Methods of Demolition

Methods of Demolition

• Hand demolition. This involves progressive demolition of a structure by operatives using hand-held tools.
• Pusher arm demolition. This is progressive demolition using a machine fitted with a steel pusher arm exerting a horizontal thrust on to the building fabric.

• Deliberate collapse demolition. Involves the removal of key structural members causing complete collapse of the whole or part of the building.

• Demolition ball techniques. This is carried out by swinging a weight or demolition ball suspended from a crane against the fabric of the structure. This can be by vertical drop, swinging in line with the jib or slewing jib.

• Wire rope pulling demolition. Involves the use steel wire ropes on which pulling tension is gradually applied.

• Demolition by explosives. Involves charges of explosives placed within the fabric of the structure and detonated to cause partial or complete collapse. It should be carried out with the advice and supervision of an expert.

Leveling sloping sites

Sloping sites should be levelled before commencing any construction work. There are three methods that can be employed to level slopes and these include:

• Cut and fill: This is where soil cut from the higher section of the sloping site is used to level the lower section there by leveling the site. It’s the most common method because if properly carried out, the amount of cut will equal to the amount of fill.

• Cut: In this method soil is cut and then ferried away so as to level the site. This method has the advantage of giving undisturbed soils over the whole site but also has the disadvantage of the cost of removing the soil from the site.

Fill: This is where soil is ferried from somewhere else and is filled on the site so as to level the slope. This method is not recommended because if the building is sited on the filled area there would be a risk of settlement at a later stage.

Storage of building materials

The type of storage facilities required for any particular material will depend upon the following factors:

1. Durability
2. Vulnerability to damage
3. Vulnerability to theft

The following are some of the building materials and how they can be stored on a building site.

• Cement and lime

These require a dry store free of draughts, which can bring in moist air. These should not be stored for long periods on site; therefore provision should be for rotational use so that the material being used comes from the old stock.

• Aggregates (sand)

These require a clean firm base to ensure that foreign matter is not included when extracting materials from the base of the stockpile. Different materials and grades must be kept separately so that the ultimate mix batches are consistent in quality and texture.

• Brick and blocks

Should be stacked in stable piles on a level and well-drained surface. Facing bricks and light-coloured bricks should be covered with tarpaulin or polythene sheeting to prevent them from discolouring by atmospheric pollution and adverse weather conditions.

• Roof tiles

These should be stacked on edge and in pairs, head to tail, to give protection to the nibs. This is because tiles have a greater resistance to load when it is imposed on the edge.

• Timber

This should be stored on a rack of tubular scaffold with a sheet roof covering to enable its moisture content remain constant. This is because timber is hygroscopic.

• Ironmongery, hand tools and paints.

These are the most vulnerable items on a building site and therefore should be kept in a locked hut/store and only issued against an authorized stores requisition.

Setting-out

This is the process of positioning a building on site. See fig 2.2a.

The stages involved in the above process are as follows;

• The first task is to establish the baseline or building line from which the rest of the building can be setout. This line is determined by the highway authorities and in urban areas it‟s approximately 8 meters from the back of the public path.

• The next step is to position the front of the building on the baseline by checking the dimensions between of the new building and the side boundaries.
• Flank walls are then set-out at right angles to the baseline often using the large builder‟s timber square or the 3:4:5 triangle (Pythagoras theorem).The builder‟s square is a right-angled triangular timber frame with sides varying in length which is used by placing it against the baseline while two pegs are driven in on the return side. By sighting across the two pegs, a third peg can be driven in the same straight line and thereafter a bricklayer‟s line can be stretched between them. If the builder‟s square is not available, a right angle can be set out based on the right-angled triangle whose sides are in the ratio of 3:4:5 (derived from the Pythagoras theorem). A peg is first driven in at the corner of the building and a distance of 3m is measured back along the baseline. A peg is driven in at this point and the ring of the measuring tape is placed over a nail driven into the top of the peg. The tape is held at the 12m mark (3+4+5=12) against the ring on the first peg and with the tape around the corner peg, the tape is stretched out to give the position of the third peg at the 7m mark. The line extended through the third peg is at right angle to the baseline.
• A check should now be made of the setting-out lines for right angles and correct lengths. This can be done by using the site square or diagonal checks as shown in fig 2.2a
• To secure permanent line markers, profile boards (see fig 2.2b) are established at corners of the building and at wall intersections. Nails or saw cuts on the profile boards demarcate the width of walls and also locate the position of the foundation trench.

The relevant tools for the work of clearing out are;

1. A steel tape for measuring
2. A large timber builders square used in corners
3. A saw for cutting profile boards
4. A hammer for nailing.