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Scaffold
1911 Encyclopedia Britannica
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SCAFFOLDING (from the O. Fr. escafaut, originally escafalt, modern echafaud, a corruption of the Italian or Spanish catafalco, a platform, especially a canopy over a bier, a catafalque; this word is composed of O. Span. catar, O. Ital. catare, to view, Lat. captare, to watch, observe, and balco, balcony), properly a platform or stage, particularly one of a temporary character erected for viewing or displaying some spectacle, and hence applied to the raised structure on which the execution of a criminal or condemned person is carried out. (See Capital Punishment, &c.). The word "scaffold" or "scaffolding" is used in a technical sense of an obstruction formed in a blast furnace by the fitting together of lumps which form a comparatively solid skeleton mass inside the furnace, preventing the charge from descending properly. The most general modern application of the word, however, is, in building, to the temporary structure of platforms erected or suspended at convenient heights to afford workmen easy access to their work. Such scaffolds may be divided into four principal classes - bricklayers' scaffolds, masons' scaffolds, gantries and derrick towers or stages. The first two are constructed with upright and horizontal poles lashed together. Gantries and derricks are built up of squared timber, and the different members are connected by iron bolts and dogs.
The bricklayers' scaffold is constructed of standards, ledgers and putlogs, and the connexions are made with lashings of rope, though wire ropes or chains are sometimes used. The Brick- standards are a series of upright fir poles 30 to 50 ft. in length, either (1) sunk about 2 ft. into the ground, (2) fixed in barrels filled with earth lightly rammed, or (3) placed upon a "sole plate" of timber with a square formed of small fillets of wood round the base to prevent movement. The standards are placed 6 to 9 ft. apart, and about 5 ft. away from the building. At every 5 ft. ledgers are tied to the standards to support the putlogs, which in turn support the platform of planks. The ledgers are poles lashed horizontally to the standards; upon these, putlogs, usually of birch wood 3 in. square in section, are laid about 3 or 4 ft. apart, with one end resting on the ledger and the other in a recess in the wall. The outer end should be lashed to the ledger. Boards are then laid upon these putlogs parallel with the face of the wall. Two thicknesses of boards are laid when the work is heavy. If the scaffold is erected in an exposed position or is more than 30 ft. high, it should be stiffened by cross braces of poles running diagonally across the face of the structure and firmly lashed to all the main timbers touched. Ties should also be taken back from the face of the scaffold through apertures in the walls of the building and firmly secured. These ties should be connected with every fourth standard and start at a height between 20 and 30 ft. from the ground. Instead of, or in addition to, these ties light shores may be taken from the face of the scaffold outwards from the building. As the work is carried up the boarding and many of the putlogs are removed to the stage above, some putlogs, however, being left tied to the lower ledgers to stiffen the scaffold. In the case of thick walls a scaffold is required inside as well as outside the building, and when this is the case the two structures are tied together and stiffened by short connecting poles through the window and door openings.
The mason requires an independent scaffold. He cannot rest the inner ends of his putlogs in the wall as the bricklayer does, for this would disfigure the stonework, so he erects another and parallel framework of standards and ledgers within a few inches of the wall-face upon which scaffold. to support them. The two portions are tied together with cross braces, and the whole of the timbering is made capable of taking heavier weights than are required in the case of the bricklayer.
Scaffolding poles are of Northern pine obtained chiefly from the Baltic ports. They consist of small trees up to 30 to 40 ft. long and of not more than 9 in. in diameter. They are sold with the bark on, but this should be removed before use. Such material forms the standards and ledgers. The putlogs are usually pieces of birch from 3 to 4 in. square in section, and 5 to 6 ft. long. In order to have the fibres uncut they should be split, not sawn. Scaffold boards are made in 8-to 12-ft. lengths, 7 or 9 in. wide, and 12 in. or 2 in. thick. They should be of yellow deal, but they are more often cut from spruce. The corners are cut off and the ends bound with stout hoop-iron to prevent splitting. The cords used for lashing are made of jute and hemp fibre. The best and strongest cords are those of white Manilla hemp. The fibres for scaffold cords are often dipped in hot tar before being made up into rope. The ropes generally used by the scaffolder are either "shroud laid," having three strands of fibres wound tightly around a core, or "three strand," which are similar but without a core.
The erection of scaffolding demands nerve and physical strength, as well as skill and discretion. The timbers near the ground are fixed by hand labour alone; the higher poles are raised Erection. by pulley and rope. The wedges used for tightening cordage are driven in between the pole and the rope. They should be of oak or other hard wood, about 12 in. long and semicircular in cross section, and should taper off from one end to the other. Practically the only tool used by the scaffolder is his hatchet, made with a hammer-head for driving spikes and wedges; the wooden handle he often uses as a lever to tighten knots and cords. Scaffolds should not be too heavily loaded, and the weight of materials should be distributed as much as possible. This applies especially to bricklayers' scaffolds, for heavy concentrated loads, even if not sufficient to cause the scaffold to fail, tend to injure the brickwork.
