1911 Encyclopædia Britannica/Shoring

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SHORING (from “shore,” a prop), an operation connected with building. It is often necessary before actual building is begun to support adjoining premises while the work of excavating for underground apartments is being carried out. The art of shoring comprises the temporary support of buildings, and may become necessary because of the failure or settlement of some portion of the structure or for the purpose of upholding the upper portion while alterations are being made in the lower. There are several different forms of shoring, each adapted to suit peculiar circumstances. Much of the shoring for ordinary cases is done with heavy, roughly sawn timbers strongly braced together, but for especially heavy work steel members may be introduced and prove of great value. There is the trouble in Connexion with their use, however, that Connexions between steel members are not made with the same facility as between pieces of timber.

Fig. 1.

The form of shore in most general use is that known as the raking shore. It consists of one or more timbers sloping from the face of the structure to be supported and bedded upon the ground, As the ground is usually of a more or less yielding nature, a stout timber plate termed a sole-piece, of sufficient area to withstand being driven into the soil, is placed to receive the base of the raking timber or timbers. A wall-plate, with the object of increasing the area of support, is fixed to the face of the wall by means of hooks driven into the wall. Where space is available an angle of 60° is the best to adopt for the main shore, the auxiliary members ranging in their slope from 45° to 75°. In many cases, especially in towns, the angle of slope is governed by outside influences such as the width of the footway. Raking shores are erected in “systems” of two or more members placed in the same vertical plane at right angles to the face of the wall. The different members rise fanwise from the sole-plate to support the wall at different points. The distance horizontally between the systems depends on the condition of the building being propped up, and also upon the spacing of its window and other openings. The usual spacing is 10 ft. or 15 ft. apart, but this distance has often to be varied according to the positions of the openings in the wall. The application of the shores should be carefully made and support given only where there is a corresponding thrust inside, such as from a floor or roof, as without this the shore is liable to act more as a destructive agent than a supporting one, and cause the wall to cave in at that point, or placed against a parapet wall it might have the effect of pushing it over. The members, therefore, should be so placed as to meet the wall at a point somewhat below the floor or roof, so that if their length were continued they would meet and support the end of the floor or roof inside. Perhaps the best idea of the positions and functions of the various component parts of a system of raking shores can be obtained from a description of the various members, coupled with some little study of the illustrations (fig. 1). The names of the different timbers are therefore set out here, and against each part is given a short description of its use and position.

Fig. 2.

Raking Shore, or Raker.—This is a piece of timber sloping up from the sole-plate to the wall-piece. For a detail drawing of the connexion between the raker and wall-plate see fig. 2. The top and longest shore is often formed in two pieces, in which form it can be more conveniently handled. The upper piece is termed the riding shore or rider, and the lower member which supports it is known as the back shore. At the junction of the rider and back shore a pair of folding wedges is introduced and driven in to give the head of the rider a firm bearing against the needle and wallplate above. The sole-piece has already been mentioned as the timber base upon which the shores take their bed or bearing. It usually consists of a piece of 11 by 3 plank, but when the ground is soft or the load supported very great it should be bedded on a platform of timber to spread the weight over a large area. The sole should be placed sloping down towards the building at something less than a right angle (say 80°) with the inside of the shore to enable the latter to be gradually levered to a firm bearing with the aid of a crowbar. Wedging should not be resorted to or the already shaky building may sustain further injury through the vibration. When in position the foot of the shore is fixed by dog-irons to the sole piece, and for additional security a cleat is spiked on the sole tight up to the shore to prevent any slipping.

Braces.—When more than one shore takes a bearing upon the sole-piece the feet of the several members are stiffened and braced either by having rough boarding nailed right across them or by being bound together with a number of rounds of hoop-iron. For further strength also braces of 1-in. boards, 6 to 9 in. wide, are taken across from the wall-plate to the topmost shore and spiked to each intervening member, binding the whole together. These braces should be fixed a little below the junctions of the heads of the shores with the wall-plate. The wall-plate has already been referred to. It is usually a deal 9 in. wide by 3 in. thick, secured tightly against the face of the wall with wrought-iron wall hooks, forming a good abutment for the shores and serving to spread the support afforded by them. Holes are cut through this plate to receive the needles (or Joggles as they are sometimes termed to distinguish them from the needles used in dead shoring, which are large horizontal

Fig. 3.

members usually of balk timber), which are pieces of wood about 1 ft. long and 4 in. square in section, cut with a shoulder to butt against the wall-plate. A portion of a brick or stone is removed from the wall and the end of the needle is passed through the rectangular hole in the wall-plate and fitted into the recess in the wall.

Fig. 4.

