Chapter 6Prefabrication Systems and Structural Schemes

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 1100

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6.1 The word ‘system’ refers to a particular method of construction of buildings using the prefabricated components which are inter-related in functions and are produced to a set of instructions. With certain constraints, several plans are possible, using the same set of components. The degree of flexibility varies from system to system. However, in all the systems there is a certain order and discipline.

6.2 The following aspects, among others, are to be considered in devising a system:

a) Effective utilization of spaces;

b) Straight and simple walling scheme;

c) Limited sizes and numbers of components;

d) Limited opening in bearing walls;

e) Regulated locations of partitions;

f) Standardized service and stair units;

g) Limited sizes of doors and windows with regulated positions;

h) Structural clarity and efficiency;

j) Suitability for adoption in low rise and high rise building;

k) Ease of manufacturing, storing and transporting;

m) Speed and ease of erection;

n) Effective utilization of available equipment, plant and machinery; and

p) Simple jointing system.

6.3 Prefabrication Systems

The system of prefabricated construction depends on the extent of the use of prefabricated components, their materials, sizes and the technique adopted for their manufacture and use in building.

6.3.1 Types of Prefabrication Components

The prefabricated concrete components such as those given below may be used which shall be in accordance with relevant Indian Standards (see Part 5 'Building Materials' of the Code) and the accepted standards [6-7A(1)], where available:

a) Reinforced/prestressed concrete channel unit,

b) Reinforced/prestressed concrete slab unit,

c) Reinforced/prestressed concrete beams,

d) Reinforced/prestressed concrete columns,

e) Reinforced/prestressed concrete hollow core slab,

f) Reinforced concrete waffle slab/shells,

g) Reinforced/prestressed concrete wall elements,

h) Hollow/solid concrete blocks and battens,

j) Precast planks and joists for flooring and roofing,

k) Precast joists and trussed girders,

m) Light weight/cellular concrete slabs/wall panels,

n) Precast lintel and Chajjas,

p) Large panel prefabricates,

q) Reinforced/prestressed concrete trusses,

r) Reinforced/prestressed roof purlins,

s) Precast concrete L-panel unit,

t) Prefabricated concrete double-T unit,

u) Prefabricated brick panel unit,

w) Prefabricated sandwich concrete panels,

y) Precast concrete foundation, and

z) Precast concrete staircase.

There may be other types of components which may be used with the approval of the Authority.

NOTE — The elements may be cast at the site or off the site.

6.3.2 Open Prefabrication System

There are two categories of open prefab system depending on the extent of prefabrication used in the construction as given in 6.3.2.1 and 6.3.2.2.

6.3.2.1 Partial prefabrication system

This system basically uses precast roofing and flooring components and other minor elements like lintels, Chajjas, kitchen sills in conventional building construction. The structural system could be in the form of in-situ framework or load bearing walls.

6.3.2.2 Full prefabrication system

In this system almost all the structural components are prefabricated. The filler walls may be of brick/block masonry or of any other locally available material.

6.3.3 Large Panel Prefabrication System

This system is based on the use of large prefab components. The components used are precast concrete large panels for walls, floors, roofs, balconies, staircases, etc. The casting of the components could be at the site or off the site.

Depending upon the extent of prefabrication, this system can also lend itself to partial prefab system and full prefab system.

Structural scheme with precast large panel walls can be classified as given in 6.3.3.1 to 6.3.3.3.

6.3.3.1 Precast Walls

6.3.3.1.1 Based on the structural functions of the walls, the precast walls may be classified as:

PART 6 STRUCTURAL DESIGN — SECTION 7 PREFABRICATION, SYSTEMS BUILDING AND MIXED/COMPOSITE CONSTRUCTION: 7A PREFABRICATED CONCRETE

a) Load bearing walls,

b) Non-load bearing walls, and

c) Shear walls.

6.3.3.1.2 Based on construction, the precast walls may be classified as:

a) Homogeneous walls — which could be solid, hollow or ribbed; and

b) Non-homogeneous walls — these could be composite or sandwich panels.

6.3.3.1.3 Based on their locations and functional requirements the precast walls may also be classified as:

a) External walls, which may be load bearing or non-load bearing depending upon the lay-out; these are usually non-homogeneous walls of sandwiched type to impart better thermal comfort; they can also act as shear walls to resist horizontal loads with appropriate design and

b) Internal walls providing resistance against vertical loads, horizontal loads, fire, etc; these are normally homogeneous walls.

6.3.3.2 Precast floors

6.3.3.2.1 Depending upon the composition of units, precast flooring units may be classified as:

a) Homogeneous floors — which may be of solid slabs, cored slabs, ribbed or waffle slabs and precast slabs with structural topping (60-75 mm thick) designed as a composite system; and

b) Non-homogeneous floors — which may be of multi-layered units with combinations of light weight concrete or reinforced/prestressed concrete, with filler blocks.

6.3.3.2.2 Depending upon the way the loads are transferred, the precast floors may be classified as one way, two way or cantilever systems:

a) One way system transfers loads to supporting members in one direction only. The precast elements which come under this category are channel slabs, hollow core slabs, channels and ties system, light weight/cellular concrete slabs, etc.

b) Two way systems transfers loads in both the directions imparting loads on the four edges. The precast elements under this category are room sized panels, two way ribbed or waffle slab systems, etc.

c) Cantilever components are supported on one

edge or two adjacent edges and other ends without supports.

6.3.3.3 Staircase systems

Staircase system may consist of single flights with in-built risers and treads in the element. The flights are normally unidirectional transferring the loads to supporting landing slabs or load bearing walls.

6.3.4 Box Type Construction

In this system, room size units are prefabricated and erected at site. Toilet and kitchen blocks may also be similarly prefabricated and erected at site.

NOTE — This system derives its stability and stiffness from the box units which are formed by four adjacent walls. Walls are jointed to make rigid connections among themselves. The box unit rests on foundation - conventional or precast.