the Week of Proper 28 / Ordinary 33
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Bible Encyclopedias
Steel Construction
1911 Encyclopedia Britannica
or The use of steel construction in the erection of large buildings is the natural consequence of the conditions imposed upon owners of property lying within sections of large cities, and the result of the introduction of new materials and devices. Apart from the aesthetic considerations to which has been due the construction of spires, towers, domes, high roofs, &c., the form and height of buildings have always been largely controlled by a practical consideration of their value for personal use or rental. The cost of buildings of the same class and finish is in direct proportion to their cubic contents, and each cubic foot constructed is commercially unprofitable which does not do its part in paying interest on the capital invested. Until the latter half of the 19th century, these considerations practically limited the height of buildings on city streets to five or six storeys. The manufacture of the wrought-iron " I " beam in 1855 made cheaper fire-proof construction possible, and, with the introduction of passenger lifts (see Elevators; Lifts or HoIsTs) about ten years later, led to the erection of buildings to be used as hotels, flats, offices, factories, and for other commercial purposes, containing many more storeys than had formerly been found profitable. The practical limit of height was reached when the sectional area of the masonry of the piers of the exterior walls in the lower storey had to be made so great, in order to support safely the weight of the dead load of the walls and floors and the accidental load imposed upon the latter in use, as to affect seriously the value of the lower storeys on account of the loss of light and floor space. This limit was found to be about ten storeys. Various devices were successively made to reduce the size of the exterior piers. In 1881 the walls of a very large courtyard were constructed by building a braced cage of iron and filling the panels with masonry, a system of construction which had been used in the early part of the century for a tall shot-tower erected in the city of New York. Subsequently several buildings were erected in which the entire weight of the floors and roofs was carried by a system of metal columns placed against the inner surface of the exterior walls. The walls thus supported no load but their own weight, and were tied to the inner cage formed by the wall columns, interior columns, girders, and floors by anchors arranged to provide for the shrinkage of masonry in drying out which always occurs to a greater or less extent. By the use of this form of construction buildings were carried to the height of eighteen or nineteen storeys.
Iron or steel as a substitute for wood for constructive purposes was long thought to be fire-proof or fire-resisting because it is incombustible, and for this reason it has not only replaced wood in many features of building construction but is also used as a substitute for masonry. In time, however, it was realized that iron by itself is not fire-proof, but requires to be protected by means of fire-resisting coverings; but as soon as satisfactory forms of these were invented their development progressed hand in hand with that of iron and steel forms and combinations.
Buildings in steel are either of " skeleton " or " cage " construction. These terms may be defined as follows: In "skeleton" construction the columns and girders are built without proper or adequate inter-connexion and would not be able to carry the required weights without the support afforded by the walls; or, as in more recent construction, the walls are self-supporting and the other portions of the building are carried on by the skeleton steelwork. " Cage " construction consists of a complete and well-connected framework of iron or steel capable of carrying not only the floors but the walls, roof, and every other part of the building, and efficiently constructed with wind bracing to secure its independent safety under all conditions of loading and exposure, all loads being transmitted to the ground through columns at predetermined points. In America under this system the walls can be built independently from any level (see fig. 4), but in England the requirements of the building acts as to the thickness of walls prevents the general use of this form of construction.
Skeleton construction is defined by the Chicago building ordinance as follows: " The term ` skeleton construction ' shall apply to all buildings wherein all external and internal loads and strains are transmitted from the top of the building to the foundations by a skeleton or framework of metal. In such metal framework the beams and girders shall be riveted to each other at their respective junction points. If pillars made of rolled iron or steel are used, their different parts shall be riveted to each other and the beams and girders resting upon them shall have riveted or bolted connexions to unite them with the pillar. If cast-iron pillars are used, each successive pillar shall be bolted to the one below it by at least four bolts not less than three-fourths of an inch in diameter, and the beams and girders shall be bolted to the pillars. At each line of flooror roofbeams, lateral connexion between the ends of the beams and girders shall be made by passing wrought-iron or steel straps across or through the cast-iron column, in such a manner as to rigidly connect the beams and girders with each other on the direction of their length. These straps shall be made of wrought-iron or steel, and shall be riveted or bolted to the flanges or to the webs of the beams or girders.
