Chapter I: Scaffolding

Scaffolding is the art of arranging and combining pieces of timber in order to enable workmen to proceed with their work, and from which, if required, to lift and carry the material necessary for their purpose. Many definitions of a scaffold have been given by authorities on building construction; some of the best known are as follows:—

Mitchell (C. F.): 'Temporary erections constructed to support a number of platforms at different heights, raised for the convenience of workmen to enable them to get at their work and to raise the necessary material for the same.’

Tredgold (Hurst): 'A scaffold as used in building is a temporary structure supporting a platform by means of which the workmen and their materials are brought within reach of their work.’

Rivington: 'Scaffolds are temporary erections of timber supporting platforms close to the work, on which the workmen stand and deposit their materials.’

Banister F. Fletcher, in 'Carpentry and Joinery’: 'A scaffold is a temporary structure placed alongside a building to facilitate its erection by supporting workmen and raising materials during the construction, or for the repair of buildings.’

Recent cases tried under the Workmen’s Compensation Act have given a wider meaning to the word, and the following definition is perhaps the most comprehensive at the present time:

A scaffold, as used in building, is a temporary arrangement of timbers combined and supported in various ways to enable the workmen to proceed with their work, and where required, to afford facilities for the lifting and carrying of the materials.

The two principal methods of scaffolding are known respectively as the North and South country systems. The northern, as indicated by the name, was at one time in use only in Scotland and the north of England, but its many advantages, more especially for the transport of material, have now caused it to become general throughout the country.

The second method is essentially the South country system, and is of greater use when power is not necessary for the construction of the building.

A combination of both methods is commonly seen, and found useful in practice.

In scaffolding, the vertical timbers are known as standards or uprights. The horizontal timbers between the standards are known as ledgers when of cylindrical section, but as transoms and runners when of rectangular section. Braces, shores, struts and ties of any section are pieces used to stiffen the structure. The putlogs, or joists as they are called when of greater length, carry the boards which form the working platform.

=The Northern System.=—This scaffolding can be divided into two parts. First, the derrick staging from which the transporting power acts; and, second, the platforms, which bring the workmen within reach of their work.

=Derrick Stagings.=—These stagings, also known as Scotch derricks and 'Scotchmen,’ are erected to carry the power required, usually a steam crane.

They consist of three or four timber towers or legs supporting a platform upon which the crane stands. The number of legs depends upon the area over which the power is required to act.

When one crane is to be erected, three legs are sufficient to carry the platform.

If the building is a large one, several such stagings may be constructed; but in some cases two cranes are required where the size of the building will not allow of two stagings. In these cases the platform is square and supported at each angle by a leg. The cranes are then fixed diametrically opposite each other.

In determining the position of the legs they must be placed where the effective range of the crane is most required, and also where they will cause the least possible obstruction to the progress of the building. The position of the tower that carries the crane, and which is known as the principal or king leg, is first fixed. The secondary or queen legs are set out from it in the form of an isosceles triangle. The distance between the king and queen legs depends upon the length of the sleepers. These run from below the engine to the lower ends of the guys, and average from 25 to 30 feet in length.

The legs, especially the king legs, if intended to rise from the earth, must have a foundation of two thicknesses of 3-inch timbering laid crosswise. This is unnecessary if there is a concrete or other solid foundation.

Apart from the necessity for any foundation, the standards should rise from a framework of balk timbers of about 12 in. by 12 in., laid on the ground, and halved at their intersection (fig. 1).

In the case of the queen legs an extra balk is placed under the framework, as shown in fig. 1.

The legs are from 6 to 10 feet square on plan, the king legs being the larger. Each leg comprises four standards, either of whole timbers or battens bolted together. The standards for the king legs should measure not less than 9 in. by 9 in.; if of battens, then three pieces 9 in. by 3 in. should be used.

For the queen legs, balks 7 in. by 7 in., or three pieces 7 in. by 2-1/2 in., are sufficient sectional area. Where battens are used they are given a lap equal to one-third of their full length, and are bolted together by 3/4-inch wrought-iron bolts.

Whole timbers are used when they can be obtained in one piece of sufficient length.

As the standards rise they are divided into bays by transoms. These are made out of about 9-in. by 3-in. deals, placed from about 6 to 10 feet apart.

The bays are triangulated by cross-braces 7 in. by 2-1/2 in.; both of these are usually placed on the outside of the bay, but one or both may be placed on the inside. The first method is the better, as the braces, by butting against the transoms, give an increased resistance to movement.

