On the Need for a Regional Policy on Seismic Resistant Design and Construction

Transcription

1 On the Need for a Regional Policy on Seismic Resistant Design and Construction H C UZOEGBO Department of Civil Engineering University of the Witwatersrand, Johannesburg UZOEGBO@civen.civil.wits.ac.za P WARNITCHAI Asian Institute of Technology, Thailand Abstract This paper discusses the occurrence of earthquakes in regions that are generally classified as earthquake-free. Southern Africa is considered an earth-quake free region in spite of several cases of reported minor earthquakes which has caused considerable damages and casualties particularly to the mining industry. Buildings and structures are not designed to resist any intensity of earthquake. The unique seismicity of Southern African region is used in support of a proposal for a mandatory seismic resistant design for certain category of structures. Relatively simple design methods which may be used to achieve seismic resistance in regions of moderate seismicity are discussed. INTRODUCTION The Southern African region has long been considered by most people as an earthquake-free zone. Most architects, engineers and builders do not consider seismic resistance as a design requirement. This is mainly because the region has not experienced any serious destructive earthquake in recent years. The most destructive earthquake in recent years appears to be the Ceres earthquake of The earthquake with a magnitude of 6.3 occurred at 8 pm on September in the Ceres-Tulbagh region of the Cape province about 100 km from Cape Town. The shocks and aftershocks were felt as far as Durban (1 175 km) and it claimed nine lives and caused damages to property worth over R20 million. The unique character of mine induced earth tremors are discussed and a review of the seismic hazard mapping was carried out. The frequency of earthquakes and the possibility of occurrence are also discussed. SOUTHERN AFRICAN SEISMICITY The study of the seismicity of Southern Africa started as early as Over the past two decades, researchers and scientist have put in a lot of work in the earthquake hazard mapping of the region (South Africa, Mozambique, Zimbabwe, Botswana, Namibia, Lesotho and Swaziland). This includes recent work published by Fernandez (1980, 1989); Fairhead (1997) and Kijko (1999) among others. The map of the major tectonic plates shown in Figure 1 indicates that the southern African region is not traversed by any of the major tectonic plates boundary. Southern Africa is located in the interior of the large African plate as shown in Figure 1. The borders of this plate to the south are located in the mid-atlantic and mid-indian ocean ridges. The continent does not appear to be affected by the

2 distant tremors of this belt. The East African rift system is a clear line of intra-plate seismicity, which has been shown to extend to Southern Africa. The zones of active seismicity in SA are in Mozambique, Zimbabwe and northern Botswana. These activities are believed to be an extension of the East African rift system. South Africa has experienced two recorded major events. One in the Orange Free State in 1912 (IX on Rossi scale) and the other was the Ceres earthquake in the Cape Province in 1969, which measured 6.3 on Richter scale. The seismicity map of recorded events in the region is shown in Figure 2. The seismicity of areas located in the interiors of the major tectonic plates is low but it is difficult to correlate with known tectonic characteristics. The SA region is located in an intra-plate area but it is associated with a rather complex seismic characteristic. Most destructive earthquakes are of a tectonic origin and the epicenters of over 90% of global natural earthquakes occur at the boundaries of the major plates. The seismicity of Southern Africa with its sporadic, scattered shallow earthquakes belongs to the type of intra-plate activity which may occasionally reach critical values such as the earthquake of 8 December 1976, in Welkom, Orange Free State which measured 5.5 on Richter scale. Figure 1: Map showing global tectonic plates (source: Fernandez and Guzman 1980)

3 Figure 2: Map of seismic hazard in southern Africa. (Source: Fairhead and Henderson,1977) Most of the seismic events in the region of magnitudes below 5M originate in the mining areas. The epicenters of mine tremors are shallow, usually lower than 2 km and are hazardous only in the immediate vicinity of the mines. The natural earthquakes have a much lower frequency of occurrence but may are much more hazardous to the region. In Table 1, a data on known seismic activities in the region up to 1986 is presented. Table 1: Parameters of known seismic activity in Southern Africa. (Source: Shapira et al) Magnitude (M L ) Number of events Data complete since Period (years) > 3 > 4.5 > 4.8 > 5.2 > 5.9 * Mostly mine tremors * The presentation of the frequency of occurrence is shown graphically in Figure 3. The graph shows a rapid drop in the frequency of occurrence with increase in the intensity of earthquakes.

