Engineering

SHORING AND UNDERPINNING IN BUILDING CONSTRUCTION

What is Shoring ?

The term Shoring’ is applied to construction of the temporary structure required to support an unsafe structure.

The main objects of shoring may be summarized as below:

1. When the walls of a building develop signs of bulging or leaning outwards, shoring is necessary to prevent further development of the defects.

2. When defective walls of a building are to be dismantled and rebuilt, shoring is resorted to for supporting the floors or roofs connected to that wall.

3. Shoring is necessary to support the super structure when large openings are required to be made in the main walls. Shoring may be essential to give support to the walls of two adjacent buildings when the intermediate buildings when the intermediate building is to be pilled down and rebuilt.

Types of Shoring used in Building Construction

Various different types of Shoring are as under:

1. Raking Shores

This is a system of giving temporary support to an unsafe wall. The construction of raking shore, also known as inclined shore, varies with the conditions of site. In all cases wall plate 23 cm. multiplied by 5 cm. to 23 cm. multiplied by 76 cm. in size is fixed against the unsafe wall with hooks. The wall plate is further secured to the wall by means of needles. The needles which are 10 cm. multiplied by 76 cm. in section penetrate inside the wall for a distance of about 10 cm. In turn, the needles are strengthened by providing wooden cleats. The top end of the inclined rakers rest against the needles. At their base the rakers are supported by a sole piece bedded in an inclined position in the ground. The rakers are secured to the sole piece by cleats and dogs. In soft ground the area of the sole piece is increased so as to distribute the pressure over large area. In places where more rakers are provided, they are bound together by means of hoop iron or braces 2.5 cm. thick and 15 cm. wide. The inclination of the outer raker to the ground should vary between 60 degree to 75 degree. The sets of shores should be usually placed at 3 m. to 4.6 m. center to center along the wall length.

2. Flying Shores

It is a system of providing temporary support to the party walls of two buildings where the intermediate building is to be pulled down and rebuilt. All types of arrangements of supporting the unsafe structure in which the shores do not reach the ground come under this category. The flying shore consists of wall plates, needles, cleats, horizontal struts (commonly known as horizontal shores) and inclined struts arranged in different forms which varies with the situation. In this system also, the wall plates are placed against the wall are secured to it. A horizontal strut is placed between the wall plates and is supported by a system of needle and cleats. The inclined struts are supported by the needle at their top and by straining pieces and their feet. The straining piece is also known as straining still is spiked to the horizontal shore. The width of straining piece is the same that of the struts.

When the distance between the walls (to be strutted a part) is considerable, a horizontal shore does not work out to be economical and a trussed frame work of members is necessary to perform the function of flying shore.

4. Dead Shore

This is the system of shoring which is used to render vertical support to walls and roofs, floors etc. When the lower part of a wall has to be removed for the purposes of providing an opening in a wall or to rebuild a defective load of bearing wall in a structure. The dead shore consists of an arrangement of beams and posts which are required to support the weight of the structure above and transfer the same to the ground on firm foundation below.

When openings in the wall are to be made, holes are cut in the wall at such a height so as to allow sufficient space for insertion of the beam of girder that will be provided permanently to carry the weight of the structure above. Distance at which the holes are cut depend upon the type of masonry and it varies from 1.2 m. center to 1,8 m. center. Beams called needles are placed in the holes and are supported by vertical props called dead shores at their ends on either side of the wall. The needles may be of timber or steel and are of sufficient section to carry the load from above. The dead shores stand away from wall on either side so as to allow for working space when the needle and and props are in position. The props are tightened up by folding wedges provided at their bases while the junction between the prop and the needle is secured with the help of dogs. Before the dismantling work is started, all the doors, windows or other openings are well strutted. In order to relieve the wall of loads of floors and roof above, they are independently supported. Vibrations and shocks are bound to occur when wall cutting is done and as such as a measure of safety, raking shores are sometimes erected before the commencement of wall cutting operation.

What is Underpinning ?

The term is applied to the building of new work underneath an existing structure without distributing its stability. Underpinning may be necessary when defective foundation of a wall is to be replaced with new foundation or when the existing foundation of a wall is required to be strengthened to enable it to carry more loads.

1. Underpinning in Building Construction

Underpinning is done in short lengths of 1.2 m. to 1.8 m. Holes are cut into the walls at regular intervals and the entire load of the wall is supported by the needles which rest on sole pieces at ground level. The section of the needle depends upon the load to be supported. The needle may be a timber beam or a rolled steel joist. Thus the structure is held well supported in a secured manner and the footings are relieved of the loads from the wall above. When the raking shore junction beams are used as needles, wooden bearing plates are inserted between the beams and the supported wall to avoid crushing of masonry. A pit of sufficient width to provide easy work ability and of the required depth is excavated along the length of the wall. When the foundation of a wall are to be replaced, the section of wall is cut to a length of 90 to 120 cm. and new work built. Again, the next 90 to 120 cm. length of a wall is cut and rebuilt. Thus the entire wall foundation is replaced. When long walls are to be underpinned, the work is started from the center of the wall and progress is made both ways. The needles are removed only when the new work is completely set and the masonry has gained full strength.

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Thalib Tantary

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