TEMPORARY SUPPORTING STRUCTURES
Temporary supporting structures are those structures which are required in the routine building construction works or repair works. During the process of concreting work, temporary structures called formworks are needed till the concrete attains its full strength.
Similarly for laying of masonry work, plastering, painting and for temporary repair, some temporary structures are used which are called scaffolding.
During the process of excavation, or temporarily supporting a weak structure some temporary structures are constructed which are called shoring.
Sometimes there may be some necessity to strengthen the existing foundation or to undertake some repairs or to increase the number of floors or introducing a basement floor, some temporary structures are constructed which are called underpinning.
In this chapter such temporary structures are discussed.
Concrete initially is in a plastic state and has to be kept within an closure of a desired shape by proper temporary supports till it gains adequate strength. This temporary enclosure is known as formwork or shuttering or simply as forms.
For circular works such as arches, doors, etc., the term centring is used generally instead of formwork or shuttering. Similarly the term moulds is used to indicate formwork of relatively small units such as lintels, cornices, cubes for testing, etc.
22.2.1 Requirements of Formwork
Irrespective of the type of material used the formwork should satisfy the following requirements:
- Adequate strength
- Smooth inner surface
- Enough rigidity
- Less leakage
- Easy removal
1. Adequate Strength
As the formwork has to bear the weight of concrete which exerts a lot of pressure on all sides when it is wet, strength is an important and essential requirement. Resistance to outward push is achieved with the help of bracings and the wire fastened on the outer side of the formwork. Further the temporary load during placing of concrete by men and machinery should also be withstood by the formwork.
2. Smooth Inner Surface
It is an essential requirement that the inner surface of the formwork to have smooth surface on all concrete structures. This is specially desired, in case, the concrete is to be exposed without plastering. All the projections, if any, are to be smoothened before concreting. Steel sheet formworks give a smooth surface. Further it is the practice to apply crude oil or soft soap solution to the inside surface of the formwork. This also further makes the removal of formwork easy.
3. Enough Rigidity
The formwork should be rigid enough to retain the shape without undue deformation. For visible finished surfaces, the allowable deformation is 1/300 of span and 1/150 of span for hidden surface. In order to use the formworks repeatedly, they should be rigid, robust and stiff.
Formwork should be designed correctly such that the required size, shape and finish are obtained. Particularly the verticality of the side formwork should be made. In order to avoid leakages through the formwork, the timber used for formwork should not have any cracks or holes.
5. Less Leakage
Adequate care is taken to see that the formwork should be so placed that there is minimum joint. Sometimes joints open once some concrete is placed. This should be avoided and the leakage should be corrected by adjusting the vertical supporting poles of the formwork.
The cost of the formwork should be minimum as the formwork is only a temporary structure and not going to contribute to the stability of the structure. This demands a careful selection of formwork material and an optimum design.
7. Easy Removal
Formwork should be placed in such a manner that it can be removed easily without causing any damage to the structure. During removal of the formwork, the nails and screws should be removed with little hammering. Such a careful removal prevents the possibility of injury to the concrete. Further such smooth removal enables the formwork to be re-used with less expense.
In general formwork should rest on strong, hard and non-yielding supports. For this the ground should be hard or the supports should rest on cured base concrete. The vertical poles supporting the formwork should be strong and should not due to the weight of concrete.
22.2.2 Materials Used
Materials generally used are timber and steel. Sometimes plywood and aluminium are also used. The choice of material to be used for the preparation of formwork depends on the nature of the job and economy.
1. Steel Formwork
If it is intended to re-use the framework material for several times it is preferred to use steel or aluminium. The initial cost of steel is very high.
Based on the requirements, the sizes of steel section are decided. For example, if formworks are prefabricated into panels or sections it is to be done as big as the handling will permit or the size of concrete. It should be noted that individuals can carry a maximum load of 35 kg.
Steel formworks are preferred based on the following advantages:
- As it can be easily installed and dismantled, there is less labour cost.
- It is feasible to use several times.
- It is of high strength and hence durable.
- Precise design can be done.
- As it does not absorb water from concrete, the formation of honeycomb surface is avoided.
- Concrete surface exposed in uniform without any undulations.
- High degree of accuracy is possible, as it does not shrink or expand.
2. Timber Formwork
Timber formwork can not be used repeatedly as that of steel formwork. Timber intended to use as formwork should be well seasoned and should be neither too dry nor too wet. The components of timber formwork depend on the design load and the type of timber available. Number of nails used in the formwork should be less and the heads projecting out for easy removal.
