IMPROVING EARTHQUAKE RESISTANCE OF BUILDINGS 269 the point of view of building response, an even more important measure of fre- quency content is the response spectrum, which is discussed below. 8.2.2 Building Response to Ground Motion When a rigid object is shaken, so-called inertia forces act on it which increase according to the acceleration of the object, and to its mass. If an absolutely rigid building is firmly tied to the ground, and shakes with the ground, then the inertia forces are transmitted from the ground into the building: the magnitude of the force is proportional to the mass of the building and varies with time in the same manner as the acceleration. This simple model is unfortunately inadequate, however, because no real build- ing is quite rigid. All buildings deform to some extent as they are shaken, and the deformation of the building substantially alters the force distribution. Small, massive buildings are relatively stiff, but as buildings become taller and lighter they tend to become more flexible. When a flexible building is shaken, the force acting on any part of it is still proportional to the mass and acceleration of that part, but the distribution of forces within the building depends on the way the building itself deforms. Depending on the mass and flexibility of the building, the accelerations within the building may be greater or less than the ground accelerations, and thus the forces may also be greater or less than if the build- ing was a rigid body. The consequences of this for building design are of great significance. The property of a building which principally determines its dynamic response to earthquake ground motion is its natural frequency. Because all buildings are flexible they will vibrate when jolted, and they will then sway backwards and forwards in a regular way. Taller buildings have lower natural frequencies (they sway more slowly) than lower buildings. A building 10 storeys high may take about a second to sway backwards and forwards in one cycle, i.e. its natural period is 1 second. A building of two storeys will take about one-fifth of a second: its natural period is 0.2 seconds. (A rough guide is that each storey adds about one-tenth of a second to its natural period.) Three- to five-storey structures are likely to have a natural period in the order of 0.3 to 0.5 seconds. High-rise frame buildings of 10 to 20 storeys have periods of between 1 and 2 seconds. And very high buildings can have period up to 4 seconds or more. If the disturbance is a short one, the swaying will continue after the disturbance has finished, but it will gradually die away. The rate at which the swaying decays after the end of the disturbance is a measure of the damping in the building’s structural system. If the building is shaken by regular ground oscillations (like the effect of a rotating machine), its response will depend on the relationship between the frequency of these oscillations and the natural frequency of the building. For ground motion frequencies much less than that of the building, the building will 270 EARTHQUAKE PROTECTION simply move with the ground, and deform very little; as the frequency of the ground motion increases, so the deformation of the building will increase, and when the two frequencies are equal the building deformation will reach a peak which may be many times greater than that of the ground. The ground motion and the building are in resonance. For frequencies of ground motion still greater than the natural frequency of the building, the deformation of the building will be less the further away it is from the resonant frequency. The relationship is illustrated diagrammatically in Figure 8.2. When a building is shaken by a real earthquake, which has a ground motion consisting of a mixture of frequencies all added together, its response will depend both on the natural frequency of the building and on the frequency content of the earthquake. A 10-storey building, with a natural frequency of 1.0 cycles per second, will be particularly affected by the component of the ground motion with this frequency, but much less by the components with higher and lower frequencies. The effect of a particular earthquake ground motion on a range of build- ings is shown by the response spectrum. The response spectrum for a particular ground motion shows what the maximum response would be to that ground motion for buildings 5 of different natural frequencies. Its shape depends on the Figure 8.2 Diagrammatic representation of the response to a 10-storey building to the frequency of ground motion vibration 5 Or, more strictly, to damped mass–spring systems, which are a useful mathematical idealisation of building structures. IMPROVING EARTHQUAKE RESISTANCE OF BUILDINGS 271 Figure 8.3 Typical response spectra and the building types they affect frequency content of the earthquake and on the degree of damping of the build- ing. Figure 8.3 shows some typical examples of response spectra. Example A is what the response spectrum might look like for a site close to the epicentre of an earthquake on firm soil or rock. It has a peak value of around 3 cycles per second. It would therefore be most damaging to low-rise buildings, but less so to taller structures, which would experience smaller forces. Example B shows a typical shape for a site at some distance from the epicentre and on a soft soil, with a peak value at about 1.0 cycles per second. This event would be especially damaging to the taller structures, but would be felt much less strongly by the low-rise structures. An example of this second type of behaviour was the 1985 Mexico City earthquake which caused ground motion in the lake bed area of the city with a period strongly concentrated around 2 seconds; the earthquake caused partic- ularly serious damage to recently constructed 10–20-storey apartment blocks, while leaving much of the older, weaker, low-rise masonry much less severely damaged. The response spectrum is commonly used in building design codes to define the design earthquake which buildings should be able to resist without damage. Codes are discussed further in Section 8.6. 