The king leg, having to carry the weight of the engine, requires greater strength. This is gained by running an extra standard up the centre of the leg. If it is whole timber, balks about 14 in. by 14 in. are required; if built up, four deals 16 in. by 4 in. are used. It should stand upon an extra balk laid with the horizontal framing at the bottom of the leg, and should rest on a solid foundation.

To prevent any tendency to flexure this extra standard should be strutted from all four of the outer standards behind each transom (see fig. 2).

Another method is to shore the central standard from the foundation, as in fig. 3.

The legs in this manner can be made to support a platform up to 120 feet in height.

The required height having been reached, the legs are connected by trussed beams in the following manner: Two balk timbers of about 12 in. by 8 in. are laid immediately above each other between the king leg and each queen leg, resting on the two top transoms, as shown in fig. 1. They are from 6 to 9 feet apart, the top bay being sometimes made slightly lower than the others.

The lower balks are connected to the centre standard of the king leg by wrought-iron straps.

FIG. 3.—SHOWING SHORING TO CENTRAL STANDARD]

The top balks project from 6 to 10 feet beyond the king leg, and are halved at their point of intersection. The projecting ends are connected to each other by pieces 8 in. by 6 in., and again to the return balk by similar pieces (see fig. 4). They are also supported by struts from the central standard, as shown in fig. 1. The upper and lower balks are connected by iron bolts about 10 feet apart, and each bay thus formed is cross-braced in the same manner as the legs.

The iron bolts are covered by pieces of the same scantling as the braces.

In the single derricks the queen legs can be connected by a trussed beam similarly formed, or by a single balk carried across and laid on the top transom.

If the span is considerable, struts can be carried from the queen legs towards the centre of the underside of the balk to prevent sagging.

On the trussed beams thus formed, joists of 9 in. by 3 in. or ordinary poles are laid about 3 feet apart.

They are laid parallel to one another, and in a direction at right angles to the truss or single beam forming the back support of the platform.

The centre joists are continued to the ends of the balks which project beyond the king leg.

The advantage of having continued the top balks can now be seen, as it gives greater area to the platform immediately round the engine.

The boards 9 in. by 1-1/2 in. are laid at right angles to the joists.

Another way of forming the platform is to cover only partially the surface between the legs. In this case two additional joists, 6 in. by 6 in., are thrown across the king leg (see fig. 4), the boards not extending beyond their length.

When this is done, the workmen reach the platform from the communicating ladder which usually passes up a queen leg, by means of a run two boards wide. It is better to lay the larger platform, as, apart from the question of safety to the men, it serves as a storage for coal for the engine, the weight of which tends to keep the erection steady. Double boards should be laid under coals or other heavy stores.

To reach the platform, ladders are fixed in different ways. They can run up inside, or be fixed to the outside of the queen legs. In either case they are nearly or entirely upright. A better method is shown in fig. 5, and should be carried out wherever possible.

The derrick sleepers, two in number, are of balk timber, and lie across the platform from beneath the engine bed to which they are connected, to the centre of the queen legs.

The guys or stays, also of balk timbers, besides being connected to the mast, are attached to the sleepers over the queen legs (see fig. 1).

To counteract the overturning force exerted by the jib and the material lifted, the guys are chained down to the timber balk at the bottom of the queen legs (fig. 1).

This balk supports a platform which is loaded with bricks or stones more than equal to double the weight that will be lifted. The chain, which works loose with the vibration of the scaffold, is tightened by means of a screw coupler fixed in its length. The arrangement is as follows:—Two lengths of heavy chain with large links at each end are required. One length is carried round the sleeper and then taken down the centre of the leg. The other length is taken round the balk which is placed underneath the staging, and carried up through the load, when the tightening screw can be applied and the correct tension brought up.

To prevent lateral motion the legs are cross-braced by poles or deals between each leg as shown on frontispiece. The poles are tied to the legs just beneath the platform and connected at their meeting point. When crossing they should be at right angles to each other.

Deals 9 in. by 3 in. can take the place of the poles if required, bolts in this case being used instead of tyings.

At the building of the new Post Office, Leeds, 1893, a different method of raising the platform for the crane was adopted. The legs, instead of being framed, consisted of a single balk of timber strutted on each side from the ground level, the sleepers and guys being firmly attached to the standards themselves.

When erecting long ranges of buildings it may be more convenient to have the derrick mounted upon a travelling bogie than to dismantle the structure in order to re-erect at another point.

Fig. 6 illustrates the system, the travelling power being usually manual. The arrangement is suitable for small derricks, and is employed where the crane is erected outside the building.

Another method of using travelling cranes is to erect a platform as shown in fig. 7.

