An explosion is a rapid release of energy in the form of light, heat, sound, and a shock wave. The shock wave travels outward, in all directions, from the source of the explosion and is the primary source of building damage considered in blast resistant design.
The duration of the shock wave is very short, measured in milliseconds rather than seconds (think of a blink of the eye), and the forces imposed on anything it its path (be it a building or a person) are enormous – many times greater than hurricanes. Shock waves impart a significant positive load on building structures followed by a much smaller negative load (or suction action). For walls, this results in inward and outward loadings and for roofs, this results in upward and downward loadings.
A number of factors contribute to how a building will respond to an explosive event, with some of the most critical being:
The size of the explosive device, the distance from the bomb to the building (i.e., standoff distance), and the orientation with respect to the building determine the magnitude of the pressure (i.e., force over area) and the duration that the pressure acts on the building element.
The type of construction is also a significant factor in how much damage a building will experience from an explosion. It is important to remember that the vast majority of existing buildings were not built with explosive loading in mind. Therefore, just because a building does not respond well to an explosion does not necessarily mean that the building was poorly designed or constructed if there is significant damage or collapse after an explosion.
Buildings are generally designed to hold up gravity (downward) loads and lateral wind loads. In earthquake regions, they are also designed to withstand forces created by ground movements. Standard buildings are not designed to withstand large, aboveground shock waves of the magnitudes associated with explosions. Very lightweight buildings and buildings built with unreinforced masonry (e.g., brick or concrete block units) tend to respond the worst to explosions, while concrete and steel framed buildings tend to respond the best.
In framed buildings, the windows and infill walls (i.e., material that fills in the space between the columns and beams) are the least resistant to blast forces, and can create hazardous flying debris. In situations where a building does not collapse from an explosion, the majority of the injuries come from flying debris.
There are many retrofit approaches out there to mitigate blast loads and their resulting hazards. However, they are by no means one size fits all. Great care must be used when designing blast effects mitigation upgrades, as the upgrades themselves could add new burdens to the structure and make the situation worse rather than better.
The following are some examples of things to be cautious of with respect to upgrades:
Potential issues such as these should be fully understood before designing blast resistant retrofits.
Learn more about blast effects and how to design buildings against them in our face-to-face class, October 19 – 23, 2015 (click here for more information).
Reinforced concrete is really a wonderful thing (the same goes for reinforced concrete masonry unit (CMU) block walls). We take two completely different types of materials (concrete and steel) and combine them to take advantage of the best of both materials. Concrete (and CMU) elements are strong in compression – like when gravity presses down along the vertical axis of a wall or column – but if you try to pull it apart or bend it, it tends to fracture. Steel could be strong in compression if you had enough cross-sectional area, but where it really shines is in tension, as when you pull it. Steel is strong in tension and can bend without breaking. Think of concrete like a pencil (if you bend a pencil hard enough, it will break) and the steel like a paperclip (you can bend it back and forth quite a few times before it breaks). The pencil is exhibiting brittle behavior and the paperclip is exhibiting ductile behavior.
So, the beauty of reinforced concrete, or reinforced CMU block walls, is that the steel (rebar) and concrete or CMU are combined so that they are strong in compression (from the concrete/CMU) but they can also bend (to a point) without breaking. You might ask yourself, when do we bend a beam, or a wall or a column. Well, there are lots of answers, but the one that we are concerned with is, Explosive Events or Blasts.
An explosion is a rapid release of energy in the form of light, heat, sound and a shock wave, and it is the shock wave that we look at most often in blast resistant design (or when determining the strength of an existing building to a blast). The shock wave does not generally apply forces in the same direction as gravity, rather it applies the load at 90 degrees to the direction of gravity (in the case of columns and walls) or opposite the direction of gravity (in the case of uplift on slabs and beams). Since reinforced concrete and reinforced CMU walls have the ability to bend, they can be designed to resist blast loads or (in the case of existing buildings) they will be able to resist more blast load than an unreinforced element.
The first photo below shows the failure of unreinforced walls from explosive forces. Note that the columns and beams that are around the walls faired much better than the walls themselves. This is because they were constructed of reinforced concrete. This next photo shows the same type of failure from the interior or occupied space within the building. The failure of the exterior, unreinforced, wall is catastrophic (that is, it didn’t just bend in and end up curved or out-of-plumb, it blew right into the room). Injuries from this type of failure can be significant and widespread. The second photo shows the same type of failure from the interior or occupied space within the building. The failure of the exterior, unreinforced, wall is catastrophic (that is, it didn’t just bend in and end up curved or out-of-plumb, it blew right into the room). Injuries from this type of failure can be significant and widespread.
All of the above is an attempt to explain why any building, wall, or structure that is supposed to be designed to resist explosions or is being built in a region of the world where there is a threat of explosion (Afghanistan, Yemen, Iraq, Somalia, Nigeria, to name just a very few) should NEVER be built with unreinforced walls.
As part of our day-to-day consulting practice, we deal with what can be termed as Extraordinary Loads. I was recently in a design meeting and when I mentioned what I consider to be a fairly low blast load, the structural engineer’s eyes widened. She said that this load would change the entire design, and she would have been correct, if we were talking about an Ordinary Load. After this, it occurred to me that perhaps the concepts of Ordinary and Extraordinary Loads bear a little more explanation.
Ordinary conventional loads are based on services loads, a common terminology within the structural engineering community, that refers to a regularly occurring load encountered by a structure. Service loads include gravity loads, live loads, wind, snow, etc.. A factored combination for these high-frequency service loads take into account serviceability and strength, resulting in structures designed to withstand these loads with a certain safety factor and within a movement limitation for the comfort of building occupants.
The magnitudes of these loads are relatively low (usually measured in pounds per square foot for uniform loads) and they are applied for long durations. Because of the relatively long or continuing duration of these loads, a static analysis approach is used in design.
On the other hand, Extraordinary Loads, including hurricanes, earthquakes, tsunamis, and explosions, have a significantly lower frequency of occurrence, and buildings designed to resist them are often assumed to experience some level of damage once the loading has passed. These loads are generally considered as a separate load case without the increase factors used for service loads. In most cases, life-safety is the main goal in design for these loads, unless the facility is deemed as mission critical and must function after an extreme event. The magnitudes of these loads can be very high (measured in pounds per square inch for uniform loads) and are applied for a relatively short period of time (measured in seconds or even milliseconds). Because of the very short duration of Extraordinary Loads, a static analysis approach would generally result in an overly conservative (and therefore cost-ineffective) design. Instead of a static analysis, a performance–based criterion utilizing a dynamic analysis approach is often used, taking into account the duration, in addition to the magnitude of the load. This distinction allows us to meet the requirements of a project, as well as provide our clients with a feasible and cost-effective design when dealing with Extraordinary Loads.
Check out our upcoming Webinars and Face-to-Face Course for more in-depth information.
