_SHORING AND UNDERPINNING_
Shoring is the term given to a method of temporarily supporting
buildings by a framing of timber acting against the walls of the
structure. If the frame consists of more than one shore, it is called a
system; if of two or more systems, it becomes a series.
The forces that tend to render a building unstable are due primarily
to gravity, but owing to the various resistances set up by the tying
together of the building, the force does not always exert itself
vertically downwards.
This instability may arise from various causes, the most common being
the unequal settlement of materials in new buildings, the pulling
down of adjoining buildings, structural alterations and defects, and
alterations or disturbances of the adjacent ground which affect the
foundations. The pulling down of an adjoining building would, by
removing the corresponding resistance, allow the weight of the internal
structure of the building to set up forces which at first would act
in a horizontal direction outwards. Structural defects, such as an
insufficiently tied roof truss, would have the same effect. Structural
alterations, such as the removal of the lower portion of a wall in
order to insert a shop front would allow a force due to gravity to act
vertically downwards.
To resist these forces, three different methods of shoring are in
general use, and they are known as flying or horizontal shores, raking
shores, and underpinning.
_Flying Shores._—Where the thrusts acting upon the wall are in
a horizontal direction, flying or raking shores are used to give
temporary support. The most direct resistance is gained by the
first-named, the flying or horizontal shore. There are, however, limits
to its application, as, owing to the difficulty of obtaining sound
timber of more than 50 to 60 feet in length, a solid body is necessary
within that distance, from which the required purchase can be obtained.
It is a method of shoring generally used where one house in a row is to
be taken down, the timbers being erected as demolition proceeds, and
taken down again as the new work takes its place.
Fig. 47 shows a half-elevation of two general systems of construction.
The framing, as at A, may be used alone where the wall to be
supported is of moderate height and the opening narrow, but larger
frames should be combined, as at B.
The framework C is for wide openings and walls of considerable
height.
The wall plates, 9 in. by 2 in. or 9 in. by 3 in., are first fixed
vertically on the walls by wall hooks. Then, in a line with the floors,
rectangular holes 4 in. by 3 in. are cut in the centre of wall plates.
Into these holes, and at least 4-1/2 inches into the brickwork, needles
(also known as tossles and joggles) of the same size are fitted,
leaving a projection out from the wall plate of 5 in. or 6 in.,
sufficient to carry the shore of about 7 in. by 7 in.
The shore, prior to being fixed, has nailed on its top and under sides
straining pieces 2 inches thick, and of the same width as the shore. To
tighten, oak folding wedges are driven at one end between the shore and
wall plate.
[Illustration: FIG. 47]
To stiffen the shore, and to further equalise the given resistance
over the defective wall, raking struts are fixed between the straining
pieces, and cleats are nailed above and below the shore. These raking
struts are tightened by driving wedges between their ends and the
straining pieces.
The cleats previous to, and in addition to being nailed, should be
slightly mortised into the wall plate. This lessens the likelihood of
the nails drawing under the pressure.
=A Raking Shore= consists of a triangulated system of timber
framing, and is used to support defective walls where the resistance to
the threatened rupture has to be derived from the ground surrounding
the building.
In its simplest form a raking shore is a balk of timber of varying
scantlings, but as a rule of square section, inclined from the ground
to the defective wall. The angle of inclination is taken from the
horizon, and should vary between 60 and 75 degrees. In settling this
the space available at the foot of the wall has to be taken into
consideration, especially in urban districts where the wall abuts on
the footpath.
[Illustration: FIG. 48]
Fig. 48 shows a raking shore in its simplest form, but usually two or
more shores are used (see fig. 49).
