INTRODUCTION

Collapse of building structures due to extreme hazards often results in a serious loss of life and property. Hence, various countries in the world assign great importance to the prevention of the collapse of building structures. For earthquake-induced collapse, after the Northridge Earthquake in the United States in 1994 and the Hanshin Earthquake in Japan in 1995, the performance-based seismic design gradually replaced the traditional seismic design. The collapse prevention of building structures under earthquakes has been studied in depth as a key performance indicator, which was detailed specified in ATC-40, FEMA273, FEMA 274 and other guidelines. However, the collapse mechanism of building structures due to accidental loads (e.g., blast loading, impact loading, fire loading, etc.) has essential differences with the collapse triggered by the horizontal seismic action on the entire structure. In most cases, accidental load is only an initiative of the collapse, which causes local structural damage, while the unbalanced gravity loads due to the local structural damage cause the subsequent global collapse. The countries of Europe and North America have studied this type of progressive collapse for over 40 years. Especially after the collapse of the World Trade Center (WTC) in New York in 2001, numerous experimental and theoretical studies have been conducted. Many specifications, such as BS8110, Eurocode 1, Eurocode 2, DoD2010 and GSA2003, have proposed design requirements for this type of collapse.

China is one of the countries that suffer the most serious losses caused by earthquakes. For example, the Tangshan Earthquake in 1976 and the Wenchuan Earthquake in 2008 resulted in tens of thousands of casualties. Therefore, China has performed extensive research on the earthquake-induced collapse. Ye et al. (2010) and Lu et al. (2012) have conducted in-depth studies on typical structures that collapsed during the Wenchuan Earthquake. The earthquake-induced collapse process and collapse mechanism of a number of important new structures that were built in recent years in China have also been studied by Lu et al. (2011, 2013a, 2013b，2013c). The corresponding research results are adopted in the Load code for the design of building structures (GB50009-2012) (MOHURD 2012), the Code for design of concrete structures (GB50010-2010) (MOHURD 2010a) and the Technical specification for concrete structures of tall building (JGJ3-2010) (MOHURD 2010b). However, there remains a lack of systematic and operational guidelines and rules to prevent earthquake-induced collapse.

In the 1990s, the progressive collapse of a six-story masonry structure in Liaoning Province occurred due to a gas explosion. This event raised concerns in the Chinese engineering community regarding the collapse induced by accidental loads. After the collapse of the WTC, Chinese researchers have performed a large number of experimental (Yi et al., 2008) and theoretical studies (Li et al., 2011) on the progressive collapse mechanism and resistance demand of various types of structures. Recently, the Load code for the design of building structures (GB50009-2012) (MOHURD 2012), the Code for design of concrete structures (GB50010-2010) (MOHURD 2010a) and the Technical specification for concrete structures of tall building (JGJ3-2010) (MOHURD 2010b) have introduced design provisions to prevent progressive collapse, but these codes are still mainly related to the conceptual design and are not detailed enough, which cannot fully meet the needs of progressive collapse prevention design.

With the development of both the economy and engineering techniques in China, huge public buildings are being constructed, and determining how to implement progressive collapse prevention design is gradually attracting the attention of the engineering community. In this background, the Architectural Society of China organized a special committee to draft the Design specification for collapse prevention of buildings. The specification has been completed and is to be published. In this specification, detailed provisions about the earthquake-induced collapse prevention design are provided in a separate chapter. The progressive collapse prevention design is divided into four chapters. One chapter describes the universal design approach, in which the type of hazard that causes the initial local damage is not considered. The other three chapters provide specific provisions for collapse induced by fire, explosion and construction errors. In this paper, the main contents and methods in the specification are described to provide a reference for relevant scientific research and engineering practice.

EARTHQUAKE-INDUCED COLLAPSE PREVENTION DESIGN

Earthquake-induced collapse prevention design includes: conceptual design, calculation and analysis, seismic action, collapse resistance assessment and anti-collapse structural measures.

For the conceptual design, the influence of the structural system on the collapse resistance is emphasized, such as structural integrity, yield and energy dissipation modes and deformation capacity.

For the calculation and analysis, more detailed provisions are specified: (1) for regular structures under 45 m or 12-stories, the “ultimate lateral load capacity” method can be used in collapse resistance analysis; (2) for regular structures under 100 m, the pushover analysis can be used; (3) for other structures, the nonlinear time history analysis method should be used in collapse resistance analysis; (4) the collapse fragility method based on the incremental dynamic analysis (IDA) can be used to check the collapse resistance of the structure; (5) the modeling requirements for nonlinear seismic analysis are provided in detail.

