SEISMIC ANALYSIS - Advances and Trends in RCC Building
Updated: Jul 27, 2021
In India, due to urbanization people are facing problems of land scarcity, cost of land. Several high rise multistory framed structures have been constructed nowadays. Due to earthquake forces, a large amount of shear force, bending moment and torsion moment are generated in buildings. So, it is mandatory to analyze of structure for all possible conditions.
To capture the real behaviour of Multistory Framed Structures with Different Soil Conditions and Different Seismic Zones, a total of 12 models was considered of three different soils types i.e., hard, medium and soft and four seismic zones II, III, IV & V are taken respectively.
These models are Zone II (hard, medium, soft), Zone III (hard, medium, soft), Zone IV (hard, medium, soft) and Zone V (hard, medium, soft). The Bureau of Indian Standards has classified regions in India into those four seismic zones on the basis of historical seismic activity. According to the Modified Mercalli scale, the seismic zone intensity are classified as zone II (low-intensity zone), zone III (moderate intensity zone), zone IV (severe intensity zone) and zone V (very severe intensity zone).
The models were analyzed in ETABS Software as per IS 1893:2016. After modelling and analysis, the result was obtained and compared concerning parameters like base shear, top storey displacement, storey drift, time period, bending moment and shear force in beam and column.
It is observed that the base shear, top storey displacement and storey drift is maximum in soft soil and minimum in hard soil. And it is decreased up to 40% and 19% in hard soil and medium soil when compared to soft soil in all zones. It is also observed that zone V has maximum base shear value, top storey displacement, and storey drift. And it is decreased up to 72%, 56%, and 33% in zones II, III and IV compared to zone V in all soil types.
Base shear is an estimate of the maximum expected lateral force that will occur due to seismic ground motion at the base of a structure. Base shear is directly proportional to zone factor (Z) and Sa/g ratio, when zone factor and Sa/g is maximum it increases the value of Ah (Design Horizontal Acceleration Spectrum value) which ultimately results in an increment of Base Shear (Vb). It mainly depends on the soil conditions at the site. Base Shear (Vb) = Ah x W (where Ah = ( Z/2)x (I/R) x (Sa/g)). Zone factor is 0.36, 0.24, 0.16 and 0.10 for zone V, IV, III, and II. And Sa/g in the formula is the Spectral acceleration due to the earth’s gravity.
Top storey displacement is the lateral displacement of the storey relative to the base. The permissible limit for roof displacement is H/500, where H – the height of the building from the base. Storey drift is the lateral displacement of one floor to the floor above or below it.
The storey drift in any storey shall not exceed 0.004 times the storey height. Maximum drift permitted = 0.004 x3= 0.012 m.
It is required to design structure according to IS 1893:2016 and should provide ductile detailing reinforcement according to IS 13920:2016, so the structure will affect minimum in the case of an earthquake.
- Aparna Rath
- Komal Singh
- Dimpal Patel
- Abdul Shakur
Mr. Sanjay Kumar Rachcha
Civil Engineering Department,
Laxmi Institute of Technology, Sarigam