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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
8/12/2019 Seismic Force Resisting Systems and Response of Concrete Building
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Department of Civil EngineeringNED University Engineering & Technology
TOPICS COVERED Seismic Force Resisting Systems Building Configuration
Response of Concrete Buildings
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Department of Civil EngineeringNED University Engineering & Technology
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
8/12/2019 Seismic Force Resisting Systems and Response of Concrete Building
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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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
8/12/2019 Seismic Force Resisting Systems and Response of Concrete Building
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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
PLAN IRREGULARITIES Torsional irregularity
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Department of Civil EngineeringNED University Engineering & Technology
PLAN IRREGULARITIES Re-entrant corners
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Department of Civil EngineeringNED University Engineering & Technology
PLAN IRREGULARITIES Diaphragm discontinuity
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Department of Civil EngineeringNED University Engineering & Technology
PLAN IRREGULARITIES Out-of-plan offsets
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Department of Civil EngineeringNED University Engineering & Technology
PLAN IRREGULARITIES Nonparallel systems
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8/12/2019 Seismic Force Resisting Systems and Response of Concrete Building
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Department of Civil EngineeringNED University Engineering & Technology
VERTICAL IRREGULARITIES Stiffness irregularity-soft story
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Department of Civil EngineeringNED University Engineering & Technology
VERTICAL IRREGULARITIES Weight (mass) irregularity
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Department of Civil EngineeringNED University Engineering & Technology
VERTICAL IRREGULARITIES Vertical geometric irregularity
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Department of Civil EngineeringNED University Engineering & Technology
VERTICAL IRREGULARITIES In-plane discontinuity in vertical lateral-force-resisting elements
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Department of Civil EngineeringNED University Engineering & Technology
VERTICAL IRREGULARITIES Discontinuity in capacity-weak story
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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
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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Department of Civil EngineeringNED University Engineering & Technology
THANK YOU FOR YOURATTENTION !