1 General

The case with which a structure can be erected and the speed of erection play an important role in determining the total cost of a building.

On expensive sites, speed of erection to permit early occupation of the completed building is the most important consideration and is sometimes the decisive factor in the choice of the structural system and the materials.

1.1 Precautions for obtaining accuracy of construction

- Setting out the positions of the foundations, walls and columns must be done accurately. The verticality of the walls and columns must be ensured. An optical plummet may be used for accurate vertical alignments.

- An important precaution that can be taken is adequate checking of dimensions. It is necessary to identify significant dimensions.

- Temporary supporting structures can be constructed with a high degree of rigidity.

- Different types of structures require different degrees of accuracy. E.g. structural steel that is normally fabricated off-site requires careful and accurate setting out procedures.

- Even in in-situ construction, some structures require high accuracies, e.g. in-situ beams cast to positions directly above window frames with no space for any adjustment.

- Some types of structures are more prone to deflection or their deflections are more obvious and therefore in in-situ construction, it might be appropriate to cast slabs with an upward camber so that they take up a horizontal position after deflecting under load.

- Accuracy is also desirable to reduce waste of materials. E.g., an increase of 3mm in 150mm thick slab is an increase of 2%.

1.2 Provisions for finishes

- In the construction of a building, it is necessary to keep in mind the requirement for finished surfaces in a given position.

This is achieved by adjusting the finish or the structure. E.g. slopes in slabs can be obtained by either using screeds or by varying structural thickness. If screeds is used, due to uneven drying out, cracking may result in the failure of the finishes.

- Top and bottom steps of stairs may be uneven due to not allowing adjustment in floor and stairs for relative thickness of the respective finishes.

2 REINFORCED CONCRETE BUILDINGS

2.1 Erection Procedure

- Analysis of formwork requirement, possible standardisation and use of systems.

- Decision as to appropriate pour sizes.

- Analysis of joint requirement and locations.

- Determination of order of erection.

- Provisions for other components of finishes.

- Precautions for obtaining accuracy of construction.

- Alternative construction techniques.

2.2 Construction Details

- formwork details; details for erection and stripping requirements, quality of concrete finish appropriate to applied finish etc.

- Joint details; for appropriate provision for transfer of stresses.

- Reinforcement details; position of reinforcement; method of assembly, adequate provision for placing concrete etc.

2.3 Safety of the structure during construction

- Normally in-situ reinforced concrete building might be considered to develop their stability along with the setting and hardening of concrete. However, this is not always so as some r.c. building achieve their stability through other components such as brick infill.

3 STEEL FRAME BUILDINGS

3.1 Erection Procedure

- The erection procedures vary according to the design method adopted for obtaining structural stability.

- Setting out and preparation of the base connections require accuracy, particularly where bolting or revetting are to be used. Generally holding down bolts are placed in the footing for connecting base of the column directly. For light construction where inaccurate positioning of holding down bolts has occurred, it may be possible to simply drill the footing to place new holding down bolts.

Accurate connection might not be possible where connections are made directly to steel members which are cast directly in bored piles; the resulting inaccuracies might require changes from bolting to welding.

- A typical method of erection would be to place shims to give correct height of base and then place the column on the holding down bolts. Nuts may be tightened to adjust the column for plumb.

an alternative method of obtaining stability and plumb is by the use of ties with adjustable turnbuckles.

It should be noted that in the process of erection, daily temperature changes can lead to inaccuracies in plumb.

- It is normal to erect columns and beams in conjunction with each other so that they assist each other in developing stability and to reduce the requirement for temporary bracing and ties.

- Ties may be used until permanent stability is obtained.

3.2 Connections

Connections between the different members of a steel frame can be classified as follows:

(i) compression connections in which the load is carried directly from one member to another and the connection serves mainly to fit the two members together;

(ii) shear connections;

(iii) tension connections

Connections are done by rivetting, bolting or welding.

- rivetting was done traditionally, but now not commonly used.

- bolting is done by using either black bolts, close tolerance bolts or high friction grip bolts.

- black bolts are made from round bar and not turned to precise diameter throughout and therefore cannot tight fit in their holes. Hence allowable stresses are low. Used mainly for secondary beams.

- close tolerance bolts are turned to be of constant diameter throughout and they fit tightly into the holes. Higher allowable stresses.

- friction grip bolts (torque controlled bolts).

- shop welding is done in factories where good quality control is possible.

- site welding is done in less than ideal conditions and hence close inspection is required to obtained adequate strength.

3.3 Coating

- Corrosion

- Fire protection.