Timber formwork claims the following advantages:
- Initial cost is less and hence cheap.
- With less modification it can be easily altered.
- It is used for small works requiring less repetitions.
- In buildings it is economical as the formwork can be used for some other building work with minimum variations.
3. Plywood Formwork
For light loads, plywood’s are used nowadays. Following are the advantages compared to timber formwork:
- Like steel it can be re-used for a few times.
- No finishing treatment is needed, as it gives surfaces which are plain and smooth.
- As large sizes of plywood are available, it is possible to cover large area and less labour cost is involved.
Typical formwork for columns and for beams and slab are shown in Figs. 22.1 to 22.3.
Figure 22.1 Formwork for different shapes of columns
22.3 DE-SHUTTERING OF FORMWORKS
Under no circumstance the forms are removed until the concrete has hardened sufficiently. Under normal weather conditions, the concrete is left in place as long as possible up to a maximum of 28 days.
Figure 22.2 Formwork for square column
Figure 22.3 Formwork for beam and slab
However, the period to which the concrete should be left in place depends on the temperature of air, the shape and position of structural member, the load condition and the type of cement used. The curing period is reduced in case of use of rapid-hardening cement, low temperature, low water-cement ratio and high loads. In such cases early removal of forms may be permitted.
In order to roughly assess the hardness of concrete, the concrete has to be struck and if a metallic sound is heard, it can be presumed that the concrete has hardened. However, for other reasons such as re-use of forms, early use of the structure and need to cool the concrete in massive concrete the forms may be removed subjected to the condition that the concrete is adequately hardened.
In normal conditions with the atmospheric temperature is above 20°C, normal cement is used and the type of structure, the forms may be removed after a curing period as recommended in Table 22.1.
Table 22.1 Type of structure and curing period
After adequate curing, forms should be stripped or removed with great care so as not to damage the concrete wedges, vertical supports etc. In order to prevent sudden application of load on the structure, the forms should be slackened gradually.
In the routine construction work, sometimes it is necessary to have some temporary structure or support to continue the work. When the height of construction exceeds about 1.5 m, temporary structures are needed. Temporary structure is erected close to the work so as to provide a safe working platform for the workers and to provide adequate space to keep the working materials. This temporary structure is known as scaffolding or simply a scaffold. Such temporary structures are used in construction, demolition, maintenance or repair work of buildings.
22.4.1 Components of Scaffolding
For a specific work of construction, a formwork is done using one or all of the following components.
These are the vertical members of a scaffold which are either supported on the ground or embedded in to the ground or rested on sand-filled bags.
These are the horizontal members of the scaffold.
These are transverse pieces which are placed on ledgers in perpendicular direction and supported on the wall.
There are putlogs whose both ends are supported on the ledgers.
These are used to bridge an opening in a wall and support one end of the putlog at the opening.
These are the cross or diagonal pieces fixed on the standards.
7. Guard Rail
This is a horizontal member provided like a ledger at the working level.
8. Toe Board
This is a board placed parallel to the ledgers and supported between the putlogs. This facility is made as a protective measure to work on the working platform.
This is an inclined support.
22.4.2 Requirements of Scaffolding
Scaffolding has to satisfy the following requirements irrespective of the material used:
- Method of erection should be easy with less wastage of material.
- As persons have to use the scaffolding at all heights, it should possess adequate strength.
- Materials needed for scaffolding should be available in all the required sizes and lengths.
- It should be feasible to interchange the material for other works with minimum wastage.
- Initial cost should be comparatively less and should have high scrap value.
- Fire-resistant scaffolding should be preferred.
- For erection purposes no skilled labour should be depended upon.
22.4.3 Types of Scaffolding
Following are the different types of scaffolding:
- Single scaffolding or bricklayer’s scaffolding
- Double scaffolding or mason’s scaffolding
- Cantilever or needle scaffolding
- Suspended scaffolding
- Trestle scaffolding
- Steel scaffolding
- Patented scaffolding
1. Single Scaffolding or Bricklayer’s Scaffolding
This type of scaffolding is commonly used and particularly in the construction of brickwork. It consists of a single row of standards placed at a distance of about 1.20 m from the wall. The spacing between the standards is about 2–2.5 m. The ledgers are fixed at a vertical distance of 1.20–1.80 m on the standards.
