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Chapter 11 temporary works


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Chapter 11 temporary works

  1. 1. 1 S.S.A.S.I.T GTU
  2. 2. 2 TIMBERING IN TRENCHES When the depth of trench is large, or when the sub-soil is loose, the sides of the trench may cave in. The problem can be solved by adopting a suitable method of timbering. Timbering of trenches, sometimes also known as shoring consists of providing timber planks or boards and struts to give temporary support to the sides of the trench. Timbering of deep trenches can be done with the help of the following methods: 1. Stay bracing. 2. Box sheeting 3. Vertical sheeting 4. Runner system 5. Sheet piling.
  3. 3. 3 1. Stay bracing. This method (Fig. 2.31) is used for supporting the sides or a bench excavated in fairly firm soil, when the depth of excavation does not exceed about 2 metres. The method consists of placing vertical sheets (called sheathing) or polling boards opposite each other against the two walls of the trench and holding them in position by one or two rows of struts. The sheets are placed at an interval of 2 to 4 metres and generally, they extend to the full height of the trench. The polling boards may have width of about 200 mm and thickness of 44 to 50 mm. The struts may have size 1OO x 100 mm for trench up to 2 m width and 200 x 200 mm for trench up to 4 m width.
  4. 4. 4 FIG. 2.31 STAY BRACING.
  5. 5. 5 2. Box sheeting. This method is adopted in loose soils, when the depth of excavation does not exceed 4 metres. Fig. 2.32 (a) shows the box like structure, consisting of vertical sheets placed very near to each other (some times touching each other) and keeping them in position by longitudinal rows (usually two) of Wales. Struts are then provided across the Wales. Another system of box sheeting, shown in Fig. 2.32(b), is adopted for very loose soils. In this system, the sheeting is provided longitudinally, and they are supported by vertical Wales and horizontal struts [Fig. 2.32 (b)]. If the height is more, braces are also provided along with struts.
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  7. 7. 7 3. Vertical sheeting. This system is adopted for deep trenches (up to 10 m depth) in soft ground. The method is similar to the box sheeting [Fig. 2.32 (a)] except that the excavation is carried out in stages and at the end of each stage, an offset is provided, so that the width of the trench goes on decreasing as the depth increases. Each stage is limited to about 3 m in height and the offset may vary from 25 to 50 cm per stage. For each stage, separate vertical sheeting, supported by horizontal wailings and struts are provided (Fig. 2.33). 4. Runner system. This system is used in extremely loose and soft ground, which needs immediate support as excavation progresses. The system is similar to vertical sheeting of box system, except that in the place of vertical sheeting, runners, made of long thick wooden sheets or planks with iron shoe at the ends, are provided. Wales and struts are provided as usual (Fig. 2.34). These runners are driven about 30 cm in advance of the progress of the work, by hammering
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  9. 9. 9 5. Sheet piling. This method is adopted when (i) soil to be excavated is soft or loose (ii) depth of excavation is large (iii) width of trench is also large and (iv) there is sub-soil water. Sheet piles are designed to resist lateral earth pressure. These are driven in the ground by mechanical means (pile driving equipment). They can be used for excavating to a very large depth.
  10. 10. 10 Shoring is the construction of a temporary structure to support temporarily an unsafe structure. These support walls laterally. They can be used under the following circumstances: •When walls bulge out •When walls crack due to unequal settlement of foundation and repairs are to be carried out to the cracked wall. •When an adjacent structure needs pulling down. •When openings are to be newly made or enlarged in a wall. Types of shoring •Raking shores •Flying shores •Dead shores SHORING
  11. 11. 11 RAKING SHORES In this method, inclined members known as rakers are used to give lateral supports to walls. A raking shore consists of the following components: •Rakers or inclined member •Wall plate •Needles •Cleats •Bracing •Sole plate
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  13. 13. 13 FLYING SHORES It is a system of providing temporary supports to the party walls of the 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. They 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 and 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 at their feet. The straining piece is also known as straining sill and is spiked to the horizontal shore. The width of straining piece is the same as that of the strut.
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  16. 16. 16 DEAD SHORES 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 been removed for the purpose of providing an opening in the wall or to rebuild a defective load 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 same to the ground on firm foundation below.
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  18. 18. 18 When opening in the wall are to be made, holes are cut in the wall at such a height as to allow sufficient space for insertion of the beam or girder that will be provided permanently to carry the weight of the structure above. Distance at which the holes are cut depends upon the type of masonry and it varies from 1.2m to 1.8m centre. 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 above.