The standards, which may be of balk timber or built up, as previously shown, are about 10 feet apart longitudinally and 20 to 30 feet transversely. They stand upon sills of the same section where the foundation is not solid. On the head of the standards, the runners are laid connecting all the standards in the same row.

Head pieces may be fitted between the standards and runners; this serves to distribute pressure. All the connections are securely made by dog irons, bolts, and straps. The stability depends entirely upon the bracing, and this, it is important to note, should be between each bay longitudinally, and at least every second bay transversely.

End Elevation

FIG. 7.—ELEVATION OF DERRICK STAGING]

Timbers placed as A in fig. 7 give rigidity to the standards by preventing flexure, and are necessary when the lengths of the uprights exceed 30 times their least diameter.

The deals used for braces are bolted to the standards; for poles, tying is resorted to.

=Working Platforms.=—The working platforms used in conjunction with overhead or overhand work depend upon the requirements of the building.

By over head or hand work is meant that the material upon which the mechanic is to be employed reaches him from over head or hand.

When no outside scaffolding is needed, the platforms are laid upon the floor joints in the interior of the building, being raised upon trestles as the work proceeds, and until the next floor is reached.

Light forms of scaffolds, as the ordinary masons’ and bricklayers’ pole scaffolds, are now frequently used as working platforms in connection with the Scotch system.

=The South Country System.=—This system is divided into two classes according to the strength required. For the first, square timbers are used; for the second, poles are employed. The scaffolds built of square timbers are known as gantries and stagings, and the pole erections are termed bricklayers’, and masons’ or independent scaffolds.

=Gantries.=—The term gantry was originally given to erections constructed with a view to the easy carriage of heavy material, but of late it has also come to mean a structure arranged to support lighter forms of scaffolding over footpaths which have to be kept open for public use.

1st. Gantries for transport of material, commonly called travellers. Figs. 8 and 8a show the general construction.

The distance between the outer rows of standards and the wall depends upon circumstances. If possible, the space should be allowed for a cart-way, as the material can thus be brought quite close to the work before being lifted. If, owing to adjacent footpaths or any other reason, this cannot be done, the uprights should be placed close to the wall on either side, the material being lifted at the end of the gantry or other convenient spot, over which the lifting gear can be brought.

The standards of square timber for the gantry are from 6 in. to 12 in. square, and are erected upon sleepers, or, as they are sometimes termed, sills laid in the same direction as the run of the scaffold. One row of standards is placed on each side of the wall. The standards are placed 8 to 10 feet apart. On the top of the standards runners are fixed connecting each standard in the same row. Sills, standards, and runners should be of the same sectional area. The runners are strutted on their underside, from the standards by pieces of, at least, half the sectional area of the supported timbers. If the struts are of equal size to the runners, double the weight can be carried.

The cleats from which the struts rise, are simply spiked to the standards, but if designed to carry excessive weights they are slightly housed in. As the space between each row of standards has to be kept open for the building, no cross bracing can be allowed except at the ends. Strutting is therefore resorted to in order to give stability. The struts, one to each standard, are bolted to the upright near the top, and again to a foot block driven into the ground. Other methods of fastening down the bottom ends of the struts are shown in fig. 9; the use of each depends upon the nature of the soil.

Struts are also fixed at the ends to prevent lateral movement. Head pieces, or corbels, as they are sometimes termed, are occasionally inserted between the standards and runners, and serve to distribute pressure.

Straining pieces spiked on the underside of the runners, for the struts to pitch against, are used when the standards are considerably apart.

Rails upon which the travelling engine or traveller can move are laid on top of the runners, and are turned up at the ends of the platform to serve as buffers to the engine platform.

Side Elevation

FIG. 10.—ELEVATION OF TRAVELLING GANTRY]

The engine platform consists of two trussed beams of timber about 3 feet apart, connected at their ends with short pieces of the same scantling, and fitted with grooved wheels to move upon the rails. Rails are also laid upon each beam and serve for the traversing motion of the crab. Movement of the traveller is obtained from the crab, which is worked either by manual or steam power, and acts through a system of shafting and geared wheels. Movement in three directions is necessary from the crab: vertically for lifting, and horizontally in two directions, transversely and longitudinally. Travellers are made up to 50 feet wide and any required length.

Another method of building travellers is shown in fig. 10.

In this case, the rails upon which the traveller moves in a longitudinal direction are fixed on sleepers on the ground level, and the standards and runners of the first example are not required. In their place is constructed a triangulated system of balk timber framing. The platform is fixed to the head pieces, and is braced as shown. Less timber is used in their construction, but owing to the greater weight a steam winch is required to impart motion.

