Function cost and reliability

The basic principle of any design is that the product should meet the owner's requirements. The owner's requirements may be reduced to just three criteria function, cost and reliability. While the terms function and cost are simple in principle, reliability concerns various technical factors relating to serviceability and safety. As the above three criteria are interrelated, and because of the normal constraints on cost, compromises with function and reliability generally have to be made. In...

Stiffness to suit required vibrational characteristics

With regard to vibrational characteristics, we note first that it would be desirable in general to avoid resonance of the structure with the dominant period of the site as indicated by the peak in the response spectrum (Figure 3.3). This is particularly true for flexible longer-period structures, while shorter-period structures with ample structural walls can be made to work on any kind of site. In the case of sites where the soil is soft and deep enough to amplify the lower frequencies,...

Introduction

The design of any new item of the built environment (e.g. structures or lifelines) provides both an opportunity and a challenge to minimize earthquake risk to people and property within the resources available. To minimize risk, designers must minimize the seismic vulnerability of whatever is being designed. Any given structure may be subject to one or more of the earthquake induced hazards listed in Section 3.1, but this chapter is restricted to design for the basic phenomenon of ground...

Steel connections Introduction

Connections as well as members should be designed to conform to the failure mode controls for the structure concerned. Thus, unless a connection is required to yield prior to the adjacent members as part of an energy-absorbing scheme, as is sometimes done with holding-down bolts, it is usual to design each connection to carry greater loads than the members entering it. In addition, the panel zones of beam-column joints should have stiffness appropriate to the assumptions made in the analysis of...

Probability of occurrence of fault displacements

In order to carry out a hazard analysis of fault displacements an investigation of the degree of activity of the fault in question, along the lines discussed in Section 4.4, may be required. The more significant the structure, the greater will be the effort that will be appropriate in the hazard study. For major structures, the study will hopefully result in recurrence intervals being associated with fault displacements of given magnitudes with an appropriate level of confidence. An example of...

Probabilistic Seismic Hazard Assessment

The determination of design earthquake ground-motion criteria from seismic hazard analyses on a probabilistic basis was formulated by Cornell (1968). The method involves two separate models a seismicity model describing the geographical distribution of event sources and the distribution of magnitudes and an attenuation model describing the effect at any given site as a function of magnitude, source-to-site distance, and ground class. The seismicity model may comprise a number of source regions...

Dynamic analysis of soilstructure systems

Comprehensive dynamic analysis of soil-structure systems is the most demanding analytical task in earthquake engineering. The cost, complexity, and validity of such exercises are major considerations. There are two main problems to be overcome. First, the large computational effort which is generally required for the foundation analysis makes the choice of foundation model very important five main methods of modelling the foundation are discussed in the next section. Secondly, there are great...

J r2 t 212

Attenuation of strong motion in interplate regions In modelling the attenuation of response spectrum ordinates (Sa), we first consider the model of Abrahamson and Silva (1997). They used 655 recordings from 58 shallow crustal earthquakes of magnitude 4.9-7.4 from active tectonic regions (i.e. interplate regions) of the world, excluding subduction zone interface events. Of their events, 49 were from the western USA and nine were from Canada, Iran, Italy, Taiwan and the USSR. The functional form...

Earthquake Risk and Hazard

In normal English usage the work risk means exposure to the chance of injury or loss. It is noted that the word hazard is almost synonymous with risk, and the two words are used in the risk literature with subtle variations which can be confusing. Fortunately, an authoritative attempt has been made to overcome this difficulty through the publication by the Earthquake Engineering Research Institute's glossary of standard terms for use in this subject (EERI Committee on Seismic Risk, 1984). Their...

Defining design events

Referring to the EERI Committee on Seismic Risk (1984), the design event is defined as a specification of one or more earthquake source parameters, and of the location of energy release with respect to the site of interest, which is used for the earthquake resistant design of a structure. Thus a design event specification may consist simply of a magnitude m and a source distance R. Design events are required when normal code loadings are inappropriate or unavailable, as often arises in the case...

Shallow foundations

The horizontal seismic shear force at the base of the structure must be transferred through the substructure to the soil. With shallow foundations it is normal to assume that most of the resistance to lateral load is provided by friction between the soil and the base of the members resisting horizontal load. Other footings and slabs in contact with the ground may also be assumed to provide shear resistance if they are suitably connected to the main resisting elements. The total available...

