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.