" If buildings are made fire-proof entirely, and have skeleton construction so designed that their enclosing walls do not carry the weight of the floors or roof, then their walls shall be not less than twelve inches in thickness; and provided, also, that such walls shall be thoroughly anchored to the iron skeleton, and provided, also, that, whether the weight of such walls rests upon beams or pillars, such beams or pillars must be made strong enough in each storey to carry the weight of wall resting upon them without reliance upon the walls below them. All partitions must be of incombustible material." With the introduction of cheap structural steel, steel cage construction came rapidly into use. The dimensions of the exterior piers ceased to control the height of the building, which was limited alone by the possibility of securing adequate foundations, and by a consideration of the amount of floor space which could be devoted without too great loss to a system of passenger lifts of sufficient capacity to afford speedy access to all parts of the building. The advantages that led to the very rapid introduction of this system were not only the power of greatly reducing the size of the piers, but the enormous facility afforded for quick construction, the small amount of materials relatively used and the proportionately small load upon the foundations, and the fact that as the walls are supported at each storey directly from the cage, the masonry can be begun at any storey independently of the masonry below it. It is a disadvantage of the system that defects of proportion, material, or workmanship, which would be of less moment in an old-fashioned construction, may become an element of danger in building with the steel cage, while the possibility of securing a permanent protection of all parts of the cage from corrosion is a most serious consideration. The safety of the structure depends upon the preservation of the absolute integrity of the cage. It must not only be strong enough to sustain all possible vertical loads, but it must be sufficiently rigid to resist without deformation or weakening all lateral disturbing forces, the principal of which are the pressure of wind, the possible sway of moving crowds or moving machinery, and the vibration of the earth from the passage of loaded vans and trolleys, and slight earthquakes which at times visit almost all localities. In buildings wide in proportion to their height it is the ordinary practice to make the floors sufficiently rigid to transfer the lateral strains to the walls, and to brace the wall framings to resist them. In buildings of small width in proportion to their height this method of securing rigidity, is generally found to be inadequate, and the frame is also braced at right angles to the outer walls to take up the strains directly. In each case all strains are carefully computed. The bracing is accomplished by the introduction at the angles of the columns and girders or beams of gusset plates or knee braces, or by diagonal straps or rods properly attached by rivet or pin connexions. All portions of the frame are united by hot rivets of mild steel or wrought iron, care being taken that the sum of the sectional areas of rivets affords in each case a sufficient amount of metal for the safe transfer of the stresses. The greatest care should be taken to see that all rivet holes are accurately punched, and if necessary that they are rhymed so that each rivet will have its full value.
For the proper and successful erection of the frame much depends upon an accurate alinement of the column bases. These should be properly tested as to position and level. The bases are either grouted with cement, or bolted to the foundations, but where cast column bases rest on masonry piers or footings any considerable grouting is not advisable. The only grouting that should be permitted in tall buildings would be in levelling up the tops of the concrete footings to receive the masonry courses, or in a very thin layer between the column pedestal and the masonry bed. The cap stones should always be brought to the most accurate bed possible, with grouting used as a thin cement and not as a backer. Accurate redressing of the cap stones after setting is much to be preferred.
All riveting and punching of the steel members is done at the shop, where also they receive the usual coat of oil or paint. This leaves the assembling and field riveting to be done on the ground, together with the adjustment of the lateral or wind-bracing, the placing of tie rods and the field painting.
After erection the steelwork should receive one or two coats of paint; two coats are to be recommended, in which case they should be of different colours. Red lead is best for the priming coat and oxide paint for the finishing coat. In German specifications it is required that the steelwork should first receive a coat of boiled linseed oil, in order that the red lead coating should be more coherent with the steel.