Earthquake Insurance

From its humble beginnings in the early part of the twentieth century, earthquake insurance has grown to a stage where it is a major factor in the management of earthquake risk. In the decade of the 1960s, when earthquake insurance began to be appreciable, total world insurance losses were US 55 million (in 1999 values), while for the decade of the 1990s the losses had increased about 430 times to US 23.3 billion (Smolka, 2000). Obviously, only a small part of this increase arises from the...

Seismic response of steel structures

The seismic response of steel structures depends mainly upon (1) the onset of instability (local or global) (2) the nature of the steel members (3) the nature of the connections and (4) the nature of other components interacting with the frame. Under ideal conditions of lateral restraint, repeatable high ductility of a very stable nature can be obtained, as shown by the hysteresis loops for bending illustrated in Figure 5.23(b), but with less restraint to webs or flanges marked loss of strength...

Criteria for critical structures or equipment

The design criteria for critical-risk construction vary widely, depending on the nature of the facility concerned. In general, they are similar in principle to those discussed above for normal-risk construction, the main difference being that the various response conditions are required to occur at much lower probabilities, as indicated by the typical criteria given in Table 8.1. Here again, terminology is varied. For instance, in the nuclear power industry the survivability event is generally...

Site response to earthquakes Introduction

As outlined in Chapter 3, there is a great variety of possible geological and soil conditions at construction sites, which give rise to a variety of responses in earthquakes. The basic response phenomena which will be considered below are modification of bedrock excitation during transmission through the overlying soils (amplification or attenuation) liquefaction of saturated cohesionless soils. The methods of analysing these responses vary in complexity, from simple empirical criteria to...

Attenuation of ground motions spectral response and intensity Introduction

Attenuation of ground motions is the term applied to the decrease in strength of shaking as the distance from the earthquake source increases, as illustrated in Figure 4.34. Attenuation is mostly modelled empirically from recorded data of one or other description, Y, of strong ground motion as listed in Section 4.6.2 (e.g. PGA), or from intensity data. Various functional forms have been used for modelling the attenuation of Y in terms of distance, x, the simplest general form being log Y b +...

Stiffness to control deformation

The importance of deformation control in enhancing safety and reducing damage and thus improving the reliability of construction in earthquakes is now well recognized (Section 8.2). The stiffness levels required to control damaging interaction between structure, cladding, partitions and equipment vary widely, depending upon the nature of components and the function of the construction, but stiff construction is obviously better than flexible in this regard. The seismic deformations of...

Caissons

Caissons are similar to piles in that they are relatively slender at least in one direction, and are used as isolated foundations spaced at intervals to support structures which may be long in plan, such as bridges or large buildings. In bridge construction the terms caissons and piers are sometimes used interchangeably. Where caissons penetrate the soil deeply, they need special consideration of soil-structure interaction effects, as discussed below for piles. Caissons may differ from piles in...

Soil Retaining Structures

As the non-seismic design of soil-retaining structures is well discussed elsewhere, little other than seismic considerations are dealt with here. The principal types of structure covered in this section are retaining walls and basement walls. The magnitude of the seismic soil pressures acting on a soil-retaining structure in part depends upon the relative stiffness of the structure and the associated soil mass. Two main categories of soil-structure interaction are usually defined (1) flexible...

Effect of workmanship and buildability on reliability

Good workmanship, complying with design requirements, obviously is fundamental to reliability. Designs which are easy to build are more likely to conform to specification than Figure 8.4 Acceleration response spectral ratio S0.2 (TR) S0.2 (2500 years) versus mean return period for a high-hazard and a low-hazard location (derived from Leyendecker et al., 2000) Figure 8.4 Acceleration response spectral ratio S0.2 (TR) S0.2 (2500 years) versus mean return period for a high-hazard and a low-hazard...

Shape in elevation

As indicated in Figure 8.6, very slender structures and those with sudden changes in width should be avoided in strong earthquake areas. Very slender buildings have high column forces, and foundation stability may be difficult to achieve. Also higher mode contributions may add significantly to the seismic response of the superstructure. Height-width ratios in excess of about 4 lead to less economical structures and require dynamic analysis for proper evaluation of seismic responses. For...

Dynamic properties of soils

Soil behaviour under dynamic loading depends upon many factors, including the environment of the soil (static stress state and water content) and the nature of the dynamic loading (strain magnitude, strain rate, and number of cycles of loading). Some soils increase in strength under rapid cyclic loading, while others such as saturated sands or sensitive clays may lose strength with vibration. Earthquake Resistant Design and Risk Reduction D. Dowrick 2009, John Wiley & Sons, Ltd This section...