In Scotland and the north of England much work is done from inside by means of platforms of boards placed upon the floor joists. When the work gets so advanced that it cannot be reached from the floor, trestles and platforms are used. For executing special external features, such as stone carving or plaster moulding, a scaffold will be thrown out on cantilevers projecting through openings in the wall and tied down inside the building. The materials are usually hoisted by derrick cranes.
"Gantry" is the term applied to a staging of squared timber used for the easy transmission of heavy material. The name has,. however, come to be used generally for strong stagings of squared timber whether used for moving loads or not.
Taking the general meaning of the term, gantries may be divided into three classes: (I) Gantries supporting a traveller; (2) Travelling gantries, in which the whole stage moves along rails placed on the ground; (3) Elevated platforms which serve as a base upon which to erect pole scaffolding.
A gantry to support a traveller (fig. I) consists of two sets of framing placed at a convenient distance apart, say 8 ft. or more, and standing independently of each other. These frames consist of standards or uprights standing upon a sleeper or sill resting in a continuous line upon the ground. The tops of the standards are levelled to receive the head or runner. Struts are taken from cleats fixed at a convenient point in the sides of the standards, and meet in pairs under the middle of the head; sometimes a straining-piece is introduced between them. Struts are also taken outwards from the uprights and bedded on foot-blocks or bolted to small piles driven into the ground. The space between the two frames must be kept free from struts and ties of any description so as to leave a free passage for the material while being lifted and moved. The different members are connected by iron dogs and bolts; dogs are used wherever possible, as they form a strong connexion and do not spoil the wood for other purposes as bolt-holes do. They should be placed on both sides of the timbers to be connected. The size of the timbers varies according to the height of the structure and the weight intended to be carried. The standards may be from 6 to 12 in. squared in section, and the heads and sills are of similar size; the struts and braces are usually somewhat smaller. The traveller consists usually of two wood girders trussed with iron rods and mounted on flanged wheels so as to run along the FIG. 2.
rails fixed to the head-piece. Along each girder also, a rail is provided upon which moves the hoisting gear; this is worked either by hand or steam power. The ends of the rails are turned up to form a stop for the traveller or crab.
A travelling gantry (fig. 2) runs along rails placed on the ground, and consists of two strong trusses braced and bolted together and supporting the two trussed girders which take the crabwinch. The latter is mounted on wheels, and by simple gearing is caused to run along the rails fixed on the upper side of the girders. This is a most useful form of gantry, and requires a very small amount of timber for its construction. The travelling frame is, however, very heavy, and such an apparatus is usually fitted with a steam winch, the power from which, besides lifting the materials, can also be applied to move the traveller. Gantries built on this principle have been used successfully in building or repairing lofty and wide-spanned steel or other roofs. After the collapse of the steel "bowstring" roof of Charing Cross station (London) in December 1905, huge travelling gantries running along rails laid upon the station platforms were employed, and these provided an efficient and economical means of access to the damaged portions; as section by section the work was removed the gantries were shifted along to the next bay. These gantries were 60 ft. in height. One, used to strip and remove the coverings of the roof, was 32 ft. deep, weighed 200 tons and moved upon 24 steel flanged wheels; the other, 40 ft. deep and with 32 wheels, weighed 250 tons and was used to take down the structural steel work of the roof. Four cranes were erected upon the staging to lower the material as it was removed. The amount of timber used in these gantries was 22,400 cubic ft.
In the erection of the Williamsburg Bridge over East river, New York, for which 19,000 tons of steel were used, "framed timber falsework" was built up of squared timber to a height of loo ft. and 90 ft. wide at the top. The span was 355 ft. The timbering was in three storeys or stages, and each "bent" had 8 vertical and 4 battering posts. The bents were 20 ft. apart and were connected Trav e ling Ganhy ' 'FIG. I.
at the top by To lines of 12-in. by 14-in. stringers, and the lower sills were 12 in. square. The cross braces were 8 by io in. and 6 by 12 in. The vertical standards or posts rested on sills, and .under each one also at its base was a timber foundation 4 ft. square. Two travelling gantry towers, 22 ft. by 25 ft. and 40 ft. high, mounted on double-flanged wheels, ran on rails at the top of the falsework and carried long derrick booms fitted with pulleys for raising the materials necessary for the bridge. Beside the cranes they carried cars with the power plant, gasoline tank, water tanks and air compressor and apparatus for the pneumatic riveting hammers.
