I SFR Systems and Response

8/13/2019 I SFR Systems and Response

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Department of Civil Engineering

NED University Engineering & Technology

SEISMIC FORCE RESISTING SYSTEMS AND

RESPONSES OF CONCRETE BUILDINGS TO

SEISMIC FORCES


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TOPICS COVEREDSeismic Force Resisting Systems

Building Configuration

Response of Concrete Buildings


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SEISMIC FORCE RESISTING SYSTEMSBasic 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 SYSTEMSA 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

systemThree 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 SYSTEMSA 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 SYSTEMSA 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 wallsMoment frames are either special or intermediate frames

Different combinations of shear walls are possible including




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BUILDING FRAME SYSTEMSA 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 analysisTwo types of detailing of walls are possible based on the effects of seismic

forces and building height




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INVERT PENDULUM SYSTEMSStructures 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 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


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PLAN IRREGULARITIESFive different plan irregularities have been identified

Torsional irregularity

Re-entrant corners

Diaphragm discontinuity Out-of-plan offsets

Nonparallel systems


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PLAN IRREGULARITIESTorsional irregularity


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PLAN IRREGULARITIESRe-entrant corners


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PLAN IRREGULARITIESDiaphragm discontinuity


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PLAN IRREGULARITIESOut-of-plan offsets


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PLAN IRREGULARITIESNonparallel systems


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VERTICAL IRREGULARITIESFive 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


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VERTICAL IRREGULARITIESStiffness irregularity-soft story


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VERTICAL IRREGULARITIESWeight (mass) irregularity


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VERTICAL IRREGULARITIESVertical geometric irregularity


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VERTICAL IRREGULARITIESIn-plane discontinuity in vertical lateral-force-resisting elements


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VERTICAL IRREGULARITIESDiscontinuity in capacity-weak story


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RESPONSE OF CONCRETE BUILDINGSA 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 RESPONSEDiaphragms 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


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DIAPHRAGM RESPONSEForces 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 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


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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


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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


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FOUNDATION RESPONSEFoundations 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 RESPONSEPiles 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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I SFR Systems and Response

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