Seismic Force Resisting Systems and Response of Concrete Building

8/12/2019 Seismic Force Resisting Systems and Response of Concrete Building

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Department of Civil EngineeringNED University Engineering & Technology

SEISMIC FORCE RESISTING SYSTEMS ANDRESPONSES OF CONCRETE BUILDINGS TO

SEISMIC FORCES


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TOPICS COVERED Seismic Force Resisting Systems Building Configuration

Response of Concrete Buildings


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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


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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


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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 seismicforces

Ordinary RC shear walls Special RC shear walls


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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


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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


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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


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BUILDING CONFIGURATIONBuildings having irregular configurations in plan and/or elevation sufferedgreater damageInelastic behaviour concentrates in certain localized regions in irregular

structureStructural elements deteriorate rapidly in these areasInelastic demand tend to be well distributed throughout a regular structureElastic analysis methods are not capable to accurately predict distribution ofseismic demand in an irregular structure

Building with regular configuration are encouraged and highly irregularbuildings are prohibited on sites close to active faults


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PLAN IRREGULARITIES Five different plan irregularities have been identified

Torsional irregularity Re-entrant corners

Diaphragm discontinuity Out-of-plan offsets Nonparallel systems


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PLAN IRREGULARITIES Torsional irregularity


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PLAN IRREGULARITIES Re-entrant corners


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PLAN IRREGULARITIES Diaphragm discontinuity


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PLAN IRREGULARITIES Out-of-plan offsets


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PLAN IRREGULARITIES Nonparallel systems


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VERTICAL IRREGULARITIES Stiffness irregularity-soft story


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VERTICAL IRREGULARITIES Weight (mass) irregularity


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VERTICAL IRREGULARITIES Vertical geometric irregularity


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VERTICAL IRREGULARITIES In-plane discontinuity in vertical lateral-force-resisting elements


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VERTICAL IRREGULARITIES Discontinuity in capacity-weak story


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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


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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 actingin 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


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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


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SEISMIC RESPONSE OF SHEAR WALLSShear walls resist gravity loads and in-plane lateral forces They are like vertical cantilever deep beamsShear force from diaphragm causes bending moment and shear force in the

plane of the wallTendency to overturn and slide is resisted by the foundationBending moment increases from top to bottom of a building and causes tensionand compression forces in the wall planeSeismic response of short stocky shear wall is governed by shear Response of taller walls is governed by flexure


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SEISMIC RESPONSE OF SHEAR WALLSFor walls with H/L between 1-2 response depends on several factors includingamount of shear reinforcingShear dominated response is characterized by inclined (x-shaped) cracking

patternThe wall can loose strength rapidly with little warning


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SEISMIC RESPONSE OF FRAMESResponse of frames is different than shear walls to lateral forcesFrame resists by being deformed by lateral forces due to the rigidity of thebeam-column joints

Beams and columns bend due to this rigidityTension stresses caused by the bending must be resisted by the reinforcementBending also causes vertical shear forces in beams and horizontal shear forcesin columnsVertical shear reinforcement is needed in beams and horizontal shearreinforcement in columns


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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


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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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Seismic Force Resisting Systems and Response of Concrete Building