A model study on affordable steel residential housing in China

Abstract

The main research purpose of this paper is to acquire a series of designed concept of “affordable housing” in key geographical areas of East China through the development of innovative, economical, flexible, reproducible and affordable residential houses using intensive steel solutions. Toward this goal, both the residential housing conditions and the development of steel residential building in China are widely investigated. Affordable housing in China is then, based on the investigation, defined as green humanized multi-storey housing comprised of medium-small type dwellings whose construction cost is not much higher than that of traditional reinforced concrete buildings and the maintenance cost is low. Taking this definition as a guiding ideology, detailed architectural and structural design of a steel affordable housing model in terms of a collective housing form with repeatable living units has been carried out. Comparisons of project cost and energy consumption between the designed steel residential housing and the corresponding reinforced concrete building show that the former is not more expensive and consumes less energy than the latter.

Keywords

affordable housing / energy-saving / duplex design / pure steel frame / economic evaluation

Introduction

Sustainable development and environmental protection have been the main subjects of China’s development in recent years. However, the construction of traditional brick masonry and reinforced concrete residential buildings has caused serious environmental problems. In contrast, as a relatively emerging industry in China, the steel residential structure can be one option of energy-saving house systems. Since November 1994 when the first steel frame residential building was built in Shanghai, dozens of steel residential projects have been completed across the country, and now steel residential building is expected to enter a fast-growing stage in China. Considering permanent housing, “global” solutions cannot be an advisable answer for the whole China, because significant differences exist for social-geographical-economic conditions in China. For example, compared to East China, West China occupies larger territory, but with a much lower population density and relatively slow economic development process. The main purpose of this paper is to achieve a new concept of affordable housing in East China and accomplish a model of affordable housing design that can promote the environmental awareness in China’s residential market and enrich the development of steel structure in multi-storey, low-cost and energy-saving residential field.

In this paper, the residential housing conditions, including housing market, traditional housing concepts, and intensive steel residential building solutions are widely investigated to obtain a concept of affordable housing which is the most appropriate for the east China. After that, a new housing model is designed based on the affordable housing concept. Finally, comparisons of the cost and energy consumption between the newly designed steel houses and traditional concrete buildings are carried out to assess the performance of the design.

Housing market in China

Housing demand

Housing demand represents housing need with affordability under certain market conditions. The process of predicting housing demand is of significant importance in housing market. Generally, housing demand is mainly caused by three parts, expansion of existing urban residents, the progress of urbanization and old housing renovation. To evaluate the potential housing market in China, housing demand from 2007 to 2020 is predicted.

First, from Fig. 1, the per-capita living space in 2020 can be conservatively predicted as 35 m², increased by 7.9 m² from 2007. The expansion demand of existing urban residents (559 million) is 4.42 billion m² in the 14 years during which it is 0.316 billion m² per year. It is found that even if the urban population keeps constant, the housing demand will increase by 30%. Secondly, China faces increasing urbanization in the long term. According to Fig. 2, nearly 55% of the population will live in urban areas by 2020, indicating that nearly 224 million rural residents will immigrate to urban areas from 2007 to 2020. Provided that the per-capita living space is 35 m² at that time, additional 7.9 billion m² building areas are required between 2007 and 2020, or an average annual demand of 0.564 billion m². Lastly, supposing the annual depreciation rate is 2%, nearly 10.8 billion m² old housing needs renovation at present. Thus the annual housing demand caused by housing renovation is 0.215 billion m². To sum up, the total annual housing demand between 2007 and 2020 would be 0.316+ 0.564+ 0.215= 1.09 billion m². Such a huge housing demand will undoubtedly provide the real estate industry a large market and a tough task for housing construction as well.

Housing affordability

The term “housing affordability” has been used to estimate the difficulties individual households face in accessing descent or adequate housing. Since early 1990s, there have been intensive discussions about housing affordability issues that commercial housing prices in urban China are too high and far beyond the affordability of the average urban household [2].

