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Example research essay topic: Earthquake Loads Resistant Design Of Buildings - 1,937 words

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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 actions without damage, yet are capable of accommodating substantial inelastic deformation without significant loss of strength or load carrying capacity. Such mechanisms have been found to generally involve the flexural response of reinforced concrete or steel structural elements or the flexural steel dowel response of timber connectors. Undesirable post-elastic response mechanisms within specific structural elements have brittle characteristics and include shear failure within reinforced concrete, reinforcing bar bond failures, the loss of axial load carrying capacity or buckling of compression members such as columns, and the tensile failure of brittle components such as timber or under-reinforced concrete. Undesirable global response mechanisms include the development of a soft-storey within a building (where in-elastic deformation demands are likely to be concentrated and therefore make high demands on the resistance ability of the elements within that zone), or buildings where the structural form or geometry is highly irregular, which puts them outside the simplifications made within the engineering models used for design. 3.

Earthquake Resisting Performance Expectations The seismic structural performance requirements of buildings are often prescribed within national building codes. For instance Clause B 1 'Structure' of the New Zealand Building Code [ 1 ] prescribes that the building is to retain its amenity when subjected to frequent events of moderate intensity, and that it is to remain stable and avoid collapse during rare events of high intensity. The Building Code of Australia [ 2 ] prescribes the performance expectations in similar rather vague terms. It is left to the Loadings Standards of New Zealand [ 3 ] and Australia interpret 'moderate' and 'high' loading intensities.

This they do by equating the 'amenity' retention as the Serviceability Limit State and collapse avoidance as the Ultimate Limit State loads and combinations of loads. Thus for compliance with the mandatory provisions of the national building codes the following requirements need to be satisfied: A. For amenity retentions (Serviceability Limit State): The building response should remain predominantly elastic, although some minor damage would be acceptable provided any such damage does not require repair. Buildings should remain fully operational. Preservation of the appropriate levels of lateral deformation to protect non-structural damage is the primary control parameter. The loading intensity for this limit state is to be relatively low (say 5 % probability of exceedance in any year).

B. For collapse avoidance (Ultimate or Survival Limit State): The risk to life safety is maintained at acceptably low levels. Building collapse is to be avoided. Significant residual deformation is expected within the buildings with both structural and non-structural members experiencing damage. Building repair may not be economical.

The loading intensity used for design can be equated to rare earthquakes with long (500 + years) return periods. This is the single most important design criterion since it relates to preservation of life. It demands that the system possess adequate overall structural ductility to enable load redistribution while avoiding collapse. There are examples in the new generation of earthquake loading specifications [ 8 ] of additional, performance orientated, limit states being introduced. For example Continued Occupancy (being somewhat beyond the serviceability limit state where although damage is minor, it will require repair but the building will be posted for continued use after the event), and Damage Control Limit State (where significant damage to both structural and non-structural elements is experienced but the building can be repaired economically to its condition before the event). Such provisions are not currently mandatory.

They are, however, available to building owners (and their insurance providers) to form the basis of performance orientated objectives. 4. Key Material Parameters for Effective Earthquake Resistant Design Compliance with the performance criteria of the various limit states outlined above requires different material properties. The serviceability limit states criteria demand that certain stiffness and elastic strength parameters be met and is primarily concerned with the linear stress / strain deformation relationships associated with elastic system response. The ultimate limit state criteria generally demand that an appropriate level of post elastic ductility capacity is available so as to avoid collapse.

There are important ramifications with this concept in regard to both the material and sectional properties assumed for members during the analysis, and also during the translation of the results derived using elastic modelling techniques into the inelastic response domain. For compliance with the serviceability limit state performance provisions, the simple linear stress / strain relationships of materials are needed. These are the conventional parameters used to assess the structural resistance to other loads. Provided the structural system remains predominantly elastic, damage avoidance can reasonably be expected and compliance thus assured. Simple elastic engineering models can be used to ascertain building response in these conditions.

Thus for concrete and masonry structures, the cracked sectional properties are appropriate for the serviceability limit state, although significant yield of the reinforcing steel (and the subsequent retention of wide residual cracks) is to be avoided. Figure 1 Post-elastic (Ductility) System Capacities For compliance with the ultimate limit state performance provisions, the post-elastic response of the structure, including large post-elastic member deformation, needs to be considered. Often traditional engineering models break down at this stage. There is thus little to be gained by using highly sophisticated engineering modeling techniques to demonstrate compliance with the ultimate limit states criteria (ie collapse avoidance) unless there is a high degree of confidence that the relationship between the elastic and inelastic structural response is realistic. The simple elastic stress / strain relationships and the elastic engineering models used to ascertain the load distribution between members within the structural system no longer apply. It is to address this particular post-elastic response condition, being the primary objective of good earthquake engineering design, that the principles of capacity design of structures were developed and subsequently introduced into many modern design standards. 5.

Earthquake Design Level Ground Motion A fundamental parameter contained within all earthquake loading standards is the earthquake induced ground motion which is to be used for design. This is generally prepared by seismologists and geotechnical engineers. It is typically presented to the structural designer in three components, namely the elastic response of the basement rock (usually as acceleration spectra), the relative seismicity at the site (commonly presented as a suite of zonation maps), and a modification function which is applied to the motion at bedrock beneath the site to allow for near surface soil conditions (presented as either a simple amplification factor or as a more complex soil property related function). 5. 1. Elastic Response Spectra Engineers traditionally have used acceleration response spectra to represent the motion induced by the design earthquake. These spectra are generally presented as a response function (acceleration, velocity or displacement) against the response period of a single-degree-of-freedom oscillator considered to represent the structure (refer Figure 2)...


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