"Elevated platforms" are generally used in conducting building operations in towns where the importance of the traffic renders it necessary to keep the footway clear. They consist of two sets of standards, sill and head, one set being erected close to the building and the other about 8 or io ft. away. These stages are formed of square timber, framed and braced in a similar manner to gantries designed to support a traveller, but, instead of external shores or braces the uprights are braced across to each other, care being taken to fix the braces at such a height as to allow free passage beneath them. Joists are placed across from head to head, and a double layer of scaffold boards is laid to form the floor, the double thickness being necessary to prevent materials dropping through the joints upon the heads of passers-by. When the gantry abuts on the road, a heavy timber fender splayed at each end should be placed so as to ward off the traffic. Sometimes the scaffold is carried up several stages in this way and is then called "staging," but more often the gantry consists of only one stage and forms the foundation upon which light pole or other scaffolding is erected. At the level of the platform a fanguard is often thrown out for a distance of about 6 ft. or more and closely boarded to protect the public from falling materials and the workmen from accident.
Derrick "gantries" or "towers" (fig. 3) are skeleton towers of timber erected in a central position on a site to support a platform at such a height as to enable an electric or steam power derrick crane placed upon it to clear p p p the highest portions of the building. The crane revolves upon a base through nearly three parts of the circumference of a circle, and in addition to this the jib of the crane is capable of an "up and down" motion which enables it to command any spot within a radius of three-quarters of the length of the jib. For a single crane, a derrick tower with three legs is built, and the crane is placed over one of these, stayed back to the other two and then counterbalanced by heavy weights. Each leg is usually from 6 ft. to io ft. square on plan, the "king" leg (that is, the leg supporting the crane) being larger than the "queen" legs. The three legs are placed from 20 to 30 ft. apart in the form of an equilateral or isosceles triangle. When two cranes are used, as is the case when important operations are to be conducted over the entire area of a circle, a four-legged square derrick tower is constructed, and a crane set upon a platform over each of two opposite legs. The ground upon which it is proposed to erect the towers must be well chosen for its solidity, and often requires to be well rammed. The foundation usually consists of a platform of 9-in. by 3-in. deals under each leg. The corner posts may be of three 9-in. by 3-in. deals bolted together, but those for the king leg may advantageously be larger. They are connected at every 8 or io ft. of their height by means of cross pieces or transoms from 9 by 3 in. to 9 by 6 in. in size, and each bay thus formed is filled in on all four sides with diagonal bracing of the same or slightly smaller timber. Up the centre of the king leg, from the bottom to the top, is carried an extra standard of timber to take the weight of the crane. It may be a balk of whole timber, 12 or 14 in. square, or may consist of deals bolted together up to 16 in. square. This central standard must be well braced and strutted from the four corners to prevent any tendency to bending.
When the towers have reached the desired height the king leg is connected to each of the queen legs by a trussed girder, the two queen legs may be connected with each other either by a similar trussed girder or by a single balk of timber which can be supported by struts if the span is considerable. For the connecting girders a balk of timber reaching from king to queen legs is placed on each of the two topmost transoms, which may be from 4 to 8 ft. apart, the depth of the top bays often being modified to the required depth of the connecting beams. Upright struts are fixed at intervals of about 5 ft. between the two balks, which are also connected by long iron bolts and cross braces filled into each bay. The top balks project 6 or io ft. beyond the king leg and form the support for a working platform of deals. Struts are thrown out from the sides of the leg to support the ends of the balks. Upon the platform are laid two "sleepers" of balk timber extending from beneath the bed of the crane and passing over the centre of each queen leg. The "mast," a vertical member composed either of a single timber or two pieces strutted and braced, is erected upon the revolving crane bed, and the "jib," which is similar in construction to the mast, is attached to the base of the latter by a pivoted hinge. The jib is raised and lowered by a rope fixed near the end of the jib and running to the engine by way of a pulley wheel at the top of the mast. The rope or chain used for lifting the materials passes over a pulley at the end of the jib and thence to the winch over a pulley at the top of the mast. In the operation of lifting it is obvious that a great strain is put upon the mast and a considerable overturning force is exerted by the leverage of the weight lifted at the end of the jib. To counterbalance this, two timber "stays" or "guys" are taken from the mast head, one to the centre of each queen leg, and there secured. From these points two heavy chains are taken down the centre of Ihr e e dctrick 91ur?in4f 10? - FIG. 3.
each queen leg and anchored to the platform at their bases, which are each loaded with a quantity of bricks, stone or other heavy material equal in weight to at least twice any load to be lifted by the crane. A coupling screw link should be provided in the length of each anchor chain so that it may be kept at a proper tension, for if allowed to get slack a sudden jerk might cause it to snap. The coupling screws should be placed in an accessible place near the ground, where they may easily be seen and tightened when necessary. The legs of the structure should be cross braced with each other, either by ties of steel bars with tightening screws, or, as is more usual, with scaffold-pole or squared timber-braces crossing each other at right angles and lashed or bolted to the framework.