Price-to-income ratio (PIR), the ratio of current housing unit price to annual income of one household obtained from the China Statistical Yearbooks, is used to analyze the affordability of Chinese households in purchasing commercial houses (Fig. 3). World Habitat Report published by the United Nations Human Settlements in 1996 suggested that: it is reasonable when PIR is within 2 and 3; when PIR exceeds 5, the majority of the families will have difficulties in affording a house. Obviously, housing price in urban China is considered too high compared with residents’ income, because the average PIR slightly changes from 9.79 to 6.45 within 1997 and 2009, which is much higher than 5. Besides, huge difference of PIRs exists within different income groups. For the low and medium-low income groups in China, the PIR is very high by any standards, and it may be impossible for them to purchase a commercial house in their whole life.

Traditional housing concept in China

Categories of housing by purchasing power of Chinese citizens

Current residential houses can be divided into four categories based on housing prices, namely, cheap renting housing, economical housing, plain commercial housing, and luxurious housing (Fig. 4). The cheap renting and economical housing are subsidized housing. Both are the most affordable houses but expected to cover only 20% of the city population at most. In addition, the luxurious housing with price highly above the average is too expensive for the majority, so its proportion would not exceed 10%. The plain commercial housing, occupying nearly 80% of the housing market, is the common object for major homebuyers in China. Furthermore, owing to high population density plus land scarcity, the most realistic choice for Chinese residents would be small-medium-sized plain commercial houses, especially in large cities.

Traditional housing types

According to statistics, development of housing construction in China can be divided into the three phases after 1950s [3–5].

The first phase: from 1950s to 1970s. Dorm-type building, small-sized flats usually with three to four floors containing only one shared washroom and one toilet on each floor, was typical city housing in that period. Despite the poor living conditions, one dorm-type building usually accommodated nearly hundreds of residents.

The second phase: from 1980s to 2000s. The most popular architectural form in that period was multi-storey brick masonry building composed of small and medium-sized units with improved living environment. Owing to the fast construction speed and low cost, it became the primary form and almost accounted for 90% of residential buildings in China since early 1980s. However, the brick masonry buildings have some severe disadvantages. Besides its brittleness and fragile deformation capacity which will result in poor seismic performance, it also causes serious environmental problems.

The third phase: 2000s to present. Since 2000s, living standard in China has been greatly improved. six-storey brick masonry building and high-rise reinforced concrete residential building, mainly in forms of concrete shear walls and concrete frames, have been the main building types in this period. The advantages of concrete building include good durability, fire resistance, and structural integrity, etc. However, owing to huge resource and energy consumption resulted from the usage of large quantity of traditional building materials such as clay brick and cement, buildings in this period are not energy-saving.

Dominating storey number of residential building

At present, six-storey residential buildings still dominate the residential housing construction. However, a binary pattern, six-storey plus high-rise residential building, has become a more and more favorable mode to real-estate developers because of its ability to increase floor area ratio and green space without raising costs. This phenomenon can be deduced from suburbanization patterns. The process of suburbanization in developed countries is expansion of urban functions in the suburbs through cars and fast transport vehicles, resulting in a relatively empty central urban area. Thus, the living space expands from urban into suburb in the low-rise and low-density pattern. In China, however, things are totally different. Living space modes in suburbs exist in the way of six-storey and high-rise building system, which is in line with the features of China’s suburbanization, namely, a centripetal mode coordinated with the bus-based transportation, salaried stratum-based population and a lower industrialization level.

Technical level of housing construction

As dominating forms, neither the multi-storey brick masonry nor the high-rise reinforced concrete residential building can meet energy saving requirements and sometimes is even detrimental to the environment. Nowadays, the in-site concrete casting and bricklaying are still the main construction methods in China. In addition, although the industry for new construction material and equipment has developed fast in China, most of them can only provide simple products, resulting in lack of technology, integration and matching competence of housing components. Lastly, the development of various technology service systems in China is just starting, and has a certain gap with those in developed countries. Hence, it is in urgent need for China to overcome the above obstacles and achieve housing industrialization.

