REGULAR AND IRREGULAR BUILDING CONFIGURATION

By: Jivani Dipak

General building characteristics: Building size Building proportion Building symmetry Plan density Symmetry Corners Perimeter resistance Redundancy

THE OPTIMUM SESMIC CONFIGURATION: Low height-to base ratio Equal floor heights Symmetrical plan shape Identical resistance on both axes Uniform section and elevations Maximum torsional resistance Short spans Direct load paths,no cantilevers Redundancy

Deviations in above point results in irregularities .

Types of irregularity

Torsion Irregularity Definition This occurs when the plan location of the center of gravity of the building does not align with the centre of resistance of the bracing structure . As per IS 1893(part 1)-2002,when floor diaphragms are rigid in their own plan in relation to the vertical structural elements that resist the lateral forces. Torsional irregularity to be considered to exist when the maximum storey drift, computed with design eccentricity, at one end of the structures transverse to an axis is more than 1.2 times the average of the storey drifts at the two ends of the structure.

Failure Example.

EI Faro apartments, Vina del Mar, Chile, 1985

Unbalanced location of perimeter walls, particulary on third, forth and fifth floors, leading to severe to torsional forces and near collapse.

Reentrant Corners Definition :The reentrant or inside corner is the common characteristic of overall building configurations that, in plan, assume the shape of an L,T,H,+ or combination of these shapes.

Seismic Effects: Produce variations of rigidity.Torsion.

Solution:

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To separate the building structurally in to simple shapes To tie the building together strongly at lines of stress concentration locate resistance element to reduce torsion

Stress concentration L shape building

Failure Example

Damage concentrated at the intersection of two wings of an L-shaped school, Anchorage, Alaska ,1964

Diaphragm Discontinuity Definition :Irregularity due to wide variation in strength and stiffeness in building element that provide seismic resistance.

Causes. - Large openings for skylights

- enclosed atrium spaces- changes in levels

- existence of a zig-zag building edge

Nonparallel systems Definition :The vertical load resisting elements are not parallel or symmetric about the major orthogonal axes of the lateral force resisting system.

Effect: High probability of torsion effect. Narrower portions of the building will tend to be more flexible than the wider ones

Failure Example

Distortion in wedgeshaped building, Mexico City, 1985

Solution: In general, opaque walls should be designed as frames clad in lightweight materials, to reduce the stiffness discrepancy between these walls and the rest of the structure.

Out of plane offsets. Definition :Discontinuities in a lateral force resistance path,such as out-of-plane offsets of vertical element.

Soft story Stiffness Irregularity-soft story:A soft story is one in which the lateral stiffness is less than 70 percent of that in the story above or less than 80 percent of the average lateral stiffness of the three stories above.

Stiffness Irregularity-Extreme soft story:A extreme soft storey is one in which the lateral stiffness is less than 60 percent of that in the story above or less than 70 percent of the average stiffness of the three story above.

Types of soft story

Mass Irregularity Definition :As per IS-1893(part 1)-2002, mass irregularity shall be considered to exist where the seismic weight of any story is more than 200 percent of that of its adjacent story. Irregularity need not be considered in case of roofs. Common conditions. Heavy roof construction Heavy equipment on the roof of a light weight framed building.

Vertical Geometric Irregularity Definition :As per IS-1893(part 1)-2002, Vertical geometric irregularity shall be considered to exist where the horizontal dimension of the lateral force resisting system in any storey is more than 150 percent of that in its adjacent story. A common example of this is the vertical setback of upper levels

Vertical setback

In-Plane Discontinuity A in plane offset of the lateral force resisting elements greater than the length of those element.

Discontinuity in Capacity-Weak story Definition :As per IS-1893(part 1)-2002, a weak story is one in which the story lateral strength is less than 80 percent of that in the story above. The story lateral strength is the total strength of all seismic force resisting elements sharing the storey shear in the considered direction.