Length in plan

Structures which are long in plan naturally experience greater variations in ground movement and soil conditions over their length than short ones. These variations may be due to out-of-phase effects or to differences in geological conditions, which are likely to be most pronounced along long bridges where depth to bedrock may change from zero to very large. The effects on structure will differ greatly, depending on whether the foundation structure is continuous, or a series of isolated...

Spatial distribution of earthquakes maps

Figure 2.1 is a plot of spatial distribution of seismic events with the lowest possible level of information, but has its uses in global tectonic plate recognition. A similar world map, 'Significant earthquakes 1900-1979', produced in the USA, uses different sizes and colours of symbols to indicate the numbers of deaths and cost of damage of the events plotted. Thus a qualitative feel for the relative seismic risk of different localities can be obtained. However, for most analytical purposes...

Field determination of fundamental period of soil

A knowledge of the predominant period of vibration of a given site is helpful in assessing a design earthquake motion (Sections 4.7.4 and 5.2.2) and the vulnerability of the proposed construction to earthquakes (Sections 8.3.6 and 5.3). Many attempts have been made to measure the natural period of vibration of different sites the vibrations measured have generally been microtremors, some arising from small earthquakes (Espinosa and Algermissen, 1972) or those induced artificially such as by...

Relative density test

The in situ relative density or degree of compaction is helpful in determining the likely settlement of dry sands and the liquefaction potential of saturated cohesionless soils in earthquakes (Section 5.2.2). As this property has a significant influence on the dynamic modulus, it indirectly relates to response analyses. Relative density for the void ratio must also be assessed to reproduce field conditions in samples which are recompacted in the laboratory for cyclic loading tests. As is well...

Degree of fault activity

Active faults include any faults which are considered capable of rupturing in the future. Because the amount and frequency of movement can vary enormously, it is important to be able to estimate the degree of activity likely to be exhibited by any fault in the region of interest. Consider the map of active faults in Figure 4.3, which is part of the computer database developed for New Zealand (Stirling et al., 2000). Each fault segment on the map has an index number linking it to its tabulated...

Global Seismotectonics

On a global scale, the present-day seismicity pattern of the world is illustrated in general terms by the seismic events plotted in Figure 2.1. Most of these events can be seen to follow clearly defined belts which form a map of the boundaries of segments of the earth's crust known as tectonic plates. This may be seen by comparing Figure 2.1 with Figure 2.2, which is a world map of the main tectonic plates taken from the highly understandable book on the theory of continental drift by Stevens...

W w tt ft

Where t t and fi are given in equations (5.35) or (5.36) and (5.37), and s is the appropriate value of the soil profile coefficient as given in Table 5.10. For a building of uniformly distributed mass and stiffness W 0.7IT (Section 5.3.4), so that equation (5.41) reduces to 0.3 + 0.75 . (5.42) In the above equations, C has a minimum value of 0.05, and the reduction in base shear due to soil-structure interaction is limited to 30 , giving the range of values 0.7 < CV < 1.0. Table 5.10 Soil...

Faults and earthquake magnitudes

In seismic hazard assessment it is important to be able to obtain good estimates with quantified uncertainties of the magnitudes of earthquakes that are likely to be generated on known faults. Models for doing this have been developed where magnitude is estimated from the fault rupture parameters of length, width, displacement and area. Traditionally, empirical models have been based on measurements of lengths and displacements made by geologists on the observed surface traces of the ruptures....

Settlement of dry sands

It is well known that loose sands can be compacted by vibration. In earthquakes, such compaction causes settlements which may have serious effects on all types of construction. It is therefore important to be able to assess the degree of vulnerability to compaction of a given sand deposit. Unfortunately this is difficult to do with accuracy, but it appears that sands with relative density less than 60 , or with standard penetration resistance less than 15, are susceptible to significant...

The characteristics of strong ground motion Introduction

In earthquakes the motion of any particle of the ground follows in general a complex three-dimensional path having rapidly changing accelerations, velocities, and displacements and a broad band of frequency content. Strong ground motion is measured by a large-amplitude type of seismograph called an accelerograph, in the form of an accelerogram, which is an acceleration history typically of the form shown in Figure 5.19, while Figure 4.19 Definition of effective duration (after Bommer and...

Y

Figure 10.15 Idealized shear deformation of beam-column panel zones may contribute up to about one-third of the inter-storey deflection in multi-storey buildings, and of this deformation about half may arise from the shear deformation of the panel zone itself. The large influence of panel zone behaviour on overall frame strength and stiffness has also been indicated by Kato (1974). If bilinear hinges were assumed in the panel zones, the ultimate shear resistance of the frame was developed only...

