Framed Structures | Concept | Principles | Materials Used | Tall Buildings | Pre-Fabricated Buildings

New technologies for vertical transportation and high socio-economic levels of urban growth have given room to tall buildings. Further, large-scale industrialisation and exhaustive land cost resulted in the expansion of the building programme.

Framed structures are comprised of series of frames with horizontal and vertical components. These framed structure concepts are effectively used in the construction of multi-storey buildings.

Although timber, steel and RCC can be used for framed structures, RCC plays the top-most role in space frame technology. The idea of space frame was originally developed for aeroplane hangers in the 1940s.

Apart from the construction of framed structures using the conventional construction techniques, the technique of pre-fabrication has also been used to economise the cost and time.

These tall buildings and pre-fabricated buildings which are discussed in the following sections with introduction of framed structures.

 

CONCEPT OF FRAMED STRUCTURES

 

Principles of Framed Structures

Framed structures are the structures which are formed by connecting a series of horizontal and vertical structural members in suitable positions. The framed structures primarily com- prise of columns and beams. Columns are connected by beams at the floor and roof levels. The floors are divided into rooms and passages of required sizes by walls.

The walls may be load-bearing walls or partition walls. The materials used for framed structures, in general, maybe wood, steel or RCC. Light framed structures are constructed of wood, steel or RCC, whereas multi-storeyed framed structures are constructed of mild steel.

An entire load of floors, roofs and partition walls are supported by the beams which transmit these loads to the columns. These columns finally carry the whole weight of the structure to the foundation.

 

Materials Used for Framed Structures

As discussed earlier, the materials used for framing structures, viz., wood, metal and RCC, are discussed below.

 

1. Timber

Use of timber for framed structures offer several advantages like beauty, versatility, durability, low cost, high strength-to-weight ratio, excellent electrical insulation and high shock absorption capacity. A variety of range of finishes can be provided to satisfy decorative and protective purposes.

In wooden frames, the walls are conventionally built with slender studs spaced about 40 cm centre to centre. In a similar way, joints and rafters which are supported on the walls are also spaced 40 cm apart. Timber studs are usually placed in walls and partitions keeping the wide faces perpendicular to the face of the wall or partition. For attachment to studs, joists and rafters, facings and decking for the required sizes are available. Wood bearing wall constructions are generally either braced to balloon frame or platform frame.

 

2. Steel

In general, the steel framework supports all loads. In this type, all columns, beams and girders in the framed structure are of the steel section. Adequate bracing is needed for these structures in order to resist the wind and earthquake forces.

For these structures, fireproof and other light materials are used for partitions and exterior walls if it is a building. Steel framed structures are capable of sustaining greater load in a given space. In steel-framed structures, circular or curved work should be avoided.

 

3. RCC

Reinforced cement concrete (RCC) forms the best material for framed structures, provided they are properly designed and constructed. As discussed earlier, columns and beams are interconnected with each other so as to form a grid of beams and girders. In order to carry various floor levels, the slabs are built monolithically. The foundations may be either isolated or combined footing or it may be a raft or mat depending on the soil and the load on the foundation.

 

Advantages of Framed Structures

Following are the advantages of framed structures:

1. If the framed structure consists of several floors, for example, in a building, it is possible to carry out different activities simultaneously. For example, finishing of lower floors can be done while the construction of the framework of upper floors can be taken up.

2. Based on the requirement, the panel walls may be changed to a different position. This brings in greater freedom in planning.

3. In a building, this helps to divide the components into two distinct categories, viz., load-bearing and non-load bearing. This enables to the construction of the non-load-bearing components using low-cost materials.

4. The framed structure construction requires thin panels which in turn leads to the larger floor area. However, outer walls are off a type which shall offer heat and sound insulation and shall withstand weather conditions.

5. Framed structures can highly resist vibrations and are ideal for seismic zones.

6. Very suitable in filled-up or soft ground compared to ordinary masonry constructions. Tall buildings are generally framed structures which are discussed in the next section.

