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Earthquake Loads & Earthquake Resistant Design of Buildings

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Earthquake Loads & Earthquake Resistant Design of Buildings

1. 1

2. Summary 2

3. Earthquake Design - A Conceptual Review 2

4. Earthquake Resisting Performance Expectations 3

5. Key Material Parameters for Effective Earthquake Resistant Design 3

6. Earthquake Design Level Ground Motion 4

6.1. Elastic Response Spectra 4

6.2. Relative Seismicity 5

6.3. Soil amplification 6

7. Derivation of Ductile Design Response Spectra 7

8. Analysis and Earthquake Resistant Design Principles 8

8.1. The Basic Principles of Earthquake Resistant Design 8

8.2. Controls of the Analysis Procedure 8

8.3. The 'Conventional' Earthquake Design Procedure 11

9. The Capacity Design Philosophy for Earthquake Resistance 11

9.1. General Approach 11

9.2. The Implications of Capacity Design 12

10. Earthquake Resistant Structural Systems 12

10.1. Moment Resisting Frames: 12

10.2. Shear Walls 13

10.3. Braced Frames 13

11. The Importance & Implications of Structural Regularity 13

11.1. General 13

11.2. Vertical Regularity 14

11.3. Horizontal Regularity. 14

11.4. Floor Diaphragms 14

12. Methods of Analysis 15

12.1. Integrated Time History Analysis 15

12.2. Multi-modal Analysis 15

12.3. Equivalent Static Analysis 15

13. Trends and Future Directions 16

14. Conclusions 16

15. References 17

1.

Summary

The primary objective of earthquake resistant design is to prevent building collapse during earthquakes thus minimising the risk of death or injury to people in or around those buildings. Because damaging earthquakes are rare, economics dictate that damage to buildings is expected and acceptable provided collapse is avoided.

Earthquake forces are generated by the inertia of buildings as they dynamically respond to ground motion. The dynamic nature of the response makes earthquake loadings markedly different from other building loads. Designer temptation to consider earthquakes as 'a very strong wind' is a trap that must be avoided since the dynamic characteristics of the building are fundamental to the structural response and thus the earthquake induced actions are able to be mitigated by design.

The concept of dynamic considerations of buildings is one which sometimes generates unease and uncertainty within the designer. Although this is understandable, and a common characteristic of any new challenge, it is usually misplaced. Effective earthquake design methodologies can be, and usually are, easily simplified without detracting from the effectiveness of the design. Indeed the high level of uncertainty relating to the ground motion generated by earthquakes seldom justifies the often used complex analysis techniques nor the high level of design sophistication often employed. A good earthquake engineering design is one where the designer takes control of the building by dictating how the building is to respond. This can be achieved by selection of the preferred response mode, selecting zones where inelastic deformations are acceptable and suppressing the development of undesirable response modes which could lead to building collapse.

2. Earthquake Design - A Conceptual Review

Modern earthquake design has its genesis in the 1920's and 1930's. At that time earthquake design typically involved the application of 10% of the building weight as a lateral force on the structure, applied uniformly up the height of the building. Indeed it was not until the 1960's that strong ground motion accelerographs became more generally available. These instruments record the ground motion generated by earthquakes. When used in conjunction with strong motion recording devices which were able to be installed at different levels within buildings themselves, it became possible to measure and understand the dynamic response of buildings when they were subjected to real earthquake induced ground motion.

By using actual earthquake motion records as input to the, then, recently developed inelastic integrated time history analysis packages, it became apparent that many buildings designed to earlier codes had inadequate strength to withstand design level earthquakes without experiencing significant damage. However, observations of the in-service behaviour of buildings showed that this lack of strength did not necessarily result in building failure or even severe damage when they were subjected to severe earthquake attack. Provided the strength could be maintained without excessive degradation as inelastic deformations developed, buildings generally survived and could often be economically repaired. Conversely, buildings which experienced significant strength loss frequently became unstable and often collapsed.

With this knowledge the design emphasis moved to ensuring that the retention of post-elastic strength was the primary parameter which enabled buildings to survive. It became apparent that some post-elastic response mechanisms were preferable to others. Preferred mechanisms could be easily detailed to accommodate the large inelastic deformations expected. Other mechanisms were highly susceptible to rapid degradation with collapse a likely result. Those mechanisms needed to be suppressed, an aim which could again be accomplished by appropriate detailing.

The key to successful modern earthquake engineering design lies therefore in the detailing of the structural elements so that desirable post-elastic mechanisms are identified and promoted while the formation of undesirable response modes are precluded.

Desirable mechanisms are those which are sufficiently strong to resist normal imposed

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