SCAFFOLDING

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SPOTTISWOODE AND CO. LTD., NEW-STREET SQUARE
LONDON

PLATE I.
[Photo by Thatcher 28 Cobourg Road, S.E.]
DERRICK STAGING.

SCAFFOLDING

A TREATISE ON
THE DESIGN & ERECTION OF SCAFFOLDS,
GANTRIES, AND STAGINGS,

With an Account of the Appliances used in connection therewith

FOR THE USE OF CONTRACTORS, BUILDERS, CLERKS OF WORKS, ETC.

With a Chapter on the Legal Aspect of the Question.

By A. G. H. THATCHER,
BUILDING SURVEYOR

SECOND EDITION, REVISED

WITH 146 DIAGRAMS AND 6 FULL-PAGE PLATES.

LONDON:
B. T. BATSFORD, 94 HIGH HOLBORN
1907.

PREFACE

Scaffolding up to quite recent years has been considered by builders and others concerned, with the exception of the actual workmen, to be a matter of small importance and consequently unworthy of study. Recent legislation, however (the Workmen’s Compensation Act, 1897, and the Factory and Workshop Act, 1901, and preceding dates), has brought it into greater prominence, with the result that more attention has lately been given to it. Special study of the subject has, however, remained very difficult owing to the lack of accessible information.

The author, in the course of considerable experience in the building trade, has had opportunities of examining a large number of scaffolds throughout the country, affording him exceptional facilities for thoroughly studying the subject, and he has been led to prepare this treatise in the hope that it may prove useful to those engaged both in the design and erection of scaffolds for building purposes. While special attention has been given to practical details, the theory has not been neglected, but has been dealt with by the use of terms well understood in the building trade. The various formulæ given have been simplified as far as possible, and it is hoped that in these days of technical education they will not be beyond the scope of the reader.

The illustrations have generally been drawn to scale, but for the sake of clearness, details are given to a larger scale where necessary.

The practice of allowing workmen to erect scaffolds without the aid of expert supervision, as is generally the case, is to be strongly deprecated. The architect, builder, or clerk of works, should in all cases be responsible for their erection—the risk of defective or unsafe work being thereby minimised, and an economy often effected in both labour and material.

The author desires to acknowledge his indebtedness to Mr. G. Thatcher, of H.M. Office of Works and Hampton Court Palace, for valuable information contributed by him, and to Mr. J. Clark, of the Factory Department of the Home Office, for his very careful reading of the proofs; while his best thanks are due to the following manufacturers:—Mr. C. Batchelor, Messrs. Bullivant & Co., Ltd., Messrs. Butters Bros., Mr. J. Fishburn, Messrs. Frost & Co., and Mr. E. Palmer, who have furnished him with particulars of their various specialities.

A. G. H. T.

London: February 1904.

NOTE TO THE SECOND EDITION

Recent alterations in and additions to the legal enactments affecting scaffolding and the persons employed in its erection have rendered necessary a second edition of this work. The author has accordingly revised the information relating to the law of the subject, and has embodied the important series of suggestions for the guidance of those engaged in building operations, published in the Annual Report of the Chief Inspector of Factories, which, if followed out, will undoubtedly be the means of reducing the number of fatal and other accidents occurring at such works.

A. G. H. T.

October 1907.

CONTENTS

LIST OF ILLUSTRATIONS

SCAFFOLDING

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]).

Fig. 1.—Elevation of Staging for Derrick Crane

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 2¹⁄₂ 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 2¹⁄₂ 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. 2.—Plan of King Leg

A, Central Standard.
B, Shorings.
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.

Fig. 4.—Plan of Top Platform partially covered

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 1¹⁄₂ 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.

Fig. 5.—Showing Method of Fixing Ladders

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.

Fig. 6.—Showing Staging mounted on Travelling Bogie

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.

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.

Fig. 8.—Elevation of Gantry for Traveller

Fig. 8a.—End Elevation of Gantry shown in Fig. 8

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.

Fig. 9.—Footing Blocks for Struts

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.

End ElevationSide 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.

Front Elevation

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 samemanner 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.

Front ElevationCross 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.

Fig. 13.—Elevation of Pole Scaffold

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.

Fig. 14.—Method of Fixing Rising Standard

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.

Fig. 15

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.

Fig. 16

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.

Fig. 17

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.

Fig. 18

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].

Fig. 19

Fig. 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.

Fig. 21.—Shores and Ties for Dependent Scaffolds

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.

Fig. 22

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.

