6. Building types and vulnerability to ground shaking
China has a population of c. 1.3 billion, spread across a land area of 9,560,975 km2. In view of China’s geographical extent, and long cultural history and development, it is hardly surprising that this population occupies a great variety of building types, ranging from cave dwellings to the most modern engineering-designed high rise buildings. These numerous building types have been condensed into 28 categories (Table 1) (Yin, 1995), which summarise building materials used together with a vulnerability index described later in this report. While mud is also used, brick is a common building material in towns and cities. Frequently brick and masonry buildings are in poor repair thus increasing their vulnerability. Increasing industrialisation has seen a corresponding increase in industrial buildings, while as wealth has increased there has been a rise in the number of taller, multi-storey, steel-frame structures. A probabilistic seismic risk analysis could be based on the all-embracing fine detail of the 28 structural categories shown in Table 1, invoking “damage probability matrices” for each specific structural category of building. In practice, however, the 28 categories have been reduced and simplified to four broadly representative groups:

This reduction from 28 to four representative categories requires detailed knowledge of the extent to which built structures can withstand strong ground shaking or ‘seismic demand’. Such an analysis of seismic vulnerability requires evaluation of building design, building materials and general characteristics of a group of buildings. This knowledge may be gained in the laboratory using shake tables and ‘pushover’ experiments, and in the field following a damaging earthquake. Ultimately if seismic demand exceeds the capacity of a building to resist then complete collapse ensues. Prior to this end-point, however, several discrete Damage States (DS) can be recognised (Table 2).

Table 1. Building type and vulnerability index range, V_index (Yin, 1995)

When several categories of building in Table 1 display similar damage states for similar levels of strong ground shaking then these categories can be grouped together into a vulnerability class, classes being A, B, C and D. This grouping is underpinned by calculation of a vulnerability index, V_index, defined by Yin (1995).

This index sums losses over all possible damage states (Table 2), averaged over the five expected damaging intensities VI to X for a specific building type or category. The outcome is that V_index provides a means to quantify and rank the earthquake resistance of built structures.

Vulnerability expressed as the probability of one of the five Damage States or grades, DSj, occurring at an intensity degree, I_i, can be compiled as a Damage Probability Matrix (DPM). An example DPM for Civil Brick Houses taken from Chen et al. (1999) is given in Table 3. The sub-division of building damage into five states or grades, each with its own attached Damage Factor (repair/replacement cost range) allows risk to be calculated i.e. likely economic losses subsequent to a forecast level of earthquake strong ground shaking or degree of intensity.

Table 2. Brief description of earthquake Damage State (DS) or Grade (Yin, 1995).

Table 3. Damage Probability Matrix for Civil Brick Houses (Chen et al., 1999)

The robust division into four building types is a large but useful simplification. However, not surprisingly when V_index is inspected in Table 1, buildings of mud and broken stone are seen to be least resistant while steel reinforced concrete buildings with shear walls etc. are most resistant to strong ground shaking. Parts of ultra-modern China have replaced the traditional brick buildings with modern high-rises such as in Pudong, Shanghai (figure 10). Nevertheless with circa “1.3 billion people spread through a land area of 9,560,975 sq km” there is an enormous and extremely varied building stock. In cities, towns and urban areas spread through China, much of the building stock is brick and is vulnerable, and as noted earlier some is in poor repair - therefore replacement, retrofit or even inventory tasks alone are formidable.

Figure 10. Modern and ultra-modern Shanghai (Photos PW Burton and SW Cole)

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