95. The direction of these operations when required is generally left to the contractor, as the responsibility for the successful carrying out of the work devolves upon him.
The architect will be wise, however, when such operations are being done in connection with work let from his office, to see that proper precautions are taken for safety, and that all beams or posts have ample strength for the loads they have to support. When heavy or difficult work has to be done, it should, if possible, be intrusted to some careful person who has had experience in that class of work, as it is almost a trade by itself.
Shoring is supporting the walls of a building by inclined posts or struts, generally from the outside, while its foundations are being carried down, or while the lower portion of the wall is being removed and girders and posts substituted.
The usual method of shoring the walls of buildings not exceeding three stories in height, especially when done for the purpose of holding up the walls while being underpinned, is shown in Fig. 53.
The props or shores are inserted in sockets cut in the wall, with their lower ends resting on a timber crib supported on the ground. At least two sets of shores should be used, one to support the wall as low down as possible and the other as high up as possible. The latter shores should not have a spread at the bottom of more than one-third of their height. The platform should be made large enough so as not to bring too great a pressure on the ground, and the shores should be driven into place by oak or steel wedges.
Only a part of the foundation should be removed at a time, and as soon as three sets of shores are in place the wall should be underpinned, as described in Section 97. As fast as the wall is underpinned the first set of shores should be moved along, always keeping two sets in place, and working under or with one set.
Shoring may often be successfully employed for holding up the corner of a building while a pier or column is being changed, and sometimes when the lower part of the wall is to be removed and a girder slipped under the upper portion. In the latter case, however, needling is generally more successful and attended with less risk.
96. Needling is supporting a wall, already built, on transverse beams or needles placed through holes cut in the wall and supported at each end either by posts, jackscrews or grillage. At least one end of the horizontal beam should be supported by a jackscrew.
Wherever a long stretch of wall is to be built up at one time, and there is working space on each side of it, needling should be employed.
The beams must be spaced near enough together so that the wall will not crack between them, and the size of the beams carefully proportioned to the weight of the wall, floors, etc. In very heavy buildings steel beams should be used for the needles, and they should be spaced not more than 2 feet apart. In three or four-story buildings the needles may be of large timber and spaced from 4 to 6 feet apart. Each chimney or pier should have one or more needles directly under it.
When the first story walls or supports are to be removed, the beams or needles are usually supported on long timbers having a screw under the lower end; or, if the wall is very high or thick, a grillage of timber is built up and the jackscrews are placed on top of the grillage, the ends of the needles resting on a short beam supported by two screws, in the manner shown in Fig. 54.
When it is desired to remove the first story wall of a building for the purpose of substituting posts and girders, or for rebuilding the wall, holes should be cut in the wall from 4 to 6 feet apart, according to the weight to be supported and the quality of the brick or stone work, and at such a height that when the needles are in place they will come a few inches above the top of the intended girder. Solid supports should then be provided for the uprights, the needles put through the wall, and posts, having screws in the lower ends, set under them, the base of the screws resting on the solid support previously provided. If the needles do not have an even bearing under the wall, iron or oak wedges should be driven in until all parts of the wall bear evenly on the needles. The jacks should then be screwed up until the wall is entirely supported by the needles, care being taken, however, not to raise the wall after the weight is on the needles.
The wall below may then be removed, the girder and posts put in place, and the space between the girder and the bottom of the wall built up with brickwork, the last course of brick or stone being made to fit tightly under the old work. The needles may then be withdrawn and the holes filled up.
97. Underpinning is carrying down the foundations of an existing building, or, in other words, putting a new foundation under the old ones.
New footings may generally be put under a one or two-story building resting on firm soil without shoring or supporting the walls above, the common practice being to excavate a space only 2 to 4 feet long under the wall at a time, sliding in the new footing and wedging up with stone, slate, or steel wedges.
Where the underpinning is to be 3 feet high or more, or where the building is several stories in height, the walls should be braced or supported by shores or needles.
The usual method of underpinning the walls of buildings where a cellar is to be excavated on the adjoining lot is shown in Fig. 53.
Pits should first be dug to the depth of the new footing, and a timber platform built as shown; the shores should then be put in place and wedged up with oak wedges.