The putlogs are placed at a horizontal spacing of 1.20–1.80 m. The details of the scaffolding is shown in Fig. 22.4. This type of scaffolding is also called as putlog scaffolding.
Figure 22.4 Single Scaffolding
In order to extend the work vertically, the platform is raised by extending, the standards by adding extra pieces. The scaffolding is removed, generally, after the completion of plastering and pointing works. After the removal of the scaffolding, the holes in the walls used by putlogs are filled immediately.
2. Double Scaffolding or Mason’s Scaffolding
This type of scaffolding is stronger than the single scaffolding. This is similar to that of single scaffolding except two rows of standards are used. Out of these two standards, one is close to the wall and the other is 1.2–1.5 m away from the face of the wall, Fig. 22.5.
Here, no holes are made in the wall for putlogs and the putlogs are supported at both ends on ledgers. Sometimes diagonal bracings and inclined supports called rakers are provided. This type of scaffolding is particularly used for stone masonry construction.
3. Cantilever or Needle Scaffolding
In this type of scaffolding the general framework may be of single or double type of scaffolding. But the standards are supported by needles or ties. These ties are projected out at floor levels or through openings or through holes provided in the masonry. Two types cantilever scaffolding are shown in Fig. 22.6.
Figure 22.5 Double scaffolding
Figure 22.6 Cantilever scaffolding
This type of scaffolding is useful under the following conditions:
- In situations where it has to be provided on a busy street so as not to disturb the traffic.
- In situations where it is difficult to fix the standards on the ground.
- In construction of tall buildings.
4. Suspended Scaffolding
This is a light type of scaffolding used only for maintenance works like pointing, white washing, etc. The working platform is suspended from the roofs. Special arrangements are made with pulleys, ropes, etc., to suspend the platform from the roof and to raise or lower based on the need. As it does not create any obstruction on the ground and only a minimum space is required, this arrangement is preferred.
5. Trestle Scaffolding
In this type of scaffolding, the working platforms are supported on tripods, ladders, etc., which are mounted on boggies, wheels or lorries. This is suitable for minor repairs or painting work within a height of 5 m.
6. Steel Scaffolding
In this type of scaffolding steel tubes are used instead of timber. Further 40–50 mm diameter tubes of 5 mm thick are used. These tubes are commercially available in suitable lengths with special couplings and set screws. Although it is strong and suitable it can be used up to a specific height. Figure 22.7 shows a typical tubular steel scaffolding.
Figure 22.7 Tubular steel scaffolding
7. Patented Scaffolding
This type of scaffolding is also referred to as ladder scaffolding. This is a modified form of double scaffolding but steel is used instead of timber. There are patented scaffolds which are readily available in the market with special types of couplings and frames. Here, the working platform is supported on a bracket which can be adjusted to any suitable height. Such patented scaffoldings are used to light works like painting or decoration, Fig. 22.8.
Figure 22.8 Patented steel scaffolding
Shoring is the means of providing support to get stability of a structure temporarily under certain circumstances during construction, repair or alteration.
Such a circumstance may arise when
- The suitability of a structure is a endangered due to removal of a defective portion of the structure.
- The stability of a structure is endangered due to the unequal settlement during construction.
- Certain alterations are required to be done in the present structure itself (e.g., re-modelling of walls, changing position of windows, introducing a beam, etc).
- Alterations are carried out in adjacent building for re-modelling, strengthen foundation, etc.
22.5.1 Installation of Shoring
Following are the points to be kept in view while installing shoring:
- For shoring, timber or steel tubes may be used. Sometimes both are used in combination. If timber is used its surface should be coated with a preservative so as to protect from wet rot.
- The shoring should be designed based on the load it has to sustain and the duration of load.
- Shoring may be installed internally or externally depending on the requirement and in certain cases they may be provided on either side of the wall to produce additional stability.
- Over-turning forces, if any, must be resisted by the shoring.
- Shoring should be installed only after getting the necessary permission, if necessary, from the local authorities.
- There is no time limit to which the shoring has to be kept. It may range from weeks to years depending on the case.
22.5.2 Types of Shoring
Shorings are classified under the following categories:
- Raking or Inclined shores
- Flying or Horizontal shores
- Dead or Vertical shores
1. Racking or Inclined Shores
In the type of shoring inclined members are adopted to provide temporary support to the external walls from the ground. These inclined members are called as rakers. An inclined shore primarily consists of rakers, braces, wall plate, needles and plank.