The head of the needle projects about 41/2 in. beyond the face of the wall-plate and forms an abutment for the head of the shore. The head of the shore is notched to fit the underside of the needle to prevent any movement sideways. If this is not done the shore is liable to be acted upon by the wind and be blown down. A small block of wood, cut somewhat after the fashion of a wedge and termed a cleat, is fixed above the needle to keep the latter quite firm. Cleats are used also in other positions to keep timbers in position. Wedges are used to obtain a tight bearing for the rider shores and are used at their base. As little force as possible must be employed in driving them as vibration is liable to injure the already weakened wall.

Fig. 5.

Horizontal shores, or flying shores as they are more often termed by the Workman, may be employed for spans up to about 35 ft. They are used to support the party walls of the houses adjoining the premises being rebuilt. They are erected during the pulling down operations and removed as the new building is raised and there is no further need for them. A system of flying shores consists of one or more horizontal timbers, sometimes known as dog shores, cut in tightly between the wall plates fixed with hooks to the faces of the walls of the adjoining buildings (fig. 3). These horizontal members are supported at each end by cleats and needles fixed in the wall-plate as described for raking shoring. The shores are supported in their length by inclined braces springing from needles fixed near the lower ends of the wall-plates and serving to strut the shore at a point about a third of its length from the wall. Corresponding braces are carried from the upper surface of the shore and abut against needles at the upper ends of the plates. Straining pieces are secured to the upper and lower faces of the shore to serve as abutments for the ends of the braces. The best angle for these braces is one of 45°, but a smaller inclination than this is frequently adopted. Wedges are inserted, usually at the end of the flyer so as to tighten this up between the wall-plates, and sometimes between the braces and the straining piece, and carefully driven to tighten up the whole and cause each timber to find a close bearing. If the adjoining premises are of considerable height and especially if it is proposed to undertake extensive excavations, the systems of flying shores may need to be somewhat complicated, each consisting of several horizontal members spaced from 10 to 13 ft. apart and well strutted one to another and to the wall-plate (fig. 4). In the application of this form of shoring, as in raking shores, the same rules apply as regards lacing the shores on the face of) the wall in a proper position to obtain a solid abutment on a floor or roof on the other side. The members should be securely dogged and spiked together to form a homogeneous framework capable of resisting the attacks of a strong wind, which in an exposed position will sometimes destroy a poorly constructed framework.

Horizontal shores should be adopted wherever possible in preference to raking shores. Besides being more economical, they are more convenient and more effectual than rakers springing from the ground, especially if the height of the building is considerable and the span at the most not much over 30 ft. Apart from the economy effected, they present a direct resistance to the thrust and are well out of the way of any building operations that may be carried on below them, so that there is no risk of their being accidentally disturbed, whereas the feet of raking shores are generally in the way of the workmen, and if not disturbed by accidental blows from materials or carts will very likely be loosened and rendered useless by the digging and pumping which is going on around them.

Fig. 6.

Needle shoring is the next method of temporary support to come under consideration. It is known also as vertical shoring and dead shoring, and is the means usually adopted to support temporarily the upper portion of the walls of a building when it is found necessary to reconstruct the foundations or to make large openings in the lower parts of the wall, as, for example, when putting a shop front in an existing building.

From a photograph by W. T. Green.
From a photograph by W. T. Green.

Fig. 7.—Shoring of the Presbytery, Exterior, Winchester Cathedral Restoration.

This form of shoring consists of horizontal members of balk timber termed needles (very different from the needles used in raking and flying shoring), which are passed through holes in the wall to be supported, at a sufficient height to allow of the insertion of any arch or lintels that may be necessary above the opening it is proposed to cut (figs. 5 and 6). The needles are supported at each end by an upright timber or dead shore, one on each side of the wall to each needle. These should not be allowed to rest upon any floor or vault but be carried down to a solid foundation and set upon and securely dogged to a timber sleeper running parallel to the wall. If it is not practicable to take the inner dead shore through intervening floors down to the solid ground in one piece, and it is necessary for its base to be set upon the floor or upon sleepers placed on the floor, the strutting must be continued in a direct line below it until a firm foundation is obtained. Between the needle and the head of the dead shores folding wedges are inserted to force the horizontal supporting balk firmly up to the underside of the masonry. Connexions between the dead shores and the needles and sleepers are made with wrought iron dogs. The spacing of the systems of dead shoring depends to a large extent upon the material with which the wall is constructed; for brickwork they should be placed at intervals not greater than 6 ft. With this form of shoring especially it is often found necessary to adopt other methods auxiliary to the main shoring. These take the form of raking or flying shores from the face of the building. All the openings in the wall above should be well strutted between their reveals to prevent any alteration of shape taking place. Inside the building vertical shores or strutting must be carried up independently in a direct line between the goofs with head and sole plates at floor level and ceiling. This strutting must start from a firm foundation at the bottom of the building and be tightly wedged up so as to relieve the wall of any weight from the floors and roof. To obviate settlement as much as possible, work done in underpinning should be built slowly with Portland cement mortar mixed in strong proportions. Before the shoring is removed at least a week should elapse to allow the work to set hard and firm. Then the needles should be carefully loosened and removed and the holes from which they were withdrawn made good. The remainder of the props can then be “struck,” leaving the raking or flying shores until the last. If possible this work should be spread over several days, an interval of a day or two being left between the removal of each portion of limbering to allow the work gradually to set on its new bearings.