Steelwork that has to come in contact with brickwork or concrete should not be painted, but should receive a wash of cement as the brickwork or concrete-work proceeds. The steelwork which is exposed to the weather should be painted about every three years, but when it is under cover an interval of five years may elapse.
To secure painting of permanent value a clean scaleless and rustless surface is first necessary. Steel plates and shapes, when delivered from the rolls which form them to the cooling beds, are largely covered with scales, which, adhering only partially to the surface, offer the intervening cracks or joints as vulnerable points for rust. After being rolled, structural steel is stored or handled out of doors for a varying period both at the mill and then again at the shop before the building is started. This period of open-air exposure allows the process of rust to start under the scales. If the rust so covered up has not begun to pit the iron the chances are that it will do no harm; but, if it is already well developed and of some thickness, it will have enough oxidizing agents in its pores to develop more oxide, and to swell up and crack the paint. The first requirement, therefore, for efficient painting is the careful removal of all mill-scale, rust, grease, or foreign substance, before even the priming coat is applied. It is agreed that the first step in the preservation of metal-work against deterioration or corrosion is the obtaining of absolute cleanness of metal before the application of paint or oil.
The following are the requirements of the New York building law in regard to the protection of iron or steelwork against corrosion, &c.: " All structural metal-work shall be cleaned of all scale, dirt and rust, and be thoroughly coated with one coat of paint. Cast-iron columns shall not be painted until after inspection by the Department of Buildings. Where surfaces in riveted work come in contact they shall be painted before assembling. After erection all work shall be painted with at least one additional coat. All iron or steel used under water shall be enclosed with concrete." The Chicago ordinance makes no mention of paint or coating to prevent rust in metal framework. The London Building Acts do not set out any special requirements, but suggestions have been made at the Royal Institution of British Architects for the regulation of skeleton buildings and they are drawn up upon a more scientific basis than the bulk of the existing acts.
In transferring the loads from the column bases to the bottom of the footings the greatest care must be taken in all systems of construction that the stresses throughout at no point exceed the safe limits of stress for the various materials used. Steel is generally used for columns in preference to cast iron, because it affords greater facility for securing satisfactory connexions, because its defects of quality or workmanship are more surely detected by careful test and inspection, and because, on account of its superior elasticity and ductility, its fibre is less liable to fracture from slight deformations. It is used in preference to wrought iron on account of its lesser cost.
Columns are generally built of riveted work of zedbars, channels, angles, plates, or lattice, of such form as will make the simplest and most easily constructed framing in the particular position in which the column is placed. The columns are sometimes run through two or more storeys and arranged to break joints at the different floors. In buildings to be used as offices, hotels, apartments, &c., it is usual in establishing the loads for the purpose of computation to assume that the columns carrying the roof and the upper storey will be called upon to sustain the full dead load due to material and the maximum computed variable load, but it is customary' to reduce the variable loads at the rate of about 5% storey by storey towards the base, until a minimum of about 20% of the entire variable load is reached, for it is evidently impossible that the building can be loaded by a densely-packed moving crowd in all of its storeys simultaneously. In the case of factories and buildings used for storage purposes the maximum variable load which can be imposed for any serious length of time on each floor must be used without reduction in computing the loads of the lower column, and proper allowances must be made for vibrating loads. In the case of very tall exposed buildings of small depth, the vertical load on the columns due to wind pressure in the opposite side of the building must be computed and allowed for, and in case the lower columns are without lateral support their bending moment must be sufficient to resist the lateral pressure due to wind and eccentricity of loading. In computing the column sections a proper allowance must be made for any eccentricity of loading. It is usual to limit the height of sections of columns without lateral support to 30 diameters, and to limit the maximum fibre stress to 12,000 lb per sq. in. The sectional areas are computed by the use of the ordinary formulae for columns and struts.
