Seismic performance of Masonry Buildings

Dr. S. K. Prasad

Professor of Civil Engineering

S. J. College of Engineering, Mysore

prasad_s_k@hotmail.com

MASONRY Advantages

• No formwork

• Greater flexibility in terms of plan forms

• Plays a dual role – functional and structural

• Economy

• Durable

Disadvantages

• 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, 3 types 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

Earthquake Protection

(Coburn and Spence, 2002)

Majority of Housing Buildings are made of Masonry and RC

Grade Damage Description

1

Negligible to slight damage (no structural damage); Hair line cracks in few walls; fall of small pieces of plaster only; fall of loose stones from upper part of building in very few cases

2

Moderate damage (slight structural damage, moderate non-structural damage), Large and extensive cracks in many walls; fall of fairly large pieces of plaster; parts of chimneys fall down.

3

Substantial to heavy damage (moderate structural damage, heavy non-structural damage); large and extensive cracks in most walls; pan tiles or slates slip off; chimneys are broken at roof line; failure of individual non-structural elements.

4

Very heavy damage (heavy structural damage, very heavy non-structural damage);serious failure of walls; partial structural collapse.

5

Destruction (very heavy structural damage) ; Total or near total collapse.European macroseismic scale (EMS)

Typical damage during earthquake

1. Cracks between walls and floor2. Cracks at corners and at wall intersections3. Out-of –plane collapse of perimetral walls4. Cracks in spandrel beams5. Diagonal cracks in structural walls6. Partial disintegration or collapse of walls7. Partial or complete collapse of building

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

House with lintel band and columns (SAMAKHYALI)

Combination of in-plane and out-of-plane failure(Samakhyali)

Separation of corner column

from the neighbouring

masonry (SAMAKHYALI)

Stone masonry building in CM – minor damage(BHUJ)

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

Corner failure (BHUJ)

Collapse of walls between openings (KHAVDA)

Rigid box like behaviour above lintel band (BHACHAU)

Collapse of RC slab and Masonry (Morbi)

Shear cracks in stair-case room of a school building (Morbi)

Damage due to a combination of in-plane and out-of-plane vibration (Samakhyali)

Out-of-plane collapse of stair case room of a school building (Morbi)

Wall flexure – RC roof on stone-in-CM

(Lodhrani)

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

Summary of types of masonry building damages

Out-of-plane collapse and or shear failure

In-plane shear and or flexure failure

Separation of wall junctions

Failure of masonry pier in between openings

Local failures

Buckling of wythes

Separation of roof from walls

Out-of-plane failure in (URM) building Out-of-plane failure in building with band

Shear failure in(URM) building Failure of masonry pier between openings

Separation of wall at junction

Buckling of wythes

Separation of roof

Out-of-plane failure and buckling of wythes

Typical X-type crack

Out of plane flexural collapse

Concepts for earthquake resistant masonry

Response of structures to 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

02 MK

EIGEN values

Natural frequencies

EIGEN vectors

Mode shapes

tfxKxCxM

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 occurrence

NO STRUCTURAL DAMAGES ADMISSIBLE, NON-STRUCTURAL DAMAGES ALLOWED BUT REPAIRABLE

MODERATE May occur once during the life-time of a structure

ALLOW MINOR BUT REPAIRABLE STRUCTURAL DAMAGES

MAJOR Very rare possibility

MAJOR STRUCTURAL DAMAGE ALLOWED, BUT STRUCTURE SHOULD NOT COLLAPSE

Design Principles

Achieve strength and ductile behaviour

Maintain structural integrity

In relatively simple and cost effectivemanner!

Basic requirements

1. Keep the structural behaviour of the building assimple as possible

2. Avoid asymmetric buildings that could lead to hugeshear stresses due to torsion

3. Avoid stiffness and mass irregularities

4. Avoid re-entrant corners, provide seismic gaps

5. Avoid horizontal and vertical geometric discontinuity

6. Avoid out-of-plane offsets and non-parallel systems

Different ways of reinforcing masonry

Different ways of reinforcing masonry

Alternate Courses of QUETTA bond

BIS CODAL PROVISIONS: IS: 4326-1993

• HORIZONTAL RC BANDS AT LINTEL AND ROOF LEVELS

• VERTICAL STEEL AT CORNERS, JUNCTIONS AND DOOR & WINDOW JAMBS

CONTAINMENT REINFORCEMENT

• Should always be accompanied by horizontal RC bands

• Containment reinforcement is a vertical reinforcement provided on both faces in a parallel manner. It may be either on the surface or hidden in 3.0 cm grooves beneath the surface

