Sunday, January 23, 2011

Structural defects

A building needs design based on two methods which are Ultimate Limit State and Serviceability Limit State. When the new building that designed non-compliance with Building Code or called as British Standard (BS), it will lead to the structural defects. Defective construction not only contributes to final cost of product but also to the cost of maintenance, which can be substantial. Hence, defective of construction may lead to the failure of structure or structural defects.

What is structural defect?

'Structural defect' means any defect in a structural element of a building that is attributes to defective design, defective or faulty workmanship or defective materials or any combination of above and that:

(a)results in, or is likely to result in, the building or any part of the building being required by or under any law to be closed or prohibited from being used, or

(b)prevents, or is likely to prevent, the continued practical use of the building or any part of the building, or

(c)results in, or is likely to result in:
(i) the destruction of the building or any part of the building, or
(ii) physical damage to the building or any part of the building, or
(d) results in, or is likely to result in, a threat of imminent collapse that may reasonably be considered to cause destruction of the building or physical damage to the building or any part of the building

Structural defects means structure of building failure in physical state such as:
(a) cracking

(b) corrosion

(c) buckling

(d) carbonation
Carbonation occurs when carbon dioxide from the air penetrates the concrete and reacts with hydroxides, such as calcium hydroxide, to form carbonates. Carbonation of concrete lowers the amount of chloride ions needed to promote corrosion.

(e) sway

(f) settlement and so on 


The loads are broadly classified as:
(a) vertical loads -dead load, live load and impact load
(b) horizontal loads - wind load and earthquake load
(c) longitudinal loads - tractive and braking forces are considered in special case of design of bridges, gantry girders etc.

1. Dead load:

Dead loads are permanent  loads which are transferred to structure throughout the life span. Dead load is primarily due to self weight of structural members, permanent partition walls, fixed permanent equipments and weight of different materials.

2. Imposed loads or live loads:

Live loads are either movable or moving loads with out any acceleration or impact. There are assumed to be produced by the intended use or occupancy of the building including weights of movable partitions or furniture etc. The floor slabs have to be designed to carry either uniformly distributed loads or concentrated loads whichever produce greater stresses in the part under consideration. Since it is unlikely that any one particular time all floors will not be simultaneously carrying maximum loading, the code permits some reduction in imposed loads in designing columns, load bearing walls, piers supports and foundations.

3. Impact loads:

Impact load is caused by vibration or impact or acceleration. Thus, impact load is equal to imposed load incremented by some percentage called impact factor or impact allowance depending upon the intensity of impact.

4. Wind loads:

Wind load is primarily horizontal load caused by the movement of air relative to earth. Wind load is required to be considered in design especially when the heath of the building exceeds two times the dimensions transverse to the exposed wind surface.
For low rise building say up to four to five storeys, the wind load is not critical because the moment of resistance provided by the continuity of floor system to column connection and walls provided between columns are sufficient to accommodate the effect of these forces.  Further in limit state method the factor for design load is reduced to 1.2 (DL+LL+WL) when wind is considered as against the factor of 1.5(DL+LL) when wind is not considered. IS 1893 (part 3) code book is to be used for design purpose.

5. Earthquake load:

Horizontal earthquakes forces ( back-and-forth shaking ) create 'whipping' forces in all parts of a building. These forces must transfer between parts of building to the foundation.

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