  19. 19. 19 The method of underpinning help to strengthen the foundation of an existing building or any other infrastructure. These involve installation of permanent or temporary support to an already held foundation so that additional depth and bearing capacity is achieved. UNDERPINNING The process of providing a new foundation below an existing foundation of an existing foundation, without endangering the stability of an existing structure.
  20. 20. 20 1. Mass Concrete Underpinning Method (Pit Method) •Mass concrete underpinning method is the traditional method of underpinning, as it has been followed by centuries. The method involves extending the old foundation till it reaches a stable stratum. •The soil below the existing foundation is excavated in a controlled manner through stages or pins. When strata suitable is reached, the excavation is filled with concrete and kept for curing, before next excavation starts. •In order to transfer the load from old foundation to new one, a new pin is provided by means of placing dry sand-cement pack. This is a low-cost method suitable for the shallow foundation.
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  22. 22. 22 Pile Method of Underpinning In this method, piles are driven on adjacent sides of the wall that supports the weak foundation. A needle or pin penetrates through the wall that is in turn connected to the piles as shown in figure-3. These needles behave like pile caps. Settlement in soil due to water clogging or clayey nature can be treated by this method.
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  24. 24. 24 Underpinning By Cantilever Needle Beam Method Figure-2 represents the arrangement of cantilever pit method of underpinning, which is an extension of pit method. If the foundation has to be extended only to one side and the plan possess a stronger interior column, this method can be used for underpinning. Advantages of Cantilever Needle Beam Method: •Faster than traditional method •One side access only •High load carrying capability Disadvantages: •Digging found uneconomical when existing foundation is deep •Constraint in access restricts the use of needle beams
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  26. 26. When wall or column or other structural member of building exceeds 1.5m, a temporary structures are needed to support the platform over which workmen can work and material can kept. These temporary structures constructed very close to wall, is in the form of timber or steel framework, called Scaffolding. SCAFFOLDING 26
  27. 27. •Standards •Ledgers •Braces •Putlogs •Transoms •Bridle •Boarding •Guard rail •Toe board COMPONENT PARTS 27
  28. 28. 1. Single scaffolding or brick layers scaffolding 2. Double scaffolding or mason’s scaffolding 3. Cantilever or needle scaffolding 4. Suspended scaffolding 5. Trestle scaffolding 6. Steel scaffolding 7. Patented scaffolding TYPES OF SCAFFOLDING 28
  30. 30. 30 Single Scaffolding Single scaffolding is generally used for brick masonry and is also called as brick layer’s scaffolding. Single scaffolding consists of standards, ledgers, putlogs etc., which is parallel to the wall at a distance of about 1.2 m. Distance between the standards is about 2 to 2.5 m. Ledgers connect the standards at vertical interval of 1.2 to 1.5 m. Putlogs are taken out from the hole left in the wall to one end of the ledgers. Putlogs are placed at an interval of 1.2 to 1.5 m.
  32. 32. 32 Double Scaffolding Double Scaffolding is generally used for stone masonry so, it is also called as mason’s scaffolding. In stone walls, it is hard to make holes in the wall to support putlogs. So, two rows of scaffolding is constructed to make it strong. The first row is 20 – 30 cm away from the wall and the other one is 1m away from the first row. Then putlogs are placed which are supported by the both frames. To make it more strong rakers and cross braces are provided. This is also called as independent scaffolding.
  34. 34. 34 Cantilever Scaffolding: This a type of scaffolding in which the standards are supported on series of needles and these needles are taken out through holes in the wall. This is called single frame type scaffolding. In the other type needles are strutted inside the floors through the openings and this is called independent or double frame type scaffolding. Care should be taken while construction of cantilever scaffolding. Generally cantilever scaffoldings are used under conditions such as •When the ground does not having the capacity to support standards. •When the Ground near the wall is to be free from traffic. •When upper part of the wall is under construction.
  35. 35. Light weight scaffolding used for repair works like painting, pointing etc. The working platform is suspended from roofs by means of wire ropes or chains. Platform can be raised or lowered at any desired level. 4.SUSPENDED SCAFFOLDING 35
  36. 36. This is also used for repair works inside the room, up to height of 5m. The working platform is supported on the top of movable contrivances like tripods, ladders may be mounted on wheels. 5.TRESTLE SCAFFOLDING 36
  37. 37. Wooden members are replaced by steel tubes and rope lashings are replaced by steel couplets or fittings. Such scaffolding can be erected and dismantled rapidly. It has greater strength, greater durability and higher fire resistance. Can be used for stone and brick masonry construction. 6.STEEL SCAFFOLDING 37
  38. 38. Market available scaffoldings. Equipped with special couplings, frames. Working platform is supported on brackets which can be adjusted at any suitable height 7.PATENTED SCAFFOLDING 38
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