4 Figure 3: Magnitude-recurrence relationship Scientists conclude that the irregular pattern of epicenters does not correlate with geological and physiographic features. It is indicated that no part of the region is free from occasional small shocks. EARTH TREMORS RELATING TO MINING ACTIVITIES It is known that up to 40 or more tremors are recorded monthly in Southern Africa. The recordings are predominantly in the places surrounding the gold mining areas like the Transvaal and the Orange Free State. Many events are recorded around the Carleton and Klerksdorp areas annually. Mine induced tremors have distinct characteristics compared with natural (tectonic) earthquakes. Milford (1997) and Ockleston (1968) reported that tremors in the region have a characteristic high peak horizontal accelerations and velocities measuring up to 0.45 g and 67 mm/s respectively. In Carletonville (1986) a peak acceleration of 0.45 g was measured. This is even higher than the peak acceleration of 0.36 g obtained from the well known El Centro earthquake of The difference between the two seismic activities is that while the El Centro earthquake caused massive damages to structures, the Carletonville tremor only caused minor damages and cracking to structures. The reason for the differences can be seen by looking at other characteristics of mine tremors. The frequency range is 10 Hz to 50 Hz. This implies that mine tremors are not likely to produce structural response from structures with natural frequencies less than 2 Hz. Most high rise buildings have natural frequencies less than 2 Hz and are therefore not expected to be affected. Figure 4 shows the estimated natural frequencies for different categories of structures. It can be seen that the single storey residential buildings fall in the frequency range of mine tremors and are expected to be affected by the tremors. Mine tremors occur at relatively shallow epicenters, typically at depths of 2-4 km. This means that the spread is over a small area. Moreover the attenuation of mine tremors is rapid. For example an acceleration of 0.39 g measured at 2.5 km reduces to 0.13 g at a 10 km distance.

5 Figure 4: Natural frequency and natural period for various types of structures. BASIC ISSUES ON EARTHQUAKE RESISTANT DESIGN The major advances made over the years in seismic design have been the outcome of better understanding of the non-linear dynamic response of structures to earthquake loading. Some relatively simple empirical rules may be used to achieve seismic resistance in concrete structures. Some of the basic requirements for such a structure are described below. Structural configuration Studies of failure of structures in previous earthquakes have shown that buildings with regular or symmetric configurations both horizontally and vertically resist earthquake forces much better than buildings with irregular configuration. Therefore the first rule is to keep it as symmetric as possible in both axes. It is also recommended that the center of rotation (CR) and the center of mass (CM) should coincide as much as possible as shown in Figure 5.

6 Figure 5: Lateral Force resisting Component Ground floor columns The ground floor columns of a tall building is the susceptible to failure. This is because it is subject to the highest applied moment and it usually has less partitions which makes it less rigid than the upper storeys. It is important that particular attention is given to the design and detailing of this part of the structure. Appropriate mechanism of post elastic deformation In the case of a strong earthquake which may result in yielding it is possible to decide by design the positions of the plastic hinges. Column side-sway should be avoided. This can be achieved by designing the elements such that the beam yields first. The failure mechanisms are illustrated in Figure 6.

7 Figure 6: Post yield failure mechanisms. Additional requirements The reinforcement should be detailed to ensure adequate ductility in the yielding regions during earthquakes. The inter-storey drift during earthquakes should not lead to excessive damage. RECOMMENDATIONS For structures located in the mining areas additional provisions should be made for the design and detailing of low rise residential buildings to minimize damages during tremors. All buildings which are more than five storeys should be designed to resist a moderate earthquake. This should be in line with zone 1 of the UBC code in the USA. REFERENCES 1. Fernandez, LM and Guzman, JA (1980), Earthquake hazard in southern Africa, Seismological series 10, Geological Survey of South Africa, Pretoria. 2. Fernandez, LM (1989), Catalogue of earthquakes in Southern Africa and surrounding oceans, Seismological series, Department of Energy and Mineral Affairs, Geological Survey, Pretoria. 3. Fairhead, JD and Henderson, NB (1977), The seismicity of southern Africa and incipient rifting, Tectonophysics 41, T19-T Kijko, A and Graham, G (1999), Parametric-historic procedure for probabilistic seismic hazard analysis, Pure and Applied Geophysics, 154, pp Shapira, A, Fernandez LM and Du Plessis, A (1989), Frequency-magnitude relationships of southern African seismicity, Tectonophysics, 167, pp , Elsevier science publishers. 6. Ockelston, AJ (1968), The effect of earth tremors on high rise buildings in Johannesburg. Forum on high rise buildings, The South African Institution of Civil Engineers.

8 7. Milford, RV and Wium, DJW (1997), Impact of seismic events on buildings in mining areas, Division of Building Technology, CSIR, Pretoria.