Fig. 23

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.

Fig. 24.—Masons’ Scaffolds: End Elevation

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.

Fig. 25.—Landing Stages

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.

CHAPTER II
SCAFFOLDS FOR SPECIAL PURPOSES

When applying the given methods for scaffolding, difficulties arise owing to the varying designs of the buildings under construction or repair.

It is impossible to deal with these cases in detail; they must be left to the scaffolder, who, while holding closely to the principles, by the exercise of ingenuity will make combinations and variations of the various systems to suit the special requirements demanded in each case. There are, however, certain types of scaffolding which occur with some regularity, and these will now be dealt with.

Needle Scaffolding.—Needle scaffolding is necessary where it is impossible or too expensive to carry the scaffold from the ground level or other solid base. It is used both for repairing and new erections.

The needles from which the scaffold takes its name are timbers (usually poles or balks) placed horizontally through and at right angles, or nearly so, to the wall of the building. The projections support a platform upon which an ordinary pole scaffold is erected ([fig. 26]).

Windows, or other openings in the wall, are utilised where possible for the poles to pass through. In other cases holes have to be made in the walls, cut as nearly as can be to the size of the needles in use.

The needles must be of sufficient scantling to carry the weight of the scaffold and attendant loads. The stability of the structure depends upon the means taken to fasten down the inner end of the needle.

The usual plan is to tie it down to a convenient joist or other rigid member of the building itself, but the method shown on the diagram is better, as resistance to movement is gained both from above and below.

Struts from the building below the needles to their outer end, give greater strength to the beam.

When erecting needle scaffolding around buildings of small area, say of a tower or chimney shaft, the needles can be laid across the building in one length, piercing the wall on opposite sides. In these cases, if the needles are wedged in, the weight of the building and the scaffold itself on the opposite ends of the needles, is sufficient to maintain equilibrium.

Fig. 26.—Needle Scaffold

The platform is formed of 9-in. by 3-in. deals, and on this is erected whatever scaffolding may be necessary.

Scaffolds for Chimney Shafts, Towers, and Steeples.—The erection of chimney shafts can be carried on entirely by the aid of internal scaffolding. As the work rises putlogs are laid across the shaft, the ends being well built into the wall. On the putlogs the platform is laid, being carried up as the work proceeds. The putlogs may be left in for the time, and struck on completion. The platform is fitted in its centre with a hinged flap door through which the material is hoisted as required.

There is some objection to this method of scaffolding where the wall is more than 1 foot 10¹⁄₂ inches thick (which is the greatest depth of brickwork over which a man can reach and do finished work), for the mechanics, in order to reach the outside joints, have to kneel on the freshly laid material, which is detrimental to good workmanship. For this reason the system of carrying up an ordinary pole scaffold externally until the height is reached where the wall is reduced to 1 foot 10¹⁄₂ inches in thickness, is to be preferred.

The walls of a chimney shaft decrease in thickness 4¹⁄₂ inches at a time, forming an internal set-back of that width at every 20 feet in height.

This set-back is of advantage to internal scaffolding when the full height of the brickwork is reached, and the cap has to be fixed. The cap or coping, when of stone or iron, does not admit of the insertion of putlogs. To overcome the difficulty, four or more standards are erected at equal distances, and standing upon the top set-back ([fig. 27]).

The standards project sufficiently to carry the pulley wheel well above the total height of the chimney, in order to give head room and to assist the workman in fixing the coping.

To stiffen the standards, short ledgers are tied across as shown in [fig. 27].

PLATE II.
[Photo by W. Cottrell
Hightown, Manchester.]
EXTERNAL CHIMNEY SCAFFOLD.
Erected for the Willesden Electric Lighting Works, under the supervision of E. Willis, Esq., A.M.I.C.E., etc.

When the chimney is to be erected by external scaffolding the ordinary mason’s or bricklayer’s scaffold is used. Owing to the small area of the erection the outside frames of the scaffold have a quick return. This makes it practically impossible for the scaffold to fail by breaking away from the building under the influence of the loads it may carry. Shoring or tying is therefore not so important. Wind pressures have, however, a greater effect, especially when the direction is not at right angles to one of the faces of the scaffold. If in that direction, the tied putlogs would offer resistance. Braces are therefore imperative, and they should be fixed at right angles to each other, each pair thus bracing a portion of the height of the scaffold equal to its width. (See plate 2.)

For the repair of chimney shafts without scaffolding from the ground level, means have to be taken to bring, first the mechanic, and afterwards his material, within reach of the work.

