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A SEMINAR ON

UNDERSTANDING EARTHQUAKE EFFECTSON RC BUILDINGS

By- Mujawar Shoyeb S.

Guide- Prof. Miss. Jadhav R. S.

Contents:

Introduction

Effects of Earthquake on Structure

Mitigation of Earthquake on Structure

Conclusion

References INTRODUCTIONIntroduction:

Earthquakes pose great threat to life and property from several decades.

RC frame construction is frequently used in regions of high seismic risk.

Special care is required to understand the challenges that earthquakes pose and ensure that appropriate features are incorporated in the architectural and structural design.

It is difficult to predict exact behaviour of structure and damage during earthquake.

EFFECTS OF EARTHQUAKE ON STRUCTUREShape Of Building: Avoid buildings with re-entrant corners. Buildings with simple geometry offer direct load paths for the inertia forces.

Recommendation:

Provide separation joint so that portions of the building are free to vibrate independently .

Pounding effect to be considered.Vertical Irregularities (Geometric): Buildings with vertical setbacks.

Buildings having fewer columns, walls in particular storey or tall column building, causes soft storey failure of that storey.

Recommendation:

Avoid hanging or floatingColumns.Structure is regular if set-back at any floor is not greater than 20% of the dimension of the plan below.Torsion:

Buildings with irregular shapes ,unsymmetrical distribution of lateral resisting elements cause building to twist.

Recommendation:

Buildings have symmetry in plan. If this twist cannot be avoided special calculations need to be done to account for this additional shear forces in the design of buildings (provided in IS 1893-2002) 8

Pounding:

When two buildings are too close to each other, they may collide during strong shaking this effect is known as pounding. The pounding effect is more pronounced in taller buildings. When building heights do not match, the roof of shorter building may pound at mid-height of the columns in the taller buildings and lead to storey collapse.

Recommendation:

Give sufficient spacing between two buildings.Short Column Behaviour:

Building on sloping ground.Buildings having mezzanine floor.Partially filled infills : Movement of lower portion of column is partially restrained by stiff walls causing shear failure (X cracking).

A classic example of short column failure (X cracking)

Recommendation:

Provide special confining reinforcement (i.e closely spaced ties) over full height of columns that are likely to sustain short column effect according to IS: 13920-1993.

Retrofitting of existing short column.Buildings With Masonry Infills:

Misconception.The frame structures are flexible and ductile, while unreinforced masonry is very stiff and fragile and may explode under the effect of only small deformations. Initially during earthquake the masonry walls carry the earthquake forces, but as shaking intensifies it causes damage of infills resulting in sudden reduction in stiffness of floor.This leads to joint damage which can occur at any floor level leading to soft storey mechanism.

Recommendation:

Isolate the infills from the frame(must be designed as ductile frames).Integrate the infills into the frame(must be designed as infilled ductile frames).

Masonary walls integrated with building frame by anchors and practical column.Soft & weak stories:

An open ground storey is a (IS 1893-2002)

Soft ground storey when the movement at the ground storey level is much larger than the stories above.

Weak ground storey when the total horizontal earthquake force resisted at the ground storey level is less than the stories above.

Recommendations:

Provide some shear walls at the open storey.

In absence of non-linear dynamic analysis the ground storey should be designed for 2.5 times the storey shear and moment obtained from the analysis of bare frame.

Also increase the size of beam and column to make the stiffness of the ground storey comparable with that of upper stories.MITIGATION OF EARTHQUAKE EFFECTSSeismic Design Philosophy:

Under minor (5-6richter scale)but frequent shaking, the main members of the building that carry vertical and horizontal forces should not be damaged; however building parts that do not carry load may sustain repairable damage.

Under moderate (up to 8)but occasional shaking, the main members may sustain repairable damage, while the other parts of the building may be damaged such that they may even have to be replaced after the earthquake.Under strong (above 9)but rare shaking, the main members may sustain severe (even irreparable) damage, but the building should not collapse.

Strong Column Weak Beam Behaviour:

Strong Beam weak column : Brittle failure leading to sudden collapse. Columns suffer severe local damage at the top and bottom of a particular storey.

Weak Beam strong column:Ductile failure .Ductile structures absorb earthquake energy through damage in beams thereby preventing sudden collapse under major earthquakes.

CONCLUSIONConclusion:

More care is necessary at the time of planning itself.

The design of the buildings should be based on seismic codes IS 1893 (Part 1):2002 and IS 13920:1993.

There should be a greater emphasis on the quality of construction.

Structures should be as light as possible.The height of the building should not be more than four times the width .

Provide ductility to the structure .Mass, strength, stiffness and ductility should be distributed evenly to avoid formation of soft stories.

Education and awareness about earthquake science,safety measures to be taken during occurance of earthquake.

REFERENCES

References:

Earthquake Tips by C.V.R.Murthy

At Risk The Seismic performance of Reinforced Concrete Frame Buildings With Masonry Infill Walls by C.V.R.Murthy, Svetlana Brzev, Heidi Faison, Craig D. Comartin, Ayhan Irfanoglu.

A manual of Earthquake resistant buildings by Heinz Fricfk & Tri Hesti Mulyani.

Conceptual investigation of short columns and masonry infill frames effect in the earthquakes by Ebrahim Khalilzadeh vahidi & Maryam Mokhtari Malekabadi.

Earthquake Hazards & Basic Concepts Of Seismic Resistant Design Of Structures by Ir.Ngu Kik Koong & Ir.Loew Ket Won.

IS 1893(Part 1):2002 &IS 13920:1993.

A Step Towards Earthquake Resistant Construction by Bhumika B. Mehta.

THANK YOU & HAVE A SAFE EARTHQUAKE