 

TALL BUILDINGS

Large-scale industrialisation has resulted in the great expansion of building programmes. Prohibitive land costs in urban areas and demand to meet large populations in urban areas have made way for the construction of tall buildings called multi-storeyed buildings. Thus, we have reached a stage now that multi-storeyed construction is essential and inevitable in urban areas. Buildings with more than five stories are called multi-storey buildings. Most of the tall buildings in cities have five to twelve stories. But in metros like Kolkata, Delhi, Mumbai and Chennai, 20- to 25-storey buildings have started coming up.

Details of RCC multi-storeyed building

 

Advantages of Tall Buildings

Following are the advantages of tall buildings:

1. Economy in the use of less land for construction.

2. Gives room for a large proportion of open space for creating a natural environment.

3. Enables better day-lighting and greater flow of air.

4. Freedom from street noises.

5. Provides a pleasant panoramic view of the city.

6. Provides amenities like common car parking, recreation centres, park, swimming pool, etc.

 

Disadvantages of Tall Buildings

Following are the disadvantages of tall buildings:

1. Density of the population is high in a small area.

2. Prevention of congestion is difficult.

3. Excessive and imbalanced load on municipal services like water supply, sewage, electricity, etc.

4. Difficult to prevent accidents due to fire, earthquake disasters, etc.

5. Poses a number of social and human problems.

 

Construction of Tall Buildings

Important aspects which are to be considered in the construction of RCC tall buildings are given below:

 

1. RCC frames are monolithic construction of columns, beams and slabs. Because of monolithic construction, deflection and bending moments are reduced which results in economical construction. Further adequate safety is ensured.

 

2. An RCC frame essentially consists of beams, columns and slabs (as floor or roof). In the case of large spans for better distribution of load, secondary beams spanning across main beams can be introduced.

 

3. A typical frame of an RCC multi-storeyed building is shown below figure. It shows the monolithic construction of columns, beams, slabs and girders.

 

4. The concreting procedure is the same as for other structures. However, a sequential procedure has to be followed. Here the formworks for different members to be cast are first installed or erected in position. Necessary reinforcement is then placed and concreted. The formwork is removed after the concrete has attained adequate strength.

 

5. The next member to be formed is taken up and formwork is fixed and concreting done. The general sequence is the construction of columns followed by beams, cross beams and slabs together.

 

6. As the entire construction can not be done in one stretch, construction joints are required to be provided at intervals. Further, in framed construction, the joints should be at the point of minimum shear.

 

7. The height of the columns is concreted so as to provide proper lap with the sides of the beams and columns in upper storeys.

 

PRE-FABRICATED BUILDINGS

A general discussion on different building units which could be pre-fabricated is made in article. Pre-fabricated buildings are constructed based on the articulated structure concept. Articulated structure means, the separation of a structure into two or more elements and join the entire structural elements such that it functions as a monolithic structure. The elements are pre-fabricated and are assembled and erected. This technique is applied to framed structures also.

 

Design and Manufacturing

While designing pre-fabricated buildings, manufacturing and effective installation techniques should be taken into account. That is, the design of structural parts, utilisation of structural parts and their joints should be installed with minimum use of materials and manpower for manufacture and erection.

In fully pre-fabricated construction, it is the practice to use larger elements while simultaneously reducing the relative mass. This is achieved by using more efficient design, light-weight concrete, synthetic heat insulation and other efficient materials.

At present, pre-fabricated concrete factories not only manufacture structural components but also assemble buildings from fabricated blocks and perform the whole complex of construction work. As pre-fabricated elements grow larger and taller, pre-fabrication makes possible speedier construction.

The erection should involve minimum consumption of labour, time and other means. Effectiveness in erection depends on efficient pre-assembly of structures relatively, equal weights of section unit, high degree of prefabrication and accuracy of manufacture, and the simplicity of the butt-joints and provision of fastening devices.

