Table of Contents
Introduction
Design serves many different purposes in in structural aspects and for buildings. As society becomes more complicated, there are more factors that have to be considered when faced with unexpected scenarios. One of the concerns that have to be addressed for more buildings that is on the rise is prevention and protection against explosions, especially for government and military based buildings. Since terrorist attack cases have increased dramatically over the last few years, they are considered dynamic loads along with natural disasters like seismic design for earthquakes and require meticulous consideration and calculation [1]. Designing for Blast means incorporating the possibility for an explosion has become a primary objective for the scope of the building and requires a series of various steps and impacts to be addressed before any decisions can be made. Although protection of people within the building would be the main concern, there are numerous other trade-offs and criteria that have to be assessed to be able to conclude with the best design [2].
Building after explosion [3] |
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Purpose
Threat
For terrorist activities, usually the two main considerations are for vehicles and weapons [2]. In terms of watching out for vehicles, the potential threat emerges from all sides of a building that is parallel to or facing a street. The vehicle is considered an air-blast load and decreases with distance so the primary danger is near the base of the building. For prevention of vehicle attacks apart from screening are intrusions such as anti-ram barriers along the perimeter of the building as well as utilizing the form of the landscape to create an obstacle course can be effective. However, oftentimes the vehicle itself is not the only threat because they can be used as a diversion or container for a secondary attack such as an explosive or hand-held weapons like a gun so further protection is needed.
There are a multitude of various weapons that produce a hazard to the lives of people but in regards to a building, the greatest pressure comes from explosives. Factors to be aware of are that the impact of explosives decrease very quickly with time and space and usually the pressure produced increases linearly with the size. Although the actual explosion only lasts a couple milliseconds, the area of the shock wave can hit the entire building and even the surroundings.
Effects of an explosion:
- The side of the building facing the initial source can be amplified up to ten times the pressure.
- Has a positive pressure phase and a negative pressure phase.
- During the negative pressure phase, regions with low pressure such as windows and sloped roofs to fail and collapse.
- Thin members like columns are impacted more by drag effects than direct pressure.
- Rebound is structures moving in the reverse direction due to vibration and can cause the exterior envelope to fall.
- Below the point of impact, an underground crater will form that damages the foundation and anything below grade.
- Any energy of the weapon that transfers through soil will create a small earthquake that can derange mechanical and electrical systems.
Vehicle weapon threat [2] |
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Sequence of air-blast effects [2] |
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Building Damage
Damage done to a building can be separated into two categories of direct air-blast and progressive collapse [2]. Direct air-blast refers to the source of the explosion and takes into account the damage caused by the high-intensity pressures near the incident. In some cases, this may cause the exterior envelope to collapse in a specific area. How severe the damage ends up being is dependent on the weapon size, distance from the building envelope, and what materials it is made of.
After the initial failure in a localized area, the extent of the damage spreads out to further elements, called progressive collapse. This may propagate laterally outwards or even vertically upwards and downwards. Not all buildings face progressive collapse since this depends on the features of the building.
Design criteria to prevent progressive collapse:
- Indirect design: prescriptive design measures, two-way systems
- Direct load: design of individual structural members to take the load
- Alternate load path: Includes possibility of losing some members yet does not effect the stability of the building
Design criteria to prevent progressive collapse:
- Indirect design: prescriptive design measures, two-way systems
- Direct load: design of individual structural members to take the load
- Alternate load path: Includes possibility of losing some members yet does not effect the stability of the building
Structural Aspects and Specifications
Walls
Walls designed to fail in a ductile method (flexure) than a brittle mode (shear) [2]
Need to resist loads transferred from doors and windows
Reduce pressure with more height because of increased distance and angle of incidence in upper floors
Poured in Reinforced Concrete
Highest level of protection [2]
Large mass to reduce deformations
Use symmetric reinforcement on both sides
Span the wall floor to floor
Avoid splices in high stress areas
Put bars between half the wall thickness and one wall thickness apart
For connections, use ductile special seismic detailing
Pre-cast Concrete
Minimum thickness of 5 inches along with bars that are reinforced two-ways [2]
For reinforcement, symmetric reinforcement is recommended
Fiber Reinforced Polymers, geotextile materials, or Kevlar are good choices for added protection
Panels should span across failed areas in either an arching manner, increased gravity connections, secondary systems, or an alternate load path
Ductile connections should be used and should be checked for rebound loads
Other Materials
Use 8-inch for CMU block walls with centered reinforcing bars in each void and horizontal reinforcement for each layer [2]
18 inches or thicker for brick walls can withstand around 10 psi
Place metal studs back to back for metal stud systems and for single stud system, use 16 gauge 6 inches deep with lateral braces
Timber should be avoided because it is too light and fragile in most cases
Curtain Wall
- Consider the flexibility of curtain walls [4]
- Capacity to withstand explosion depends on performance of various elements considered
- Attachment to floor slab and spandrel beams need to be monitored closely
| Blast resistant sacrificial wall [5] |
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Roof
- Main load distributed by the roof is the downward air-blast pressure [2]
- Exterior bay roof system is most vulnerable
- Low roof systems more at risk than higher
- Roof bay dimensions should be 30 feet or less
- Try to use cast-in place ductile reinforced concrete with beams running in two directions
- Shear ties should be placed close together across the system and have a minimum 135 degree angle
- Pre-cast, pre-tensioned, post-tensioned, lightweight (steel deck and wood frame), and concrete slab systems are all not desirable
