SHORING

Excerpt from "Traité pratique de charpente" by E. Barberot, architecte(s.c.),
Paris, Librairie Polytechnique, 1911. Translation by F. LeBlanc 2001

The word "shore" designates the pieces of wood used to temporarily support constructions or parts of constructions, or the earth from an excavation. These pieces have different names according to their purpose, their function and their location.

"Sheeting" (“blindage” in French), boards or thick planks joined or placed at variable intervals according to the consistency of the ground, and that forms a shield for a dig in loose ground, gravel or sand when it is feared that these materials may cave in. The boards are placed in the direction of the length of the trench. Plates or platforms press this sheeting against the ground and are held in place by raking shores (contre-fiches).

"Dead shore or standard" (chandelle) when the piece is placed vertically plumb under a piece that must be supported horizontally.

"Trestle" (chevalement), group of pieces forming two large supports, composed of pieces angled in opposite direction, topped by a crossbeam, and resting on a platform. If horizontal movements are expected transversely to the shored construction, then the trestle is cross-braced.

"Raking shore" (contre-fiche), piece placed at an angle against a wall in danger of collapse, against a vertical piece of wood to resist to the forces thrusting against it, or against earth to prevent it from caving in.

"Sole plate" (couche), flat piece of wood placed under the foot of a shore to create impalement, that is, a support on a large area of ground, or placed against earth that requires support to achieve the same result.

"Sole piece, Wall piece" (couchis), a relatively long horizontal or vertical piece of wood that receives the top or the foot of a raking shore; it offers a variable point of support to the raking shore according to its angle.

"Shore" (étançon), the vertical or slightly angled pieces used to support a portion of a construction or excavated ground; it is a form of dead shore of large dimension that is seldom longer than two or three meters.

"Dot shore" (étrésillon), name given to pieces of wood placed at an angle between tow walls in danger of collapse, between the two jambs of an opening or between the lintel and the sill to prevent horizontal or vertical deformation. These pieces rest on platforms, they may form a zigzag, St. Andrew's cross or even be aligned, that is, become dead shores. They may also be used between the walls of a trench to prevent caving in of the ground.

"Cross-bracing" (étrésillonnement), all the arrangements taken to prevent two parts of a construction to come closer to one another, or the banks of a dig to cave in.

"Platform, plate or sole plate" (plate-forme, sole ou patin), these words are used to describe large pieces of wood, thick planks or boards that are placed under the foot of shores or raking shores to ensure a sufficient area of support.

Pieces of wood that are straight, not sawn, and relatively light, should be used as much as possible for shores, raking shores and trestles, and generally for all pieces under compression along their length. Fir, for example, will make the work easier because of its lower density.

On the contrary, for pieces that will support compression stress perpendicular to the wood fibers, such as platforms, shims, sole pieces, etc., it is preferable to use hardwoods such as oak.

Shoring the walls of an excavation -- When it is not possible to shore the opposite walls of an excavation using pieces of wood that span across the excavation because it is too large as we can see in fig. 804, or because the excavation is made on a hillside, the method illustrated in fig. 802 can be applied; the earth is supported by raking shores.

For this example, we have selected the least favorable case; we have assumed an essentially loose ground, sand and gravel. In this instance we have to sheet the ground that has been cut, that is, cover it with strong boards and thick planks where necessary, according to the active earth pressure and the distance between the raking shores. According to the more or less lose nature of the ground, the boards are placed at a certain distance from one another, or they are placed edge to edge, as is the case in this example for sand or gravel, which sometimes requires filling the joints between the boards with plaster.

Sole plates, pieces of wood on which abut the raking shores that are held in place by wedges fixed to the sole plates with pins, are placed against the sheeting. On the ground, the raking shores rest on platforms where they are held in place by shims in the shape of wedges.


Fig. 802 – Shoring of an excavation

More or less raking shores are needed depending on the height of the excavation wall; for ordinary cases, more than three will very seldom be necessary. One should be concerned with preventing the platforms from sliding on the ground; to achieve this, piles on which the platforms abut are driven into the ground.

