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Configurational Problems in

PreliminaryDesign of Structural System

Structural configuration system has played a vital role in catastropheduring earthquake

IS 1893 (Part 1): 2002 recommended configuration systems in Section 7for the better performance of RC buildings during earthquakes.

Building configurations - regular or irregular

Regular building configurations are almost symmetrical (in plan andelevation) about the axes and have uniform distribution of the lateral force-resisting structures such that, it provides a continuous load path for both

gravity and lateral loads

A building that lacks symmetry and has discontinuity in geometry, mass, orload resisting elements is called irregular. These irregularities may causeinterruption of force flow and stress concentrations. Asymmetricalarrangements of mass and stiffness elements may cause a large torsionalforce (where the centre of mass does not coincide with the centre of rigidity)

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Irregularities in Structural Systems

Section 7 of IS 1893 (Part 1): 2002 enlists the irregularity in buildingconfiguration system

These irregularities are categorised in two types

Vertical irregularities referring to sudden change of strength, stiffness,geometry and mass results in irregular distribution of forces and/or

deformation over the height of building and

Horizontal irregularities which refer to asymmetrical plan shapes (e.g. L-, T-,U-, F-) or discontinuities in the horizontal resisting elements (diaphragms) suchas cut-outs, large openings, re-entrant corners and other abrupt changes

resulting in torsion, diaphragm deformations, stress concentration

Note: There are numerous examples enlisted in the damage report ofpast earthquakes in which the cause of failure of multi-storied reinforced

concrete buildings is irregularities in configurations systemof 28 3


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Vertical Irregularities

Vertical Discontinuities in Load Path/ Load Transfer

Load path: earthquake forces, which originate in all the elements ofthe building, are delivered through structural connections to horizontal

diaphragms. The diaphragms distribute these forces to vertical

resisting components such as columns, shear walls, frames and other

vertical elements in the structural system which transfer the forces into

the foundation. The diaphragms must have adequate stiffness to

transmitting these forces.

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Vertical Irregularities

Seismic forces on the elements ofshear wall building system

Load path:

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Examples of Failure due to Vertical Irregularities

Floating box construction in residentialbuilding in Ahemedabad, India

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Discontinuous shear wall

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Irregularity in Strength and Stiffness: Weak

and Soft Storey

A weak storey is defined as one in which the storeys lateral strength is

less than 80 percent of that in the storey above

A soft storey is one in which the lateral stiffness is less than 70% of that

in the storey immediately above or less than 80% of the combined stiffnessof the three stories above,

This discontinuity is caused by of lesser strength, or increased flexibility,

the structure results in extreme deflections in the first storey of the

structure, which, in turn results in concentration of forces at the secondstorey connections. The result is a concentration of inelastic action.

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Stiffness irregularities - soft storey

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(a) Design earthquake spectral acceleration

(Sa) versus period (Tn)

(a)

Stiffness irregularities - soft storey

(b)

(b) Design earthquake spectral displacement

(Sd) versus time period (Tn)

It is also recognised that this type of failure results from the combination of

several other unfavourable reasons, such as torsion, excessive mass on

upper floors, P-( effects and lack of ductility in the bottom storey.of 28 10


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Examples of Failure due to Irregularity in Strength

and Stiffness

(a) Apollo Apartment at Ahmedabad, ground

floor was completely collapsed

(a)

Soft storey failures in reinforcedconcrete buildings

(b)

(b) A weakstorey mechanism developed in thebottom storey of five storey building under construction during Kocaeli, Turkey earthquake,1999

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Mass Irregularities

Mass irregularities are considered to exist where the effective mass of anystorey is more than 200% of the effective mass of an adjacent storey

Mass irregularity in building

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Mass Irregularities

Excess mass can lead to increase in lateral inertial forces, reduced ductility

of vertical load resisting elements, and increased tendency towards

collapse due to P-delta effect

Irregularity of mass distribution in vertical and horizontal planes can result

in irregular responses and complex dynamics

The characteristic-swaying mode of a building during an earthquake

implies that masses placed in the upper stories of building produce

considerably more unfavourable effects than masses placed lower down

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Mass Irregularities

The centre of gravity of lateral forces is shifted above the base in the case

of heavy masses in upper floors resulting in large bending moments

M

assive roofs and heavy plant rooms at high level are therefore to bediscouraged where possible

Where mass irregularities exist, check the lateral-force resisting elements

using a dynamic analysis for a more realistic lateral load distribution of the

base shear

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Examples of Failure due to Mass Irregularities

Failure in reinforced concrete buildings due to structural irregularity: Totalcollapse of half portion of A-Block ofMansi Complex

Mass irregularity in building

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Vertical Geometric Irregularity

Vertical Geometric Irregularity

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Examples of Failure due to Vertical Geometric

Irregularity

Failure of a setback building along aplane of weakness in Kobe earthquake,1995

The general solution of a setback problem is total seismic separation in plan

through separation section, so that portions of the building are free to vibrate

independently. When the building is not separated, check the lateral-force-

resisting elements using a dynamic analysis.

