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Example research essay topic: Earthquake Loads Resistant Design Of Buildings - 2,074 words

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... Spectra are developed by calculating the of response a single mass oscillator (usually with 5 % critical damping present) to the design level earthquake motion. Engineers traditionally have shown a preference for acceleration spectra, since the resulting coefficient, when multiplied by the seismic mass, results in the lateral base shear for the building. In Australia [ 7 ] and the Uniformed Building Code used in the western USA [ 9 ] these spectra are presented as a simple uniform coefficient followed by an exponential decay. The New Zealand Loadings Standard [ 3 ] prescribes an elastic response spectrum, derived using a uniform risk approach, for each soil class.

The modern trend as indicated by the European Earthquake Standard [ 10 ] and also in the proposed National Earthquake Hazard Reduction Programme (NEHRP) specification [ 8 ] is to acknowledge that the response spectra is building period dependent. This is achieved by publishing the design spectra in parametric form where the ordinates of each parameter and the characteristics of the curve between them are read from a series of seismic zonation maps of the region. Figure 2 A Form of Parametric Acceleration Response Spectrum 5. 2. Relative Seismicity The current generation of earthquake loading standards uses a single seismic zonation map with iso-sensual contours to represent the relative seismicity between locations. An example of one for New Zealand is shown as Figure 3. The product of the zone factor, Z, and the lateral acceleration coefficient derived from the design spectrum is used for design.

The next generation of earthquake loading standards are expected to specify spectral acceleration as a function of the response period and also design event return period. The simple linear scaling of a standard spectral shape will no longer be acceptable. Instead we may expect, for example, a suite of three series of maps to reflect different probabilities of exceedance (0. 05 (20 year return period) 0. 002 (500 year return period) and 0. 0005 (2000 year return period) ). Each set will comprise 4 maps each with spectral ordinates for periods of perhaps T = 0, T = 0. 2 seconds, T = 1 second and T = 2. 5 seconds).

The complete suite may therefore comprise 12 regional maps which will enable the development of different shaped elastic response spectra for different return periods. Figure 3 Typical Seismic Zonation Map (interpolation between iso-seism als is acceptable) This approach is likely to have significant impact on regions of low to moderate seismicity. Reference to Figure 4 indicates that while, as expected, the peak ground acceleration is much higher in high seismicity regions than it is in low seismicity ones (ratio of 3. 5: 1), the differential is markedly reduced as the probability of exceedance increases (2: 1 for 0. 0005 probability of exceedance) with the PGA being approximately equal to that of the normal design event within a high seismicity area. It is likely that important key facilities of the future will be required to survive earthquakes with exceedance intervals of this order.

The design requirements may well be quite similar regardless of regional seismicity in such events. Figure 4 Variations of PGA against Probability of Exceedance with Seismicity (from Play & Priestley [ 11 ]) 5. 3. Soil amplification Earthquakes are usually initiated by rupture over a fault rupture plane, often deep within the earth's mantle. The ground motion experienced on the surface results from the transmission of energy waves released from that bedrock source transmitted first through bedrock and then undergoing significant modification by soil layers as the energy waves near the earth's surface. Typically rock sites experience high short period response but more rapid decay. Thus, short duration high intensity motion may be expected in such locations.

Conversely soft soils, particularly when they extend to moderate depths (> 50 metres) are likely to filter out some of the short period motion and usually amplify longer period response, particularly in cases where the soil mass has a natural period similar to the high energy component of the earthquake. While such resonance effects can be taken into account when site specific spectra are being developed, it is usually impractical to include such effects in a loadings standard. Soft soil response spectra have a flatter, broader plateau (refer Figure 5). Figure 5 Typical Spectral Response Curves Modified for Soil Effects. 6.

Derivation of Ductile Design Response Spectra Most modern earthquake design standards acknowledge the reality that buildings will experience damage when they are subjected to severe earthquake attack. Attempts are made to quantify the post-elastic capacity of different building and material types by including some form of ductility based adjustment factor. This has the effect of reducing the elastic response coefficient down to a more convenient level below which elastic response with little or no damage is expected, but beyond which some damage is accepted while collapse avoidance is to be assured. Figure 6 Basis for Translation of Elastic Response Spectra to Inelastic Design Spectra Earthquake standards differ in how they translate the elastic response spectra derived for the site (which includes both the seismic zonation factor and the local soil factors) into inelastic spectra which can be used as the basis for structural design. The two most common methods are to use a combination of structural ductility and structural performance factors. Within the New Zealand Loadings Standard [ 3 ] this is a combination of the ductility factor, & # 61549; , and the structural performance factor, Sp.

The European Earthquake standard, EC 8 [ 10 ] combines these as a structural behaviour factor, q. The earthquake standards of Australia [ 7 ] and the UBC [ 9 ] used in the western USA use a structural response factor, Rf... Both q and Rf are period independent and are therefore direct scaling factors of the site response spectra. The various inelastic response spectra published within the New Zealand Standard introduce period dependency with equal energy concepts being applied to short period structures and equal displacement to long period ones, with a transition zone in between (refer Figure 6). For very long period structures, a constant displacement response can be expected. The ability of the structure to sustain levels of inelastic deformation implicit in these ductility values is dependent on the material and detailing used.

