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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

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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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