• It is generally provided every 1.0m in the horizontal direction and also next to door and window jambs

• It is not needed at corners

(a) Un-reinforced (b) core-reinforced (c)Containment reinforcement

Performance of brittle masonry to flexure

CONTAINMENT REINFORCEMENT

Ideal choice: Stainless steel flats and rods

Containment reinforcement in grooved blocks

• Reinforcement on both faces to be held by ties going through the wall in alternate courses or once in 3 courses

• Following materials are possible GI wire – 3.0 to 4.0 mm

Corrosion resistant steel ~ 6.0mm

Stainless steel – 3.0 to 4.0 mm

Bamboo

Timber

• Function is to prevent growth of flexural cracks

• Experiments show good flexural ductility

Reinforcingmaterial

Remarks

Mild Steel rods 6mm rods available, very ductile, liable to corrosion if exposed and hence has to be eithercoated with non-corrosive paints or covered with plasterAlternatively 20-25mm wide, 3mm thick MS flats could also be used, holes could be madeat regular intervals to insert links/bolts to tie the flats provided on both faces of the wall

Galvanized Iron (GI) wires

Any dia wire available hence easy for handling, good ductility, can corrode and hencehas to be protectedAlternatively 20-25mm wide, 3mm thick GI flats could be used as mentioned above

Stainless Steel Perfectly suitable for containment reinforcement, 3mm to 4mm wires could be used at1.0m spacing, no need of coating, plastering etc.

Timber battens Good quality (teak wood, sal wood etc.) reepers of size 50mm x 25 mm could be used at1.0m spacing, the pair of reepers on either face of the wall could be tied together at twopoints at the base and two points at the top by boring a hole and inserting a bolt; surfacehas to be painted especially when exposed to wetness; has to be maintained regularly toprevent rotting; care to be taken to prevent it from catching fire

Bamboo Pairs of bamboo or half bamboos could be used at about 1.0m to 1.5m interval; the polescould be tied at two points at the base and two points at top by using GI wires; less life;can catch fire hence has to be protected

Ferrocement strips

Thin ferrocement strips (about 150mm wide) with sufficient amount of reinforcing materialsuch as chicken mesh, expanded metal, weld mesh etc.; can be used at about 1.2mspacing; the strips have to be bonded to the masonry wall by using grouted hooks.

Aluminum Wires, rods and flats are readily available, durable and has good resistance to corrosion,however strength and modulus is less and hence large quantity is needed

Materials for “Containment Reinforcement”

Comparison

Contained Masonry Confined concrete

Basic material is URM Basic material is reinforced concrete

Prevents growth of flexural tensile cracks

Prevents brittle failure of concrete in compression

Ductile behaviour with lot of cracking but no collapse

Ductile behaviour with few cracks inside confined core

Masonry building with horizontal bands and Containment reinforcement

Naliyawali Deewal (JHADAWAS, RAPAR Taluk)

Sill, lintel and roof band with vertical containment reinforcement for Masonry building (Bhuj)

Sill level RC band and vertical containment reinforcement

Plinth, sill, lintel and roof bands,

ferro-cement roof with containment

reinforcement

51

Horizontal bands in masonry building improves resistance to earthquake

Building with nohorizontal lintel bandCollapse of roof and walls

52

Building with horizontallintel bandNo Collapse

Performance of buildings with & without band

Bending and pulling in lintel bands

Cross section of lintel band

53

Bands must be capable of resisting the above forces

Horizontal bands in masonry buildings

54

RC Bands are the BEST

Different type of roofs

55

Flat Roof

Hipped Roof

Earthquake response of a hipped roof building

56

No vertical reinforcement is provided

57

Horizontal sliding at sill level – No vertical reinforcement

Vertical reinforcement prevents sliding in walls

Earthquake response of a flat roof building

Cracks in building

with no corner

reinforcement

58

No cracks in building

with vertical reinforcement

and bands

Cracks in corners of masonry buildings

Schematic of the wall section

Thick walls that split into two vertical layers

59

Separation of unconnected adjacent walls at junctions

Proper bond in stone masonry

Use of through stones or bond stones in stone masonry walls prevents wall separation

60

Horizontal lintel band is essential in random rubble stone masonry wall

CONCLUSIONS Masonry buildings in mud or lime mortar are

prone to severe damage during earthquake dueto poor bond strength.

Round stones in wythes without through bondstones can further aggravate the problem.

The major failure of masonry walls is due to out-of-plane flexure.

Use of roof band, lintel band and verticalreinforcement in corners & junctions of walls asper IS:13828-1993 appear to improve ductilityand prevent complete collapse of building.

THANK YOU