In the case of a three-towered gantry it is necessary to ballast only the two queen legs. The weighting of the king leg, as is sometimes done, is quite unnecessary, and even injurious, for in soft or moderately hard ground the added weight combined with that of the crane engine and load may cause a serious settlement. With a square gantry having four legs, all four should be weighted, and in calculating the ballast necessary for the crane towers the weight of the engine should be considered. Access to the platform is obtained by ladders fixed either inside or outside one of the queen legs. With the exception of the boards forming the working platform, which are usually spiked down, the timbers of a tower gantry should all be connected by screw bolts and nuts.
Swinging scaffolds are useful for executing light repairs to a building. Perhaps the simplest form of swinging scaffold is the Cradles " boatswain's boat," so called from its being chiefly and used for the painting or examination of the sides of swinging ships, but it is dangerous to work from and a light wind scaffolds, w i ll cause it to swing to and fro, and owing to the extremely awkward position occupied by the workman there is difficulty in doing good work from it. A better, safer and more comfortable arrangement, the "painter's boat" (fig. 4), is suspended by blocks and falls from two cantilever "jibs" fixed in the upper part of the building. The positions of the jibs are altered as required. The ends of the suspension ropes are fastened securely to the cradle, and by altering their length the workmen can adjust it to the proper height for working. These boats are usually constructed with a framework of iron and fitted with edge boards and guard rails all round. Like the "boatswain's boat" they sway considerably in the wind.
An improved form of cradle has been patented which is swung on block runners working along a tight wire cable stretched between two jibs. Block tackle is used to raise or lower the cradle, and horizontal movement also is obtained by light guy lines working over pulleys at the jibs and secured to the tops of the suspension ropes. All adjustments can be made from the cradle with perfect safety. The guy lines steady the boat to some extent and prevent it from swinging in the wind.
Tall chimney shafts may be erected by internal scaffolding only, or by a combination of external and internal staging. The latter Scaffold- method is often adopted when the lower part of the shaft affold Scfor is designed with ornamental brickwork, string courses, chimneys. panes a ef l ull &finished. An external scaffold is therefo eocardrieedbe up until plain work not more than 2 or 21 bricks thick is reached, when the remainder can be completed by "overhand" work from an internal scaffold. The offsets made in the brickwork on the inside are used to support the timbering. For the repair of tall chimneys, light ladders are erected one above the other by a steeplejack and his assistants, each being lashed to the one below it and secured to the brickwork by dog-hooks driven in the joints. When the top of the chimney is reached balk timbers are raised by pulleys and laid across the top. From these are swung cradles from which the defective work is made good. If the work or weather demand a more stable scaffold, a light but strong framework of putlogs held together with iron bolts is fixed on each side of the shaft with iron holdfasts, and a platform of boards is laid upon them. For circular chimneys pieces of timber cut to a curve to fit the brickwork are clamped with iron to the putlogs to prevent them from bending when the bolts connecting the two frames are screwed up.
In England, the Factory and Workshop Act of 1901 empowers the secretary of state to make regulations respecting any Accidents. dangerous "machinery, plant, process, or descrip t i on of manual labour." No regulations affecting the building trade have been made, however, but a memorandum was issued in 1902 by the Home Office with the following suggestions for the prevention of scaffold accidents: I. All working platforms above the height of io ft., taken from the adjacent ground level, should, before employment takes place thereon, be provided throughout their entire length, on the outside and at the ends, (a) with a guard rail fixed at a height of 3 ft. 6 in. above the scaffold boards. Openings may be left for workmen to land from the ladders and for the landing of materials; (b) with boards fixed so that their bottom edges are resting on or abutting to the scaffold boards. The boards so fixed should rise above the working platform not less than 7 ins. Openings may be left for the landing of the workmen from the ladders.
2. All "runs" or similar means of communication between different portions of a scaffold or building should be not less than 18 in. wide. If composed of two or more boards they should be fastened together in such a manner as to prevent unequal sagging.
3. Scaffold boards forming part of a working platform should be supported at each end by a putlog, and should not project more than 6 in. beyond it unless lapped by another board, which should rest partly on or over the same putlog and partly upon putlogs other than those upon which the supported board rests.
In such cases where the scaffold boards rest upon brackets, the foregoing suggestion should read as if the word bracket replaced the word putlog.
N.B. Experiments have shown that a board with not more than a 6 in. projection over a putlog can be considered safe from trapping or tilting.
4. All supports to centring should be carried from a solid foundation.
5. In places where the scaffolding has been sublet to a contractor, the employer should satisfy himself, before allowing work to proceed thereon, that the foregoing suggestions have been complied with, and that the material used in the construction of the scaffold is sound.
See J. F. Hurst, Tredgold's Carpentry; A. G. H. Thatcher, Scaffolding. (J. B'r.)
Copyright Statement
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Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Scaffold'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/s/scaffold.html. 1910.