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Proximity of Adjacent Building

Pounding damage caused by hitting each other of two buildings

constructed in close proximity

Pounding may result in irregular response of adjacent buildings of different

heights due to different dynamic characteristics.

(a) Anand building, Bhuj, damage resultingfrom pounding in Bhuj earthquake, 2001

(a) (b)

(b) Pounding between a six storey building anda two storey building in Kocaeli, Turkey

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Plan Configuration Problems

The lateral-force-resisting elements should be a well-balanced system that

is not subjected to significant torsion. Significant torsion will be taken as

any condition where the distance between the storey centre of rigidity and

storey centre of mass is greater than 20% of the width of the structure ineither major plan dimension..

Torsion irregularities with stiff diaphragm

Torsion Irregularities

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Examples of Failure due to Torsional Irregularities

(a) Unbalanced location of perimeter wall

leading to severe to torsional forces and

near collapse in Alaska earthquake, 1964

(a) (b)

(b) Torsional collapse of a building inMexico

city, 1985

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Re-entrant Corners

The re-entrant; lack of continuity or inside corner is the common

characteristic of overall building configurations that, in plan, assume theshape of an L, T, H, +, or combination of these shapes occurs due to lack

of tensile capacity and force concentration

According to IS 1893 (Part 1): 2002, plan configurations of a structure and

its lateral force resisting system contain re-entrant corners, where bothprojections of the structure beyond the re-entrant corner are greater than

15% of its plan dimension in the given direction

The re-entrant corners of the buildings are subjected to two types of

problems

First is that they tend to produce variations of rigidity, and hence differential

motions between different parts of the building, resulting in a local stress

concentration at the notch of the re-entrant corner. Second problem is

torsion. of 28 22


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Re-entrant Corners

Example of buildings with planirregularities

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Examples of Failure due to Re-entrant Corners

Damage concentrated at the intersection oftwo wings of an L-shaped school, Alaskaearthquake, 1964

To avoid this type of damage, either provide a separation joint

between two wings of buildings or tie the building together strongly in

the system of stress concentration and locate resistance elements to

increase the tensile capacity at re-entrant cornerof 28 24


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Nonparallel Systems

Non-parallel system

The vertical load resisting elements are not parallel or symmetrical about the

major orthogonal axes of the lateral-force resisting system

This condition results in a high probability of torsional forces under a groundmotion, because the centre of mass and resistance does not coincide

This problem is often exaggerated in the triangular or wedge shaped buildings

resulting from street inter-sections at an acute angle. The narrower portion of the

building will tend to be more flexible than the wider ones, which will increase the

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Examples of Failure due to Non-parallel System

Distortion in wedge shaped building, Mexicocity, 1985

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Diaphragm Discontinuity

The diaphragm is a horizontal resistance element that transfers forces between

vertical resistance elements

The diaphragm discontinuity may occur with abrupt variations in stiffness,

including those having cut-out or open areas greater than 50% of the gross

enclosed diaphragm area, or change in effective diaphragm stiffness of more than

50 % from one storey to the next

(a) Diaphragm discontinuity

(a) (b)

(b) Failure resulting from diaphragm flexibility in

Loma Prieta earthquake, 1989of 28 27


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Summary and Recommendations

The multi-storeyed reinforced concrete buildings with verticalirregularities like soft storey, mass irregularities, floating box constructionshould be designed on the basis of dynamic analysis and inelastic

design

The proper effect of these irregularities can be accounted by 3Dmathematical modelling of the building and dynamic analysis

The ductility provisions are most important in such situations

More care is necessary at the time of planning for reducing irregularities

The torsional effects in a building can be minimised by proper location ofvertical resisting elements and mass distribution

Shear walls should be employed for increasing stiffness wherenecessary and be uniformly distributed in both principal directions.

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