Seismic performance of Masonry Buildings - sjce.ac.in · Seismic performance of Masonry Buildings...
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Seismic performance of Masonry Buildings
Dr. S. K. Prasad
Professor of Civil Engineering
S. J. College of Engineering, Mysore
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
Collapse of RC slab and Masonry (Morbi)
Shear cracks in stair-case room of a school building (Morbi)
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
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
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
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
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
• 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
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
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.