The standard sections in use are numerous and varied, and from time to time a steel user has occasion to design a new steel shape because no existing section is suitable. The experiments given by Professor Burr indicate that a closed column is stronger than an open one, but practice does not always support theory, and many other questions besides mere form arise in connexion with the choice of a section; special considerations in the use of columns in buildings sometimes call for a form very different from the circular section, and such include the transfer of loads to the centre of the section, the maximum efficiency under loading, and the requirements for pipe space around or included in the column form. Lattice bars, fillers, brackets, &c., add just so much more weight without increasing the section, and must be allowed for; the method of riveting the sections together must also be taken into account.
For girders of small spans " I " beams or channels are generally used, but for greater spans girders are built of riveted work. in the form of boxes with top and bottom plates, side plates, and angles with proper stiffening bars on the side plates, or " I's," or lattice, or other forms of truss work. In girders and beams the maximum fibre stress is usually limited to 16,000 lb. In very short girders the shear must be computed, and in long girders the deflexion, particularly the flexure from the variable load, since a flexure of more than of the length is liable to crack the plastering of the ceilings carried by the girders. The same necessity for computing shear and flexure applies to the floor beams. The floors between the girders are constructed of " I" beams, spaced generally about 5 ft. between centres; their ends are usually framed to fit the form of the girders, and rest either upon their lower flanges, or upon seats formed of angles riveted to their webs, being secured to them by a pair of angles at each end of the beam riveted to its web and to the web of the girder. Sometimes the beams rest upon the girders, and are riveted through the flanges to it; in this case the abutting ends of beams are spliced by scarf plates placed on each side of the webs and secured by rivets. A similar construction is followed for flat roofs, the grades being generally formed in the girder and beam construction, and a flat ceiling secured by hanging from them, with steel straps, a light tier of ceiling beams. The floor beams are tied laterally by rods in continuous lines placed at or above their neutral axis. It is usual in both girders and beams to provide not only for the safe support of the greatest possible distributed load, but for the greatest weight, such as that of a safe or other heavy piece of furniture which may be moved over the floor at its weakest points, the centres of the girders and beams. It must always be borne in mind that the formulae for the ultimate strength of the " I " beams only hold good when the upper chord or flange is supported laterally.
Considerable improvement has been made in the design of rolled steel shapes; for example the rolling of a 16-in. joist was formerly deemed a remarkable achievement, though now there are several works producing 24-in. joists with flanges 7 and 7 in. wide. The Broad Flange Differdange Beams are claimed by the manufacturers to be stronger and to minimize weight for use as girders; they are made in twenty-one different sizes with flanges from 88 to i 2 in. wide.
The introduction of steel construction has simplified many details of architectural treatment, such as projections for cornices, bay windows and galleries. These may be supported by bracket-angles attached to the columns with a system of anchors to tie them back; the material must be carried in such a manner as to make it independent of the general structure, and must be constructed as light as possible. If the supporting member is a floor beam or girder the girder should be rigidly connected to the floor system to prevent any twisting due to the weight of the projection.
The arrangement of the building and floor framings is in a great measure governed by the architectural effect sought and by the arrangement and proper planning of the interior according to the intended uses; the positions of columns, girders and floor beams are usually the result of particular requirements, and unless complicated and expensive framing is to be expected the distance between columns must be kept within the limits of simple girder construction. The position of the columns having been determined, the girders must next be located; these serve to support the floor beams which transfer the loads direct to the columns, and also to brace the columns during erection. The spacing, or distance from centre to centre of the floor beams, will depend upon the type of fire-proof flooring employed; it also depends to a considerable extent upon the amount and character of the floor load and the length of span. If the loads to be carried are largely stationary, and if the span is small, the floor joists can be readily proportioned by means of tables given in the handbooks issued by many steel companies. The distance between joists should be limited to 5 or 6 ft.; horizontal bracing by means of diagonal rods is sometimes used, but should be avoided. The following are the usual assumptions made in good practice for superimposed loads: Floors of dwellings and offices.. 70 lb per sq. ft.
