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Example research essay topic: Earthquake Loads Resistant Design Of Buildings - 2,097 words

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... se dynamic response may also introduce twisting and torsional effects into the response, either directly as a function of the input ground motion, or because of variations in the spatial distribution of seismic mass, or because of the structure being irregular in plan. Measures employed to counter these effects typically involve distributing the lateral load resisting systems about the building plan and attempting to limit the plan profile to being reasonably regular and compact. Most modern earthquake loading standards require the designer to assess the Centre of Rigidity (CoR) of the structural system, and the centre of mass (CoM) of the uniformly distributed seismic mass. The eccentricity is typically increased by 10 % of the building width to allow for unexpected variations in torsional effects with the magnitude of the resulting torsional action (being the product of mass and linear eccentricity between CoR and CoM) increasing accordingly. Such approximations tend to be based upon the response of the structure within the elastic response domain and may provide little security against collapse once the deformations have progressed into the inelastic domain.

Paulay [ 13 ] has recently proposed an elegant means of directly addressing post-elastic torsional effects. He postulates that the post elastic torsional demand can be met, satisfied and indeed controlled by rigorous detailing of lateral load resisting elements so as to ensure their displacement ductility demands are met. Provided this is achieved the effects of torsion are readily accommodated. The preferred method of minimising torsional effects is to select floor plans which are regular and reasonably compact. Wide separation of horizontal lateral load resisting systems is encouraged. Plan forms with re-entrant corners such as 'L' and 'T' plan layouts should be avoided, or, where these plan forms are dictated by other constraints, seismic separation joints should be introduced between rectangular blocks.

Such joints must be designed to accept the post-elastic dynamic response of the building parts, which may be responding with disparate phases. Contact and hammering between blocks is to be avoided. 10. 4. Floor Diaphragms The floor system, in additional to supporting the live loads induced by the building contents, can also be designed as a floor diaphragm which links the lateral load resisting systems at each level. Other horizontal loading mechanisms such as horizontal trusses or deep beams can be used where the floor system is interrupted by penetrations or openings. The diaphragm action of floors is often taken for granted during design.

It is important that designers allow for the concentrations of horizontal stress within the floor diaphragm around openings and penetrations through the provision of diagonal corner steel within the flooring. Care should also be taken to ensure that the interconnection between the floor diaphragm and the vertical lateral load resisting system is sufficiently robust to transmit the required shear between elements. Precast flooring systems using slender cast inside toppings can be quite vulnerable in such circumstances. 11. Methods of Analysis Earthquake engineering design techniques have advanced greatly with the advent of modern computing techniques.

The prudent designer, however, is wise not to lose sight of the primary objective of earthquake design (ie collapse avoidance) and to remember the level of uncertainty present in several of the key input parameters. Little may therefore be achieved by using highly sophisticated analysis techniques when neither the input ground motion nor the post-elastic response of the structure are well understood. The selection of regular building configurations and the application of sound detailing principles are more likely to provide the required level of security against collapse than detailed refinement of the analysis techniques. 11. 1. Integrated Time History Analysis Integrated time history analysis techniques involve the stepwise solution in the time domain of the multi-degree-of-freedom equations of motion which represent the actual response of a building. It is the most sophisticated level of analysis available to the earthquake engineer.

Its solution is a direct function of the earthquake ground motion selected as the input parameter for the specific building. Such records are seldom available directly for a given site and either synthetic ground motion or modified real free-field records are generally used. The modelled representation of the structure itself is required to realistically represent both the elastic and post-elastic response characteristics of the building. Since this detail of information is seldom available when commencing the design process, this analysis technique is usually limited in its application to checking the suitability of assumptions made during the design of important structures (i. e. the onset sequence and deformation demands of inelastic plastic hinge zones) rather than a method of assigning lateral forces themselves. 11. 2.

Multi-modal Analysis Multi-modal analysis is an elastic dynamic response analysis technique which involves first the determination of the structural response of each mode of vibration of the building followed by the combination of the resulting forces for each significant response mode. For such assessments it is usually convenient to consider the structural mass to be concentrated at each floor level which results in one degree of freedom for each floor provided torsional effects are ignored. When the building is torsionally susceptible, lateral and torsional response will need to be considered thus doubling the number of possible response modes. The procedure involves determining both the response period and mode shape, the determination of the lateral shear coefficient for each response mode (from the design spectra using the modal period) and the distribution of the resulting base shear according to the response shape at each floor. The contribution of each response mode is then combined with an allowance being made for the time variance between different response modes. Thus a square-root-sum-of-squares (SRSS) method of combining lateral forces is generally used, although other combination methods such as the complete quadratic combination (CQC) method may be required where the response modes are close together.

A static analysis using the resulting equivalent forces is usually used as the basis for determining the forces and displacements of the overall structure. While this technique takes into account allowance for the true response characteristics of the building (i. e. does not assume only first mode response) it should be remembered that it is still assessing the structural response while it remains elastic. Collapse avoidance, with the implied onset of controlled damage (i. e.

post-elastic deformations), requires many assumptions to be made to arrive at the inelastic response. In addition many of the structural member properties needed for the analysis are unknown until after a preliminary analysis has been undertaken. Thus the sophistication of the model often adds little to the final design. 11. 3. Equivalent Static Analysis The equivalent static analysis procedure is also essentially an elastic design technique, although some consideration of the post-elastic response enters into the selection of the determination of the lateral force coefficient (item 2 below). It is, however, simple to apply than the multi-model response method, with the implicit simplifying assumptions being arguably more consistent with other assumptions implicit elsewhere in the design procedure. The equivalent static analysis procedure involves the following steps: 1.