The following table from Mr. Stock’s book[1] shows the general rule and
also the scantlings to be used:
For walls from 15 to 30 feet high 2 shores are necessary in each system
For walls from 30 to 40 feet high 3 shores are necessary in each system
For walls from 40 feet high and upwards 4 shores are necessary in each
system
[Illustration: FIG. 49.—EXAMPLE OF RAKING SHORE]
Taking the angle of the shore at from 60 to 75 degrees:
For walls from 15 to 20 feet high 5 in. by 5 in. may be the scantling
for each shore
For walls from 20 to 30 feet high 6 in. by 6 in. may be the scantling
for each shore
For walls from 30 to 35 feet high 7 in. by 7 in. may be the scantling
for each shore
For walls from 35 to 40 feet high 8 in. by 8 in. may be the scantling
for each shore
For walls from 40 to 50 feet high 9 in. by 9 in. may be the scantling
for each shore
For walls from 50 feet and upwards 12 in. by 9 in. may be the scantling
for each shore
In the greatest length, the beams are 12 in. by 9 in. to give increased
rigidity, which prevents any likelihood of sagging.
The wall plate is the first timber put into position. It is placed
vertically down the face of the wall, and held in its position by wall
hooks. Note should then be taken of the position of floors. If the
floor joists run at right angles to the wall, the shore should abut on
the wall in such position that it points directly below the wall plate
carrying the floor joist. If the joists run parallel to the wall, the
shore should act directly on a point representing the meeting of lines
drawn down the centre of the wall and across the centre of the floor
(see fig. 50).
[Illustration: FIG. 50]
To enable the shore to fulfil this condition, the needle (of 4 in. by 3
in.) should be let through the plate 4-1/2 inches into the wall below
the point in question. To strengthen the needle cleats are nailed, and
slightly let into the plate immediately above.
The footing, or sole piece, has next to be laid. It consists of a
timber 11 in. by 3 in., and long enough to take the bottom ends of
the required number of shores. Attention should be paid to the ground
in which it is to be bedded, and if this is at all soft, additional
timbers should be laid under, and at right angles with it, to give
greater bearing.
The sole piece should not be laid at right angles to the shore, but
its face should form, with the outside line of the top shore, an angle
somewhat wider, say of 93 degrees. The advantage of this will be seen
presently.
The shore itself has now to be prepared. Its top end should be grooved
sufficiently (fig. 51) to receive the needle. This will prevent lateral
motion when under pressure.
[Illustration: FIG. 51]
The bottom end should be slightly slotted, in order to receive the end
of a crowbar (see fig. 52).
It is now placed in position, and gently tightened up by the leverage
of a crowbar acting in the slot, and using the sole piece as a fulcrum.
The advantage of the sole piece not being at right angles to the shore
can now be seen, as if it were so laid no tightening could be gained
by the leverage. This system is an improvement upon the tightening up
by wedges, as the structure is not jarred in any manner. If the frame
is to have more than one shore, they are erected in the same manner,
the bottom shore being the first put up, the others succeeding in their
turn. When in position the shores are dogged to the sole piece and a
cleat is nailed down on the outer side of the system. The bottom ends
are then bound together by hoop iron just above the ground level. To
prevent the shores sagging, struts are fixed as shown on fig. 49.
[Illustration: FIG. 52]
Besides preventing the sagging these struts serve the purpose of
keeping the shores in position. They may be fixed as nearly at right
angles to the shores as possible, or at right angles to the wall; in
any case they should reach to the wall plate at a point just below the
needle. The struts should be nailed to the shores and wall plate. If
the latter is wider than the shores, it should be cut to receive the
struts.
It sometimes occurs that the timbers are of insufficient length to
reach from the sole piece to wall plate. To overcome this difficulty,
a short timber is laid on the sole piece against and parallel to the
next middle raker, and on this short timber a rider shore stands
reaching to its position on the wall plate (see fig. 49).
When this is done the top middle raker should be stiffer to resist the
increased cross strain. Stiffness is gained by increasing the depth. A
rider shore is tightened by oak folding wedges driven between the foot
of the shore and the short timber which supports it.