„ „ churches, theatres and ball-rooms 125 „ „ warehouses. ... 200 to 250 „ „ for heavy machinery. .. 250 to 400 „ The relation between the velocity of wind and the pressure exerted upon surfaces must be considered in steel construction, and designers differ in regard to the forces to be resisted and the material to be used. Every building offers its own peculiar condition; the height, width, shape and situation of the structure, and character of the enclosing walls, will determine the amount of wind pressure to be provided against, and the internal appearance and the planning of the various floors will largely influence the manner in which the bracing is to be treated. There are many and varied forms of bracing, each designer adopting methods peculiar to his own ideas. One form consists of adjustable diagonals, rods or bars, properly fastened to the columns in the building; these diagonals may run through one floor and be attached to the columns at the floor above. Another form is known as portal bracing; this is usually braced between adjacent columns in halls or passage-ways and extends from the foundations up from floor to floor to such a height that the stability of the building itself is sufficient to resist the assumed wind pressure. In general, if the building is square or nearly so wind-bracing should be placed close to the corners. In case neither of the above methods can be applied, brackets should be used at each floor level or a continuous deep beam or girder carried all around the building. Some architects depend solely upon partitions, and a building with a well-constructed iron frame should be safe if provided with brick partitions or if the exterior of the iron framework is covered with well-built masonry of sufficient thickness.
Truss rods, portals, or lattice or plate girders constitute the more definite types of wind-bracing ordinarily employed; the bracing must reach to some solid connexion at the ground. The greatest wind pressure to which a building is subjected is that from a horizontal wind. The maximum pressure is not uniform from the ground level to the roof but is greatest at the centre; it is diminished near the ground level by the frictional resistance of the ground, and at the eaves by the eddies formed by the air escaping over the roof. The change in direction of the air when striking a flat surface such as the side of a building will form a cushion to diminish the effects of impulses and shocks from local gusts.
The building laws of the city of New York require the following provisions as regards wind forces: " All structures exposed to wind shall be designed to resist a horizontal wind pressure of thirty pounds for every square foot of surface thus exposed, from the ground to the top of the same, including roof, in any direction. In no case shall the overturning moment due to wind pressure exceed seventy-five per centum of the moment of stability of the structure. In all structures exposed to wind, if the resisting moments of the ordinary materials of construction, such as masonry, partitions, floors and connexions, are not sufficient to resist the moment of distortion due to wind pressure, taken in any direction on any part of the structure, additional bracing shall be introduced sufficient to make up the difference in the moments. In calculations for wind pressures, the working stresses set forth in the code may be increased by fifty per centum. In buildings under one hundred feet in height, provided the height does not exceed four times the average width of the base, the wind pressure may be disregarded." The steel used throughout the entire structure should be subjected to the most thorough chemical and mechanical tests and inspection, first at the mill and subsequently at the fabricating shops and the building, ensure that Used. g p g?
it shall not contain more than o
08% of phosphorus or o
06% of sulphur, that it shall have an ultimate strength of between 60,000 and 70,000 lb per sq. in., with an elastic limit of not less than 35,000 lb per sq. in., and an elongation before fracture of not less than 25% in 8 in. of length, and that a piece of the material may be bent cold 180 over a mandril equal to the thickness of the piece tested without fracture of the fibres on the outside of the bend. At least two pieces are taken from each melt or blow at the mill, and are stamped or marked, and all the various sections rolled from the melt or blow are required to bear a similar stamp or mark for identification. All finished material is carefully examined to see that it possesses a smooth surface, and that it is free from cracks, seams and other defects, and that it is true to section throughout. Rivets are either of wrought iron or of extra soft steel, with an ultimate tensile strength of 55,000 lb per sq. in. The material must be sufficientIy tough to bend cold 180° flat on itself without sign of fracture. The greatest care is taken that no steel is left in a brittle condition by heating and cooling without proper annealing. All abutting joints in riveted work are faced to exact lengths and absolutely at right angles to the axis of the piece, and are spliced by scarf plates of proper dimensions adequately secured by rivets. The work should be so accurate that no packing pieces are necessary. If the conditions are such that a packing or filling piece must be used, the end of one piece is cut to a new and true surface, and the filling piece is planed to fill the space accurately. Where cast iron is used it must be of tough grey iron free from defects. In testing it pieces 14 in. long and i in. square are cast from each heat and supported on blunt knife edges spaced 12 in. apart; under a load in the centre of the piece of 2500 lb the deflexion must not exceed - in.
