The increasing threat of terrorist attacks has made security-focused site planning and structural design essential. Ensuring the safety of occupants requires careful consideration of the physical layout, security measures, and surveillance. By implementing strategic design elements, buildings can be made more resistant to blast effects and progressive collapse, reducing potential damage and casualties.
Site Planning Strategies
Access Controls and Physical Barriers
One of the primary security measures is controlling access to the building through barriers, fencing, and security checkpoints. These measures limit unauthorized entry and prevent potential threats from approaching the structure.
Restricted Parking Near Buildings
Restricting parking near high-risk structures minimizes the risk of vehicle-borne explosive attacks. Designated parking areas should be placed at a safe distance from the building to reduce potential blast impact.
Deep Buried Structures for Enhanced Safety
Underground structures provide increased protection from explosions. Walls with soil cover are particularly effective, as soil absorbs and dissipates blast energy. Military applications have successfully employed buried roofs for protection, a strategy that can also be adapted to civilian infrastructure for enhanced security.
Architectural Configuration for Security
Preferred Building Shapes
Convex-shaped buildings are preferable over concave shapes. Concave structures, such as U- or L-shaped buildings, have re-entrant corners that trap shock waves, amplifying blast effects and increasing damage.
Functional Planning for Safety
Grouping similar functional areas within a structure enhances security and efficiency. Designating protected zones for critical operations ensures that essential functions remain operational during an attack.
Air Intake System Protection
Air intake systems should be elevated to prevent contamination from ground-level threats, such as chemical or biological attacks. Secure air filtration systems can further enhance building resilience.
Structural Design for Terrorist Attacks
Objectives of Structural Design
A secure building should exhibit high stiffness and significant lateral and vertical strength to withstand explosive forces.
Key Structural Features for Building Resilience
a) Large Mass & Stiffness: Buildings with higher mass and stiffness respond better to blast loads by delaying oscillation and minimizing deformations. Lightweight structures, on the other hand, are more vulnerable.
b) High Redundancy in Load Resisting Systems: A structure should have multiple load-resisting elements to redistribute forces in case of localized damage, preventing complete failure.
c) Capacity Design Concept for Member Strengths: Structural elements must be proportioned to absorb energy and prevent brittle failure. Members should undergo ductile flexural failure before experiencing shear failure.
d) Resistance to Reversed Loading from Blast Overpressure: Blast waves generate both positive and negative pressure phases. To counter this, roofing systems should be bolted down to prevent lift-off.
e) Strong Connections Between Structural Elements: Connections should be robust enough to allow redistribution of loads. Reinforced concrete (RC) construction has demonstrated excellent resilience in military applications.
Avoidance of Load-Bearing Structural Configurations
Structural configurations reliant on load-bearing walls should be avoided, as they are more susceptible to progressive collapse under explosive forces.
Progressive Collapse Analysis
Understanding Progressive Collapse
Progressive collapse occurs when damage to a single structural element leads to the failure of adjacent components, resulting in the complete collapse of the building. This makes preventing progressive collapse a priority in security-focused designs.
Prevention Strategies
a) Structural Analysis by Removing Key Load Elements: Engineers should simulate scenarios where critical elements (e.g., columns or beams) are removed and assess whether the structure remains stable.
b) Evaluating Alternative Load Paths: The design should provide multiple pathways for load redistribution in case of structural failure.
c) Ensuring Structural Stability Against Progressive Collapse: The building should be tested under different failure conditions to confirm its ability to withstand localized damage without a complete breakdown.
Design Methods for Blast Resistance
Indirect Method
Prescriptive guidelines improve structural integrity by selecting robust systems, ensuring proper load distribution, and detailing members for enhanced ductility.
Alternate-Load-Path Method
This method designs buildings to withstand the loss of a critical structural component by redistributing loads using remaining members.
Specific Local-Resistance Method
This comprehensive approach factors in both blast loading and the nonlinear response of structures. It considers the location of blasts, such as under the building or near the façade, to optimize design against potential attacks.