Development of steel residential buildings

Advantages and difficulties of steel residential buildings

Compared with traditional masonry and reinforced concrete structures, steel residential buildings have the following advantages: 1) self-weight of steel residential buildings is about half of that of concrete buildings of similar volume due to the high strength-to-weight ratio of steel material; 2) the cross-section of load-carrying members in steel buildings is usually smaller than that of concrete ones, usually resulting in more than 4% usable floor area; 3) steel structures possess better seismic performance than concrete structures; 4) the assembly erection mode of steel structure makes the removal much easier; 5) fast erection mode of steel structure reduces the labor force on the site and shortens the construction period; 6) steel structure is more sustainable with less construction waste and environmental pollutions.

Although steel residential building is a relatively new building form in China, no essential technical difficulties exist in structural design. The real challenges include the architecture design, function satisfactory and cheap wall and roof material, construction equipment and construction techniques, which are different from traditional concrete building. Consequent solutions to these problems will directly affect the development of steel residential buildings in China.

Steel residential building market investigation

China has been the largest producer of steel in the world since 1996 when China’s crude steel production exceeded 100 million ton for the first time (Fig. 5). From then on the government started to encourage the application of steel in civil engineering industry. With construction steel processing industry growing day by day, steel structure construction in China has stepped into a high-speed developing period.

At present, only 1% residential buildings are steel structures in China, while the corresponding proportion in developed countries is about 40%~50%. Provided that 20% of the 1.09 billion m² newly built residential buildings each year are constructed using steel, the steel consumption will attain 10.9 million ton, with a steel consumption of 0.05 t/m². With the government policy of encouraging construction steel usage as well as the large demand for housing industry, the prospect for steel residential building is very promising and pending for further development.

Steel residential building forms in China

In November 1994, an eight-storey steel frame residential building completed in Shanghai marked the starting point of steel residential structure development. Since then, based on researches and practice over the preceding few years, dozens of steel residential projects have been completed across the country, from which important knowledge and experience have been accumulated. At the beginning of this century, steel structure residential buildings in China have attracted more attention from both the government and industry [6–7]. Typical residential steel structural systems adopted in Chinese market are listed as follows.

Light-gauge steel structure system

Light-gauge steel structure system is supported by C-shaped cold-formed or small sized hot rolled load-bearing steel elements with thickness between 1~3 mm and intercolumniation between 400~600 mm. Such assembled steel structure system has a number of advantages, including industrialized manufacture, completely dry construction mode, less steel consumption, energy and labor saving, etc.. However, due to its slender load bearing members, light-gauge steel structure system can only be used in low-rise structures such as villas which are too expensive for most Chinese households.

Pure steel frame system

The pure steel frame system uses steel frames to sustain the vertical and lateral forces in both longitudinal and transverse orientations. With the advantages of fewer member types, clear loading transfer path, short construction period and small lateral stiffness, steel frame system is often used in low and multi-storey buildings. When the structural height or story level increases, lateral braces must be added to increase the lateral resistance capacity of the frame structure.

Steel braced frame system

Steel braced frame system includes eccentrically braced frame and concentrically braced frame. The vertical bearing system of steel frame-brace system is steel frame, and the lateral resistant system is co-acted by steel frame and steel braces, thus a higher lateral resistance capacity than the pure steel frame system is achieved. However, construction difficulties of steel frame-brace system, especially eccentrically braced frame, are relatively greater. Furthermore, wall panels are difficult to match this structural system due to larger cross-section of braces, which therefore results in certain problems in interior decoration at the same time.

Steel board frame system

Steel board frame system is a composite structure system based on steel frames but embedded with several steel plate shear walls or reinforced concrete walls. It can be used for structures not higher than 12 stories in earthquake zones, even where the seismic intensity reaches 8 or 9 degrees. In this case, the vertical loads are carried by the steel frames, while the horizontal loads are undertaken by the steel frame and shear walls together according to their lateral stiffness ratio.

Possibility for demonstration of steel residential building in China

Being the largest steel producer in the world, the Chinese government has encouraged the application of steel in civil engineering industry since 1996. Housing industry has become a new hot consumption point and played a more and more important role in the national economic system. With great support from the government, application of steel residential building is an inevitable trend. In one word, the market future for steel residential building is very promising and the demonstration of affordable housing is extremely valuable.