The wall plate is fastened to the wall by means of square needles. The needles penetrate into the wall for a depth of about 15 cm and prevent the wall plate from sliding against the wall. The wall plate distributes the pressure evenly (Fig. 22.9). Further the needles in turn are strengthened by providing wooden cleats. The inclined rakers are interconnected by timber braces. The feet of the rakers are tied together by braces and hoop iron. They are in turn connected to sole plate by means of iron dogs or dog spikes.
Figure 22.9 Raking or Inclined shoring of a wall
The details shown in Figure 22.9 is a simple raking shore for a building height of 10 m. For multi-storeyed building and building on road side, special rakers have to be used. While dealing with road side buildings, the bye-laws in force of the locality have to be followed.
2. Flying or Horizontal Shoring
In this type of shoring horizontal supports are provided parallel to the walls which have become unsafe due to some reason (Fig. 22.10). This arrangement is called a simple flying shore.
Figure 22.10 Details of single flying shore
This type of flying shore comprises of wall plate, needles, cleats, struts, straining pieces and folding wedges. As in inclined shores, here also the wall plates are secured against the wall by means of needles and cleats. The horizontal shore is kept in the required position by means of wedges, needles and cleats to the wall plate. The inclined struts are supported by the needles at there one end and straining sill at the other end. In turn the straining is fixed to the horizontal shore.
This type of simple flying shoring can be adopted for a maximum distance of about 9 m between the adjacent parallel walls. When the distance is from 9 to 12 m, a compound or double flying shore, shown in Fig. 22.11, may be provided. It is to be noted that both the horizontal shores are symmetrically placed with respect to floor levels.
Figure 22.11 Details of double flying shore
3. Dead or Vertical Shoring
These shores are placed vertically and are used for the temporarily supporting the wall while the lower part of which are to be removed for repairs. By this arrangement the whole load of the roof is supported by these shores (Fig. 22.12).
Needles are used to transfer the load from the walls. These are first inserted into small wall opening which is made just sufficient to fit. Horizontal beams are laid along the floors. This beam supports the dead or vertical shore and distributes the load evenly on the base. The dead shores are installed in between the beams and the needles by means of wedges. In order to avoid any damage, the usual walls are supported with props before removing the desired portions. In the same way, the windows or other openings are duly strutted.
Underpinning is the method of supporting structures while providing new foundations or strengthening the foundation or carrying out repairs and alterations without affecting the stability of the existing structures.
Figure 22.12 Dead-shores used for repairs
22.6.1 Uses of Underpinning
Underpinning techniques are adopted under the following situations:
- To strengthen the existing shallow foundation of a building when an adjoining building has to be constructed with a deep foundation.
- To safeguard the existing structure from the danger of excessive or differential settlement.
- To deepen and widen an existing foundation to increase the bearing capacity of the foundation soil.
- To build a basement floor to an existing building.
- To lift a building fully or partly to alter the foundation so as to prevent from waterlogging.
22.6.2 Precautionary Measures Before Underpinning
The following general measures should be undertaken before starting the underpinning operations:
- The existing strength of the building should be ascertained before resorting to underpinning (e.g., inferior material used or use of construction method which is not standard or poor workmanship).
- Temporary support should be provided by adequate shoring and strutting wherever needed.
- During underpinning of buildings, a watch on possible movements should be checked and rectified then and there.
- No damage should be caused to the adjoining structures.
- It should be ensured that no obstruction is created to the passage of people or vehicles in the adjoining areas.
22.6.3 Methods of Underpinning
Several methods are available for underpinning foundations but four routine methods which are used in general are:
- Pit Method
- Pile Method
- Pier Method
- Chemical Method
1. Pit Method
Generally underpinning by pit method is carried out by excavating slowly in stages. For this the existing wall is divided into suitable width of about 1.20–1.50 m. Holes are made in the existing wall in the required points. Needles with bearing plates are then inserted through these holes and supported on jacks, Fig. 22.13. This is followed by excavation, first up to the existing foundation level and then to the required depth.
Figure 22.13 Pit method
During the process of excavation, the following precautions are to be taken to avoid formation of crakes or settlement:
- Excavation should be done one at a time preferably starting at the middle and progressing sideways.
- Proper timbering is to be provided to the excavated trenches.
- Instead of central needle beam, cantilever needle beam may be adopted as shown in Fig. 22.14.
- After completion of excavation, foundation concrete is laid.
- A suitable connection between the old foundation and new foundation has to be made using vertical mild steel bars.
- After the new foundation is completely set, then only the needle beams and raking shores are removed.