272 EARTHQUAKE PROTECTION 8.3 How Buildings Resist Earthquakes Many small buildings are so stiff that they can be assumed to be rigid in a first estimate of earthquake forces. If a horizontal shaking occurs, the forces on each element of the building can be found by assuming that it is static, but has a horizontal force acting on it (through its centre of gravity) proportional to the ground acceleration and to the mass of the element, but in the opposite direction. This is what is referred to as the inertia force. The effect of vertical shaking is similar. The resistance of this stiff building is principally determined by the ability of the structure to transmit these large and rapidly varying inertia forces to the ground without failure. Consider first a single-storey building, consisting of four walls (with window and door openings) and a flexible roof which sits on two of the walls, but does not tie them together, see diagram A in Figure 8.4. The effect of a primarily vertical ground shaking will be to increase or decrease the vertical forces, but as the structure is capable of carrying substantial vertical gravitational forces under normal conditions, it can usually accept extra vertical forces without difficulty. The effect of a horizontal shaking parallel to two of the walls will be to set up horizontal inertia forces on each wall in proportion to their mass: the forces on the walls parallel to the direction of the shaking (the in-plane walls) will be along their length, while those on the perpendicular walls (the out-of-plane walls) will be at right angles to them. The force on the roof will also cause an additional horizontal force to be transmitted on to whichever wall supports it. The principal effect of out-of-plane forces is to cause the walls to bend (i.e. deform out of Figure 8.4 Response of single-storey masonry building to earthquake ground shaking IMPROVING EARTHQUAKE RESISTANCE OF BUILDINGS 273 their plane), which can cause damage to brittle masonry structures even under low levels of loading. Wall elements tend to be stronger under in-plane forces: these cause in-plane shear forces which are easier to resist in a solid wall or can be provided for by bracing or other means. In the same building the effect of a horizontal force in the direction perpen- dicular to that just described would be to exchange the responses of the walls, those previously out-of-plane becoming in-plane and vice versa. Thus under a real earthquake shaking, with horizontal shaking in all directions, all walls are subjected to both out-of-plane bending and in-plane shear simultaneously. This type of building tends to have little resistance to earthquake forces. If instead the roof is constructed in such a way as to tie the tops of the walls together as a rigid diaphragm, the behaviour will be different, as in diagram B in Figure 8.4. The unresisted out-of-plane bending of diagram A will be prevented, as the out-of-plane wall will be connected to the roof diaphragm member, which is then able to transfer the forces involved to the tops of the stiffer in-plane walls, and then to the ground. In addition the continuity of the roof will also tie the corners together, inhibiting corner cracking. Under shaking in the other plane, the behaviour is the same in reverse. Thus these elements – the stiff vertical shear wall in each direction, to carry the loads to the ground, and the stiff horizontal diaphragm, to transfer the earth- quake forces at this level to the appropriate wall – form the basis of an effective earthquake-resistant structural system. The same system can be used as effectively in multi-storey construction, in which case the horizontal loads to be transmitted by the shear walls increase (as do the vertical gravitational loads) from top to bottom of the building, so that the ground floor walls are required to transmit to the ground the horizontal forces acting on the whole building. However, the use of extensive shear walls can often create serious limitations on the planning of a building, and the equivalent shear strength can also, in some cases, be achieved by means of alternative vertical elements such as braced frames and moment-resisting frames (Figure 8.5). In the braced frame, the bracing members transmit the horizontal forces in tension and compression; such frames can be very stiff but are often appropriate only on the external walls of a building. In the moment-resisting frame,the horizontal forces are transmitted by bending moments in the columns and in their framing beams. The moment-resisting