Transcript I SFR Systems and Response

NED University Engineering & Technology
Department of Civil Engineering
SEISMIC FORCE RESISTING SYSTEMS AND
RESPONSES OF CONCRETE BUILDINGS TO
SEISMIC FORCES
NED University Engineering & Technology
Department of Civil Engineering
TOPICS COVERED
 Seismic
Force Resisting Systems
 Building Configuration
 Response of Concrete Buildings
NED University Engineering & Technology
Department of Civil Engineering
SEISMIC FORCE RESISTING SYSTEMS
 Basic structure systems that may be used to resist earthquake forces include
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Moment-Resisting Frame Systems
Bearing Wall Systems
Dual System
Building Frame System
Inverted Pendulum System
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
PLAN IRREGULARITIES
 Five different plan irregularities have been identified
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Torsional irregularity
Re-entrant corners
Diaphragm discontinuity
Out-of-plan offsets
Nonparallel systems
NED University Engineering & Technology
Department of Civil Engineering
PLAN IRREGULARITIES
 Torsional irregularity
NED University Engineering & Technology
Department of Civil Engineering
PLAN IRREGULARITIES
 Re-entrant corners
NED University Engineering & Technology
Department of Civil Engineering
PLAN IRREGULARITIES
 Diaphragm discontinuity
NED University Engineering & Technology
Department of Civil Engineering
PLAN IRREGULARITIES
 Out-of-plan offsets
NED University Engineering & Technology
Department of Civil Engineering
PLAN IRREGULARITIES
 Nonparallel systems
NED University Engineering & Technology
Department of Civil Engineering
VERTICAL IRREGULARITIES
 Five different vertical structural irregularities have been identified
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Stiffness irregularity-soft story
Weight (mass) irregularity
Vertical geometric irregularity
In-plane discontinuity in vertical lateral-force-resisting elements
Discontinuity in capacity-weak story
NED University Engineering & Technology
Department of Civil Engineering
VERTICAL IRREGULARITIES
 Stiffness irregularity-soft story
NED University Engineering & Technology
Department of Civil Engineering
VERTICAL IRREGULARITIES
 Weight (mass) irregularity
NED University Engineering & Technology
Department of Civil Engineering
VERTICAL IRREGULARITIES
 Vertical geometric irregularity
NED University Engineering & Technology
Department of Civil Engineering
VERTICAL IRREGULARITIES
 In-plane discontinuity in vertical lateral-force-resisting elements
NED University Engineering & Technology
Department of Civil Engineering
VERTICAL IRREGULARITIES
 Discontinuity in capacity-weak story
NED University Engineering & Technology
Department of Civil Engineering
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
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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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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
NED University Engineering & Technology
Department of Civil Engineering
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