=Gantries which serve as a base for lighter forms of scaffolding.=—These erections are in reality elevated platforms, and allow of a clear way for a footpath where required. They are constructed of two frames, placed apart according to the width of the path over which the platform stands (fig. 11).

The method of erection, so far as the side frames are concerned, is the same as for the first example of travelling gantries. Stability is, however, gained by cross-bracing as shown in figure, thus making strutting unnecessary. The platform can be laid by placing short boards 9 in. by 3 in. across the runners when the platform is narrow. It is more usual, however, to place joists 10 in. by 2 in. across, and on these to lay the boards longitudinally. The joists average 2 to 3 feet apart, the braces are about 2 in. by 7 in. On the outside of the scaffold, parallel to the sills, balk timbers are placed forming a 'fender’ to prevent the vehicular traffic from injuring or disturbing the erection.

End Elevation

FIG 11.—GANTRY OR ELEVATED PLATFORM OVER FOOTPATHS]

=Stagings.=—Stagings are erected in a manner similar to travelling gantries, but are carried more than one storey high (fig. 12). It is a form of scaffolding rarely seen, more especially since the introduction of the Scotch derrick system. The timbers are erected to the height of the first runner in the same manner as the frames in fig. 11. In order to carry the scaffold higher, horizontal pieces are laid across the scaffold, over the standards, and are made to project 9 or 10 feet on each side of the runners.

On these beams, uprights, as in the first tier, are raised, being connected in like manner, longitudinally by transoms. The rising tiers of standards are strutted by timbers A A, rising from the projecting portion of the beam called the footing piece, which serves in the same manner as a footing block. The footing piece is supported by struts, B B, rising from the lower standards. The struts B B are in two pieces, being bolted to the sides of the footing pieces and uprights. This allows the shores A A to pass between them.

Cross Section

FIG. 12.—EXAMPLE OF STAGINGS]

Strutting within the bays formed by the standards is carried out on each tier with the exception of the top, where braces are fixed, as shown at C.

On the top runners rails are laid for a traveller.

In constructing the foregoing square timber erection, note should be taken of the following points:—

That the uprights of the upper tiers should stand immediately over those of the lower tiers, in order to prevent cross strains on the runners.

That the timbers should fit as evenly as possible, as thereby the whole erection is rendered more stable.

That joints between the runners should occur immediately over the standards.

The several parts of this structure, if for temporary purposes, can be connected by dog irons; if for a more permanent use, by bolts and straps.

POLE SCAFFOLDS

=Bricklayers’ Scaffolds.=—A bricklayer’s scaffold consists of a series of upright poles or standards, to which are lashed horizontal poles, termed ledgers. The ledgers and the wall of the building carry the putlogs, on which boards are laid to support the workman, his material, and tools (fig. 13).

The standards are first erected, and may stand singly or in pairs. In a repairing job, unless of great height, and where there is no great weight of material, single poles are sufficient.

Where double poles are required, the first pair are erected of different lengths.

The short pole is termed a puncheon. The difference of length allows of a lap in connecting the succeeding poles.

The lap should equal half of the full-length pole. The standards are placed 6 to 8 feet apart, and from 4 to 6 feet away from the building.

The butt-ends are embedded about 2 feet in the ground, which affords some resistance to overturning. If they cannot be embedded, they should be placed on end in barrels filled with earth tightly rammed. As the building rises additional poles are added, being lashed to the standards already erected.

If the standard is a single pole, the second pole, having a lap of 10 or 15 feet, stands upon a putlog placed close to the first pole for that purpose (fig. 14).

The inner end of the putlog is securely fastened down to the scaffold or inserted into the building.

If the standard is double, the rising pole is placed upon the top end of the puncheon, and afterwards others are placed on end upon the lowest free end of the standards already fixed.

As the standards rise, they are spliced or 'married’ together with band ties.

At a height of 5 feet, this distance being the greatest at which a man can work with ease, a ledger is tied across the standards to form a support for the working platform.

Where a single pole is insufficient in length to form a continuous ledger, two are joined in one of three ways.

In the first they are lapped over each other as fig. 15. This method gives a strong connection, but prevents the putlogs being laid evenly.

The second way provides that the ledgers shall lap horizontally side by side. This allows of evenness of line for the putlogs, but is not so strong (fig. 16).

In both of these methods the lap should cover two standards, and not as shown in fig. 17.

The third manner of connection (fig. 18) is the best. The ledgers butt end to end. Underneath, a short pole is placed crossing two standards. The tying at the standard embraces the double ledger. A band tie is run round the supporting pole and the ends of the ledgers where they butt.

Great strength is obtained in this way and the putlogs can be evenly laid.