According to the nature of the ground, the walls of the excavation are sloped to allow the soil to hold, at least temporarily, for the time to install the shoring.

The example in fig. 803 is similar to the preceding one except that a parallel dig was made to butt the foot of the raking shores. Since varying stresses may occur in the same ground and that certain raking shores could bear more stress than others and therefore support a greater fatigue than others, they are stiffened by double members, as indicated in our drawing. They resist buckling that could occur towards the ground or the sky. It is sometimes necessary to install double members lengthwise for the same reason, e.g. perpendicular to the raking shores.


Fig. 803 – Shoring of an excavation

It should be noted that we don't show any wedges at the foot of the raking shores; this is due to the fact that the shores, which have been cut with two saw cuts, rest perpendicular to, or almost perpendicular to, the lower bed plate and have therefore no tendency to slip.


Fig. 804 – Shoring of an excavation

When the excavation is on flat ground, that the sides are almost of the same height, and that the distance is not too great for the wood available, it may be advantageous to install shores that bridge the dig as shown on fig. 804. Temporary digs of the bottom of the excavation as shown in fig. 803 are avoided, and also, if enough space can be left beneath the lower shores to allow sufficient height for workers to pass, the work at the bottom of the excavation will be facilitated.

The banks are sheeted according to the more or less loose nature of the ground.

"Bracing ditch or trenches" (étrésillonnement de tranchées ou rigoles) -- Trenches are cut vertically, or at an angle and covered with sheeting depending on the fact that the ground holds together or caves in.

When the ground is compact, and for average depth excavation, bracing is not usually called for, but if the ground on which one intends to build is susceptible to caving in, one must take the necessary precautions.


Fig. 805, 806 – Bracing of trenches

As per the excavations that we have just seen, the sides are sheeted with boards on which wall pieces are applied at a distance varying between two to three meters, and trench braces are placed in an inclined position as we show on fig. 805.

The arrangement shown in fig. 806 is sometimes used but it has the inconvenient to require purposely cut timber that can only be reused for another trench of the same width, while, in the arrangement in fig. 805, the pieces of wood can be used for excavations of varying width because all that is required is to modify their inclination.

"Pit boarding" Pit excavations are sometimes done to great depth and it would be very unwise not to take the necessary precautions to avoid collapse that could be the cause of the worse accidents.

Pits of cylindrical shape are most commonly used and when the ground does not have sufficient consistency, one should proceed in the following manner:

On the pit's circumference, a sheeting of boards of length varying from 1,5 meter to 2 meters is placed vertically (fig. 807), then they are pressed against the cylindrical face by a flexible metal ring of 5 cm X 9 mm of which the ends overlaps by approximately 30 cm (fig. 808), for pits of diameters ranging from 1,2 m minimum to allow a worker to work, to 1,5 m maximum for normal pits.

Once the planks are in place, some longer, already forming part of the upper circle, the ring is set to the diameter that will be required to tighten firmly the sheeting. It is put in place and the saddle fittings are held in place with iron wedges.


Fig. 807: Pit boarding -- Fig. 808: Ring joint -- Fig. 809: Pit boarding

The ring is then placed at an angle, slightly bent in the shape of an ellipse while the two ends are in their final position, and the sheeting, is tightened by bringing the circle to the horizontal position, hammering it until all the staves are tight.

The technique for adjusting the rings requires a certain skill that is quickly acquired by pit diggers.

The roughing-in with plaster of 4 cm to 5 cm thickness is sometimes used to replace sheeting but it is not as safe and it represents a small saving that may sometimes prove to be very expensive.

To avoid possible sliding, it is prudent to support the cross pieces with wedges nailed to the sheeting (fig. 809).

"Shoring of walls with raking shores" -- The most common situation requiring shoring is found in walls loosing their plumb and being in danger of collapse, sagging, cracking or buckling.