Estimate the first mode response period of the building from the design spectra. 2. Use the specific design response spectra to determine that the lateral base shear of the complete building is consistent with the level of post-elastic (ductility) response assumed. 3. Distribute the base shear between the various lumped mass levels usually based on an inverted triangular shear distribution of 90 % of the base shear commonly, with 10 % of the base shear being imposed at the top level to allow for higher mode effects. 4. Analyse the resulting structure under the assumed distribution of lateral forces and determine the member actions and loads. 5. Determine the overall structural response, particularly regarding the inter-storey drifts assessed for the elastically responding structure. (For the assessment of the post-elastic deformation, design standards typically magnify the elastic deformed shape by the structural ductility to determine the overall maximum deformation - typically at roof level. The introduction of a non-linear response profile to allow for local rotation at plastic hinge zones is often required when determining the inter storey drifts. ) 12.

Trends and Future Directions Considerable technical effort is being expended on refining the models used to determine earthquake hazards throughout the world. Although this is one area of uncertainty, modern structural design practices, particularly the application of capacity design techniques, are robust enough to overcome such deficiencies. It is the author's view that this effort may perhaps be better directed towards refining understanding of the post-elastic demand capacities of different structural systems and to devising techniques where the performance of new or innovative solutions can be realistically assessed. The international trend towards prescribing building performance expectations has gone some way towards raising public awareness as to what performance is expected from buildings within their community. This enables some rational cost / benefit decisions to be made regarding insurance and business continuance planning. However, the engineering fraternity is somewhat tardy in tackling the thorny issue of realistic whole-building performance under rare events such as earthquake attack.

The concept of various intermediate performance levels is being presented in modern design specifications [ 8 ] where four levels of performance are stipulated which range from survival through continued occupancy to no-damage. This may be anticipated as being the target for future standards and more work is required to ensure the performance targets are matched by the design procedures employed. Displacement based design appears to offer some solutions in this endeavour. Here the engineering convenience of translating the earthquake motion into forces in order to execute the design is avoided. Instead the acceptable deformation limits which limit damage are addressed directly, with the collapse avoidance prerequisite achieved by using capacity design techniques [ 14 ]. The elegance of displacement based design is its directness in addressing the deformation control aspects necessary for performance based design, and it is expected to see the introduction of this technique into modern standards over the next five years.

The changes in design methodology and the necessity for material standards to provide guidance on damping values for different structural system can each be seen as challenges to be addressed. 13. Conclusions Modern buildings can be designed to be safe under extreme earthquake attack with collapse being avoided. Current earthquake design practices achieve this by dictating the post-elastic response of the building, locating and detailing zones within the structure where high post-elastic deformations are acceptable, rigorously detailing these zones so they can dependably resist the imposed actions while other, less desirable, post-elastic mechanisms are suppressed. Simple procedures for achieving these objectives is included in the paper and their implications on material property (particularly their over strength ratios) are outlined. The importance of achieving a regular building plan layout, with a well distributed lateral load resisting system and each with a uniform structural elevation is highlighted.

The value of placing design emphasis on achieving the required post-elastic deformation control through careful detailing is highlighted, as is the futility of highly sophisticated, complex analysis when it is based on inherently unreliable loading models. 14. References 1 New Zealand Government Print, 1992. Regulations to the Building Act, Wellington. 2 Australian Building Codes Board. 1996. Building Code of Australia. CCH Australia for the ABCB.

Canberra. 3 Standards New Zealand. 1992. Loading Standard. NZS 4203. Wellington. 4 Standards Australia. 1988. Dead and live loads and load combinations. AS 1170. 1.

Homebush, Sydney. 5 Standards Australia. 1989. Wind loads. AS 1170. 2. Homebush, Sydney. 6 Standards Australia. 1992. Snow loads. AS 1170. 3.

Homebush, Sydney. 7 Standards Australia. 1988. Earthquake loads. AS 1170. 4. Homebush, Sydney. 8 Building Safety Standards Committee. 1997. Draft National Earthquake Hazard Reduction Programme (NEHRP) Recommended Provisions for Seismic Regulations for New Buildings and Other Structures. National Science Foundation.

Washington. 9 International Conference of Building Officials. 1997. Uniform Building Code. Whittier, California 10 European Pre standard (ENV) 1994. Eurocode 8 Design provisions for earthquake resistance of structures ENV 1998 - 1 - 1. European Committee for Standardisation, Brussels. 11 Paulay T. and Preistley M.

J. N. 1992. Seismic Design of Reinforced Concrete and Masonry Buildings. John Wiley & Son Inc. New York. 12 Canadian Concrete Association. 1994. Design of Concrete Structures for Buildings.

CAN-A 23. 3 -M 84. Rexdale, Ontario. 13 Paulay T. 1997. A Review of Code Provisions for Torsional Seismic Effects in Buildings. New Zealand National Society for Earthquake Engineering Bulletin. Wellington Vol 30 (3) pp 252 - 264. 14 Priestley, M.

J. N. 1993. Myths and Fallacies in Earthquake Engineering - Conflicts between Design and Reality. New Zealand National Society for Earthquake Engineering Bulletin. Wellington Vol 26; 329 335.


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