Define affordable housing concept in China

Based on the above investigations, the affordable house suitable for East China can be defined as a green humanized multi-storey house system comprised of medium-small-type dwellings with a cost not much higher than that of traditional reinforced concrete buildings and low maintenance cost. This definition could be the guiding ideology of the future residential housing design.

First, occupying more than 90% of China’s housing market, the six-storey brick masonry and reinforced concrete buildings had been the most common architectural forms of residential buildings in China in the 20th century. However, neither of them meets the environment-friendly and energy-saving requirements owing to the serious environmental pollutions caused by these traditional building materials and the insufficient energy-saving design. Hence, it is urgent to establish a green building system which is re-usable, recyclable, re-movable and most importantly energy-saving.

Secondly, the majority of Chinese people belong to middle and lower income classes, who can only afford small-medium type dwellings. Thus affordable house in China should exist in forms of small-medium type. Besides, only basic living requirements are satisfied in such dwellings at present. To improve this situation, special attention should be paid to the design of more comfortable and humanized residential houses.

Thirdly, it is a big challenge to reduce the cost of steel structure to the same level of that of traditional reinforced concrete buildings because steel is usually much more expensive than concrete. Hence, if steel structures are so designed to be no more expensive than the corresponding concrete building, they can be categorized as affordable housing.

Finally, a four-storey steel building that meets all these conditions is designed in this paper as a pilot project. For the aim of generalizing such housing system, a corresponding six-storey steel building is also analyzed.

Architectural and structural design

Architectural design

With respect to architectural design, the concept of affordable housing is realized by adopting a collective housing form comprised of repeatable living units, where affordable housing is considered the hybrid of privatization, collectiveness and urbanism (Fig. 6). As shown in Fig. 7, each living unit, with two floors, contains two bedrooms and two bathrooms, providing a typical family with relatively comfortable living condition. Due to the duplex design, the common corridor is placed on every other level, saving half of the public area, and at the same time, creating a foyer in two-storey height. Through the connection between the lower south living room and the upper north facing terrace, an ideal natural ventilation condition for each living unit is formed, where the southern wind can travel through the living room, dining room, stairwell, and all the way to the north terrace. Furthermore, these two storey units can be easily assembled to accomplish various combinations of plan and vertical arrangements, taking advantages of gallery-style architectural form and steel structures.

To make the housing system more energy-saving, planted green roof is adopted in the architectural design. It not only brings green space to the building increasing amusement to the residents, but also improves effectively the performance of a building’s insulation. According to Wang [8], green roof can reduce the temperature of roof surface by 20°C–40°C during the summer and decrease the room temperature by 4°C–6°C, saving 50% of electricity used for air-conditioning, and at the same time extending the roof life expectance by three to five times. Therefore, even if the initial investment is slightly increased, in the long run, roof greening is still a promising solution.

Structural design

The pure steel frame system rigidly connected in both vertical and horizontal directions is employed as the main structural system in this paper because it is the most economical and effective structural system for the low-rise small-spanned residential building with regular arrangements. In addition, its well-performed seismic behavior and relatively small cross-sections are very crucial to affordable housing. Using cast-on-site concrete slabs, the structural integrity may be greatly enhanced. Furthermore, no other complicated construction is required; hence the projected cost is reduced.

Generally, one critical problem of steel residential building design is how to combine the arrangement of structural and architectural functions perfectly so as to take the maximum advantages of both and at the same time reduce the project cost. To solve this problem, a three-column steel frame is adopted here (Fig. 8). A four-storey as well as six-storey pure steel frame systems are analyzed to meet all requirements including seismic action and wind load. Due to the rational structural arrangement, the steel consumptions of the four-storey and six-storey steel frames are only 32.5 kg/m² and 35.5 kg/m², respectively (including steel members and connection parts). Both are much lower than most of the completed steel structures in China, brightening the future of this affordable housing system.

