Applied Knowledge

Innovative Extreme Loading Mitigation Measure Approaches

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.

GSA Announces Ambitious Federal Courthouse Building and Renovation Plans

The GSA has announced a $947 million plan to improve the federal courthouse facilities in the US. The plan covers courthouses in Tennessee, Ohio, North Carolina, Iowa, South Carolina, Alabama, Georgia, Texas, and Pennsylvania.

Here is a link to the announcement

Here is a link to more information on the individual courthouses.

Blast Resistant Retrofit Concepts

When leasing space in or buying an existing building, chances are that the building was not designed and constructed with blast resistance and protection in mind. As such, buildings located in high threat environments (see our previous post) most likely require some type of blast resistant retrofits.

The scope and selection of retrofits varies depending on the type and size of the threat. However, the basic concepts underlying the majority of blast resistant retrofits are simple and generally fall into one of the following categories:

  • Vacate the premises and move personnel to a more secure location,
  • Keep the threats as far away as possible from the people and buildings,
  • Install specially designed systems to catch hazardous debris,
  • Upgrade the capacity of structural members to reduce the amount of hazardous debris,
  • Upgrade connections securing building elements to each other (tie the building together better).
  • Protect the building using a shield structure,
  • Position personnel away from hazards within the compound and within the buildings, or
  • A combination of the above approaches.

An example of this can be seen in window retrofits. Most existing windows are not designed to resist blast loads and can create significant flying glass hazards to people, in the event of an explosion.   These existing windows require upgrades to reduce hazards to people who are occupying the buildings. One retrofit approach is to install Shatter Resistant Film (also known as Blast Film or Anti-Shatter Film) to the inside face (protected side) of the window glass. While this is a good first step, it really only provides a baseline of protection.   Where the threat is larger, the distance to the threat is smaller, or the amount of required protection is higher, the film can be combined with a catch system to provide additional protection.  There are many types of catch systems for windows, two typical systems are horizontal catcher bars or vertical cables. See the figure below for before-and-after photos of a window with film and a catcher bar.

catch_film

In looking at these photos, you can see significant damage. At first glance, this may seem to be a failure of protection. However, in many instances, this would be considered to be successful protection. Why? The Post-Blast photo shows that the shatter resistant film has held the shattered glass together as a single sheet rather than multiple shards that could cause serious injury. Additionally, the catcher bar has stopped the glass/film sheet from being thrown into the protected space. Thus, the hazard is isolated to the area directly in front of the windows and the potential for injury has been greatly reduced.

Another retrofit concept to remember is that when upgrading a building element (i.e., a window, a door, beams, etc.), the capacity of the connections and structure supporting that element must also be checked to ensure that they can resist the blast loads. The system is only as strong as its weakest link.

For example, if the glass of an existing window is strengthened to resist a specific blast load, but the existing connection between the window frame and the supporting wall is only designed to resist conventional wind loads the weakest link has been moved from the glass to somewhere else. In this example, if an explosion occurs, the glass might not fly out, but the window frame might break free from the structure and fly into the room.  Below is a picture of a room after a blast event.

catch_system

In this case, the existing windows were retrofitted with shatter resistant film in combination with a catch system consisting of cables spanning the full floor-to-floor height, at the interior side of the windows. The blast caused the glass to break, but not shatter. However, the anchors connecting the window to the wall failed, and the entire window frame was thrown into the occupied area. Fortunately, the catch system was able to stop the frames before they could fly very far into the room. As a result, the hazard was isolated to the area near the windows. This also highlights the importance of personnel location during a blast event. Hazards can be significantly reduced by locating people away from exterior walls and windows.

For information like this, and more, check out our upcoming face-to-face course: Protective Knowledge – Protection in High Threat Environments.   This course is being offered in New York City from May 23 – May 25, 2016, and is focused on security and design professionals, owners, and contractors who work in high threat environments.