Bhfc

Mu design resisting moment (ultimate (MNm)) As total area of vertical reinforcement (mm2) fy yield strength of vertical reinforcement (MPa) h horizontal length of shear wall (mm) c distance from extreme compression fibre to neutral axis (mm) b thickness of shear wall (mm) Nu design axial load (ultimate), positive if compressive (N) f. characteristic cylinder compression strength of concrete (MPa) j1 0.85 for strength up to 27.6 MPa and reduced continuously at a rate of 0.05 for each 6.9 MPa...

Impediments to Earthquake Risk Reduction

There are many impediments to earthquake risk reduction (as discussed in various parts of this book, e.g. Section 1.4), which may be divided into two types finding out what to do physically to the environment and implementation. While steady progress is being made in research on what to do, the most intractable impediments arise in getting that knowledge effectively implemented. Even in relatively wealthy communities with well-developed bureaucracies implementation meets impediments,...

Probabilistic vs Deterministic Seismic Hazard Assessment

In the assessment of seismic hazard the terms deterministic and probabilistic do not have exact or unequivocal meanings in the way they are commonly used. This is recognized by the frequent use of the terms semi-deterministic or semi-probabilistic, where the word semi means (vaguely) part rather than (precisely) half. As discussed earlier in this chapter, the probabilistic approach to seismic hazard assessment accounts quantitatively for the uncertainties surrounding the values of the...

Earthquake Risk Management in Developing Countries

The management of earthquake risk in developing countries deals with the same problems in principle as are faced in developed countries. But the scale of the problems is worsened considerably because developing countries have less available resources of all kinds (physical, financial, educational and administrative) per capita. The problems are magnified by rapidly increasing populations (especially in cities), uncontrolled urban development spreading into marginal and more hazardous areas, and...

The Design and Construction Process Performance Based Seismic Design

Earthquakes provide architects, engineers, constructors and enforcers with a number of important considerations foreign to the non-seismic design and construction process. As some of these criteria are fundamental in determining the form of the 'structure', it is crucial that adequate attention is given to earthquake considerations at the correct stages in the process. To this end, a simplified flowchart of the design and construction process for earthquake resistant infrastructure is shown in...

Cyclic triaxial test

This test (Figure 3.6) is one of the best laboratory methods at present available for determining the shear modulus and damping of cohesive and cohesionless soils for use in dynamic response analyses (Sections 5.2 and 5.3). In this test, cyclically varying axial compression stress-strain characteristics are measured directly. The compressive modulus E so obtained is converted to the shear modulus G using the relationship where v is Poisson's ratio. The damping ratio may also be obtained from...

Estimating Casualties

As demonstrated forcefully by Table 1.1 and Figure 1.2, human casualties are a major problem in earthquakes. For managing the risk from a future strong earthquake centred in a populated area, realistic estimates of the number and nature of casualties are needed by various bodies, particularly emergency services, healthcare providers and insurers. An example of such an event is provided by the case of the large earthquake on the Wellington fault for which one component of the damage costs has...

Equivalent viscously damped response

Because of the complexity and expense of rigorously computing the effects of radiation damping in the foundation, an equivalent viscously damped response spectrum technique would be desirable. For estimating an equivalent viscous damping for a soil-structure system, the foundation damping (radiation plus hysteretic) is not directly additive to the structural damping, as described above. For structures with more uniform stiffness and mass distributions the equivalent viscous damping concept may...

Uniform and continuous distribution of strength stiffness and mass

This concept is closely related to that of simplicity and symmetry. The structure will have the maximum chance of surviving an earthquake if the following conditions are satisfied Very slender buildings may have excessive horizontal deflections Effects of fa ade setbacks cannot be predicted by normal code equivalent-static analyses Figure 8.6 Simple rules for elevation shapes of seismic buildings (only with dynamic analysis and careful detailing should these rules be broken) (1) The...

Site Investigations and Soil Tests

For any construction project it is normal to carry out some investigations of the site, generally using fairly standardized operations in the field and in the laboratory such as drilling boreholes and carrying out triaxial tests. In this section, only those investigating techniques related to the seismic response of soils are discussed. The scope of the site investigations will depend upon the site and on the budget and importance of the project, but in general it will be desirable to examine...

Models of the earthquake process

The following discussion was kindly contributed by David Rhoades. The earthquakes in a given region are the result of an ongoing physical process, and models may be made of (1) the physics of the phenomena producing the earthquakes and (2) statistical regularities in the series of times of occurrence, magnitudes, and locations of the resulting events. These two separate approaches are complementary both are needed to improve our understanding of the earthquake generation process. Wyss et al....