The filling between the girders and floor beams consists of segmental arches of brick, segmental or flat arches of porous (sawdust) terra-cotta, or hard-burned hollow terra- - cotta voussoirs, or various patented forms of con crete floors containing ties or supports of steel or iron. In all cases it is customary to fill on top of the arches with a strong Portland cement concrete to a uniform level, generally the top of the deepest beam; the floor filling is constructed and carried to this level immediately upon the completion of each tier of beams, for the purpose not only of stiffening the frame laterally, and of adding to its stability by the imposition of a static load, but also to afford constantly safe and strong working platforms at regular and convenient intervals for use throughout the entire period of the construction. In cases in which the lateral rigidity of the floors is depended upon to transfer the horizontal strains to the exterior walls which are framed to resist them, no form of floor construction should be used which is not laterally strong and rigid. With very rapid building, no method of construction of floors furrings, or partitions should be adopted which will not dry out with great speed. In flat forms of masonry floor construction the level of its bottom is placed somewhat below the bottom of the " I " beams and girders, so that when it is plastered a continuous surface of at least an inch of mortar will form a fire-proof protection for the lower flanges of the beams and girders. Where the width of the flange is considerable it is first covered with metal lath secured to the under side of the floor masonry. Girders projecting below the floor are usually encased in from z to 2 in. of fireproof material, 2 or 4 in. of which is also put on all columns. Such fire-proof coverings, and also interior partitions, are composed of hollow, hard-burned terra-cotta blocks, of porous (sawdust) terra cotta, or various plastic compositions applied to metallic lath, many of which are patented both as to material and method of application. The most simple test for the value of a system of fire-proof coverings, and of partitions and furrings, is to erect a large sample of the work and to subject it alternately to the continued action of an intensely hot flame which is allowed to impinge upon it, and to a stream of cold water directed upon it from the ordinary service nozzle of a steam fire engine. It is important in all fire-proofing of columns and girders, and in all floor construction, furring and partitions, that there shall be no continuous voids, either vertical or horizontal, which may possibly serve as flues for the spread of heat or flame in case of fire. All furrings and partitions must be started on the solid masonry of the floors to prevent the possible passage of fire from the room in which it may occur. The failure to make this provision has been the cause of very serious losses in buildings which were supposed to be fire-proof.
Steel construction possesses great advantages in time required for erection. When once the site is cleared and the foundations prepared and set, work can be pushed on the walls at different storeys at one and the same time, and often main cornices and filling-in work are fixed before special details and ornamentation. In the Commercial Cable Building, New York, seven complete tiers aggregating 7000 tons were erected in nine weeks. In the Unity Building, Chicago, of seventeen storeys, the metal framework from basement columns to finished roof was accomplished in nine weeks. In the Fisher Building, Chicago, the entire steel skeleton above the first floor, nineteen storeys and attic, was erected in twenty-six days.
Owing to the low price of steel it is possible to make a steel column of equivalent strength cheaper than one in cast iron. The question of cost is purely a commercial one, but the cost of the raw material will practically never determine the relative cost between various forms, as the expense of manufacture and the detail and duplication of members will all influence the ultimate cost to a much greater extent than the simple cost of the plain materials. The steelwork for a building of any considerable size is almost invariably rolled to order.
| Mewes & Davis, Architects. | FIG. I. - THE MORNING POST BUILDING, LONDON. Waring White Building Co. Ltd., Contractors.
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