Seismic force resisting systems and responses of concrete buildings to seismic forces
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SEISMIC FORCE RESISTING SYSTEMS AND RESPONSES OF CONCRETE BUILDINGS TO SEISMIC FORCES PowerPoint PPT Presentation


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SEISMIC FORCE RESISTING SYSTEMS AND RESPONSES OF CONCRETE BUILDINGS TO SEISMIC FORCES. TOPICS COVERED. Seismic Force Resisting Systems Building Configuration Response of Concrete Buildings. SEISMIC FORCE RESISTING SYSTEMS.

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SEISMIC FORCE RESISTING SYSTEMS AND RESPONSES OF CONCRETE BUILDINGS TO SEISMIC FORCES

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SEISMIC FORCE RESISTING SYSTEMS AND RESPONSES OF CONCRETE BUILDINGS TO SEISMIC FORCES


TOPICS COVERED

  • Seismic Force Resisting Systems

  • Building Configuration

  • Response of Concrete Buildings


SEISMIC FORCE RESISTING SYSTEMS

  • Basic structure systems that may be used to resist earthquake forces include

    • Moment-Resisting Frame Systems

    • Bearing Wall Systems

    • Dual System

    • Building Frame System

    • Inverted Pendulum System


MOMENT-RESISTING FRAME SYSTEMS

  • A structural system with complete space frame for gravity loads

  • Lateral forces are resisted by flexural action of frame members

  • Entire space frame or portion may be designated as seismic-force-resisting system

  • Three types of detailing of frames are possible based on the effects of seismic forces

    • Ordinary RC frames

    • Intermediate moment frames

    • Special moment frames


BEARING WALL SYSTEMS

  • A structural system without complete space frame for gravity loads

  • Bearing walls provide support for gravity loads

  • Lateral loads are also resisted by the bearing walls acting as shear walls

  • Two types of detailing of walls are possible based on the effects of seismic forces

    • Ordinary RC shear walls

    • Special RC shear walls


DUAL SYSTEMS

  • A structural system with the following features

    • Complete space frame for gravity loads

    • 25% base shear resisted by space frames

    • Resistance to lateral force is provided by the shear walls

  • Moment frames are either special or intermediate frames

  • Different combinations of shear walls are possible including

    • Ordinary RC shear walls

    • Special RC shear walls


BUILDING FRAME SYSTEMS

  • A structural system without complete space frame for gravity loads

  • Lateral loads are resisted by the shear walls

  • No interaction between the shear wall and frames is considered in the lateral load analysis

  • Two types of detailing of walls are possible based on the effects of seismic forces and building height

    • Ordinary RC shear walls

    • Special RC shear walls


INVERT PENDULUM SYSTEMS

  • Structures that have a large portion of mass concentrated near the top

  • Essentially one degree of freedom

  • Little redundancy and overstrength

  • Inelastic behaviour concentrated at the base

  • Less energy dissipation capacity than other systems


BUILDING CONFIGURATION

  • Buildings having irregular configurations in plan and/or elevation suffered greater damage

  • Inelastic behaviour concentrates in certain localized regions in irregular structure

  • Structural elements deteriorate rapidly in these areas

  • Inelastic demand tend to be well distributed throughout a regular structure

  • Elastic analysis methods are not capable to accurately predict distribution of seismic demand in an irregular structure

  • Building with regular configuration are encouraged and highly irregular buildings are prohibited on sites close to active faults


