EARTH QUAKE RESISTANT

STRUCTURES

J RAHUL VENKAT 11011D2004

IntroductionThe worst of all natural disasters,

earthquake have become more pronounced and have claimed a large number of lives from the start of the history. This is a subject of growing concern among civil engineers and architects.

This is important because it is not the earthquake that kills the people but it is the buildings they live in.

This paper involves the various techniques that can be adapted to make the buildings more resistant to the earthquakes.

Earthquake in General & Analysis

Earthquake and Seismic Energy:◦ Earthquake is a violent shaking of the Earth when large

Elastic Strain Energy released spreads out through seismic waves and through the body and along the surface of the Earth

Earthquake Forces Analysis:◦ Determination of design earthquake forces is computed

as follows: Equivalent static lateral loading Dynamic Analysis

The basic principle of design of earthquake resistant structures is by◦ Enhancing the ductility (rotation capacity) of the

structural members.◦ Increasing the energy dissipation capacity of the

structure

Dynamic AnalysisDynamic analysis may be

performed either by the Time History Method or by the Response Spectrum Method

Time history method of analysis is performed using accepted principles of dynamics based on the appropriate ground motion. Response spectrum method of analysis is performed using a site-specific design spectrum value

General Design PhenomenonBeam Column Effect:

◦Moving for higher zones strong column and weak beam design proves better. Since damage of beam will cause localized effect but whereas when a column damages it leads to entire structural damage named globalize damage

Inverse Pendulum Effect:

◦ It is analysed to have more flexibility hence too weak to carry earthquake force due to its collapse at ground storey and this effect termed as inverse pendulum effect.

◦ Open ground storey buildings are inherently poor systems. In the current practice, stiff masonry walls are avoided and bare frames are considered in design calculations. In practical, steel sections will be raised as vertical reinforcement and hollow blocks will be hoisted as partitions. Thus, the inverted pendulum effect is not captured in design

Short Column

◦ During past earthquakes, reinforced concrete (RC) frame buildings that have columns of different heights within one storey, suffered more damage in the shorter columns as compared to taller columns in the same storey.

◦ Poor behavior of short columns is due to the fact that in an earthquake, a tall column and a short column of same cross-section move horizontally by same amount.

◦ Stiffness of a column means resistance to deformation – the larger is the stiffness, larger is the force required to deform it. This behavior is called into the columns vertically above. Short Column Effect. As per Indian Standard the reinforcement must extend beyond the short column

Basic Structural Concepts

Extra attention to analysis and details is not likely to improve significantly the performance of a poorly conceived structural system

In precise it is to have a check on ◦Control over mass and stiffness, ◦Continuity in load transfer ◦Regularity of the system ◦Redundancy of the structural element◦Damping of the building

Lateral Resisting System

Horizontal DiaphragmsThese are horizontal resistance elements, generally

floors and roofs that transfer the lateral forces between the vertical resistance elements (shear walls or frames).

Shear Wall:Shear walls are vertical walls that are designed to

receive lateral forces from diaphragms and transmit them to the ground

In a simple building with shear walls at each end, ground motion enters the building and creates inertial forces that move the floor diaphragms. This movement is resisted by the shear walls and the forces are transmitted back down to the foundation.

Moment resisting frames

◦When seismic resistance is provided by moment resistant frames, lateral forces are resisted primarily by the joints between columns and beams.

◦At joints, to avoid damage as shown below we can go for the anchorage of the bars at the ends and to avoid congestion micro concreting can be done.

Recent trends in Seismic Design

Concrete Repair –Epoxy Resin Injection

Used for the reuse of damaged concrete

Plot Placing Epoxy Injection

Hidden Beams

◦ Beams which have their depth equal to that of the slab

◦These beams are designed for negative bending moment caused due to load reversal during earthquake

◦When provided along longer span, load carrying capacity increase to 135% with an economical increase of just 0.4 – 0.5%

Pre Tensioning Technique

◦In case of domes and shell structures, the lateral thrust experienced will be more. This fault is answered well by pre tensioned concrete. In case of huge structures like nuclear rectors, large spanning domes we will be having a thin walled cylindrical tube of diameter about 10 to 15 cm and steel rods will be packed tightly

Techniques to Adopt on Sky Scrappers1. Rubber Bearings:

◦Rubber bearings are made from layers of rubber with thin steel plates between them, and a thick steel plate on the top and bottom. The bearings are placed between the bottom of a building and its foundation

2. Viscous Dampers:◦They consist of a closed cylinder

containing a viscous fluid and a piston having small holes in its head. As the piston move in and out of the cylinder oil is forced in and out causing friction

3. Friction Dampers:◦The damper is made up from a set of

steel plates, with slotted holes in them, and they are bolted together. At high enough forces, the plates can slide over each other creating friction causing energy dissipation

4. Cross Bracings

◦In this the entire building will be laid in a cross horizontal bracing rather than placing it directly on foundation. It will distribute the load to joints and through foundation finally.

REFERENCES

Disaster Management – S L Goel

Design of reinforced concrete elements – P C Varghese

Abrahamson, N. and Silva, W. (2008), Summary of the Abrahamson and Silva NGA

ground--motion relations, Earthquake Spectra, 24(1), pp. 67-97.

2. ADB-WB. (2005), Pakistan 2005 earthquake: Preliminary damage and needs

assessment, Technical Document, Asian Development Bank and World Bank,

Islamabad, Pakistan.

3. Ahmad, N. (2011), Seismic risk assessment and loss estimation of building stock of

Pakistan”, PhD Thesis, ROSE School-IUSS Pavia, Pavia, Italy.

4. Ahmad, N., Crowley, H., Pinho, R. and Ali, Q. (2010), Displacement-based

earthquake loss assessment of masonry buildings in Mansehra city, Pakistan, Journal

of Earthquake Engineering, 14(S1), pp. 1-37.

5. Ahmad, N., Ali, Q., Ashraf, M., Naeem, K. and Alam, B. (2011), Seismic structural

design codes evolution in Pakistan and critical investigation of masonry structures for

seismic design recommendations, International Journal of Civil, Structural,

Environmental and Infrastructure Engineering Research and Development, 1(1), pp.

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