x 16 ft.; this area being further subdivided by a cross girder. The floor plate, which thus has an unsupported span of 16 ft. in both directions, is 7 in. thick on the first floor, and 5 in. thick on all other floors. The brickwork veneer on each story is supported by a projecting ledge of concrete on the wall plates, 3 in. wide, and bonded to the concrete by projecting galvanised-iron wires, set 18 in. apart, horizontally, and every sixth course, vertically. The whole magnificent building has given every appearance of complete success.
The successful applications of re-inforced concrete are so varied, so numerous, and so interesting that I have found it very difficult to make a characteristic, though reasonably limited, selection from the examples to which I have access. One case of interest is that of stairways, and the next picture (plate 10) will show an example of this. This is a winding staircase constructed in the Palace of Fine Arts in the Paris Exhibition, 1900. It was designed for a full loading of 154 lbs. per square foot, and carried an enormous traffic most successfully.
Plate 11 is an example of a daring piece of architectural enterprise which will, I think, interest you. The view represents the design prepared for the erection, in an existing building, of two floors of re-inforced concrete in lieu of the existing floors, which were of timber. The problem was to carry out this proposal without removing the roof, or dismantling the remaining three upper floors.
This was accomplished in a simple manner by hanging up the upper floors by a temporarily rigged truss, formed by adding tension rods to the posts, as will be seen from the photograph (Plate 11) which shows the work during actual progress.
The work in question was carried out in the town of Lethmathe, in Westphalia.
I have not yet referred to the artistic side of this form of construction. It must not be supposed that there is nothing to be said under this head. On the contrary, for reasons, which a little consideration will make evident, the existence of ribs and other projections depending from the ceiling affords special opportunities for bold and handsome ceiling effects. Here, again, examples are so numerous that it is difficult to make a choice for illustration.
[From among several views shown, of decorated ceilings, as applied to various arrangements of ribs, plate 12 has been selected for reproduction.]
Plate 12 shows the ceiling of the Commercial National Bank, Baltimore, United States of America. This photograph, it will no doubt interest you to know, was taken after the fire. This floor proved to be absolutely uninjured. The three upper floors, being non-fireproof, were completely demolished by the fire, while this floor not merely checked the progress of the fire in that building, but also sustained the shock of the falling debris, without cracking the ornamental plastering.
As I have now touched upon the question of the fire-resisting qualities of re-inforced concrete construction, I will bring my paper to a close by selecting for description, out of many, one example of its actually observed behaviour in a large conflagration.
[The pictures referred to have not been reproduced, but will be found in the 1904 number of the periodical "Cement".]
The general scheme of construction adopted in the Bond and Trust building of the United States Fidelity and Guarantee Company, in Baltimore, was as follows:- The building consists of five stories, 20 in. x 20 in. columns in ground floor, diminishing in upper floors, the spans for main girders are 21 ft., the spacing laterally of columns is 11 ft., the floors are slightly arched, being 3 1/2 in. thick at centre.
The whole construction, except the outer walls, was of monolithic re-inforced concrete, supported on "wall columns". The external walls were 18 in. thick, of brick throughout, and stood independently of the monolithic frame, and not as a veneer, previously described.
It is stated that during the fire, in February last, temperatures exceeding 2,000 deg. Fahr. were attained. It is a fact that the metal in the fireproof safes of many buildings was melted. During the fire, the building adjacent to the Bond and Trust building completely collapsed, and in its fall tore away a considerable portion of the brickwork in the front, and one of the side walls of this building. The fire had therefore complete access to the whole of the contents of the third and fourth floors, which were completely consumed.
I will now show you a photograph of the building after the fire.
This was taken ten days after the fire. The debris of the collapsed neighbouring building had been cleared away, and the demolition gang had already made considerable efforts to tear down this building also, with ropes and winches. After pulling at it for several days, and pulling out a further portion of the side walls, it was decided that the structure was so secure that it offered no danger, and it was permitted to stand.
About three weeks afterwards a test, of which I also have a photograph, was made by the United States Government, who detailed an officer of the Engineer Corps of the United States Army, to investigate the matter. The floor, which was designed for 150 lbs. per sq. ft., was loaded to the extent of 300 lbs. per sq. ft., and the load left on for twenty-four hours. The greatest observed deflection was 1/16 in. A minute inspection of the building made by Mr C. G. Edwards, building surveyor, showed that two columns were damaged on the corners for a length of 2 ft. exposing the corner rods. It could not be determined whether this was due to heat, or to a blow. The top floor showed several slight cracks on the under side of the floor plates, but not enough to impair their efficiency. It is further stated by Mr. Edwards that the depth to which the fire affected the concrete was nowhere more than one-eighth of an inch.
I must now conclude what I fear has been a wearisome catalogue of single instances, with making my most grateful acknowledgment to Mr. Anketell Henderson for his great labour in preparing and exhibiting the lantern views, and to yourself, Mr. President, and to you, gentlemen, for a very patient hearing.
After the reading of the papers the President thought it unwise to discuss them that night owing to the lateness of the hour, and suggested that the discussion be at the next meeting, on July 26. The suggestion was adopted.
D. C. Askew (F.) moved, and R. J. Haddon (F.) seconded, a vote of thanks to the authors of the papers, which was supported by Mr. D. A. Swanson (President of the Master Builders' Association) and carried amidst applause. After brief responses by Messrs. Monash and Kerr, the meeting concluded.
