09 ms 07

8/12/2019 09 ms 07

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Guided By:

Prof. D. G. Panchal

Prof. K. N. Sheth

Prepared By:

Pooja Mistry

M.Tech 1st sem

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Disadvantage:

Seismic resistance capacity is low compared

to RCC and steel.

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Dead Load

Live Load

Wind Load

Earthquake Load Snow Load

Most building codes and engineering practice standards

specify that wind and earthquake may be assumed never to

occur simultaneously.

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Earthquake is a vibration or oscillation of

the earth’s surface.

Earthquake forces on a structure are

considered a function of the mass andstiffness of the structure.

Earthquake force = f (m, k)

EARTHQUAKE

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Ground vibration

cause inertia

force.

These forces are

travel through

roof to walls and

thoroughfoundation.

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Material is brittle and its strength degradation due to

load repetition is severe.

Masonry has great weight because of thick walls.

Consequently the inertia forces are large. Quality of construction is not consistent because of

quality of the locally manufactured masonry units,

unskilled labor etc. that lead to large variability in

strength.

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A wall topples down

easily if pushed

horizontally at the top

in a direction

perpendicular to itsplane, it is called OUT-OF PLANE failure.

It offers much

resistance if pushed

along its length. It iscalled IN-PLANE failure.

BEHAVIOUR

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When ground shakes theinertia force causes the

small sized masonry piers to

disconnect from the

masonry above and below.

These masonry sub-unitsrock back and forth,

developing contact only at

the opposite diagonals.

Which crush the masonry at

the corners.

BEHAVIOUR

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BEHAVIOUR

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Determination of lateral load based on IS 1893 (part – I):2002

Distribution of lateral forces on the basis of flexibility ofdiaphragms

Determination of rigidity of shear wall by considering theopenings.

Determination of direct shear forces and torsi anal shearforces in shear walls.

Determination of increase in axial load in piers due tooverturning.

Check the stability of flexural wall for out-of-plane forces.

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Determination of lateral loads:

Most important lateral loads are EARTHQUAKE

loads. They are sudden, dynamic and can be of

immense intensity. Magnitude of lateral loads depends upon type of

soil, seismic zone and ground condition.

The design base shear is computed and then be

distributed along the height of the building. The design lateral force is obtained at each floor

level shall be distributed to individual lateral

load.

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Design seismic base shear:

Vb = Ah * W (cl-6.5.3)

Ah = Z*I*Sa / (2*R*g) (cl – 6.4)

Z= zone factor, I= importance factor, Sa/g =

avg. coefficient, R = Response reduction factor. W= seismic Weight of building.

The value of Ah for any structure T<= 0.1 sec.

Time period of building :

Ta=0.09*h/d^(1/2)

as per clause – 7.6.2

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Vertical distribution along the height of

the building as per clause – 7.7

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Distribution of lateral forces:

Earthquake forces, which originate in allelements of building, are delivered through thetransverse wall of building & it is bent between

floors. Lateral loads transmitted from these transverse

walls to side shear wall by horizontal floor androof diaphragms.

Diaphragms distribute these forces to verticalresisting components such as shear walls andvertical resisting elements.

The shear wall, which transfer the forces to thefoundation.

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The diaphragms must have adequate stiffness

and strength to transmit the forces. The distribution of lateral forces in masonry

will depend upon flexibility of horizontaldiaphragm.

Types of diaphragms : Rigid diaphragm : when its midpoint

displacement, under lateral load, is less thantwice the avg displacements at its ends.

Flexible diaphragm : when the midpointdisplacement, under load, exceeds twice theavg displacements of the ends supports.

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Determination of rigidity of shear wall:

Rigidity of shear wall depends on its

dimension, modulus of elasticity and modulus

of rigidity and support condition. Assumptions:

Points of contra flexure is assumed at the

mid points of the piers

Rotational deformations of the portionsabove & below the openings are much

smaller than those piers between the

openings are neglected.

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Cantilevered Pier or wall:

R = Em*t/(4*h/d)^3 + 3(h/d)) Fixed pier or wall :

R = Em*t/((h/d)^3 + 3(h/d))

Effect of aspect ratio on deflection due to shear

Aspect Ratio(h/d)

% deflection due to shear

Cantilevered wall Fixed end wall

0.25 92 98

1 43 5

2 16 43

4 5 16

8 1 4.3

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Determination of direct shearforces and torsional shear forces:

Direct shear forces: The lateralloads will distributed to wall inprportion to their relative stiffness.

R = ki / Σ k1+k2+…….kn

Vd = R*P

Torsional shear forces :When centreof mass and centre of rigidity do

not coincide, torsional shear forceswill be induced on the wall.

horizontal load P, will be at thecenter of mass, thus a torsionmoment Mt; is induced.

Mt = Py * ex

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Center of mass:

Xm = [ (Wr+Ws+Wn)*L/2+We*L]/ΣW

ΣW=Wr+Wn+Ws+We+WwYm = [(Wr+Ws+Wn)*B/2+We*B]/ΣW

W = weight of wall in respective direction

Center of rigidity: Xr = Re * L /(Rw + Re)

Yr = Rn * B / (Rn + Rs)

R = stiffness of wall in respective direction

Torsional eccentricity:

ex = Xm – Xr

ey = Ym – Yr.

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Determination of increase in axial load in piers

due to overturning :

The lateral load create severe overturning

moments in buildings. It induce high compression in walls that they

may increase axial load.

O t i t t 2nd

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Over turning moment at 2nd

floor

Movt2 = Vr * ( h2 + h3) + V3

* h2 Total over turning moment

Movt = Movt2 + total V *

Distance to the 2nd floor

from 1st storey. Povt = Movt * liAi / In

Where li = distance from CG

of wall section

= ΣliAi / Σai

Ai = c/s of wall

In = moment of inertia

= Σaili^2

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Building should not be slender.

Bricks should be stronger than mortar.(clause – 8.2.1.2 and 8.2.6 IS- 4326 : 1993)

Good interlocking at junctions and toothedjoint. (clause – 8.2.4)

wall thickness – not less than 1 brick (forsingle storey)

wall thickness – not less than one and halfbricks (up to three storey).

Horizontal reinforcement ( clause – 8.4.1)

Vertical reinforcement ( clause – 8.4.8 ).

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Bands are the

most important

earthquake

resistancefeatures.

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For safety in seismic zone reinforcing bars have to be embedded inbrick masonry at corners of all rooms and the side of the dooropenings.

Window opening more than 60 cm in width will also need suchreinforcing bars.

These vertical bars have to be started from foundation concrete andwill pass through all seismic bands where they will be tied to bandreinforcements using binding wires and embedded in to the ceiling.

REINFORCEMENTS

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