For the seismic action, a very important characteristic of this specification is that it summarizes the lessons from the severe damage of several strong earthquakes (beyond the maximal considered earthquake (MCE) level) that occurred in recent years in China, and the specification provides recommended collapse resistance requirements for an extremely rare earthquake (i.e., mega-earthquake) that is stronger than the MCE. The scaling factors between the peak ground acceleration (PGA) of the extremely rare earthquake and that of the MCE are listed in Table 1.

Table 1. Scaling factors

Note: * is the PGA for a 10% exceedance probability in 50 years of the corresponding seismic fortification intensity.

For the collapse resistance assessment, according to current engineering requirements, three different levels of structural damage criteria are defined: (1) damage criteria based on the member section level, which are similar to the provision in ASCE/SEI-47, but some coefficients are adjusted according to Chinese engineering practice; (2) damage criteria based on the material level. Because the fiber beam model and the multi-layer shell model, which are established based on the material stress-strain constitutive law, are widely used in current structural nonlinear analysis, the strains of the components can be easily obtained from the nonlinear analysis results. Thus, the damage degree of the components can also be determined based on the maximal strains of the components; (3) collapse criteria based on the structural system, i.e., when the collapse fragility method based on the IDA is used to verify the structural collapse resistance, the acceptable earthquake-induced collapse possibility shall meet the requirements in Table 2.

Table 2. Acceptable earthquake-induced collapse possibility of the structure

Based on the experiences of earthquakes in recent years in China, this specification also provides the seismic structural measures for reinforced concrete (RC) structures and masonry structures, including the design of the staircase, the RC frame structures with a small amount of shear walls, the braced frames with a small amount of braces and so on.

PROGRESSIVE COLLAPSE PREVENTION DESIGN

In this specification, the threat-independent progressive collapse prevention design adopts the tie force method, the alternate load path method and the strengthening key element method, which are common methods in European and American specifications. The detailed analysis of the entire progressive collapse process, which especially considers the action of the accidental loads, is also recommended for use when necessary. The latest research outcomes are included during the implementation of these methods.

For the global conceptual design, the specification recommends some conceptual design methods that are in favor of progressive collapse prevention as well as some measures that can improve the structural integrity and redundancy, for example, the use of a structural system with larger degree of statistical indeterminacy and the use of an integral roof.

For the tie force method, the specification describes different design strategies according to the different collapse resistant mechanisms in the different regions of the structure. For example, for internal and edge continuous beams, the beam mechanism under small deformation and the catenary mechanism under large deformation are observed in experiments and analysis (see Figures 1a and 1b). The specification assesses the collapse resistance under these two mechanisms. However, for the corner beams and the beam perpendicular to the edge beam, they can resist progressive collapse only through the beam mechanism, thus only the beam mechanism is evaluated in these regions (see Figure 1c). In addition, considering the dynamic effect in the progressive collapse process, the dynamic amplification factor is used in the tie force calculation.

Note: q is the standard value of the uniform line load obtained from the quasi-permanent combination acting on the horizontal member; F_T is the tie force of the horizontal member, M_b is the moment at the end of the horizontal member and L_i and L_j are the spans of the horizontal member.

For the alternative load path method, the specification recommends both the use of either the linear or nonlinear modes and either the static or dynamic methods in the analysis, which is similar to the existing European and American specifications. However, the state equation of structural collapse is theoretically established based on the energy principle, in which the gravitational potential energy (i.e., the work performed by the unbalanced gravity G) equals to the structural energy dissipation (see Figure 2). The dynamic amplification factor (DAF) and the internal force reduction factor (IFRF) are assessed via the analysis of the state equation, with the statistical results of a large number of Chinese structures. In addition, the specification also recommends the removal locations for large-span spatial structures.

Figure 2. Assessment of DAF and IFRF based on the energy principle.

FIRE-INDUCED COLLAPSE PREVENTION DESIGN

For building structures under fire, the specification requires the structure to resist fire for a sufficiently long time (i.e., the fire resistance) without collapse. The specification recommends three methods to design and verify the structural safety against fire, namely: the simplified component method, the alternative load path method and the advanced analysis for entire fire process (see Figure 3).

Figure 3. Analysis strategies for buildings to prevent fire-induced collapse.