frame can be designed (using steel or reinforced concrete) to be as strong as required, but frame structures will tend to be rather more flexible than braced or shear wall structures. Similarly it is not always necessary (especially in a small building) for a fully rigid diaphragm to be provided at each level. Cross-bracing of a framed floor (steel or timber or trusses), along with the provision of a ringbeam in concrete or even timber, may in some cases be an adequate alternative. Where a flexible, moment-resisting frame is to be used, care also needs to be taken with the additional bending moments in the columns which arise from 274 EARTHQUAKE PROTECTION Figure 8.5 Alternative earthquake-resistant structural forms: shear wall structures, moment-resisting frames and braced frames the relative displacement of their ends. This so-called P-delta effect can be the cause of rapid material breakdown and collapse if adequate provision has not been made for it. 6 8.4 Structural Form and Earthquake Resistance The simple elements of an earthquake-resisting structure described in the pre- vious section can be provided in a great variety of ways. But simply providing these elements is unfortunately not sufficient to guarantee good performance in an earthquake. The static force analogy presented above fails to explain the complex behaviour of real structures subjected to the unpredictable, large and rapidly vary- ing forces of real earthquakes. In addition, there are certain principles of overall structural design which need to be observed. Structures should be symmetrical, continuous, small in plan, not elongated in plan or elevation. Experience has repeatedly shown that simple structures symmetrical in plan perform much better in earthquakes than complex and unsymmetrical ones. The force distribution in complex and unsymmetrical structures under earthquake loading is extremely difficult to predict; torsional forces are liable to be set up if the centre of mass is not coincident with the centre of resistance, and this can cause local failures. Adequate design of members and details with complex arrangements and under complex force systems is much more difficult than for simple cases. The same applies to re-entrant plan shapes even if they are symmetrical. Uniformity and continuity of structure are of equal importance, because changes in cross-section, either in overall elevation or in one particular element, cause concentrations of stress which are very damaging. 6 For further details see Dowrick (1987), Penelis and Kappos (1997) or Booth (1994). IMPROVING EARTHQUAKE RESISTANCE OF BUILDINGS 275 Experience has shown 7 that a structure will have the maximum chance of surviving an earthquake if: • the load-bearing members are uniformly distributed; • the columns and walls are continuous and without offsets from roof to foundation; • all beams are free from offsets; • columns and beams are co-axial; • reinforced concrete columns and beams are nearly the same width; • no principal members change section suddenly; • the structure is as continuous (redundant) and monolithic as possible. The concept of redundancy implies that any applied load can find many alter- native routes (load paths) to the ground. Given the unpredictable nature of earthquake motions and the real chance of local overload, a structure designed so that if one element fails others will be able to carry its load must evidently have a better chance of survival in an earthquake. Avoid Soft Storeys One particular type of discontinuity is worth elaborating on. Very commonly multi-storey frame buildings are provided with cross-walls or frame infilling in residential upper storeys, but these are omitted or partially omitted on the ground floor to provide open commercial or car-parking space; this is often the cause of a serious weakness on this floor. This has been the cause of the disastrous failure of the ground floor of many buildings such as that illustrated in Figure 8.6. The effect of setbacks in elevation is similar and these should also be avoided for the same reason. Plan Size and Slenderness Limitation Limiting the size of a building size in plan is important because earthquake forces vary rapidly in both time and space and a long building is likely to have different ground movements applied to it at each end, coupled with ground distortion along its length. Where a long building is needed for planning reasons, it is likely to perform better if subdivided into separate short lengths of structures with movement gaps between them. 8 The slenderness of a building should also be restricted to limit horizontal deformations: a height/width limitation of 3 or 4 has been proposed, 9 although this can be exceeded with good design. 7 Dowrick (1987). 8 The legendary survival of Frank Lloyd Wright’s very large Imperial Hotel in the 1923 Kanto (Tokyo) earthquake has been partly attributed to its separation in this way. 9 Dowrick (1987). 