seismic resitant design of masonary structure-sgs

Structural Dynamics &

EarthquakeEarthquake Engineering-2722010

Dr. Siddharth G. ShahHead & Associate Professor

Faculty of PG studies & ResearchMEFGI-Rajkot

Earthquake resistant Earthquake resistant features of masonry features of masonry

structuresstructures

Lightness

Since the earthquake force is a function ofmass, the building shall be as light as possibleconsistent with structural safety and

functionalrequirements. Roofs and upper storeys ofbuildings, in particular, should be designed aslight as possible.

Continuity of Construction

As far as possible, the parts of thebuilding should be tied together in such amanner that the building acts as one unit.

Projecting and Suspended Parts

Projecting parts shall be avoided as far aspossible. If the projecting parts cannot beavoided, they shall be properly reinforced andfirmly tied to the main structure, and theirdesign shall be in accordance withIS 1893 : 1984

Building ConfigurationIn order to minimize torsion and stressConcentration, plan of building should be simple

geometric shape like square, rectangular or circle be symmetrical both with

respect to mass and rigidity so that the centersof mass and rigidity of the building coincidewith each other in which case no separationsections other than expansion joints arenecessary

Building Configuration

If symmetry of the structure is not possible in plan, elevation or mass, provision shall be made for torsional and other effects due to earthquake forces in the structural design or the parts of different rigidities may be separated through crumple sections. The

length of such building between separationsections shall not preferably exceed three timesthe width.

Building Configuration

As an alternative to separation section toreduce torsional moments, the centre of

rigidity of the building may be brought close or coincident to the centre of mass by adjusting the locations and/or sizes of

columns and walls.

Building Configuration

Buildings having plans with shapes like,L, T, E and Y shall preferably be separated

intorectangular parts by providing separationsections at appropriate places.

IS 13828:1993

Improving Earthquake Resistance of Low Strength Masonry Buildings-Guidelines

Two types of constructions are included herein, namely

a) Brick construction using weak mortar and

b) Random rubble and half-dressed stone masonry construction using different mortars such as clay mud lime-sand and cement sand.

 Precautions should be taken to keep the rain water away from soaking into the wall so that the mortar is not softened due to wetness. An effective way is to take out roof projections beyond the walls by about 500mm.

Use of water-proof plaster on outside face of walls will enhance the life of the building and maintain its strength at the time of earthquake as well.

Stone Masonry (Random Rubble or Half-Dressed)

The mortar should be cement-sand (1:6), lime sand (1:3) or clay mud of good quality.

The wall thickness ‘t’ should not be larger than 450mm. Preferably it should be about 350mm, and the stones on the inner and outer wythes should be interlocked with each other.

Height of the stone masonry walls should be restricted as follows, with storey height to be kept 3 m maximum, and span of walls between cross walls to be limited to 5 m.

For convenience of constructions, builders prefer to make a toothed joint, which is many times left hollow and weak.

To obtain full bond it is necessary to make a sloping (stepped) joint by making the corners first to a height of 600mm and then building the wall in between them otherwise, the toothed joint should be made in both the walls alternately in lifts of about 45cm

Typical detail of Masonry

1. Openings to be located away front the inside corner by a clear distance equal to at least 1/4 of the height of openings but not less than 60cm.

2. The total length of openings not to exceed 50 % of the length of the wall between consecutive cross walls in single storey construction, 42 % in two storey construction and 33 % in three storey buildings.

3. The horizontal distance (pier width) between two openings to be not less than 1/2. or the height of the shorter opening but not less than 60cm.

4. The vertical distance from an opening to an opening directly above it not to be less than 60cm nor less than 1/2 of the width of the smaller opening.

5. When the openings do not comply with requirements 1 to 4, they should either be boxed in reinforced concrete all around or reinforcing bars provided at the jambs through the Masonry.

Recommendation regarding openings in bearing walls

Size and positioning of openings

Strengthening measures for Low strength masonry buildings

General Recommendations

The height of the construction should be restricted to one storey of Category I and II buildings and two storeys of categories III and IV buildings. The height of a storey should be kept as low as 2.5m but not more than 3.5m.

The minimum wall thickness shall be one brick in one storey construction and one brick in top storey and 1 ½ brick in bottom storey of up to three storey construction. It should also not be less than 1/16 of the length of wall between two consecutive perpendicular walls.

The unsupported length of a wall between cross walls should be limited to 7m.

For longer walls, buttresses should be used at intermediate points not farther apart than 3m. The size of buttress may be kept as: thickness = top width = t and base width = h/6 where t. = thickness of wall and h = actual all height.

The mortar should be lime-sand (1:3) or clay mud of good quality. Where horizontal steel is used between courses, cement-sand mortar (1:3) should be used with thickness so as to cover the steel with 6mm mortar above and below it.

Openings should be as small and as centrally located as practicable.

GTU QUESTIONS1. Discuss in detail the advantage of horizontal

bands and vertical reinforcement in the masonry buildings.

2. Explain failures of masonry structures observed in past earthquakes & how will you improve performance of masonry building.

3. Earthquake resisting features of unreinforced brick masonry structure.

4. Explain the earthquake resistance feature of masonary structures