- Sloped roofs are more of a hazard than flat roofs and should be avoided when possible
| Roof members during blast loading [6] |
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Ground
Below Grade
Effects of shock through the ground are generally secondary [2]
Placing secured areas below grade is a good protection
Roof still has to fit air-blast pressure levels
Keep in mind perimeter security barriers need deep foundations
Underground garage should be adjacent and not directly below building
Floor Slabs
- Reinforced-concrete, flat-plate structural slab for the floor is good for buildings with limited height [4]
- Put more emphasis on detail of lower floors, exterior bays, and spandrel beams
- Drop panels and column-capitols can be used for more shear resistance
- For vertical clearance, embed shear heads in the slab
- Including beams will help the framing system transfer lateral loads to shear walls
- Should be able to prevent punching shear failure that may result in progressive collapse
| Floor slabs [4] |
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Windows/Fenestration
- Use either laminated glass or apply anti-shatter film to glazing [4]
- Frame around window should exceed capacity of glazing, known as "glass fail first criteria"
- Designed to first resist conventional loads, then for explosive loads/ balance [2]
- Typically the weakest part of a building, balanced design means it does not lose to frames and wall systems
- Minimize number and size of windows
- For new construction, use laminated annealed glass with structural sealant
- Glass can break and retained by frame or only falls within 3-10 feet from the frame
Table 1. Performance Conditions for Windows [2] | |||
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| 1 | Safe | None | Glazing does not break. No visible damage to glazing or frame. |
| 2 | Very High | None | Glazing cracks but is retained by the frame. Dusting or very small fragments near sill or on floor acceptable. |
| 3a | High | Very Low | Glass cracks. Fragments enter space and land on floor no further than 1 meter (3.3 feet) from window. |
| 3b | High | Low | Glazing cracks. Fragments enter space and land on floor no further than 3 meters (10 feet) from the window. |
| 4 | Medium | Medium | Glazing cracks. Fragments enter space and land on floor and impact a vertical witness panel at a distance of no more than 3 m (10 feet) from the window at a height no greater than 2 feet above the floor. |
| 5 | Low | High | Glazing cracks and window system fails catastrophically. Fragments enter space impacting a vertical witness panel at a distance of no more than 3 meters (10 feet) from the window at a height greater than 0.6 meters (2 feet) above the floor. |
Structural Layout
Site
In terms of the site, it is more beneficial to place the building further from the property lines; both away from streets as well as neighboring buildings [2]. One way to do this is to design a plaza that sits in front of the building in question that also wraps around the sides and back. But in this case, the two sides and the back will be more vulnerable as well as considerations for the envelope below grade. There can also be careful planning of naturally occurring obstructions in the surroundings such as bollards, trees, and street furniture [1].
| Schematic layout of site for protection against bombs [7] |
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Architecture
The amount of damage to the structure also depends on the general shape [2]. Bad inclusions for the architecture would be reentrant corners and overhangs that have the potential to strengthen the air blast by withholding the shock from the explosion. When necessary, it is better to include larger and more gradual reentrant corners or overhangs in comparison to short and sharp ones. For the exterior of the building, convex shapes, such as a circular building, are preferred over concave, like flat or u-shaped buildings. Also, arches and domes are more resilient to blast shock than cubicle forms [1]. In most cases, be aware that single-story buildings are safer than tall multi-story buildings. If terraces are considered, careful planning for framing and details are needed to minimize damage to beam supports [2]. Lobby and loading dock areas, which are frequently targeted in an attack and may portray progressive collapse should be planned so that they are exterior to the main section. Soil can be a great obstruction to shock waves such as utilizing bermed walls and buried roofs, but do not design parking overhead. In summary, less complicated geometric shapes should be the first option for design and unnecessary decoration should be avoided whenever possible.
Internal
The internal layout of the building should be arranged so that the section with the greatest 'value' is the most protected and therefore furthest away from the threat [1].
- Foyer areas surrounded with reinforced concrete walls
- Double doors and facing the corridors eccentrically
- Entrance should be separate from building and closely controlled
- No parking below building
Something else that has to be considered is that a fire could occur after the explosion and generate even more damage so all the members inside should be fire-resistant.
| Internal planning of a building [1] |
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Bomb shelters are designated areas within a building that offer maximum protection to the inhabitants from a bombing. They should be designed large enough to fit all people inside the building and can be easily accessed at all times. The location is important and needs to be away from any openings such as doors and windows and unless the roof is strong, it should not be on the top floors. It is necessary for the walls to be fully concrete and not share the same space with parking, gas tanks, light curtain walls, conference rooms, and even bathrooms. A basement would be a good choice provided the building does not collapse on it.
- Accommodate everyone inside
- Easy communication with outside world
- Adequate sanitation/ventilation
- Blast pressure controlled to smaller than rupture pressure of the eardrum
- Alternate escape routes available
Issues and Considerations
- First, choose factors that will dictate the design [7]
- For one particular bomb size and distance or multiple possible scenarios
- Consider conventionalized loading
- Type of structure; completely enclosed or open
- Decide a maximum for allowed response for corresponding conditions
- Can be elastic or deflect to the extreme or mix of both
- Considered for every part of the structure
- After choices are made, the yield-point resistance is computed
- Recognize there are always unpredictable factors; uncertainty up to 25%
- Blast pressure, duration, structural properties, and more
- Refining and editing calculations is super important and crucial to reduce uncertainty as much as possible
- After structure is completed, engineers should only perform accurate physical and pressure measurements during testing
Additional Reading and Resources
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