How to Prevent Progressive Collapse & Catastrophic Failure of Structures Due to Explosions

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Progressive collapse is a catastrophic chain reaction of a structure failing when a vertical load-bearing element of the structure is lost, causing damage to either a portion of the structure or the entire structure (Fig. 1). Blast-resistant building design evaluates the potential progressive collapse in new and existing buildings to prevent the overall collapse of the building and fatal damages. The progressive collapse potential of a gravity-load designed mid-rise reinforced concrete building was evaluated using 3D non-linear dynamic push-down analysis in SAP2000 software.

The alternate load path method is mainly recommended by the U.S. Department of Defense (DoD, 2007) and General Services Administration (GSA, 2003). The philosophy of this method is to permit the occurrence of local damage; however, the collapse of a large portion of the structure is avoided by providing alternate load paths in the neighboring elements to redistribute the loads that were applied on the damaged components if they have been designed sufficiently.
Progressive collapse can be prevented in several ways, one of which is to offer an alternative path over the failed column, vertical cables that run parallel to the columns are hung from a steel hat-braced frame that is positioned on top of the structure. In new buildings, the cables are embedded in the columns; in existing buildings, the cables are connected to the ends of the beams (Fig. 2).

Progressive collapse can also typically be avoided for steel and reinforced concrete structures if the depth of the beams around the removed column is more than beam span L /15 (for steel structures) and L/12 (for concrete structures) [14,15], [where L is the original span (without column removal)],as shown in Figure 3.

A new way to prevent the progressive collapse of floors, as investigated by Astaneh-Asl, is to place steel cables inside the concrete floor slabs for new construction or add the cables under the slab for existing structures as a measure of retrofit. The main role of these cables is to prevent progressive collapse of the floor in the event of loss of one of the columns.

Figure 4 shows the application of this concept in a building. When a single column is removed and the floor starts to collapse, the steel cable prevents the collapse and transfers the load of the floor to neighboring columns and rest of the structures. Since cables are used in every floor, the loads of all floors above the removed column will be transferred to the adjacent columns. As a result, although the floors might have relatively large vertical deformations on the order of 40 to 60 centimeters (16 to 24 inches), the full progressive collapse and pancaking of the floors are prevented.

Norazman et al. suggested the use of a damage evaluation form in evaluating damaged structures due to various reasons such as an act of terrorism. This form is effective and gives a detailed inspection view that could be used as a guide for decision making and planning for rehabilitation work. See the online version of this article for a blank damage evaluation form and how it would be filled out according to the progressive collapse example presented hereafter.

The following is a step-by-step example showing how to perform progressive collapse analysis of a three-story administrative concrete building exposed to a corner column removal scenario due to explosion (Fig. 5). The evaluation of a blast-damaged concrete building follows the GSA guidelines (see references online).

Gravity Loads were taken as:

Superimposed Dead Load (Ceiling)= 31.2 psf
Walls = 907.1 plf acting on beams
Live Load= 62.4 psf (for administrative building)
Dimensions of Beams & Columns as follows:
Exterior Beams 14 × 24 in.
Interior Beams 14 × 22 in.
Corner Columns 22 × 22 in.
Exterior Columns 22 × 22 in.
Interior Columns 26 × 26 in.

Per GSA guidelines, progressive collapse analysis is completed by removing a single-story load bearing column on the perimeter of a building per following different scenarios, as shown in Figure 6. For this example, the first-story corner column has been removed (Fig. 7).

The nonlinear dynamic collapse analysis is needed to observe the formulation of plastic hinges throughout the structure and to determine which elements are failing. Many studies proved that nonlinear dynamic collapse analysis is the best choice to achieve accurate results compared to nonlinear static analysis.

Step 1: Perform traditional design of the concrete building with floor, beams, and columns sized to gravity and lateral loads. Prepare 3D analytical computer model.
Step 2: Define and assign plastic hinges according to ASCE 41 to the beams and columns. Beam hinges to be located along the beam length at the following intervals: each end, midspan, and approximately third points.
Step 3: Define analysis loading. For gravity loads, Dead Load + 0.25 x Live Load is the recommended load combination per GSA. Non-linear dynamic case automatically defined by SAP 2000.
Step 4: Perform progressive collapse analysis (Fig. 8). Note, SAP2000 v21 TRIAL Version can perform dynamic collapse analysis to model progressive collapse.
Step 5: Observe the hinge formation status for all frame members once the select column has been removed.
Step 6: Compare observed hinge formations with damage limits. According to FEMA-356, when plastic hinge rotations for any member exceed 0.025 radians, the member is considered a collapse hazard or in other words, the member has exceeded the collapse prevention (CP) state.

Also, according to Egyptian Specifications for Blast-Resistant buildings, the permissible damage area due to the loss of an external column must be smaller than 753.474 ft2. For the example above, the damaged area of the slab panel above the removed column is 27.8 x 27.88= 777 ft2.

The plastic hinges are spread in all beams and columns as shown in Figure 8. The value of most plastic hinge rotations for most members in this scenario exceed 0.025 radians and hence, collapse will occur. Consequently, overall progressive collapse is expected for this structure. ■

About the Author

Ibrahim M. Metwally, PhD, PE, is a professor of concrete structures at the Concrete Structures Research Institute at the Housing and Building National Research Center, Giza, Egypt. He is licensed by the Wyoming Board of Professional Engineers in the U.S. and registered as a senior structural consultant at DRSO of the Ministry of Housing of Egypt.

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Examples