DEMOLITION OF BUILDINGS

I.INTRODUCTION

Demolition is the process of destroying down or dismantling or collapsing down of large buildings after its useful life period. The process of demolition is carried out with the help of some equipment or other methods with legal procedure followed by the consent of the local authority. We know that every structure is designed for a specific life period generally from 70 to100 years. The existence of the structure after the service life period is over is very dangerous to its occupants and surrounding buildings. The purpose of demolition is to prevent the accidental collapse of any part of the building and to ensure safety of workers, public and neighboring properties.

• Prior to carrying out any building demolition, detailed building appraisal by means of surveys and appropriate assessments shall be required which shall include a building survey and a structure survey.
• Increased concern for environmental protection has led governments to introduce legislation to encourage the use of construction demolished waste after recycling it. Demolition sites are the sources of large amounts of solid waste, which today is being used as mere landfill.
• On the other hand, building practices are such that reusable materials also become mixed with rubble, stone and soil, reducing their value and making recycling difficult or uneconomical. A building waste recycling as aggregates is a modern approach for preventing environmental pollution through reducing the stocks of waste. The reuse of building waste is a relatively new issue for the world despite the existing considerable quantity of building waste.

Demolition work is one of the most important operations in construction engineering field. If demolition activity is not done carefully it may result into severe consequences. The various causes of consequences of demolition of buildings are as follows -

• Injury to human workers due to the difficulty of accessing into or working inside a building which is under demolition.
• Falling of smaller objects or debris from the demolishing building.
• Falling of partially demolished structure.
• Collapse of unstable structure due to original structure being disturbed.
• Employing inappropriate methods to demolish.
• Collapse of heavy demolition equipment due to inadequate support of the partially demolished structure.
• Collapse of the partially demolished structure due to the accommodation of large amount of unclear debris.
• Congested site environment that easily cause damages to human workers or to the third parties that are situated nearby the demo1ition site.
• Difficult access for workers entering into a building under demolition
• Heavy machinery used in demolition may have risk of collapse due to insufficient support.

To avoid these consequences from demolition work, it is necessary to understand complete process of demolition of building.

II. DEMOLITION OF BUILDINGS

Demolition is the tearing-down of buildings and other structures. Demolition contrasts with deconstruction, which involves taking a building apart while carefully preserving valuable elements for re-use. Demolition methods can vary depending on the area where it will be held on, time available, the building material, the purpose of the demolition and the way that debris is going to be disposed. Times saving methods are more expensive than the slower ones.

2.1. Pre-planning of demolition activity

The different steps before the start of a demolition process are -

2.1.1. Surveying of Site

Study of different parameters with different views of the structure and its surroundings with structural point of view is carried in surveying. The two types of surveying which are mainly conducted are building surveying and structural surveying.

2.2.2. Removal of hazardous materials

Before starting demolition of building hazardous materials if any, such as asbestos containing materials, petroleum contamination and radioactive contamination, etc exist in the building are removed. Further investigation and removal of such hazardous material or contamination by experts shall be done.

2.2.3. Preparation of plan for demolition work

Demolition Plan and strategy is prepared which includes the following -

• The building location to be demolished.
• Topography of the site with its ground level contours and sections of the slopes and ground supported by the building where appropriate in detailed.
• Details of ground removal and/or backfilling.

2.2.4. Required stability report

The Stability Report shall include the following parts -

• A report on the stability of the building to be demolished during all stages of demolition.
• In the case when powered mechanical plants or equipment are used, structural calculations for all temporary supports and bracings.
• A report on the stability of neighboring buildings, adjoining properties.

III. METHODS OF DEMOLITION

The demolition sequence will depend on things like the type of construction, location, and demolition methods selected. Buildings and structures should generally be demolished in reverse order to their construction, that is, by „sequential demolition?. The different methods of demolition are as below –

3.1. Demolition of building by Top Down- Manual Method

Manual methods are carried out top down, proceeding, in general, from the roof to ground. The particular sequence of demolition may vary, depending on site conditions and structural elements to be demolished.

3.1.1. Demolition of Cantilevered Structures and Balconies

Cantilevered structures, balconies and canopies may project out of the building over the pedestrian footpath or in some cases over a portion of the traffic lane. Figure 1.Illustrates the demolition of cantilevered slab. The general sequence of dismantling cantilevered slabs and beams are described in the following –

• The exterior wall shall be demolished first.
• Any structure or dead load supported by the cantilevered system shall be removed prior to demolishing the cantilevered slabs and beams.
• The concrete shall be broken down gradually starting from the exterior edge of the cantilevered floor, working inwards and toward its supporting beams.

Figure:. Demolition of cantilevered RCC slab (Manual method)

3.1.2. Demolition of Floor Slabs

Figure: Demolition of slab

3.2. Demolition of building by Top Down- By Machines

The sequence of demolition by machine is typically the same as the top down manual method, except that most of the demolition is done by mechanical plant. The demolition begins with the lifting of the mechanical plant on to the building top floor. When rope or tie wire is used for pulling, the workers shall be protected or stay away from the area within reach of the rope or tie wire. The concrete shall be broken away first before the cutting of reinforcement. Alternatively the reinforced concrete slab may be cut by saw cutting. Demolition sequence would be as shown in following figures.

The wrecking ball application consists of a crane equipped with a steel ball. The destruction of the building is by the impact energy of the steel ball suspended from the crawler crane. Recommended techniques for the wrecking ball operations include

• Vertical Drop - free falling of the wrecking ball onto the structure and
• Swing in line - swinging of the ball in-line with the jib.

3.4. Demolition of building by implosion

Implosion is the strategic placing of explosive material and timing of its detonation so that a structure collapses on itself in a matter of seconds, minimizing the physical damage to its immediate surroundings. The technique weakens or removes critical supports so that the building can no longer withstand the force of gravity and falls under its own weight. Implosion are discussed in the following -

• Pre-weakening of the structure shall be designed to ensure the structural stability before the implosion.
• To minimize the dispersion of building debris into adjoining land after implosion, a trench or bund wall shall be installed outside the building to contain the debris, unless a basement exists.
• A good design will cause the structure to fall towards the centre of the building and/or within the protected area.