Additional ledgers are fixed as the work proceeds.

On the ledgers, and at right angles to them, putlogs are laid, resting outwardly on the ledgers and inwardly on the wall, where header bricks have been left out for their reception.

The putlogs, which are placed about 3 or 4 feet apart, should be tied to the ledgers and fastened by wedges into the wall. This is not often done, but at least one putlog to every tying between standard and ledger should be so treated.

Where the putlogs cannot be carried by the wall owing to an aperture in the building, such as a window, they are supported by bearers fixed as shown in figs. 19 and 20.

By wedging the inner end of the putlog into the wall, some stability is given to the scaffold, but the connection cannot be considered satisfactory, as the putlog would draw under very little strain. Greater stability can be gained if the outer frame of the scaffold is supported by one of the three methods given as follows.

A shore or tie can be fixed between the erection and the ground as shown in fig. 21, or, if there are openings in the wall, supports can be fixed as ties shown in the same diagram.

The ties or struts should be placed to every third or fourth standard at about 25 feet from the ground, and their fastenings made good. Additional ties should be carried within the building at a greater height where possible. The stability of the scaffold under loads and cross wind pressure, depends greatly upon the ties or shores, and their fastenings should be well made and kept in good order. The historical instance of the mechanic who, to escape a shower of rain, stood upon the inner board of the platform, and by leaning against the building pushed the scaffold over, should have no opportunity of recurrence.

To stiffen the scaffold longitudinally braces are tied on the outside of the scaffold in the form of a St. Andrew’s cross (see fig. 13).

They start from the lower end of one standard and rise obliquely across the scaffold to near the top, or some distance up a standard in the same run. Tied at their crossing-point, connections are made to all the main timbers of the scaffold with which they come in contact. Braces are fixed in all exposed situations, and generally where the scaffold is more than one pole (30 feet) in height.

The only exception to effective bracing being carried out is where the building, being of irregular form, creates many breaks and returns in the scaffolding. It is obvious that where a scaffold butts against or breaks with a return wall, the tendency to lateral motion is lessened.

The boards, which are placed longitudinally across the putlogs, can be laid to lap or butt at their ends. When lapping, one putlog only is required to carry the ends of two series of boards (fig. 22).

When butting, two putlogs are required placed about 4 inches apart (fig. 23).

The second method is the better, as the boards are not so likely to lose their place or to trip the workmen. If heavy work is in progress the boards are laid double. As the building rises, the boards are carried up to each successive platform, but each tied putlog is left in its place.

=Masons’ Scaffolds.=—Masons’, or independent scaffolds differ from the bricklayers’ in that they have to be self-contained. Owing to the different material of which the building is erected, the putlogs cannot rest upon the wall. If openings were left for them, as in brickwork, the wall would be permanently disfigured, more especially when ashlar fronted.

In order to gain the necessary support two parallel frames of standards and ledgers are erected along the line of wall to be built (fig. 24).

They are from 4 to 5 feet apart, the inner frame being as close to the wall as possible. As a heavier material has to be dealt with, the standards are placed closer together, say from 4 to 5 feet.

The ledgers and braces are placed as before, the putlogs now resting on ledgers at each end, and not on the wall at the innermost end, as in the bricklayer’s scaffold.

To prevent cross movement of the scaffold, an additional method of bracing is available in this system. An inner and outer standard are connected by short braces across each bay, as shown in fig. 24.

This method of cross-bracing can be continued to the top of the scaffold, and the braces should be put in longitudinally, about 20 feet apart.

The platforms laid on all pole-scaffolds are from 4 to 5 feet wide. It is usually necessary, on anything but the smallest jobs, to keep this width free for the workman and his material.

In order, therefore, to provide a platform on which the material can be landed, it is convenient to erect, on the outside of the scaffold, an additional platform from 5 to 10 feet square (fig. 25).

It is constructed of standards, ledgers, and braces, in like manner as the scaffold to which it is attached.

The face-boards, as shown in this figure, should be fixed wherever material is being hoisted, to prevent any projection of the load catching under a ledger and upsetting.

=Connections.=—The members of pole scaffolds are connected by cordage. The names of the various knots are given in Chapter V.

The arranging of the various timbers used in erecting scaffolds is a dangerous occupation, and one requiring skill and considerable nerve on the part of the workmen. In the majority of cases, the timbers on the ground level are placed in position by manual labour only, shear legs being used to facilitate matters. When the scaffold rises, advantage is taken of any rigid member on which pulley wheels can be hung, and by this means the succeeding poles, &c. are raised, manual dexterity and strength being responsible for their final position.