Both the structural ductility and the structural performance factors depend on both the structural form selected and the materials used. As such they need to be prescribed within the seismic provisions of the material design standards along with the specific material detailing provisions which ensures that the inelastic deformation implicit in the ductility assumed can be attained. 7. Analysis and Earthquake Resistant Design Principles 7. 1. The Basic Principles of Earthquake Resistant Design Earthquake forces are generated by the dynamic response of the building to earthquake induced ground motion. This makes earthquake actions fundamentally different from any other imposed loads. Thus the earthquake forces imposed are directly influenced by the dynamic inelastic characteristics of the structure itself.

While this is a complication, it provides an opportunity for the designer to heavily influence the earthquake forces imposed on the building. Through the careful selection of appropriate, well distributed lateral load resisting systems, and by ensuring the building is reasonably regular in both plan and elevation, the influence of many second order effects, such as torsional effects, can be minimised and significant simplifications can be made to model the dynamic building response. Figure 7 Loading Pattern and Resulting Internal Structural Actions Most buildings can be reasonably considered as behaving as a laterally loaded vertical cantilever. The inertia generated earthquake forces are generally considered to act as lumped masses at each floor (or level). The magnitudes of these earthquake forces are usually assessed as being the product of seismic mass (dead load plus long-term live load) present at each level and the seismic acceleration generated at that level.

The design process involves ensuring that the resistance provided at each level is sufficient to reliably sustain the sum of the lateral shear forces generated above that level (ref Figure 7). 7. 2. Controls of the Analysis Procedure A schematic of the earthquake design process in presented in Figure 8 below. The essential features of the process are as follows: 1) Structural designers are usually given the site location and intended occupancy of the building as part of their brief. 2) The national building code normally includes the requirements for the following; o the design philosophy acceptable for buildings (Limit States or Working Stress Design) o the performance objectives for the prescribed occupancy class o the structural importance classification (which transcribes into the acceptable design event return period) and o the proportion of live load considered to be present during a rare event (such as a major earthquake). Figure 8 Schematic of the Earthquake Design Procedure 3) Derive the peak ground acceleration (i. e.

elastic response spectrum for T = 0) for the design intensity earthquake ground motion from consideration of the seismicity of the region (selected to match the design event return period) modified by the near surface soil modification factor (refer Section 5). 4) Select a suitable structural configuration with consideration for the following parameters; o the characteristics of the various lateral load resisting structural forms available (refer Section 9) o the desirability of matching the strength and stiffness of the structural frame to that expected under the dynamic loading of the building itself (ie strength and stiffness decreasing uniformly up the height of the building). (Note this will influence the distribution of the base shear over the building height and as such may dictate the method of analysis acceptable for the building so as to ensure soft-storey collapse is avoided - refer Section 10. 2) o the desirability of a regular building plan with well balanced lateral load resisting systems, evenly distributed about the building plan. (Note: Irregular plan will usually require three dimensional analysis and may experience severe torsional response - refer Section 10. 3), o the material from which the structural system is to be constructed and thus the post-elastic curvature (ductility) which can be accommodated through specific detailing. 5) Determine the level of design required. (Note: There will be many normal occupancy buildings in regions of low seismicity which do not require any specific earthquake resistant measures to be introduced. Other levels of design involve a) simply tying elements together so as to ensure a continuous, rational load path exists for earthquake induced lateral loads; or b) detailed analysis of the building subjected to both gravity induced loads and a rationally derived lateral loading pattern which reflects the earthquake generated forces. ) 6) Ascertain the fundamental period of response of the building based on assumed member sections and properties (Note: Several empirical formula are available as the basis for determining the fundamental period of buildings. It is generally preferable to assess the building response based on a realistic distribution of seismic mass at each level up the building and appropriate inter-level structure stiffness - for concrete a cracked section is appropriate. ) 7) Ascertain from the horizontal regularity of the structure (refer Section 10. 3) whether a simple two dimensional or the more complex three dimensional analysis model required. 8) Ascertain by consideration of the vertical regularity of the structure (refer section 10. 2) whether the structural response will be dominated by the first mode response of the structure (in which case the simplified equivalent static design procedure can be used) or whether, because of vertical structural irregularity, multi-modal analysis is required to enable the base shear distribution to be established. 9) If equivalent static analysis (refer section 11. 3) is acceptable then; o calculate the design level base shear force from the product of the seismic mass and the lateral force coefficient (derived from the inelastic response spectra) o distribute the base shear to each level of the building and between lateral load resisting systems in accordance with horizontal and vertical regularity of the structure o use elastic analysis techniques to determine actions induced on members from load combinations which include earthquake forces. 10) If multi-modal analysis (refer section 11. 2) is required then; o ascertain the period and deformed shape for each mode o ascertain the contribution of each mode from the base shear of each mode (derived from the elastic response spectra lateral acceleration at each respective modal period) distributed between levels according to each mode shape o combine the contribute...


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