Preventing Fire-Induced Collapse of Tall Buildings

Introduction

In this chapter, how to design a building to prevent fire-induced collapse will be discussed. The collapse mechanism of a building in fire and methods for mitigating the collapse of a tall building will be introduced, all based on the existing research and fire-induced collapse incidents. As steel-framed buildings are more vulnerable in terms of fire-induced collapse, this type of buildings will be focused in this chapter.

Design Objective and Functional Requirement for Structural Stability in Fire

Although the first priority of a fire safety design is to save lives of occupants rather than preventing the collapse of buildings (Fu, 2016a), any collapse of the buildings will cause huge economic loss. In addition, collapse of a building can also have a heavy impact on life safety design target. Therefore, building regulation of New Zealand, building Act 1991, specifies a design to have the following requirements:

C4.I STRUCTURAL STABILITY DURING FIRE OBJECTIVE

  • • Safeguard people from injury due to loss of structural stability during fire
  • • Protect household units and other property from damage due to structural instability caused by the fire.

C4.2 FUNCTIONAL REQUIREMENT

  • • Buildings shall be constructed to maintain its structural stability during fire to
  • • Allow people adequate time to evacuate safely
  • • Allow fire service personnel adequate time to undertake rescue and firefighting operations
  • • Avoid collapse and consequential damage to adjacent house units or other properties.

Importance of Collapse Prevention of Tall Buildings in Fire

From Section 7.2, it can be seen that in a fire safety design, the primary purpose of a building’s structural stability is to save the lives of occupants in the event of fire, rather than preventing collapse Currently, the major objective of structural fire design in most design codes is to ensure load- bearing capacity of the building to continue to function until successful evacuation of the occupants, rather than prevent collapse of the building. Therefore, thus far, there is no clear guidance for designing a building to prevent fire-induced collapse across the world.

However, as introduced in Chapter 1, there are several incidents of fire- induced collapse of tall buildings such as World Trade Center (WTC1, WTC2, and WTC7). As explained in Section 7.2, even for the purpose of saving lives of the occupants, preventing or delaying the collapse of a building in fire is also essential in a fire safety design.

Collapse Mechanism of Tall Buildings in Fire

All the collapse of a building starts from a local failure of structural members. As introduced in Chapter 2, there are four major failure modes of structural members in fire discovered by Cardington tests:

  • • Beam buckling and yielding
  • • Column buckling and yielding
  • • Connection failure
  • • Slab failure.

These structural members are not working independently; they have impacts on each other. Under thermal expansion and subsequent contraction in the cooling stage, the interaction between the structural members causes extra stress and deformation to them, which makes it difficult to predict the actual failure mode of the whole building. However, as introduced in Chapter 2, it is found from WTC1 and WTC7 collapse incidents that the column buckling is the key reason for the collapse of these two buildings. The column buckling will trigger the failure of the floor above. The floor failure in both WTC1 and WTC7 caused further failure of surrounding columns in the horizontal direction and triggered progressive failure of the whole building. Therefore, it is worth investigating the response of individual structural members first.

Factors Affecting Thermal Response and Failure Mechanism of Individual Members

The thermal behavior of structural members in fire is a complex problem, and the response of individual members in a tall building subjected to fire loading is affected by the following parameters:

  • 1. Temperature profile of the member
  • 2. Degree of thermal restraint offered by the surrounding members
  • 3. Degradation in material properties with increasing temperature
  • 4. Capacity of deployment of alternative load-carrying paths for adjacent members.

To better understand the behavior of the structural member in fire, a 3D finite element model of a typical single-storey composite frame is set up in Abaqus® by the author, as shown in Figure 7.1. To facilitate further discussion on the modeling result, this model is designated as Model 1 in this chapter.

Model I of a typical composite floor in Abaqus®. (Abaqus® screenshot reprinted with permission from Dassault Systemes.)

Figure 7.1 Model I of a typical composite floor in Abaqus®. (Abaqus® screenshot reprinted with permission from Dassault Systemes.)

In the analysis, parametric fire temperature is used for gas temperature, Short-hot fire scenario is adopted. Figure 7.1 shows the temperature distribution of slabs, beams, and columns after analysis. The edge columns and beams and columns are fire protected. Therefore, it can be seen that the temperature of the slabs is higher than that of beams and columns due to their fire protection.

 
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