Earthquake Resistant Design Of Structures

BEHAVIOR OF MASONRY STRUCTURES

Dr. G. P. ChandradharaProfessor, Dept. of Civil EngineeringS. J. College of Engineering, Mysore

Email: chandu_gpc@yahoo.com

MasonryAdvantages

• No formwork

• Greater flexibility in terms of plan forms

• Plays a dual role – functional and structural

• Economy

• DurableDisadvantages

• Structurally very complex

• Brittle

• Heavy

• Too many variables!

Masonry is a composite construction consisting of:

Masonry units

Adobe (Sun dried mud blocks)

Stone, Laterite blocks

Burnt clay bricks

Concrete blocks (solid or hollow)

Calcium silicate bricks

Stabilized mud blocks (SMB)

Fly-ash gypsum blocks

Based on method employed in production, three varieties of burnt clay bricks are available in India viz.

Country brick

Table moulded brick

Wire-cut brick

Mortar

•Mud mortar•Lime sand mortar•Cement, lime, sand mortar•Cement sand mortar•Composite mortars( cement,lime,soil,sand and additives)

Reinforcement •Metallic•Non-metallic

Multi-storeyed Un-reinforced masonry tower, Tanjavur, India

S Raghunath, BMSCE, Bangalore 19

Un-reinforced masonry arches and aqueducts

(Ref: KS Jagadish and BP Sinha)

S Raghunath, BMSCE, Bangalore 19

Monadnock Building, Chicago, 1891

1.82m thick wall at basement!

(Ref: BP Sinha)

S Raghunath, BMSCE, Bangalore 19

16 storeyed load bearing masonry

(Ref.: BP Sinha)

S Raghunath, BMSCE, Bangalore 19

Masonry is

• Weaker than the brick

• Weaker than the mortar (?)

Example

• Block strength 40.0 N/mm2

• Mortar type H1, strength 8.0 N/mm2

• Masonry strength is only 3.05 N/mm2

S Raghunath, BMSCE, Bangalore 19

Typical “Masonry Prisms”

S Raghunath, BMSCE, Bangalore 19

Masonry Walls: Slenderness and Eccentricity

Stocky wall, eccentric loadReduced strength

eSlender wall, eccentric loadMuch more reduced strength

S Raghunath, BMSCE, Bangalore 19

Structural Walls have 3 functions:

1. Resist vertical compression

2. Resist out-of-plane bending from eccentric vertical loads (gravity loads), and/or transverse loads such as wind loads and earthquake loads

3. Resist in-plane forces (in-plane shear and in-plane bending) attracted by the masonry building as a whole

S Raghunath, BMSCE, Bangalore 19

Earthquake Protection

(Coburn and Spence, 2002)

Majority of Housing Buildings are made of Masonry and RC

Cross Wall

Shea

r W

all

Cross Wall

Earthquake Motion

Shea

r W

all

S Raghunath, BMSCE, Bangalore 19

Masonry Walls

TYPES OF DAMAGE DURING EARTHQUAKE

•Cracks between walls and floor•Cracks at corners and at wall intersections•Out-of –plane collapse of perimetral walls•Cracks in spandrel beams•Diagonal cracks in structural walls•Partial disintegration or collapse of walls•Partial or complete collapse of building

Figure below shows the deformation and typical damages suffered by a simple masonry building subjected earthquake ground motion.

Out-of-plane collapse of sandstone in lime mortar masonry wall (MORBI)

Combination of in-plane and out-of-plane failure

(Samakhyali)

Separation of corner column

from the neighbouring

masonry (SAMAKHYALI)

Out-of-plane failure of wall leading to collapse of lintel band (BHUJ)

Corner failure in presence of corner reinforcement

(BHUJ)

Collapse of walls between openings (KHAVDA)

Shear Cracks in an unreinforced brick masonry building from 1993 Kilari earthquake

Separation of wall at junctionBuckling of wythes

Separation of roof

Out-of-plane failure and buckling of wythes

Concepts for earthquake resistant masonry

Response of structures during earthquake depends on

1. Natural frequencies of the structure (which is dependent on Mass (M) and Stiffness (K)

2. Frequency content of earthquake

3. Amplitude of earthquake

4. Duration of earthquake

5. Ductility

6. Damping characteristics (energy dissipation capacity)

7. Structural integrity

STRUCTURAL DYNAMICS & MASONRY BUILDINGS

QUASI-STATIC RESONANT

INERTIAL

Quasi static behaviour: fundamental frequencyof building is below the range of frequencies in ground motion

Resonant behaviour: fundamental and other higherfrequencies of building are within the range offrequencies in ground motion

Inertial behaviour: fundamental frequency of buildingis above the range of ground frequencies

period range of masonry buildings

DESIGN PHILOSOPHY

TYPE OF EQ. CRITERION

MILD Frequent occurrenceNO STRUCTURAL DAMAGES ADMISSIBLE, NON-STRUCTURAL DAMAGES ALLOWED BUT REPAIRABLE

MODERATE May occur once during the life-time of a structureALLOW MINOR BUT REPAIRABLE STRUCTURAL DAMAGES

MAJOR Very rare possibilityMAJOR STRUCTURAL DAMAGE ALLOWED, BUT STRUCTURE SHOULD NOT COLLAPSE

Design Principles

Achieve strength and ductile behaviour

Maintain structural integrity

In relatively simple and cost effectivemanner!

Different ways of reinforcing masonry

1. Horizontal reinforcement

2. Vertical reinforcement

(a) Core reinforcement

(b) Containment reinforcement (Surface

reinforcement)

1. Horizontal reinforcement

• Typically through the bed joints of masonry

• Easy to provide

• Provides integrity – ties the cross walls and

shear walls

• Also known as seismic bands

• Improves strength and ductility in the

direction parallel-to-bed-joints

IS 4326 – 1993 (re-affirmed 1998), “Earthquake resistant design of buildings –codes of practice”, BIS, New Delhi

Different ways of reinforcing masonry

CONTAINMENT REINFORCEMENT

Ideal choice: Stainless steel flats and rods

Masonry building with horizontal bands and ‘Containment reinforcement’

Naliyawali Deewal (JHADAWAS, RAPAR Taluk)

Sill, lintel and roof band with vertical ‘containment reinforcement’ for SMB building (Bhuj)

CONCLUSIONS

Masonry buildings in mud/ lime mortar areprone to severe damage during theearthquakes due to poor bond strength. Roundstones in wythes without through bond stonescan further aggravate the problem. The failureof such masonry walls is due to out-of-planeflexure.

CONCLUSIONS Contd…

Use of roof and lintel band and verticalreinforcement in corners and junctions of wallsas suggested by IS:13828-1993 appears tohave prevented complete collapse of buildingbut does not seem to improve the ductility tothe desired levels

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