Upper and lower bounds on vulnerability

Math Magic Tricks Card Number

The mean damage ratio data for buildings and equipment from the various studies referred to above are plotted in Figure 6.23, and lines representing approximate upper and lower Figure 6.23 Mean damage ratio data from New Zealand earthquakes, for buildings and equipment as a function of intensity, with approximate upper and lower bounds (from Dowrick, 2003) Figure 6.23 Mean damage ratio data from New Zealand earthquakes, for buildings and equipment as a function of intensity, with approximate...

Regional Seismotectonics

In attempting to understand and then quantify seismic activity, whether it be interplate or intraplate in origin, working in the framework of global tectonics (Section 2.2) we try to relate seismicity to quantifiable deformational features such as faulting, tilting, warping or folding, or to major geological structures such as basins, grabens and platforms, which are basement rock features. The nature, age, location and movement history of these features need to be known. The magnitude and...

Criteria for reliability of performance General serviceability and safety criteria

The term reliability is used here in its normal language qualitative sense, and in its technical sense, where it is a quantitative measure of performance at given limit states, as stated in terms of probabilities (of failure or survival). Aspects of the probabilistic ingredients of reliability control are discussed in other parts of this book, notably the evaluation of seismic hazards and the question of acceptable risks. The required reliability is achieved if enough of the elements of the...

Tt 0 101016

Eccentric Braced Frame After Earthquake

10.2.6 Steel frames with diagonal braces Diagonally braced frames are discussed under two classifications, depending on whether the braces create perfect triangulation or not, namely concentrically braced frames (CBFs) and eccentrically braced frames (EBFs). Reliable ductility under cyclic loading is much more readily obtained from EBFs than CBFs, as will be evident from the following discussion. Concentrically braced steel frames Introduction The general characteristics of CBFs have been...

Developing Countries

Developing countries of course have special difficulties in reducing earthquake risk, arising from inadequate knowledge and physical resources. The fact that this is the most important chapter of this book (Section 8.1) is especially true for developing countries. People from such places should take particular note of Sections 8.3 and 8.7 above. A wise seismic structural form, tied together by means of so-called intermediate technology, and with basic but thorough enforcement of these simple...

Construction and the Enforcement of Standards

It is obvious that the standard of construction should match the standard of the design by meeting the requirements of the drawings and specifications for any given project. As described above, in relation to Figure 8.1, the design should be buildable and within the skills and experience of the constructors and fabricators. This often requires the involvement, to some degree, of the contractor in the design process. In larger and more complex projects, this may be automatic when the contractor...

Low Damage Structures Damage Avoidance Design

Much work has been done in recent years to find economical ways of creating structures which suffer low damage in design-level shaking, using some of the devices discussed above as well as subsequent ideas. For example, efforts have been made to increase the numbers of ways in which to use rocking mechanisms in columns and walls, with or without use of supplementary energy absorbers and vertical prestressing to recentre the vertical member after the shaking (e.g. Mander and Cheng, 1997). The...

Hybrid structural systems

Structures are often built in which the lateral resistance is provided by more than one of the above methods. The most common of these hybrid systems are those in which moment resisting frames are combined with either structural walls or diagonally braced frames. While hybrid systems are often unavoidable and can provide good seismic resistance, care must be taken to ensure that the structural behaviour is correctly modelled in the analysis. Interaction between the different components can be...

Concentrically braced frames

Concentrically braced frames are here defined as those where the centre-lines of all intersecting members meet at a point (Figure 10.11). This traditional form of bracing is, of course, widely used for all kinds of construction such as towers, bridges, and buildings, creating stiffness with great economy of materials in two-dimensional trusses or three-dimensional space frames. Concentrically braced frames are constructed from steel, timber and concrete, and composite forms are frequently met...

Structural walls shear walls

The term structural walls or shear walls refers to structures in which the resistance to horizontal forces is principally provided by walls. These walls are usually constructed of concrete, masonry, timber or steel, while other lesser structural materials such as gypsum, or composites are also encountered. As mentioned earlier in this chapter, the great advantage of structural walls is the protection their natural stiffness offers to non-structure through limiting inter-storey deflections....

Framed tube structures

The framed tube system is a special case of the moment resisting frame, which usually consists of closely spaced wide steel columns combined with relatively deep beams. These frames are usually, but not only, located on the perimeter of the structure, and introduce more stiffness to overcome the problems of excessive horizontal deflection of orthodox moment resisting frames, at the expense of a reduction in ductility. They have been widely used for tall buildings in high wind regions since the...

