Strengthening a building to resist extreme loads (such as explosions, ballistics, and forced entry) that were not anticipated in the original design of the structure has a unique set of challenges and demands. These include not only the ability to resist the extreme load itself, but also constructability restrictions that come from trying to retrofit existing structures, the limitations of the existing materials and configurations, code requirements, and durability demands. Techniques and materials used to strengthen buildings and structures can generally be divided into two categories: Conventional and Innovative.

Conventional mitigation techniques and materials include structural element enlargement and the addition of supplemental supports using reinforced concrete and/or steel materials.  These approaches have proven themselves to be effective solutions but they often pose constructability and compatibility challenges along with possible negative impacts from the added weight and loads on the existing building.   These challenges can cause unanticipated time and cost implications for the retrofit project.

Innovative mitigation approaches and materials are alternative solutions to their conventional counterparts that can provide high strength-to-weight ratios, easier installation, resistance to complex threats and loads, and more effective dissipation of induced energy into the original structural systems.  While we term these approaches ‘innovative’ they are not ‘shoot-from-the-hip’ solutions.  They have been investigated and validated with testing and engineering modeling and are increasingly used to mitigate the challenges that conventional techniques and materials cannot adequately address.  Examples of these materials include:

• Fiber reinforced polymer or polyurea composites systems,
• Rubber-based composite systems,
• Micro or fiber reinforced high-performance concrete systems,
• Energy absorbing anchoring systems, and
• Catch systems.

It is critical for the design engineer to understand the strengths and limitations of these innovative products and techniques, including the appropriate load ranges for the different threats and the required levels of protection.   Implementation should be based on the published data and applicable guidelines and specifications.

As an example, externally applied composite materials such as Fiber Reinforced Polymers (often referred to as FRP) have proved to be effective in increasing the bending strength of masonry walls and concrete slabs, and with providing axial confinement improvement when wrapped around columns.  However, this material has unique limitations and special considerations that must be understood and accounted for in the design to prevent pre-mature and possibly catastrophic failures.

These considerations include:

• Concrete/epoxy bond limitations and the tensile forces that can be transferred into the composites,
• Possible membrane action of the composites or concrete/masonry,
• The effect of additional composites on the overall effective stiffness of the slabs or wall,
• Boundary support conditions, and
• Termination points of the applied systems and their anchorage details

Overall, conventional or innovative materials and approaches have many potential applications in protecting people and property from extreme loads.  However, their use requires a qualified engineer with a substantial understanding of mitigation approaches, dynamic behavior of the different types of materials, their physical properties, benefits and limitations in order to arrive at an effective and optimized mitigation solution.