Economic evaluation

The impression that “steel is more expensive than concrete” hinders the development of steel residential structures. In fact, to judge whether a structural system is economical or not, all factors that affect the cost should be considered, not only the material cost of the main components alone. To show the advantages of our design, detailed economic comparisons between the pilot building and the corresponding RC frame is carried out, where the stress ratios are kept at the same level to guarantee the comparability.

Project cost comparison

Two important items are used to evaluate the project cost, namely, the direct project cost and the total project cost. The direct project cost refers to site development cost plus construction and installation cost; while the total project cost includes direct project cost, management fee and taxes. The calculation of project cost is based on Shanghai construction project quota 2000 and Shanghai decoration project quota 2000, with a consulting of the latest market information of construction price in Shanghai in November 2009. The market price may change with time, so will the project cost of our design.

Table 1 shows the direct project cost of the four-storey steel frame system and the corresponding RC frame system. It is observed that the direct project cost of the former (1392.7CNY/m²) is almost the same as the latter (1394.4 CNY/m²) and is much lower than that of common steel structures, mainly due to its low steel consumption. A similar trend can be found regarding the total project cost, which is 1530.55 CNY/m² and 1528.85 CNY/m² for the steel and RC frames respectively.

A comprehensive comparison

Compared with the concrete frame system, residents of the steel frame system can enjoy extra usable floor area, which is 24.3-9= 15.3 m² (Table 2). Assuming the housing price is 8000 CNY/m², the total benefit of the increased area is 15.3 × 8000/2909= 42.08 CNY/m². Table 3 illustrates a comprehensive economic evaluation of the project cost. The resulting larger usable floor area not only compensates the raised part of steel components cost but also makes the total cost of steel frame 2.71% lower than the traditional concrete structure.

Energy-saving design

There are many advantages of the energy-saving design proposed in this project. The combination of steel frame system and ALC panel greatly reduces the process of wet construction and environmental pollution, and at the same time obtains better energy-saving and thermal insulation performance. The green roof, the natural lighting and ventilation all help to save energy effectively. Furthermore, the application of steel structure has been proved to be more environment-friendly than the traditional concrete structure from all aspects. The steel frame structure is more energy-saving than the concrete structure in terms of unit building area, although the energy consumption in steel production is relatively higher than that for concrete. Comparison results suggest that energy consumption of the concrete frame system is 38.6% higher than that of the steel frame system (Table 4).

Conclusions

To obtain steel “affordable housing” concepts in China, a detailed model study of steel residential housing is completed in this paper. The following important conclusions are drawn.

• Based on the investigation of Chinese housing conditions, the affordable housing in China is defined as a green humanized multi-storey house system comprised of medium-small-type dwellings whose cost is not higher than that of traditional reinforced concrete buildings and the maintenance cost is low. The definition contains the guiding ideologies of the architectural and structural design.

• The architectural system in terms of a duplex unit model with corridors is considered a hybrid concept of privatization, collectiveness and urbanism. The gallery style apartment, double deck unit type and planted roof guarantee the energy-saving capacity and better application of structural system.

• The main structure system is a pure steel frame with the transverse frame consisting of three columns. The steel consumption is 32.5 kg/m², lower than the present average level of steel residential structures in China.

• A comprehensive economic assessment shows that the direct project cost is about 1400 CNY/m², which is also lower than the average level.

• Comparisons of cost and energy consumption indicate that our proposal is not expensive and consumes less energy than the traditional concrete building.

It can thus be concluded that our proposal well meet the requirements of affordable housing, and the so designed steel building may qualify as affordable housing.

References

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[3] China Steel Construction Yearbook, 2005, China Steel Construction Society

[4] China Real Estate Statistics Yearbook, 2008. Beijing: China Statistics Press

[5] China City Statistical Yearbook, 2008. Beijing: China Statistics Press

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[7] Du S. Technical Study of Steel Structure Residential Building. Dissertation for Ph.D. degree, Tsinghua University, 2003 (in Chinese)

[8] Wang X M. Current Status and Development on Green Roof in China and Abroad. Beijing Association of Green

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[11] Environmental management-life cycle assessment- life cycle impact assessment. <patent>ISO 14042</patent> 2000

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