Specific Structural Forms for Earthquake Resistance

In the preceding sections of this chapter, we have considered the principles underlying good earthquake resistant design, which should be applied to the specific structural forms utilized for a given project. The various structural forms in use around the world all have their strong and weak points, conforming better to some of the above principles than others. The main structural forms suitable for earthquake resistance are (3) structural walls (shear walls) (4) concentrically braced frames...

Substructure and failure mode control

Although the form of the substructure must have a strong influence upon the seismic response of structures, little comparative work has been done on this subject. The following notes briefly summarize what appears to be good practice at the present time. The basic rule regarding the earthquake resistance of substructure is that integral action in earthquakes should be obtained. This requires adequate consideration of the dynamic response characteristics of the superstructure and of the subsoil....

Nonstructure and failure mode control

Non-structural elements have an important role in the reliability or predictability of seismic response of any given type of construction. In considering the form of a structure, it is important to be aware that some items which are normally non-structural become structurally very responsive in earthquakes. This means anything which will interfere with the free deformations of the structure during an earthquake. In buildings the principal elements concerned are cladding, perimeter infill walls...

Choice of construction materials

Hinge Beam Earthquake

Reliability of construction in earthquakes is greatly affected by the materials used for the constituent elements of structure, architecture, and equipment. It is seldom possible to use the ideal materials for all elements, as the choice may be dictated by local availability of local construction skills, cost constraints, or political decisions. Purely in terms of earthquake resistance, the best materials have the following properties (2) high strength-weight ratio (5) ease in making full...

The Strength of Earthquakes Magnitude and Intensity

During earthquakes the release of crustal stresses is believed generally to involve the fracturing of the rock along a plane which passes through the point of origin (the hypocentre or focus) of the event (Figure 4.25). Sometimes, especially in larger shallower earthquakes, this rupture plane, called a fault, breaks through to the ground surface, where it is known as a fault trace (Figure 4.41). The cause and nature of earthquakes is the subject of study of the science of seismology, and...

Performancebased seismic design

From around the 1980s there was a growing awareness that building codes, in general, provided a good level of life-safety protection, but were significantly less reliable in minimizing property damage in moderate and even small earthquakes. This concern led to the birth of performance-based seismic engineering, or performance-based seismic design. Work by the Structural Engineers Association of California (SEAOC) resulted in the publication of tentative design guidelines on this subject as an...

Info

In equations (5.77) and (5.78), Svn and San are the spectral velocity and spectral acceleration for mode n, and are as defined in equations (5.54) and (5.56). Equations (5.77) and (5.78) enable the maximum response in each mode to be determined. As the modal maxima do not necessarily occur at the same time, nor necessarily have the same sign, they are best combined on a probability basis. Various approximate formulae for superposition are used, the most common being the square root of sum of...

Simplicity and symmetry

Earthquakes repeatedly demonstrate that the simplest structures have the greatest chance of survival. There are three main reasons for this. First, our ability to understand the overall behaviour of a simple structure is markedly greater than it is for a complex one - for example, torsional effects are particularly hard to predict on an irregular structure. Secondly, our ability to understand simple structural details is considerably greater than it is for complicated ones. Thirdly, simple...

Ei Fu

Figure 5.27 Horizontal seismic shear diagram for lift core of 20-storey hotel building showing effect of brick partitions above the fourth floor To fully simulate the earthquake response of an infilled frame, a complex non-linear time-dependent finite-element dynamic analysis would be necessary, as provided by computer codes such as DRAIN-2DX. Such are the problems and effort involved in using such software, e.g. in modelling the structural behaviour of normal masonry infill, that more...

G

Where vs is the velocity of the shear wave and G is the shear modulus for the soil. A limiting value for v reached when the shear-wave stress r equals the shear strength of the soil c, and using the relationship vs *fG p, equation (4.26) becomes Thus the maximum particle velocity is a simple function of the mechanical properties of the local soil or rock. For a strong rock such as limestone or basalt (Table 5.3), using E G 3, E 105 MN m2, E c 600, p 2700 kg m3, it follows that vmax 1800 cm s....

Damping

The second key dynamic parameter for soils is damping. Two fundamentally different damping phenomena are associated with soils, namely material damping and radiation damping. Material damping (or internal damping) in a soil occurs when any vibration wave passes through the soil. It can be thought of as a measure of the loss of vibration energy resulting primarily from hysteresis in the soil. Damping is conveniently expressed as a fraction of critical damping, in which form it is referred to as...