Figure 22.14 Support by cantilever needle
2. Pile Method
Pit method may be impracticable or uneconomical in the following cases:
- Waterlogged area
- Heavy loads on existing structures
- Loads to be transferred to a deeper depth
In such situations pile method may be adopted.
In this method, the piles are driven along both the sides of the existing wall. Then needles in the form of pile caps are provided through the existing wall as shown in Fig. 22.15. Thus, the existing wall is relieved of the loads coming on it.
Figure 22.15 Underpinning by piles
This method is particularly useful in clayey soils and for waterlogged areas and for walls bearing heavy loads. In case of light structures piles are driven along the structure and then brackets or cantilever needles are provided.
In another approach, pre-cast piles are installed beneath the existing foundation using jacks and digging approach pits. The materials surrounding the pit is removed by water jets or air lifts and then filled with cement concrete.
3. Pier Method
This method is generally resorted to in dry ground. This is a simple method of underpinning. It involves the installation of piers under foundations of structures through the approach pits. Filling the pits with concrete and finally wedging up to transfer the loads to the new piers, Fig. 22.16.
Figure 22.16 Underpinning by pier
4. Chemical Method
In this method the foundation soil is consolidated by employing chemicals.
Perforated pipes are driver in an inclined direction beneath the foundation, Fig. 22.17. The slopes are provided such that the entire area under the existing footing corners under the area used to be strengthened.
Figure 22.17 Chemical method
After the pipes are installed, solution of sodium silicate in water is injected through the pipes. This is a two-injection method. The pipes are withdrawn and at the time of withdrawal of pipes, calcium or magnesium chloride is injected through the pipes. Chemical reaction takes place between these two chemicals and the soil is strengthened by consolidation. This method is suitable for granular soils.
- Temporary supporting structures are those structures which are required in the routine construction of building or repairs works.
- Concrete is placed in a timber or steel casing during the period of curing. Such a casing is known as shuttering, formwork or forms.
- Generally for circular works, such as arches, domes, etc., the term centring is used instead of formwork.
- The term moulds is used to indicate formwork of relatively small units such as lintels, cornices, cubes for testing, etc.
- Formwork should satisfy by the following requirements:
(i) Adequate strength,
(ii) Smooth inner surface,
(iii) Enough rigidity,
(v) Less leakage,
(vii) Easy removal and
- Materials used for the formwork are steel, timber, aluminium and plywood.
- Temporary structure erected close to the work so as to provide a safe working platform for the workers and to provide adequate space to keep the working materials is known as scaffolding or scaffold.
- The component parts of scaffolding are: standards, ledgers, putlogs, transoms, bridges, braces, guard rail, toe board and raker.
- Types of scaffolding are: single scaffolding or bricklayer’s scaffolding, double scaffolding or mason’s scaffolding, cantilever or needle scaffolding, suspended scaffolding and trestle scaffolding.
- Shoring is the means of providing support to get stability of a structure temporarily under certain circumstances during construction, repair or alteration.
- Types of shoring are: raking or inclined shores, flying or horizontal shores and dead or vertical shores.
- Underpinning is the method of supporting structures while providing new foundations or strengthening the foundation, or carrying out repairs and alterations without affecting the stability of the existing structures. The methods are: pit method, pile method and tier method.
- What are the requirements of formwork? Explain.
- Discuss the materials used for formwork.
- Enumerate the general rules to be observed to avoid the failure of formwork.
- Explain the circumstances which are favourable for timber centering and steel centering.
- Explain the requirements of shuttering with respect to quality and leakage.
- How de-shuttering of forms are done?
- What are the essential requirements of scaffolding?
- Name different types of scaffolding. Explain any one in detail.
- Detail the method of scaffolding provided to plaster the outer walls at first floor level.
- What is meant by shoring?
- It is proposed to make an opening of 6.5 m × 4.0 m height in front wall of an existing two storey building. The floor to floor height is 3.6 m. Suggest a suitable temporary structure to execute the work.
- Distinguish between raking and flying shores.
- What is underpinning? Briefly explain any one method of underpinning.
- It is intended to add are more floor on an existing two-storey building. The foundation was designed only for two floors. Explain with sketches the operation of carrying out new construction and strengthening of the existing foundation.
- Explain how will you handle the following field situation. Draw neat sketches wherever needed.
(i) A basement floor has to be provided for an existing three-storied building.
(ii) In a public building a large opening has to be made to use as a passage.