Note must be taken that the outer raker is not carried too near the top
of the building, or else the upward thrust of the shores, which always
exists with raking shores, might force the bond or joints.
Fir is the best wood for shoring owing to the ease with which it can
be obtained in good length. Another advantage is its straightness of
fibre; although, as it is more easily crushed by pressure across the
grain, it does not answer so well as oak for wedges, sole pieces, &c.
In erecting flying or raking shores, notice should be taken of the
following points.
The systems should be placed from 12 to 15 feet apart if on a wall
without openings, otherwise on the piers between the openings.
In very defective walls it is an advantage to use lighter scantlings,
the systems being placed closer together. Heavy timbers handled
carelessly may precipitate the collapse which it is the intention to
avoid.
Wedge driving and tightening should be done as gently as possible. It
should be remembered that support only is to be given, and not new
thrusts set up, which may result in more harm than good.
=Underpinning.=—Underpinning is necessary to carry the upper
part of a wall, while the lower part is removed; for instance, the
insertion of a shop front, or the repairing of a foundation. It is only
kept in position until a permanent resistance to the load is effected.
Underpinning is, as a rule, unnecessary when the opening to be made
is of less width than five feet. This method of shoring is a simple
operation, but yet requires great care in its execution.
The first thing to be done is to remove from the wall all its attendant
loads. This is accomplished by strutting from the foundation floor
upwards from floor to floor until the roof is reached (see fig. 53).
Header and sole plates 9 in. by 2 in. are put in at right angles to the
joists in order to give bearing to the struts.
The portion of the wall to be taken down having been marked out, small
openings are made, slightly above the proposed removal, at from 5 to 7
feet apart, and through these, at right angles to the face of the wall
itself, steel joists or balk timbers 13 in. by 13 in., called needles,
are placed. These are supported at each end by vertical timbers 13 in.
by 13 in., called dead shores, which again rest upon sleepers.
The sleepers serve as a bed to the dead shores to which they are
dogged, and by distributing the weight over a larger area, they prevent
the dead shores sinking under the pressure. The dead shores, if well
braced, may be of smaller scantling.
Where it is impossible to arrange for the dead shores to be in one
length, the lower pieces are first fixed. They must be of uniform
length, and across their top end a transom is carried to support the
upper pieces, the bottom ends of which must stand directly over the top
ends of the lower pieces (see fig. 53).
Having placed all the timbers in position, and before the tightening up
takes place, the windows or other openings in the wall are strutted
to prevent any twisting which may take place. This is done as shown on
fig. 54, but small windows do not require the centering.
[Illustration: FIG. 53.—EXAMPLE OF UNDERPINNING]
The tightening up is caused by the driving home of oak folding
wedges placed in the joints between the needles and the dead shores.
This position is better than between the shore and sleeper, as any
inequality of driving here would have the tendency to throw the shore
out of the perpendicular. For a similar reason the wedges should be
driven in the same line as the run of the needle, as cross driving, if
unequal, would cause the needle to present an inclined surface to the
wall to be carried.
[Illustration: FIG. 54]
In carrying out these operations note should be taken of the following
points:—
1. That the dead shores should not stand over cellars or such places.
It is better to continue the needle to such a length that solid ground
is reached, and the needle can then be strutted from the dead shore.
2. That extra needles should be placed under chimney breasts, should
there be any in the wall which is to be supported. The same applies
where corbels, piers, &c. occur.
3. Sleepers and shores should be so placed that they do not interfere
with the proper construction of the new foundations or portions of the
building.
4. The inside shores should be carried freely through the floors until
a solid foundation is reached.
5. The removal of the shores after the alterations have been made is
one requiring great care. It should be remembered, that while the work
is new it cannot offer its greatest resistance to its intended load.
Time should therefore be given for the work to well set, and then
the timbers, eased gradually by the wedges being loosened, should be
finally taken out.
6. The raking shores, if used in conjunction with underpinning, should
be left to the last.