PLAN IRREGULARITIES

  • Five different plan irregularities have been identified

    • Torsional irregularity

    • Re-entrant corners

    • Diaphragm discontinuity

    • Out-of-plan offsets

    • Nonparallel systems


PLAN IRREGULARITIES

  • Torsional irregularity


PLAN IRREGULARITIES

  • Re-entrant corners


PLAN IRREGULARITIES

  • Diaphragm discontinuity


PLAN IRREGULARITIES

  • Out-of-plan offsets


PLAN IRREGULARITIES

  • Nonparallel systems


VERTICAL IRREGULARITIES

  • Five different vertical structural irregularities have been identified

    • Stiffness irregularity-soft story

    • Weight (mass) irregularity

    • Vertical geometric irregularity

    • In-plane discontinuity in vertical lateral-force-resisting elements

    • Discontinuity in capacity-weak story


VERTICAL IRREGULARITIES

  • Stiffness irregularity-soft story


VERTICAL IRREGULARITIES

  • Weight (mass) irregularity


VERTICAL IRREGULARITIES

  • Vertical geometric irregularity


VERTICAL IRREGULARITIES

  • In-plane discontinuity in vertical lateral-force-resisting elements


VERTICAL IRREGULARITIES

  • Discontinuity in capacity-weak story


RESPONSE OF CONCRETE BUILDINGS

  • A reliable load path is necessary to transfer lateral forces to the foundation

  • Earthquake forces are resisted by either walls or frame elements

  • Foundation components transfer the force to the earth

  • Key elements of the load path through the structure include

    • Diaphragm

    • Walls

    • Frames

    • Foundations

  • Connections are also important components of the chain

  • Resistance of building is as strong as the weakest link in the path


DIAPHRAGM RESPONSE

  • Diaphragms typically span between shear walls of concrete

  • Respond like deep beams bending in their own plane under lateral forces

  • Forces produced at the diaphragm edge include

    • Shear

    • Tension or compression

  • Seismic forces acting perpendicular to the long side produce shear forces acting in the opposite direction

  • Shear forces are transferred to the shear walls

  • Tension develops in the chord and compression develops on the side on which seismic forces act


DIAPHRAGM RESPONSE

  • Forces similar to chord forces also develop around openings

  • Openings may need to be reinforced with additional longitudinal steel

  • Shear forces at the diaphragm edge are transferred through shear-friction

  • Another mechanism of shear transfer is dowel action

  • The assumption here is that reinforcement acts as anchor bolt in shear


SEISMIC RESPONSE OF SHEAR WALLS

  • Shear walls resist gravity loads and in-plane lateral forces

  • They are like vertical cantilever deep beams

  • Shear force from diaphragm causes bending moment and shear force in the plane of the wall

  • Tendency to overturn and slide is resisted by the foundation

  • Bending moment increases from top to bottom of a building and causes tension and compression forces in the wall plane

  • Seismic response of short stocky shear wall is governed by shear

  • Response of taller walls is governed by flexure


SEISMIC RESPONSE OF SHEAR WALLS

  • For walls with H/L between 1-2 response depends on several factors including amount of shear reinforcing

  • Shear dominated response is characterized by inclined (x-shaped) cracking pattern

  • The wall can loose strength rapidly with little warning


SEISMIC RESPONSE OF FRAMES

  • Response of frames is different than shear walls to lateral forces

  • Frame resists by being deformed by lateral forces due to the rigidity of the beam-column joints

  • Beams and columns bend due to this rigidity

  • Tension stresses caused by the bending must be resisted by the reinforcement

  • Bending also causes vertical shear forces in beams and horizontal shear forces in columns

  • Vertical shear reinforcement is needed in beams and horizontal shear reinforcement in columns


FOUNDATION RESPONSE

  • Foundations can be shallow or deep

  • Shallow foundations are supported by vertical pressure of earth

  • Foundation types include

    • Square or rectangular spread footings

    • Continuous strip footings

  • Deep foundations consists of piles made of

    • Wood

    • Steel

    • Concrete

  • Piles can be poured in place or driven piles


FOUNDATION RESPONSE

  • Piles are supported by end bearing and skin friction

  • Connected together by ties, grade beams or slabs on grade

  • Shear forces are transferred from walls and frames to the foundation

  • Dowels in foundation must match the vertical reinforcement in walls and frames


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