Fig. 27

The preliminary process of kite-flying is now rarely seen, except for square-topped chimneys, and even in these cases the delay that may arise while waiting for a suitable steady wind is a drawback to its practice. The kites used are about 10 feet long and 8 feet wide. They are held at four points by cords which continue for a distance of about 16 feet, and then unite into one. Near this point on the single rope another cord is attached, which serves to manipulate the kite into position.

Stronger ropes or chains are then pulled over the shaft, after which a workman ascends, and the necessary pulley wheels and timbers to form a regular means of ascent are sent up after him.

A light line carried up in the interior of the shaft by a hot-air balloon is another means of communication.

The most certain and safest method of ascent is to raise on the exterior of the shaft a series of light ladders, which are lashed to each other and firmly fixed to the chimney as they ascend.

The ladders have parallel sides, and are used up to 22 feet in length.

One method of fixing is as follows:—

A ladder is placed against the shaft on its soundest side. It rests at its top end against a block of wood slightly longer than the width of the ladder, and which keeps it from 7 to 9 inches away from the wall. This space allows room for the workmen’s feet when climbing. The ladder is then fixed by two hooks of round steel driven into the wall, one on each side immediately under the blocks, the hooks turning in and clipping the sides of the ladder ([fig. 28]). The hooks, which have straight shanks of ⁷⁄₈-inch diameter with wedge-shaped points, are driven well home, as the stability of the erection depends upon their holding firmly.

Above the top end of the ladder a steel hook is driven into the wall on which a pulley block can be hung, or instead, a pin with a ring in its head can be so fixed. A rope from the ground level is passed through this block or ring, and reaches downward again for connection to the ladder next required. The connection is made by lashing the rope to the top rung and tying the end to the seventh or eighth rung from the bottom; this causes the ladder to rise perpendicularly. The steeplejack who is standing on the already fixed ladder cuts the top lashing as the hoisted ladder reaches him, and guides it into its place as it rises. When the rung to which the rope is tied reaches the pulley block, the ladders should overlap about 5 feet. They are at once lashed together at the sides, not round the rungs.

Fig. 28

The workmen can now climb higher, driving in hooks round the sides, and under the rungs of the ladder alternately, lashings being made at each point. A wooden block is placed under the top end of the last ladder and fixed as before. The hoisting rope, which has been kept taut meanwhile, is now loosened and the process repeated.

The ladders rise in this manner until the coping of the shaft is reached. Here, owing to the projection of the cap which throws the ladders out of line, it is impossible to lash the top ladder to the lower. To overcome the difficulty, the wall is drilled in two places immediately over the topmost fixed ladder, and expansion bolts are fitted therein. To these bolts the lower end of the top ladder is tied. The hoisting rope is then tightened sufficiently to hold the ladder, and by this means the workmen are enabled to reach the top of the shaft.

A variation of this method of climbing is to replace the wooden blocks by iron dogs with 9-inch spikes, which should be driven well into the wall. Short ladders of about 10 feet in length are then used, these being lashed to the dogs as they rise.

Another method of fixing the ladders is shown in [fig. 29].

Fig. 29

In this case eye-bolts are driven horizontally into the wall in pairs, rather wider apart than the width of the ladders.

Iron rods hook into these and are fastened to the ladder sides by thumb screws.

The ladders rise above each other and are connected by 3-inch sockets.

When fixed, they stand about 18 inches from the wall. This is an advantage, as it enables the workmen to climb on the inside of the ladders, thus lessening the strain on the eye-bolts, and the ladder can more easily pass a projecting chimney cap.

On the other hand, the whole weight of the ladders rests upon the bottom length, so that if through any cause it gave way, for instance under accidental concussion, the entire length would most certainly collapse.

This danger could be avoided if the ladders were supported on brackets as [fig. 30]. No reliance should be placed upon the thumb screws, as they may work loose under vibration. Danger from this source would be avoided if the slot in which the ladder peg moved was made as shown in [fig. 30].

Fig. 30

The necessary repairs can be carried out by means of boats, cradles, or scaffolding.

Cradles and boats are swung from balk timbers laid across the top of the shaft, or from hooks where the design of the chimney permits, as shown in [fig. 31].

