SEISMIC FORCE RESISTING SYSTEMS AND RESPONSES OF CONCRETE BUILDINGS TO SEISMIC FORCES

1. SEISMIC FORCE RESISTING SYSTEMS AND RESPONSES OF CONCRETE BUILDINGS TO SEISMIC FORCES

2. TOPICS COVERED • Seismic Force Resisting Systems • Building Configuration • Response of Concrete Buildings

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. PLAN IRREGULARITIES • Five different plan irregularities have been identified • Torsional irregularity • Re-entrant corners • Diaphragm discontinuity • Out-of-plan offsets • Nonparallel systems

11. PLAN IRREGULARITIES • Torsional irregularity

12. PLAN IRREGULARITIES • Re-entrant corners

13. PLAN IRREGULARITIES • Diaphragm discontinuity

14. PLAN IRREGULARITIES • Out-of-plan offsets

15. PLAN IRREGULARITIES • Nonparallel systems

16. 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

17. VERTICAL IRREGULARITIES • Stiffness irregularity-soft story

18. VERTICAL IRREGULARITIES • Weight (mass) irregularity

19. VERTICAL IRREGULARITIES • Vertical geometric irregularity

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

21. VERTICAL IRREGULARITIES • Discontinuity in capacity-weak story

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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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