 

Delivery and Storage of Pre-fabricated Structures

Structural units are delivered to erection zones by most effective mechanised procedures and allowing for haulage distance, availability of approach roads and conditions of in-site roads. Depending on the character of the application of structural units, they are transported from the place of manufacturing to construction sites and unloaded at a pre-assembly area. A pre-assembly area is a site storage area or a zone identified in an erecting area.

Pre-fabricated structural units are generally transported by trucks with two-axle trailers, tractor trucks with semi-trailers and panel transporters. Delivered units to the construction site should correspond to assembly lists which specify the name, the type and the number of pre-fabricated elements intended for placement in a specified area of the building.

 

Pre-assembly of Pre-fabricated Concrete Structural Units

It is difficult to haul large-size buildings because of their bulkiness and large mass. Thus, these structural units have to be made as transportable components which could be assembled by erection units at the construction site.

Reinforced cement concrete structural units are manufactured at areas within the radius of truck haulage. They are generally transported as complete units by means of special transport vehicles. Sometimes bulky reinforced concrete structures are delivered to construction sites as sub-components. These are then assembled into heavy columns, beams, arches, shells and other large constructions.

Reinforced concrete trusses are pre-assembled in a horizontal position on special racks with the use of jigs securing elements in precise positions or in the design position on special stands. The assembly of trusses from two half-trusses in the design position is carried out on special-purpose universal stands.

Pre-assembly can also be done in the zone of erection at ground level with the use of movable stands or along the line of erection work. Structural units may be cast for assembling frame structures to form a multi-storey building. The erection of structural units to form multi-storeyed pre-fabricated buildings is discussed in the next section.

 

ERECTION OF PRE-FABRICATED

 

MULTI-STOREY FRAME BUILDINGS

Multi-storey buildings may be built of standardised construction with column spaces based on some modular grid (say 6 m × 9 m or 6 m × 6 m). Based on dimensions, multi-storey buildings are erected by means of tower or derrick cranes placed on one or both sides of the frame (below figure).

 

When two cranes are put into use, they are to be arranged such that there is no dead area (i.e., areas not covered by the cranes). Further, another requirement is that the booms or loads they hoist should not interfere with one another. This is achieved by positioning the cranes such that they are spaced more than two radii of reach apart so as to provide a space sufficient for their safe operation. Sometimes the cranes operate in sequence with one lagging behind the other.

Diagram of locating crane for erecting high-rise building

 

When cranes are placed outside the building, erection should be carried out one storey after another. Based on this procedure, the erection of a storey is started only after completion of all the structures of the preceding storey have been completed. Further, the erection braces are placed so as to ensure the longitudinal stability of the building. When cranes are located within the building cross-section, the vertical break between adjacent cells (units) of the framework should not exceed one tier.

Columns on the ground floor are placed on the heads of foundation columns or in foundation pockets. Columns of the subsequent storey are mounted using group jigs which are intended for the erection of four or six columns (below figure). A group jig consists of a box-type metal structure with collars for securing columns and a wooden working platform for the erectors. Further, a jig carries three collars for each column. The bottom collar is attached to the jig projecting caps of the underlying storey columns. The jig is aligned in the centre lines with the aid of a special frame. Further, it is secured to erection parts of intermediate floors of bracing, using screw jacks and the jig is levelled.

After the jig has been placed and secured to the caps of columns of all underlying storeys, all four new columns are installed, secured and trued by adjacent screws.

 

Single-tier jig for trueing four columns

Using a theodolite, the verticality of the columns are checked. The admissible deviation of longitudinal axes of columns is ±10 mm for columns less than 4.5 m and ±15 mm for columns 4.5–15 m high. Once columns are trued and secured to jigs, their joints are then grouted with concrete (above figure).

Crossbars of the ground floor are placed then. Once the crossbar has been positioned correctly, its cast-in fittings are then welded to column brackets. After securing the crossbar throughout the width of the building, reinforcement projections are welded. The cast-in-fittings of crossbars and column brackets are permanently welded. The joints are then grouted with concrete.

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