Shoring should be the subject of careful calculations to ascertain the most suitable sizes of timbers and to determine the most appropriate points of support. This is not always done, however, and much work of this character is carried out by rule of thumb methods. The usual result is that the timber used is of a much greater size than is really necessary, although as the material is not much injured and is available on removal for re-use this fact is not of great consequence. Such methods perhaps work very well for ordinary buildings, but in special cases they may very well lead to shoring being constructed in too fragile a manner, with serious results; Some rules which experience has shown to work satisfactorily for ordinary work are given below, together with the approximate scantlings of the timber required.

Rules and Sizes for Raking Shores.—Walls 15 ft. to 30 ft. high should have 2 shores to each system; if 30 ft. to 40 ft. in eight, 3 shores each system; if 40 ft. or more in height, 4 shores, with an additional shore for each 10 ft. increase. Shoring is rarely seen more than 5 shores high. The angle of the main shores is usually about 60°, and none of the timbers should exceed an angle of 75°. Some of the lower shores will slope much less than this, at angles between 40° and 60° The systems should not be placed at a greater distance apart than 15 ft. It is often found convenient to place them at the piers between window openings. As regards the sizes of the timbers used for walls 15 ft. to 20 ft. high, the shores may be 4 in. or 5 in. square in section; for walls 20 ft. to 30 ft. high, 6 in. by 6 in., or 9 in. by 41/2 in.; for walls 30 ft. to 35 ft. high, 12 in. by 6 in., or 8 in. by 8 in.; for walls 40 ft. to 50 ft. high, 9 in. by 9 in.; for walls above this height 12 in. by 9 in.

For Horizontal or Flying Shores.—for spans not exceeding 15 ft. the principal strut may be 6 in. by 4 in., with raking struts 4 in. by 4 in.; for spans exceeding 15 ft. but not exceeding 35 ft. the size of the principal strut should be from 6 in. to 9 in. square, and the raking struts from 6 in. by 4 in. to 9 in. by 6 in.

From a photograph by W. T. Green.
From a photograph by W. T. Green.

Fig. 8.—Shoring of the Presbytery, Interior, Winchester Cathedral Restoration.

Interesting examples of shoring on a large scale may frequently be seen applied to large buildings in the course of repair or restoration. The rebuilding of the foundations of the retro-choir and lady chapel of Winchester cathedral which was carried out in the autumn of 1906 necessitated the erection of a very elaborate and complicated arrangement of shoring to uphold the masonry while the work of underpinning the walls was being carried on. The foundations of the eastern portion of the cathedral were found to be dangerously insecure, being in fact laid upon a bed of soft marl only 10 ft. below the surface of the ground, in spite of the fact that at a depth of 16 ft. a hard solid stratum of gravel, at least 6 ft. thick, is arrived at. The medieval builders without doubt entertained suspicions as to the sustaining power of their proposed foundation, and so to ensure stability, as they thought, strengthened it by placing below the masonry horizontal layers of beech trees, filling up the interstices with hard chalk and flints. These contrivances were not sufficient to prevent the gradual sinking, through succeeding centuries, of the heavy mass of masonry. This not only affected the footings of the building, but caused fissures of an alarming nature in the vaulting and walls. Under the direction of Mr T. G. Jackson a carefully designed arrangement of shoring was applied, consisting of raking shores, flying shores and needling, for the purpose of the underpinning, with specially designed timbering to support the arches and vaulting while they were undergoing repair. The foundations were found to be much undermined by water, which filled the excavations made for the underpinning in such quantities that it was necessary to employ a diver to deposit cement concrete in bagfuls upon the gravel bed to which the new foundations are taken down. The illustration (fig. 7) will readily explain the external shoring above described, while fig. 8 shows the interior shoring of the presbytery.

Authorities.—The principal works of reference on this subject are: C. H. Stock, Shoring and Underpinning; T. Tredgold, Elementary Principles of Carpentry; J. Blagrove, Shoring and its Application.  (J. Bt.)