A piece of wood on which will rest the part to support is sealed into the wall; it should be as much as possible at right angle to the raking shore (fig. 810). In order to resist the raking shore's thrust, this should be done at the point that seems to be the most favorable for shouldering, but preferably at the level of a floor. Then if the sheeting is not sufficiently strong, a bed plate is built on top of it; the foot of the raking shore will rest on it and will be held in place with shims nailed in place as per previous examples. Both ends of the raking shore are beveled as shown in our various illustrations. Raking shores are generally of square section to resist buckling in both directions. This timber section has the advantage of letting the compression stress pass through the middle of the piece while with a rectangular section, only one side is compressed and the piece tends to buckle.


Fig. 810 – Shoring of a wall with raking shores

The use of a single shore is a makeshift solution; to obtain a good shoring, it is necessary to double the shore, that is to say, place two shores in the same plane, as we show in fig. 811.

The shores must be aligned; every effort must be made to create a triangle or a portion of a triangle because this will produce a figure that keeps its shape. The shores are kept together by splicing them with smaller pieces that will resist buckling of the wood fibers that never are equally compressed, and if there are gang shores, they can be held together by splices to resist buckling in the other direction, especially for longer pieces; this should be done in a way that interferes as little as possible with the underpinning work to be done later.

If splices aligned with the wall to be supported cannot be used, a shore can be built with pieces having splices placed to form a triangle of which the base would be the ground; the two shores in fig. 811 would then be replaced by two trestle supports, angled, and held together by splices aligned and at right angle to the wall. This should create a solid and safe frame.


Fig. 811, 812 – Shoring of a wall with raking shores

According to the situation, the triangle created by the raking shores can have its vertex at the top, as we have just seen, or at the base as shown on fig. 812, when it is necessary to support the wall at different heights.

Fig. 813 illustrates several different shoring methods for a facade and a gable in danger of collapse; they don’t appear in danger in the drawing, but lets assume that they do.


Fig. 813 – Shoring

The windows above the breastsummer are braced by placing vertical platforms or wall pieces maintained apart and held against the jambs by wooden dot shores, angled in opposite directions and forming a zigzag; as much pressure as needed can be applied by forcing the dot shore closer to the horizontal position. The breastsummer is supported by trestle supports. It can also be supported by a cast iron or stone column that will stay in place indefinitely. The upper portions and the gable are propped up by simple or double raking shores according to the load to be supported and will be stiffened by raker splicing members if necessary.

It is usual to line the platforms on the ground with lime putty. This does not add anything to the strength of the shore, but it is a good tell tale that reveals if any movement has occurred and if the base on which rests the shore has sagged. In fact, if sagging occurred, cracks would appear in the plaster and this would be a precious warning.

"Trestle" (chevalements) -- A trestle is an assembly of wooden pieces meant to support masonry that is being repaired by underpinning or when a ground floor is being transformed into a storefront and a portal must replace the inter-fenestration.


Fig. 814 – Trestle

The trestle is always placed at right angle to the wall it supports (fig. 814). Because of the considerable load it may have to carry and to avoid having to use crossbeams of very large dimensions, the top of the trestle's pieces are brought closer together as much as possible, without interfering with the work nor the installation of pieces, lintels or portals that will replace the masonry pieces removed.


Fig. 815, 816 – Trestle

The true trestle is always made up of four legs and looks like a sawhorse (fig. 815 and 816). It is the best shoring method to use to support a wall when a fillet or a portal is to be installed to make a large opening.

First, the wall is pierced above the area where the portal will be installed at the distance corresponding to the trestles spacing, by making holes through which the crossbeams are inserted, then, the platforms are installed on the ground and the legs of the trestles are put in place slightly inclined from the vertical.

The foot of every trestle leg is cut to fit the sole plate and rests on a wedge to allow for tightening; the top is notched to support the crossbeam and hold it vertically, then both legs are bolted together squeezing the crossbeam.

If it is necessary to support the wall in the space between two trestles, it is possible to install joists aligned with the wall and rest them on the crossbeams. The joists can then support intermediary crossbeams inserted in holes pierced into the wall in the same manner as those that form part of the trestle.

