A.ABOUT EARTHQUAKES
B. BUILDING RESPONSE
C. DESIGN PRINICIPLES
D. CONSTRUTION TECHNOLOGY
E. EPILOGUE
A:ABOUT EARTHQUAKES
GENESIS
Earth has four main layers
1. CRUST:
5-10 Km thk. bellow sea – Basalt type
& 35 -70 Km thk. bellow land mass –
Granite type
It is thinner than apple skin. There are
cracks in the crust dividing it in to 7
Major & 13 minor plates covering
globe.
MANTLE:
2900 Km thick, a mixture of molten
& plastic rocks – Magma having
Convectional Currents triggering
volcanic action and imparting
movement in Crustal plates.
3. OUTER CORE: 2200 km thk. having
liquid metal.
4. INNER CORE: 2900 km across
having solid Metal,
mainly iron.
Chains of volcanic ridges encircle the earth beneath its oceans.
Magma in ridges exudes through cracks in the old sea bed.
It then cools & hardens in to new crust.
Earth’s ever widening sea floors have clad it with tectonic plates.
These drifting slabs of crust perpetually collide, tug apart, or grate each other.
This interaction causes almost all the million or more quakes that occur each year.
The worst natural disaster in south-east Asian history at western java, has wrought havoc.
It has also driven home some key lessons in predicting and preparing for earth-quakes.
It has vividly confirmed a theory for forecasting quakes in “seismic gaps”.
These are zones at the boundaries between the jostling crustal plates where violent tremors are overdue.
It yielded the best record yet of the terrifying bounce and role of the ground in a mighty trembler (8.9) as well as the previously unheard devil dance of the Tsunami.
Drifting of these slabs caused by movement of magma currents below them & resultant interaction between them releases energy which is sometimes more than a thousand times as much energy as Hiroshima bomb.
Seismic waves, traveling in pulses spherically, at the speeds of 25,000 km / hour, fan out from epicenter & travel around globe.
Heaving earth which spills ocean water from its beaches, creates an ocean swell called tsunami, creating waist high serf & traveling at 1000 km / hour speeds for thousands of kilo meters, release their energy at shores of even distant lands.
There is also another most feared secondary effect of a quake – FIRE.
None of these disasters can be accurately predicted given our present knowledge.
But much of the appalling loss of life could have been avoided.
Prewarning mechanism
was non-existent.
Preparedness for the aftermath of a quake was absent.
We know for certain that such disasters will strike again ……………
and again ……………….
and that nations, especially those bordering the “RING OF FIRE” must learn to detect their approach & better prepare for their assaults.
Earth-quakes do not kill –
THE BUILDINGS DO.
Earth-quakes do not over run you –
THE TSUNAMIS DO.
Earth-quakes do not roast you –
THE FIRES DO.
B. BUILDING RESPONSE
There are 4 basic causes of earthquake induced damage:
ground shaking, ground failure, tsunamis & fire.
As a quake rumbles through a metropolitan area a kind of natural selection determines which building will survive.
1: A structure whose long axis is
parallel to ground motion will
sway less than another whose axis
is perpendicular.
2. When such buildings are joined a
rupture occurs.
3.Likewise a tower rising from a lower structure may fracture at the point of connection.
4.”Soft supports” such as in a building whose first story is composed of only tall columns can be the recipe for destruction.
5. A building can resonate with
frequency of ground waves. The
force applied at the bottom may
be multiplied As many as 5 times
at the top & result is destruction..
6. Building close to one another that
begin to sway may collide, a
phenomenon known as ‘pounding’.
As their masses are great, even a
slight touch can be disastrous.
7. Even though a building is strong
enough to survive ground
oscillations of a quake its innards
could become deadly shambles.
C: DESIGN PRINCIPLES
D: CONSTRUCTION TECHONOLOGY
A PRIMER FOR SURVIVAL
In designing for survival in quake-prone areas architects should pay more attention to local geology.
Solid rock foundations transmit seismic waves as short jolts.
Built on rock a relatively tall building can have extra reinforcement at structural joints.
This allows strength with stability
Uncompacted sub-strata transmit rocking motions similar to those produced by an ocean swell.
On soft ground a short, stiff building will not resonate to long ground waves. Diagonal members brace the structure, adding rigidity.
Buildings should be planned to make them less prone to collapse. The study of damages gives us the answer.
Buildings can be divided into two general categories –
Engineered and Non Engineered
Planning Aspects
Configuration & planning of any seismically sound building depends on many features like shape, size, scale, slenderness ratio, aspect ratio, stiffness, ductility and symmetry.
SHAPE
Simple shapes are called CONVEX because a line connecting any two points in the figure, always falls within the boundary.
A convex shape in Plan or Elevation promotes easy load transfer through direct path.
Simple shapes in Plan Simple shapes in Elevation
Complex shapes are called CONCAVE because a line connecting any two points in the figure, may fall outside the boundary. Complex or irregular shapes have re-entrant corners which are vulnerable due to concentration of stresses.
Complex shapes in Plan Complex shapes in Elev.