JOHN LITTLE (F.) in opening the discussion, said the illustrations appearing in the last number of the "Proceedings" suggested to his mind a petrified timber construction. It was most probable therefore that any new material in building construction would be used practically upon the old well-known lines. Great stress, he insisted, should be laid upon securing good design (in which it seemed that the use of a particular formula mattered little, as the same result was obtained by the use of many formulae); good material, in which the ingredients were properly proportioned and unsuitable material - especially limestone if fire-resisting properties were to be secured - avoided; and good labour, especially in the fixing of the centreings [sic] or forms, and particularly in the supervision. The more prominent qualifications for the use of re-inforced concrete were that it was fire-resisting, cheap, and permanent, whilst great speed was attainable in the erection of structures in which it was largely employed. He referred to the difficulties of using a colouring matter in the cement facing to give a pleasing appearance - all previous attempts having resulted in failure. He did not like the application of paint, because of its constant renewal. He considered it was a blot upon the city of Melbourne Building Act in that so little, if any, provision was made for the use of new materials; and further, in its administration, there seemed to be too great a love for the use of old systems. The Institute should certainly move in the direction of securing an amended Act, containing provision for the use of modern methods of construction.
A. Henderson (V.P.) thought everyone was pleased with the paper, which contained matter new to most of them. The paper had not gone far enough, however, to bring up subject matter for discussion. He hoped that at an early date Mr. Monash would deal with the scientific presentation of the subject, and lay before the Institute the calculations of the various formulae, and also the results of experiments. He had carefully read the reports of the Baltimore fire. In one of these it was stated that the re-inforced concrete buildings were not in the burnt area, which, he concluded, meant the area not exposed to the greatest heat. All the reports, however, were unanimously in favour of re-inforced concrete on account of its fire-resisting properties. The tile arches, upon which great reliance had been placed, failed, and increased the spread of the flames by the falling of safes from the upper, through the lower, floors. Re-inforced concrete deserved to be experimented with under more diverse conditions than it had yet been subjected to. Their appetites had been whetted, and therefore they desired some scientific data presenting before them. Some curious revelations had come under his notice. A foreign professor, for instance, in showing - more or less clearly - four ways of getting one result, had shown - most clearly - his ignorance of the subject. He (the speaker) had recently obtained the sanction of the official referees of Melbourne to use a 3-inch concrete plate for a span of 9 feet. The use of concrete in the manner illustrated by the paper was not permissible under the existing regulations.
R. J. Haddon (F.) thought it was hardly fair to ask Mr. Monash to tell "everything he knew", because, as a specialist, the lecturer could not be expected to give them the whole of his knowledge. Mr. Little had said limestone was unsuitable in reinforced concrete. He (the speaker) desired to learn what materials were most suitable. He personally favoured the use of clinker, which, he maintained gave first-class results, whilst coke was frequently used for lintels and beams. He thought the paper one of the most interesting to which they had listened. What was its use, however, unless our out-of-date Building Act were amended, so as to permit of the use of modern methods?
F. J. Smart (F.) was not present at the reading of the paper, but wished to observe however, that when they used thin ceilings the ironwork ceased to be protective in case of fire. The surface became fractured and the iron heated. Terra-cotta was a first-class material to use in fireproof construction, and he (the speaker) narrated some experiments he had witnessed proving it to be a material vastly superior to brick. Cement, on account of its density, he thought, was liable to fracture.
Mr. MONASH, in reply, said very few questions had been raised in the debate. Mr. Little was quite correct in his contention that good design and workmanship were necessary, but good design implied provision for factor of safety which would allow for variations in workmanship. All the materials had their known factors, and were proper to use in re-inforced construction. If they knew the nature, property, and application of concrete in foundations, they also knew it when used in the superstructure as re-inforced concrete. Mr. Little had stated that a cement exterior was unattractive but it was usual to apply tile or brick to produce a "facing". This endeavour to secure a pleasing exterior was of course, unnecessary in the case of factories, stores, etc., but he thought the "colour" difficulty could always be solved by the use of a brick casing. Regarding the request of Mr. Henderson, reiterated by Mr Haddon, he would be happy to give the theories and calculations. He regretted that the literature on the subject in English books was far behind that of France and Germany. Various doctrines were maintained by different practitioners in different languages, which he would be glad to review, and to point out the good and weak points in the various systems. The technical journals of France and Germany published much that was valuable, and also much that was fallacious, being based upon incorrect or ill-considered data. The American journals were now beginning to take up the subject, but they were as yet far behind some of the Continental authorities. The main principles, however, were now well understood and generally accepted, and, in the end, they had their working formulae. Many practical users found they could afford to let the mathematics go. Concerning building regulations, the most progressive municipalities had now provided for the use of re-inforced concrete. He had no objection to tell "all he knew"; he had been forced to study the subject, and was willing to make everything as clear as possible. Regarding the composition of aggregates, his experience was mainly confined to basaltic bluestone, which both as metal and screenings, was in common use in Victoria. He could not say whether similar results were attainable by the use of coke or clinkers. Mr. Smart's remarks concerning the cracking of concrete were opposed to his (the speaker's) experience, and he had shown that in the Baltimore fire, after a ten days' endeavour to pull down a certain building, it was decided to let it stand. In Berlin, some recent fire tests showed equally satisfactory results. In a building erected on purpose, the very high temperature of 1,500 deg. was attained, and then the building was flooded with water. In temperatures attained in ordinary fires the concrete was not affected one-quarter of an inch beneath the surface. The American journals seemed to hold the opinion that, in the Baltimore fire, re-inforced concrete had more than held its own. He thanked the Institute for the opportunity of making these later remarks upon the paper he had read at the last meeting.
Other papers by Monash on this web site.