Effects of microzoning on damage to houses as a function of MM intensity

Figure 6.19 plots the mean damage ratios (from Table 6.5) for one-storey weatherboard houses, (excluding chimney damage) in Greymouth at intensity MM7.5, with the two types of foundation, and on the four different ground classes described above. Two very different patterns are seen. First, houses with concrete foundations have steadily increasing damage levels as the ground becomes more flexible. This pattern follows the well-established trends of peak ground acceleration and peak spectral...

Crustal Strain and Moment Release

Considering any given area of the earth's crust, Kostrov (1974) expressed the relationship between strain rate tensor s and seismic activity in the form of the sum of the earthquake moment tensors, Mn, as Earthquake Resistant Design and Risk Reduction D. Dowrick 2009, John Wiley & Sons, Ltd where is the rigidity of the crust, A is the area under consideration, Hs is the thickness of the seismogenic crust, and m is the number of earthquakes within the volume over the time T. Thus, the volume...

Spatial patterns of ground motions

Rupture Distance

The spatial pattern of ground motions in the MW 6.7 Northridge, California, earthquake of 1994 is shown in Figure 4.25, as expressed by Dewey et al. (1995) in terms of Modified Mercalli intensity. The variability of the intensities at any given distance from the centre of the source is seen to be considerable. This irregularity is made particularly apparent as the isoseismals have been drawn without much smoothing. This inherent variability in the pattern of ground motions arises because the...

Survivability in extreme events

In addition to the two earthquake limit states discussed above, we sometimes need to consider that of survivability in more extreme events. While this has long been a required design condition for critical facilities (notably nuclear power plants and large dams), there has been a growing tendency to consider the survivability of other types of construction with acceptable risk levels much nearer to the norm. This practice has developed for a variety of reasons, in particular the following (1)...

Ground Classes and Microzones

As soil types and thicknesses, and to a lesser extent rock, vary widely from site to site in a region and worldwide, many different ways of classifying sites exist. Fortunately, as knowledge of site response to earthquakes has grown in recent years, there has been Total number of records analysed 104 Total number of records analysed 104 Soft to medium clay and sand - 15 records _ ' Deep cohesionless soils (> 250 ft) - 30 records ' Stiff soil conditions (< 150 ft) - 31 records Rock - 28...

Sources of accelerograms and response spectra

Earthquake engineers experienced at working outside basic code requirements have developed sources of information of their own, through government and university organizations specializing in seismology and earthquake engineering. As the problem of availability of information varies so widely from place to place and as the situation is changing so rapidly, this section will simply discuss a few of the chief sources of data presently existing. (i) Accelerograms of real earthquakes. The major...

Piled foundations Introduction

Horizontal Spring Stiffness Raking Piles

The reliable design of piles for earthquake loads is difficult because of the uncertainties involved in determining the design deformation state of the piles. This is partly due to the uncertainties involved in assessing lateral soil-pile interaction, and partly to the complexity of behaviour of pile groups. As indicated in Figures 9.1, 9.4 and 9.5, high bending moments may occur at various locations up the pile. In addition to the locations of high bending moments indicated by these idealized...

References

American Petroleum Institute (various years) API Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms. American Petroleum Institute, Washington, DC. Amin NR and Louie JJC (1984) Design of multiple framed tube high rise steel structures in seismic regions. Proc. 8th World Conf Earthq Eng, San Francisco V 347-354. Applied Technology Council (1982) An Investigation of the Correlation between Earthquake Ground Motion and Building Performance, ATC-10. Berkeley,...

Indian Plants Kasuda

Berrill J and Yasuda S (2002) Liquefaction and piled foundations Some issues. J Earthq Eng 6 (Special Issue) 1-41. Bhattacharya S (ed.) (2007) Design of Foundations in Seismic Areas Principles and Applications. National Centre of Earthquake Engineering, Indian Institute of Technology, Kanpur. Blaney GW, Kausel E and Roesset JM (1976) Dynamic stiffness of piles. 2nd Int Conf on Numerical Methods in Geomech, Blacksburg, USA, ASCE. Broms BB (1964a) Lateral resistance of piles in cohesive soils....

Designing for fault movements

Surface fault displacements in large earthquakes, i.e. of magnitude 7.5 or more, can be very large, horizontal and vertical displacements of 10 m and more having been measured in various parts of the world. For example, horizontal displacements of up to c. 13 m have been found by measuring the offsets of streams crossing the surface trace of the 1855 Wairarapa, New Zealand, earthquake (Grapes and Downes, 1997). While designing against such extreme displacements would seldom be contemplated, the...

Effect of soil on bedrock excitation

The presence of soil overlying bedrock modifies the excitation in a complex manner, with conflicting effects dependent on dynamic characteristics of the soil layers and the strength of the excitation. In many earthquakes, the degree of damage to structures situated on soils has been reported as worse than that occurring on adjacent bedrock sites. Measured on the subjective intensity scales, the intensity may increase by 1 or 2 units (or occasionally more) compared with bedrock, depending on the...