SIZE & SCALE
Taller a building is, greater is its vertical cantilever from ground. Transverse deflection may become a problem, especially in tall chimneys & high rise structures.
In long buildings, if spatial wavelength of seismic waves is lesser than plan dimension then it can induce distress. Ground motion reaches different parts of building at different times by different magnitudes & may break building in parts.
SLENDERNESS RATIO
Slenderness ratio which is height divided by least lateral dimension, & aspect ratio which is length divided by width, must be a small value to prevent over turning.
Thus a square building is unlikely to fall or topple. Unsupported floor spans decide how load will be transferred to its vertical elements.
FOOTPRINT of a Building
This is structural plan density, a percentage of total vertical elements like wall & column to its gross floor area. This factor is about 50% in older monumental buildings, where transfer of forces is through direct load path & works out to be about 3% in modern multistoried construction.
A structure with many columns and beams is safer because if a member fails, there is a redistribution of force. Hence a high level water storage tank on a concrete shaft is vulnerable because there is no redistribution available if the shaft fails.
STIFFNESS
Performance characteristics that determine superiority of a structural system are stiffness & strength. Stiffness refers to load required to cause deformation under elastic condition & strength refers to max. load it can carry or resist before failing.
Buildings with large stiffness have smaller periods of vibration. Buildings with large natural periods undergo excessive deformation, therefore, stiffness should be increased.
DUCTILITY
When a material fails only after considerable amount of plastic deformation, it is called ductile. Design & ductile detailing of load resisting system must be done so that it dissipates energy, & fails but does not collapse.
RESONANCE
It has been observed that if natural period of vibration coincides with that of the seismic wave, then resonance occurs. In Mexico city only building between five & fifteen stories height had collapsed due to excessive resonant vibration.
SOFT STOREY AND WEAK STOREY
These occur due to sudden change in strength & stiffness. Along building height if this happens, it creates a soft storey where plastic deformation occurs first & building columns collapse. This happens at ground floor parking area where open columns are left without infill panels or there is no continuation of shear walls from upper floors.
When a weak storey lies between two adjacent stronger stories, then it forms plastic hinges & pancakes between the two. Main problem is due to discontinuity in direct load paths, especially when shear walls are not logically continued down to foundations.
STRONG COLUMN WEAK BEAM PHILOSOPHY
In a weak column strong beam system, failure of columns is due to plastic hinges created in it. This is catastrophic to its vertical load carrying capacity & leads to collapse of building.
Whereas a strong column weak beam system has plastic hinges in beam, but not necessarily resulting in collapse of structure.
DIAPHRAGM ACTION OF FLOORS
In-plan rigidities of floor transfer forces to ground thro. its diaphragm action. Any recesses or cutouts in slab reduce effectiveness of this action.
Structural symmetry in plan & elevation
Center of mass & center of stiffness or center of resistance should coincide. If they are not concentric, a net moment couple is developed called Torsion.
SYMMETRICAL ASYMMETRICAL
PERIMETER STIFFNESS
Perimeter of building should be stiff. Variations like setbacks & openings cause torsion. Providing shear walls, reducing aspect ratio & designing moment resistant frames along perimeter are some solutions.
LOCATION OF CORE
Solid walls of service cores often render them very stiff. So location of lift wells & service shafts is very critical.
RE-ENTRANT CORNERS
An interior corner creates concentration of stresses & problems to transfer loads in plan. Vertical overhangs or inverted setbacks create large overturning moments because of an indirect path to load transfer to ground.
When vertical configuration is discontinuous, lower portion has high ductility resulting in failure & collapse.
SOLUTION
If architectural plan demands re-entrant corners then solution is: 1. Relieving stress at corners
through splays
2. Structural separation with gap,
expansion joints
Structural separation for attaining symmetry
3. Balancing stiffness by reducing
forces at notches
4. Tying two parts together with
bracing, connectors.
5. Providing additional structural
elements to stiffen critical portion
with shear walls.
IDEAL ASEISMIC BUILDING
Attributes of an ideal aseismic building are:
Small mass
Low height to base ratio
Low center of mass relative to ground
Balance lateral resistance
Direct load paths
Symmetrical plans
Uniform section and elevation
Uniform floor heights
Short spans
Maximum torsional resistance
Emphasis must be on Simplicity, Stiffness & Torsional Regularity
Urban areas exhibit rapidly increasing constructional activities, & inconspicuous rural activity has a vast expanse.
Most of prone population is in less developed areas & is ignorant of any effective measures & so constructive prevention is a purposeful strategy to avoid loss. Mitigation is always difficult, costly, time consuming & traumatic.
Proper construction techniques should be propagated to grass roots level. Stringent rules should be formulated by local authority & be strictly adhered to during implementation.
This can reduce irreparable loss of life & property.
Therefore constructive prevention is much more effective than any post disaster mitigation.
EPILOUGE
In designing Earth-quake resistant buildings cost escalation of up to 25% has to considered. In this case, every building cannot be made EQ resistant.
Priorities can be set up to distinguish important buildings such as Govt. offices, Hospitals, Schools & Colleges and Heritage buildings.
THANK YOU