276 EARTHQUAKE PROTECTION Figure 8.6 Collapse of reinforced concrete buildings in Adapazari, Turkey, in the 1999 Kocaeli earthquake Columns Stiffer than Beams In framed buildings, additional important rules of design must be observed. One requirement is that columns should be stiffer than the beams which frame into them. If this is the case, the beams will fail before the columns, limiting failure to the area supported by the beam and enabling the beams to be used as energy absorbers; where the columns begin to fail first, failure tends to occur very rapidly, under their vertical load. Infill Panels The use of stiff infill panels in framed buildings as cladding or as internal or external partitions presents serious problems: often they are not treated as a part of the structure and are themselves weak. However, in an earthquake they tend to attract load initially, because of their stiffness. When they fail, this will be a brittle type of failure, which can cause serious damage to the main structure, as well as injury to occupants, and result in serious economic loss to the building, even if the main load-bearing structure is unharmed. Thus infill panels either should be treated as a fully integral part of the structure (making it a shear wall not a frame structure) or should be totally separated from it by movement joints which allow the frame to move independently. This latter approach presents detailing problems if the structure is expected to support the infill panel under normal conditions. Infill panels can have an equally disastrous effect if they are IMPROVING EARTHQUAKE RESISTANCE OF BUILDINGS 277 discontinuous in either elevation or plan. The effect of this is to create regions of high stress concentration in a structure for which it was not designed, causing local failures. Separation Between Buildings Individual buildings need to be provided with adequate separation, to prevent damage caused by pounding when they deform in earthquakes, which has been a serious cause of damage, even of collapse in recent earthquakes. The mini- mum separation gap depends on the height and flexibility of the building. The gap between buildings should exceed the expected cumulative maximum drift (lateral displacement) of all storeys added together with an extra allowance. Separation can be a particularly difficult problem to deal with where a tall building of complex or large plan is divided into smaller separate structural elements for reasons discussed above. The gaps created then generally need to be bridged to preserve functional continuity, but it is essential that any bridging should be designed not to transmit forces, so as to maintain structural separa- tion. 10 Alteration to Existing Buildings Stress concentrations are very frequently caused by supposedly non-structural alterations carried out on existing buildings when their function changes. Not only the addition or removal of partitions, but also the positioning of windows, doors and staircases can significantly affect the earthquake performance of a building. Vertical or lateral building extensions, particularly where new materials are to be used, can be equally damaging. Non-structural Elements Finally, to achieve good earthquake performance, it is essential to pay attention to the non-structural elements of a building. 11 In recent earthquakes a high propor- tion of the damage was unrelated to the main structure of the building. Heating and cooling plant, fuel, electricity and water supply mains, elevator equipment, etc., need to be secured to resist earthquakes, otherwise serious damage includ- ing fire outbreaks can occur. Heavy furniture and equipment such as bookstacks need to be properly secured. Flying glass is a serious hazard in urban areas and for flexible high-rise buildings detailing for movement is needed. Cupboards and bottles containing hazardous chemicals have to be specially designed to avoid spillages. 10 Solutions have been discussed by Arnold and Reitherman (1982) and Dowrick (1987). 11 Lagorio (1991). 278 EARTHQUAKE PROTECTION The jamming of doors in buildings as a result of deformation is a serious hazard as it may prevent escape or rescue; doors should be detailed so that some movement of the structure can occur without causing them to jam. Foundations Foundations, particularly for large buildings, should equally be kept as simple as possible. Only one type of foundation should be used for the whole building (or any structurally independent part of it). Separate column or wall foundations should be interconnected so as to achieve an integral action, and should all rest at the same level. Foundations should be loaded approximately uniformly under vertical load, and where possible sites with large variations in subsoil conditions should be avoided. 8.4.1 Engineering Techniques for Improving Earthquake Resistance Some new engineering techniques for modifying the structure to achieve better earthquake resistance are available, and can be expected to become more widely used in the future. The most important of these techniques are base isolation, and the use of energy absorbers. Base Isolation The principle of base isolation is to introduce some form of flexible support at the base of a building so that earthquake forces transmitted to the building are much lower than if the building is firmly fixed to the ground. The simplest form of base isolation is a frictional sliding layer, which will slip if the force exceeds a certain proportion (perhaps 3–5%) of the weight of the building. As such slip is likely to result in permanent displacements, a spring system is normally preferable. Spring systems will transmit forces proportional to the relative movement of the ground and the building, and incorporating them will increase the natural period of vibration of the building, hence (for most earthquakes) considerably reducing the forces the building experiences. 