Elastic or viscoelastic halfspace

Modelling the foundation as a homogeneous linear elastic or viscoelastic half-space in which the stiffness and damping are treated as frequency-dependent provides a very useful means of allowing for the radiation damping effect. Various numerical and partly closed-form formulations of the theory have been made, such as those of Luco and Westmann (1971) and Veletsos and Wei (1971), and others as noted in the following discussion. Consider a rigid circular plate of radius R on the surface of an...

Steel beams

Moment Column

In this section, the behaviour and design of beams acting primarily in bending will be considered. In most beams axial forces are small enough to be neglected, but where large axial forces may occur column design procedures should be employed. Moment-curvature relationships for steel beams under monotonic loading For the adequate seismic design of the steel beams, and the associated connections and columns, the moment-curvature or moment-rotation relationship should be known. A long stable...

Springs and dashpots at the base of the structure

Coefficient For Rectangular Footing

The most rudimentary method of modelling the soil is to use only springs, located at the base of the structure, to represent the appropriate selection of horizontal, rocking, vertical, and torsional stiffnesses of the soil Figure 5.11 . An increase in the rigorousness of the model may be effected by adding dashpots at the same location. In the system shown in Figure 5.11 b , the stiffness of the individual vertical springs must be chosen to sum to either the global rocking stiffness or the...

Liquefaction of saturated cohesionless soils

Epicentral Distance Liquefaction

During earthquake shaking, some saturated granular soils may compact, increasing the porewater pressure, thereby decreasing the effective stress which results in a loss of shear strength. This phenomenon is generally referred to as liquefaction. It is usually confined to sands and cohesionless coarse-grained silts, and is more severe in looser, uniformly graded soils, and those with more rounded particles. Also, in truly undrained conditions, gravelly soils can be susceptible to liquefaction....

Earthquake Risk Reduction Actions

To reduce earthquake risk, each country needs to examine its strengths and weaknesses, build on the strengths, and systematically take actions which reduce or eliminate the weaknesses. An example of such an approach comes from New Zealand where a list of weaknesses was identified Dowrick, 2003 . Over a score of weaknesses were identified there in a preliminary list of weaknesses of a wide range of types. The weaknesses have been initially divided into two main categories, named strategic and...

Shear modulus

For soils the stress-strain behaviour of most interest in earthquakes is that involving shear, and, except for competent rock, engineering soils behave in a markedly non-linear fashion in the stress range of interest. For small strains the shear modulus of a soil can be taken as the mean slope of the stress-strain curve. At large strains the stress-strain curve becomes markedly non-linear so that the shear modulus is far from constant but is dependent on the magnitude of the shear strain Figure...

Stiff structures versus flexible

The terms 'stiff' and 'flexible' are relative ones, and must be interpreted with care. Some of their effects depend in part on the height of structure concerned. Table 8.2 summarizes some of the comparative merits of stiff and flexible construction, some of which have been discussed in the earlier parts of this section. A few further points of comparison are highlighted by discussing fully flexible structures. Fully flexible structures may be exemplified by many modern beam and column...

Damage due to liquefactioninduced differential ground deformations

Bird et al. 2005 have developed a straightforward analytical method of predicting building response to liquefaction-induced differential settlement and differential lateral deformation of foundations of buildings. An outcome of the method is sets of vulnerability fragility curves of the type shown in Figure 6.26. The vertical line across the probability plots on the figure shows that for buildings subjected to a different settlement Afv 200 mm, a single building would have a 35 probability of...

Seismic soil pressures

Coulomb Active Earth Pressure

In the present state of knowledge, the recommended method of obtaining seismic soil forces is that using equivalent-static analysis. Only for exceptional structures would dynamic analyses using finite elements seem warranted. In the equivalent-static method, a horizontal earthquake force equal to the weight of the soil wedge multiplied by a seismic coefficient is assumed to act at the centre of gravity of the soil mass. This earthquake force is additional to the static forces on the wall. In...

Eccentrically braced frames

Deformed Braced Frame

Traditional design of trussed structures lays great importance on keeping the forces in the structure to axial only, avoiding moments by ensuring that the centre-lines of all intersecting members meet at a point, i.e. concentrically Section 8.4.4 . However, starting in the late 1970s, the concept of using deliberately eccentric bracing for earthquake resistance purposes has been found to have certain advantages, so far principally for steel structures, with major structures being designed this...