The common method of fixing light scaffolds round a chimney or steeple is shown in [fig. 32]. They are most easily fixed to square or other flat-sided erections. The scaffolder having by means of ladders or boats reached the desired height, fixes a putlog by means of holdfasts to one of the walls. Another putlog is then fixed on the opposite side of the building at the same level. The two are next bolted together by 1-inch iron bolts of the required length. The bolts are kept as near to the wall as possible. The process is repeated again about 6 feet higher on the building. The boards for the platforms are next laid. The first are placed at right angles to the putlogs and project sufficiently to carry the boards which are laid parallel to the putlogs. To prevent the boards rising when weight is applied at one side of the scaffold, iron plates bolted together ([fig. 33]) are fixed at the corners, and clips ([fig. 34]) connect them to the putlogs.

Fig. 31

Fig. 32

Fig. 33

The stability of these scaffolds depends upon fixing at least two sets of putlogs, connected by means of stays as shown in [fig. 32]. Bracing is unnecessary if the putlogs and bolts tightly grip the building. When these scaffolds are used on circular chimneys, chucks have to be fitted on the inside of the putlogs to prevent them being drawn by the bolts to a curve. The chucks ([fig. 35]) can be fastened to the putlogs before they are fixed, if the curve of the building is accurately known. When this is not the case, the putlogs are fixed by a holdfast at their centre. The chucks are then placed in position, and clamped to the putlogs as shown in [fig. 36].

Additional holdfasts are then driven into the wall immediately under the chucks, so that the putlogs are kept level.

Fig. 34

Fig. 35

The putlogs are fixed on edge, and when not exceeding 16 feet in length are 7 in. by 3 in. Above that length they are 9 in. by 3 in. The stays should be 4 in. by 2 in., and connected to the putlogs by ⁵⁄₈-inch iron bolts. The platform is usually of three boards 11 in. by 2 in.

Fig. 36

Fig. 37

Hollow towers are erected or repaired in the same manner as chimney shafts, except that climbing ladders are not often required. External or internal scaffolds may be erected. Towers being usually of larger area than chimney shafts, the putlogs for internal scaffolding are often of short poles from 6 to 8 inches diameter. Even these may require extra support. This is gained by carrying standards from the ground level or other solid foundation and tying to the putlogs. If of great height the standards may be unable to carry their own weight. For the cases where danger might be apprehended from this cause, [fig. 37] shows a system of framing, which, being supported by the set-back in the thickness of the wall, will carry the upper standards.

Steeples are generally built by the aid of external scaffolds, which, as in the case of chimney shafts, should be well braced. The lower portion may also be repaired in this way, the standards rising from the ground level, or, if so designed, from the top of the tower. A series of needles could be arranged for the higher portions.

Fig. 38

Domes and arches.—The scaffolding for domes and arches consists of a series of standards standing upon the area covered by the building, and connected by ledgers and braces in directions at right angles to each other. The platform is laid on the top ledgers.

When the building is of large span square timbers are often used, balks for standards and runners, and half timbers for struts and braces.

[Fig. 38] shows a design for repairing roofs and arches where a roadway has to be kept below.

Swinging scaffolds. Painters’ boats or cradles.—Painters’ boats are useful scaffolds for the repair of buildings, more especially where the work is light. [Fig. 39] shows the general construction. They are suspended from jibs, fixed usually on the roof for outside work, and by means of blocks and falls they can be moved in a vertical direction by the workmen when in the boat.

Fig. 39

The boats are fitted with guard boards and rails, and their safety, providing the jibs are well fixed by balancing weights, is in their favour. They are not self-supporting, and there is a distinct danger of their running down if the sustaining ropes are not securely fastened off. The wind causes them to sway considerably, and their use is confined chiefly to façade work. An improved cradle is now in general use, which is slung by head blocks from a wire cable running between two jibs (see [fig. 40]). By the aid of guy lines movement in this case can be also obtained horizontally, which removes the necessity of shifting the jibs or employing a greater number of boats as in the older method.

Fig. 40

Fig. 41

Another cradle as shown in [fig. 41] has advantages which cannot be ignored. It has steel cables with a breaking weight of 15 cwt. instead of fibre ropes, and the cradle is raised and lowered by means of gearing and a drum fixed in the gear case A. It is self-supporting, and therefore safer than the cradle mentioned above. The lower ends of the cable are fastened to the drum, and the gearing gives sufficient mechanical advantage for one man to raise the scaffold by turning the handle B. The uprights and rails are of angle steel or barrel and will take apart and fold.

Fig. 42

The boatswain’s boat (see [fig. 42]) is useful under some circumstances, especially for making examinations of buildings for possible damage. It is dangerous and awkward to work from, and is also acted upon considerably by the wind.