It is sometimes necessary to cross-brace the trestle legs with splicing members or St. Andrew’s crosses according to the situation and this could interfere greatly with the installation of the portal. There are several alternatives:

1. Lay the lintel at the foot of the wall before the installation of the trestles.

2. If the lintel is made up of two pieces and it is not acceptable to leave them on the ground where they may interfere with the circulation, they can be suspended to the crossbeams and double members can be installed directly underneath the crossbeams at right angle to the wall and joining the two trestle legs. At the bottom, as soon as the demolition work is completed, the feet of the legs may be cross-braced with St. Andrew’s crosses.

3. When it is indispensable that there be no movement of the trestle in the direction perpendicular to the façade, this can be achieved by letting the crossbeams extend beyond and installing diagonal ties to consolidate the angles.

4. Finally, it is possible to create more free space for the workers by increasing the distance between the feet of the trestle with a longer and supported crossbeam or rather squared by tie beams towards the interior. If the load on the crossbeam is too heavy, it can be reinforced with I-beams joined by strong bolts at every 40 cm to 50 cm, as we will see in figures 821 and 822.

In certain cases, the wall may only require to be supported vertically and a shore may sometimes be sufficient, at least provisionally.


Fig. 817, 818 – Trestle

The trestle shown in figures 817 and 818 differs slightly from the previous one. The legs of the trestle are parallel in a plane at right angle to the wall requiring support.

Used mainly for short-term repair work, this trestle can be braced with cross-braces on all four sides.


Fig. 819, 820 – Trestle

When it is necessary to repair the corner of a building made up of two walls that meet at right, obtuse or acute angle, for the first two cases, the trestle can be placed according to the bisecting line of the angle created by the two walls (fig. 819); if the angle is acute as in figure 820 it is preferable to position the trestle at right angle with the bisecting line as shown in the drawing.

When the legs of the trestle are far apart, the cross section of the crossbeam increases considerably. In order not to reach too large wooden beam dimensions, steel I-beams are used. Their size and shape depends on the load to bear and the span to bridge.


Fig. 821, 822 – Crossbeams or caps

A beam can be sandwiched between two I-beam (fig. 821), and bolted at intervals of 40 cm or 50 cm; or the I-beam can be embedded between two pieces of wood (fig. 822), and bolted in the same way.

Shoring of floors. -- It is often necessary to shore wooden or steel floors to either repair them or allow them to temporary support an excessive load for which they were not designed or finally to relieve a wall in danger of collapse on which they rest.


Fig. 823, 824 – Shoring of a beam

If the floor rests on a beam and it is the beam that is bending, or if the end resting on masonry may fail, this beam must be shored according to the drawing in figure 823. This shore is made up of a sole piece resting properly on the ground with which it has perfect contact, then, after having lifted the beam, if it has sagged, with a jack or even a simple lever, the shore or dead shore is installed to maintain it at the desired height.

To avoid any lateral movement of the support, brackets or purlin, cleats are nailed at the top and bottom of the shore on the relieved beam and the sole plate as shown on our drawing.

This shore is placed near the beam's load if it is the wall that threatens to fail or somewhere along the beam where excessive stress can be predicted because exaggerated bending can be observed.

If lateral displacement can be anticipated or if the dimensions of the wood available are not sufficient for the load to bear, it is possible to proceed according to figure 824. This shore comprises two inclined shores that meet the beam to support it; they are notched at the top to create a rest and resist torsion or rotation; on the ground, they rest on a sole plate and the feet are sawn at an obtuse angle. It is possible to tighten, if necessary, or even to raise the beam with the use of a crow bar. Shims nailed to the sole piece maintain the shore in place. Even though their inclination or especially the load that they bear prevents the shores from slipping from the beam, still, they are tied together by ties nailed or bolted; often this is done with ordinary boards. Lag bolts are also used to secure the top or the shores to the beam as shown in our drawing.

With these types of shores, the load must always be spread on an area of ground that is sufficient to ensure complete safety; to achieve this, horizontal sheeting that spreads the pressure is first constructed on well-compacted and leveled ground.

In the case where there are several superimposed floor levels, shoring of the top level should be directly above the one underneath and successive loads add up to compress the bearing area on the ground.