The simplified component method verifies the bearing capacity of each component individually when it reaches the fire resistance limit. In this process, the effect of temperature on the bearing capacity is considered, but the redistribution of the internal forces in the entire structure is not considered. To simplify the calculation, the standard temperature fields of different sections under standard fire action and the performance parameters of the material under high temperature are recommended in the specification.

The alternative load path method under fire action must create the FE model of the entire structure and verify its bearing capacity when the fire resistance limit is achieved. The development of the fire needs not to be considered. In the verification process, the degradation of the bearing capacity (stiffness) and thermal expansion under high temperature of the components should be considered. When the bearing capacity of the component is lower than its internal force, the component will be removed from the entire structure, and the analysis will be continued. In the end, whether unacceptable damage of the structure occurs is determined according to the collapse criteria (the area and proportion of the collapse region).

Based on the alternative load path method, for the advanced analysis method of the entire fire process, the damage and failure of the structures due to the dynamic effect of fire (such as the spread of fire and the heating and cooling of local fires) is considered. The collapse criteria (i.e., the threshold values of deformation and the deformation rate) of different components (e.g., beams or columns) are specified. The collapse criteria of the entire structure is the same as that in the alterative load path method.

It can be seen that the complexity and accuracy of the above three methods gradually increase. In the specification, buildings are divided into three different levels according to their importance and fire risk. For an important building with a high fire risk, a relatively more complicated and accurate method shall be adopted for the analysis. In contrast, for a generic building with low fire risk, an easier method shall be adopted.

COLLAPSE PREVENTION DESIGN IN THE CONSTRUCTION, RECONSTRUCTION AND DEMOLITION PROCESS

The collapse prevention design in the construction, reconstruction and demolition process is usually ignored in existing design codes, especially the latter two, which only occasionally occurs in engineering practice. In the specification, the conceptual design method and the structural analysis method are proposed to prevent the structural collapse at this stage.

In the conceptual design, time-varying structural systems in the construction, reconstruction and demotion process must meet the vertical and horizontal continuity requirements. For the temporary structure, its connectivity with the main structure is required in the specification. For the structure in the reconstruction process, mismatch of the new structure to the original old structure is most likely to cause structural collapse. Thus, the specification describes several categories of engineering structures that require special attention, such as the arched roof (floor) with large internal compressive stress constructed on the column (masonry wall) with low lateral resistance, the heavy roof (floor) placed on the column (wall) with low stiffness or weak resistance and the mismatch of the super structure to the base system. For the demolition of the structure, the specification provides the demolition principles, including the removal order, the slenderness limits of the cantilever columns forming in the demolition process and the thickness ratio limits of the separate wall.

For the structural analysis, the incomplete main structure during the construction process must meet the special requirements of the lateral bearing capacity and deformation. In addition, the over-turning resistance of its support system must be verified.

EXPLOSION-INDUCED COLLAPSE PREVENTION DESIGN OF MAINTENANCE STRUCTURES AND EXTERIOR FIXTURES

Because the specification is primarily for civil buildings and the terrorist attack risk is relatively low in China, the prevention of explosion-induced collapse is mainly achieved by improving the maintenance structures, and there is no specialized explosion prevention design for the main structure. The layout and structural requirements of the perimeter maintenance wall to prevent explosion are provided in the specification. Detailed provisions for the windows, the glass curtain walls and the skylights, which easily produce fragments, are also specified. For the exterior fixtures, which are prone to damage and forming secondary injury in the explosion, the specification recommends a reasonable placement and connection performance.

CONCLUSION

The Design specification for the collapse prevention of buildings refers to the successful experience of collapse prevention of building structures in the world while combining the research results of collapse prevention in China. Because the current world-wide interest on the structural collapse is just raised at the beginning, many further research studies will be conducted in China. In the first edition, relatively mature research results of the collapse induced by earthquake, accidental loads, fire, explosion and construction errors are included in the specification. The ongoing collapse studies in China, such as impact-induced collapse, collapse of composite structures and new anti-collapse structural systems, are not included in the current edition. The committee will continue to focus on the domestic and foreign research progress. With the continued in-depth study and the feedback of the first edition specification applied in engineering practice, the specification will be improved and expanded for new related content.

ACKNOWLEDGEMENT

The authors are grateful for the financial support received from the National Basic Research Program of China (973 Program) (No. 2012CB719703), the National Nature Science Foundation of China (No. 51222804), the Key Technology R&D Program of the Ministry of Railway (No. 2012G003-j) and the Program for New Century Excellent Talents in University (NCET-10-0528).

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