Demolition of buildings.ppt

Demolition of buildings

Necessity of demolition • • • • •

When the building is too old and cannot be put to further use Structural changes Development of the city Structural failure of the building Expansions

Safety Aspects Causes of Accidents • Falls of persons on site • Falling materials • Unintentional collapse • Lifting operations • Electric shocks • Fire

Safety Aspects Precautions during demolitions • People involved in the demolition work may not know the structural part of the building, so guidance in this aspect must be competent and continuous. • Plans of the structure must be checked before demolition. • First step is to ensure all the existing services like electricity have been disconnected. • Windows and door opennings must be boared up. • Internal entrances to lift shafts etc must be barricaded. • Balconies and cantilevers must be taken down first. • Stair must be kept free from debris. • Must not stand on the brickwork which is being demolished.

Sequence of demolition 1. First task in the site is that the services to the buildings have been disconnected. 2. Demolition happens in the reverse order of construction. 3. The preliminary stripping out process such as roof coverings, fittings pipework and generally all non structural part of the building. 4. All rubble and debris must be lowered to ground constantly.

Methods of types of demolition

Demolition by hand

Pulling down by rope

Mechanical demolition - Demolition ball and Pusher arm machinery

Deliberate collapse

Explosives

Methods used in demolition of various types of buildings Bricks and masonry buildings • Mortar breaks away very easily from the bricks, where bricks often remain in one piece and thus are easily salvaged. • Demolition by reverse order of construction. • Building must be demolished by hand to wall plate level. • Careful removal of members in tension must be done. • After careful demolition the site should be 'back bladed' to ensure there are no pieces of bricks or concrete is left on the site.

Methods used in demolition of various types of buildings Masonry and brick arches • Care should be taken as there is a serious possibility of structural failure. • Single span arches can be demolished by hand by carefully taking narrow strips, say 230mm wide, from the side parallel to the line of the edge of the bridge until only a norrow portion remains which can be easily be collapsed. • Other method is – Spandrel infilling is removed down to the springing line – The arch rings are lifted out and removed – the abutments are demolished.

Methods used in demolition of various types of buildings Steel frames and components

• Dismantling of roof trusses is done by the following procedure, – The structure must be carefully braced temporarily. – The roof claddings must be removed and lowered to ground. – The truss must be supported by a suitable crane or roped up. – If the span is more then the lifting might cause compressive load on the member therefore the member must be braced with heavy timber girders splinted along the component. – The truss must be lowered to the ground and dismantled.

Methods used in demolition of various types of buildings Bridges and gantries of steel construction

• The first stage of demolition for all the bridges is to remove the dead load of road metalling or railing tracks and ballast. • Cutting points must be carefully chosen so that the ends cannot slide off the abutments. • If crane lifting method is adapted then the entire structure can be lifted out to flat ground and then cut down to smaller parts.

Methods used in demolition of various types of buildings Reiforced Cast In-Situ Structures

• It is the most expensive sector of the demolition industry as the amount of salvageable materials is very low. • Knowing the details of placement of reinforcements in the component through drawings is key step. • Structural stability must be checked.

Methods used in demolition of various types of buildings Suspended Reinforced concrete slabs

• Reinforced concrete slabs should be demolished by cutting with compressor guns, hydraulic busters and oxy-acetylene equipment, parallel to the line of main reinforcement, thus reducing the slab to small sections. • When possible the cutting work must be carried out from an independant platform. • Strip cut away should be no more than 300mm wide at one time.

Methods used in demolition of various types of buildings Reinforced Concrete Beam • • •

All loads must be removed before demolishing the beams. Suppoting rope must be fixed for each beam. next the reinforcement must be cut at one end and that end lowered to the ground followed by the cutting the other end.

Reinforced concrete columns • • • •

After removing all the superimposed load, two wire ropes must be fixed onto the top of the column to provide support. Then the reinforcing bar can be exposed near the base of the column using compressor guns . These must be cut in the opposite side of the column from the rope which has been selected for pulling. Then the column is pulled over.

Methods used in demolition of various types of buildings Reinforced Concrete walls • Can be best dealt with by cutting them into manageable sections and then treating these as a series of columns. Structure made of post-tensioned units • In fact when the load is removed by demolition, the beams can fail in an upward direction due to the forces inherent in the highly tensioned tendon. • The following three methods have been postulated. – Reach the tendons and hols them, then cut through the concretre to formsmall sections and gradually allow the tendons to take up a shorter path. – Apply heat so that the tendons weaken and expand. This allows the concrete to be broken up progressively. – In some places the tendons are held up in ducts created by spaces between the top and bottom flanges and the tendons form a cured outline. In such cases we can break the ducts and gradually allow the tendons to rise and take a shorter path, thus relieving the tensile stress.