Shoring of floors without beams is done on the joists.

When preparing the ground base for shoring, consideration must be given to the load that will result from supporting all the floors, and consequently all these loads adding up, the shores on the ground floor must bear all the accumulated loads of the upper levels.

The shoring at one level can be made up of smaller shores than the one of the level immediately beneath it. For example, lets assume a four story building and a row of shores supporting 3 m x 5 m x 400 kilograms = 6,000 kilograms, each floor will represent a 6,000 kg load. Therefore, the ground will bear 6,000 kg x 4 = 24,000 kg; the ground floor, 6,000 kg x 3 = 18,000 kg; the second level 6,000 x 2 = 12,000 kg; and lastly the third level a single floor load of 6,000 kg.


Fig. 825 – Shoring of floors

Figure 825 shows the general layout. Sole plates are installed at the base and pieces of identical shape and resistance are placed on the ceiling, and in between, shores that have been beveled at their ends and installed at an angle in order to press at the top and bottom when straighten.

In summary, the shores must be installed plumb and as much as possible, one on top of the other above and below the floors of the various levels, especially if the sole plates are not thick enough to spread the loads; shores of less strength can be used on higher levels.

Shoring of openings. -- Openings in walls are weak points where deformations may occur. When movement happens in a wall, the window jambs that are not supported are the first elements to bear the consequences. Therefore, we always begin by shoring the openings unless there are more pressing areas that require immediate attention for shoring.


Fig. 826 – Cross-bracing of window Fig. 827 – Cross-bracing of opening

It is seldom necessary to cross-brace in the vertical direction, though it sometimes happens. When that is the case, the approach only differs in the direction and length of the pieces and therefore, it is not necessary to illustrate it with a drawing.

The most common solution for cross-bracing an opening is shown in figure 826. Wall pieces are placed on the jambs or lateral sides of the windows and then cross-braces are installed, inclined in an alternate way, forming zigzag; their ends are cut beveled and contact the wall pieces only at the edge of the cut that is located in the middle of the width.

Sometimes, wall plates are installed on three sides of openings as in example 827 and sometimes on all four sides and the opening is fully framed.

Fully framed openings and vertical shores are sometimes used, or fully framed openings and one or several vertical props and St. Andrew's crosses in each of the voids. This method braces in all directions even in diagonal.

When the walls are very thick, the shoring with cross-braces can be doubled, or one is placed in the reveal of the opening and the other in the frame.

Shoring of vaults. -- A very complete study of vault shoring is presented further on in this book that covers almost all cases that may come up. Here, we only cover one example that we present in figure 828.


Fig. 828 – Shoring of a vault

A tie beam or cross-beam is installed at the beginning of the arched portion of vaults that need to be repaired or for which the piers need to be repaired by underpinning; this piece is held in place by inclined shores that will permit stiffening, and their feet rest on platforms.

Above the tie beam, wall pieces are installed to form a polygon and the various segments are shimmed to support at all points the masonry or individual stones; the wall pieces are held in place by inclined braces and the pieces are shimmed as illustrated in the drawing in a way to avoid any slippage that would compromise the solidity of the ensemble.

LEGEND:

  • Figure 802 – Shoring of an excavation
  • Figure 803 – Shoring of an excavation
  • Figure 804 – Shoring of an excavation
  • Figure 805, 806 – Bracing of trenches
  • Figure 807 – Pit boarding
  • Figure 808 – Ring joint
  • Figure 809 – Pit boarding
  • Figure 810 – Shoring of a wall with raking shores
  • Figure 811, 812 – Shoring of a wall with raking shores
  • Figure 813 – Shoring
  • Figure 814 – Trestle
  • Figure 815, 816 – Trestle
  • Figure 817, 818 – Trestle
  • Figure 819,820 – Trestle
  • Figure 821,822 – Crossbeams or caps
  • Figure 823, 824 – Shoring of a beam
  • Figure 825 – Shoring of a floor
  • Figure 826 – Cross-bracing of window
  • Figure 827 – Cross-bracing of opening
  • Figure 828 – Shoring of vaults
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