Thursday, 7 April 2016


                                         STRUCTURAL STABILITY


All buildings have to serve two important functions. They have to carry the loads (structural requirement) and provide livable environment to the occupants to enable them to be comfortable in the use of the building (functional requirement).

The building has to carry its own weight and external occupancy loads and those caused by the environment such as those caused due to wind or earthquake. The elements that bear these loads and transmit them to the foundation are termed as “structural elements”. A good performance of the structural elements such as beams and columns is of vital importance from the safety and stability points of view. Hence, they are designed and constructed with prescribed safety margins as per requirements specified in the Bureau of Indian Standard specifications. Their failure will cause cracking, yielding of steel and even ultimate collapse of the building.

The serviceability of a building depends on the good performance of the structural and the functional elements such as flooring, doors and windows, glass panels and partition walls. The functional elements are generally not intended to carry structural loads. Their failure to perform will only affect the use of the building and not its strength or stability. Their good performance is also equally important.

In a metropolitan city such as Chennai, most apartment buildings are made of reinforced concrete frames. In this type of construction, the structural elements comprise beams and columns which carry the loads. The partition walls and infill walls in these buildings are treated as non-structural and hence are not generally designed and constructed to carry structural loads. In such buildings, for good performance it is necessary to detail the junction structural and non-structural members with proper care. The deformation of the structural members under load should not be restricted by non-structural partitions or in-fills.

For good performance of both beams and partitions, a proper sequence of construction is essential. First building the infill or partition and that casting the concrete beam on top will make the dead weight of the concrete beam load the partition which is not correct. The partition is not supposed to carry the weight of the beam. Moreover, when the additional live load comes on the floor the deflection of the beam will crack the brick infill or partition. 

To avoid this problem, first the frame should be completed. This should be followed by the erection of partitions and in-fills. Care should be taken to see that a structural gap exists between the frame and the in-fill. For functional convenience this gap can be filled by inert flexible material such as thermocol or pith.

Under no circumstance a structural member should be cut or chased or damaged in any way for any purpose including for taking services like electrical conduits, concealed wiring etc. This will weaken the element and may cause its collapse leading to the collapse of the whole building. However the services may be taken through the non-structural partitions or infills.

This type of construction is undertaken in semi-urban and rural areas.  All the main walls carry loads and also serve to provide functional requirement. In these buildings, though the partition walls do not carry structural loads, they cannot be either built or demolished without due consideration for overall stability of the building. Some times their weight may be required to provide stability against overturning failure. The main walls invariably provide the structural stability and carry loads. Hence, they cannot be damaged or dismantled for carrying conduits for electricity, etc.

Bearing wall buildings though safe for vertical loads are weak under lateral loads. They are vulnerable for failure under out of plane bending during lateral loading. Therefore, to make the building act as one integral unit, they should be constructed with continuous plinth and continuous lintel beams. No brick pillar should be less than 600 mm in width. All openings should be reinforced on all sides. These simple precautions will ensure good performance of bearing wall buildings under lateral loads caused by wind or earthquakes.

In many sites the brick wall is first raised and then used as shuttering to cast the column or beam adjoining it. This will make the column or beam weak because the pores in brickwork will absorb the moisture from wet concrete making the w/c ration in the beam and column uncertain. The weakness will manifest in the cover concrete of the beam or column, leading to early onset of corrosion in the rebars.

The cracking in the walls of the buildings are many times attributed to following wrong constructions practices. It is important to pay attention to the details at the site of construction to ensure long and trouble free service of the infrastructure created. A conscious distinction should be made between the structural and non-structural items in a building and their requirements and performance. These defects when permitted during construction are difficult and costly to rectify later.

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