12 Laminated rubber springs are the materials most widely used; they have a much lower stiffness in the horizontal direction than in the vertical direction, and thus are effective only to reduce the damag- ing horizontal forces. A lead core is incorporated to provide energy absorption through damping, thus further reducing the earthquake loads experienced by the building. 13 12 Key (1988), p. 70. 13 Base isolation techniques have been discussed by Key (1988) and by Buckle and Mayes (1990). [...]... Although welded joints can be a source of weakness and have resulted in some failures in recent earthquakes 280 EARTHQUAKE PROTECTION reinforcement, suitably placed, they can be made to perform in a semi-ductile manner, making them suitable for earthquake- resistant construction Since the extra forces resulting from an earthquake are proportional to the mass of the structure, structural materials which are... models for the codes of practice used in other countries In California, the level of resistance aimed for in design has, since the late 1970s, been based on the concept of an ‘acceptable risk’ The objectives are:20 1 To resist minor earthquakes without damage 2 To resist moderate earthquakes without significant structural damage, but with some non-structural damage 3 To resist major or severe earthquakes... other aspects of the earthquake- resistant quality of the building (regularity of form, low eccentricity, good damping, redundancy); • the reduction in base force with increased natural frequency of the building Figure 8.7 shows the general shape of the standard elastic spectral response curves for design of structures defined in the 1997 US Uniform Building Code The actual curve for any location is generated... Z (for the seismic zone applicable) and a soil profile type.24 The class of the soil is derived 24 Bachman and Bonneville (2000) 284 EARTHQUAKE PROTECTION 2.5 Ca 0.9 0.8 Spectral acceleration (x g) 0.7 0.6 Cv/T 0.5 0.4 Ca 0.3 0.2 0.1 0 0 1 2 Period, T (seconds) 3 Figure 8.7 A typical response spectrum for use in design (using design response spectral formula given in 1997 Uniform Building Code for. .. consequences for earthquake resistance is poor bonding of the stonework or other 26 Coburn and Hughes (1984) IMPROVING EARTHQUAKE RESISTANCE OF BUILDINGS 287 Figure 8.9 Common defects in traditional stone masonry construction at wall–roof junctions that increase earthquake vulnerability (after Coburn and Hughes 1984) 288 EARTHQUAKE PROTECTION Table 8.2 Examples of low-cost and no-cost measures to reduce earthquake. .. (1986) et al (1995) 292 EARTHQUAKE PROTECTION Figure 8.12 Increasing levels of earthquake resistance with increasing levels of cost and building skills required for traditional stone masonry building, Eastern Turkey (after Coburn 1986a) IMPROVING EARTHQUAKE RESISTANCE OF BUILDINGS 8.7.3 293 Manuals for Strengthening Traditional Construction In an attempt to reach those whose earthquake safety is not... loads and so on) required for design are not specified, but left to be decided by national authorities through a National Application Document An important principle is complete compatibility between this code and the independent Eurocodes for steel, concrete, timber and masonry Similar codes have been formulated for application in many countries, but in some of the poorer earthquake- prone countries,... Low-strength unreinforced masonry materials (such as rubble, stone and adobe) have an extremely poor seismic performance and should be avoided whenever possible However, where there is no economic alternative to their use, even these materials can, with suitable reinforcement of timber, steel or reinforced concrete, be made to behave in a semi-ductile fashion which will significantly improve their performance... experience in the 1994 Northridge earthquake in the United States (EEFIT 1994) and the 1999 Kocaeli earthquake in Turkey (EEFIT 2002b) 18 Some rules for detailing such structures have been proposed by Smith (1988) IMPROVING EARTHQUAKE RESISTANCE OF BUILDINGS 281 is not well preserved, its deterioration can be a problem, and it contributes to the risk of fire damage in earthquakes Heavy roofs supported... wave velocity or penetrometer test values in the top 30 metres Design response spectra for the codes applicable to other countries are structured in a similar way Codes also specify procedures for distributing the total load thus computed between the different levels of the building, and for combining the earthquake forces with the other types of loading which the structure experiences (dead load, live . Northridge earthquake in the United States (EEFIT 199 4) and the 199 9 Kocaeli earthquake in Turkey (EEFIT 2002b). 18 Some rules for detailing such structures have been proposed by Smith ( 198 8). IMPROVING. Imperial Hotel in the 192 3 Kanto (Tokyo) earthquake has been partly attributed to its separation in this way. 9 Dowrick ( 198 7). 276 EARTHQUAKE PROTECTION Figure 8.6 Collapse of reinforced concrete buildings. discussed by Arnold and Reitherman ( 198 2) and Dowrick ( 198 7). 11 Lagorio ( 199 1). 278 EARTHQUAKE PROTECTION The jamming of doors in buildings as a result of deformation is a serious hazard as it