Life safety is the ultimate design goal
By Michael G. Paczak, P.E.; Warren Duvall, P.E.; and Judith Cosby, P.E.
The 1995 attack on the Oklahoma City Murrah Building served as a major impetus to raise government interest in blast protection for facilities in the United States. In response, the federal Interagency Security Committee (ISC) addressed the issue promptly by developing a blast-resistance standard outlining new criteria for design. Subsequently, the horrific structural collapses of Sept. 11, 2001, refocused attention and emphasis on design for extraordinary loads.
Structural engineers who may not have experience in blast-resistant design need to recognize how these complex requirements interface with typical structural design, and coordinate them with architectural requirements.
Experts at HDR Inc. have worked on numerous blast-resistant designs and are familiar with fundamental blast criteria issues, both for new projects and for reinforcement of existing buildings.
Establishing blast design criteria Blast design criteria for federal projects are based upon the threat level set by the government’s established criteria.
These criteria are established prior to the structural engineer’s involvement.
On the other hand, non-federal governmental entities, such as county, municipal or private clients that lack the experience and technical resources of the federal government, may need input from security planners, blast consultants, or the structural engineer.
Together, they would establish the design criteria as it relates to the facility’s specified threat level.
Federal guidelines define three threat levels that delineate blast protection:
A high threat level entails a verified high threat of attack. These projects typically are buildings of high importance, buildings whose loss will have high consequences, or those that are cultural icons.
A medium threat level consists of a verified threat of attack. These buildings may be regional symbols, or their loss will highly impact governing powers.
A low threat level constitutes a suspected threat. These buildings may be regional symbols, or their loss will have moderate consequences.
Without experience in this relatively new design field, most structural engineers do not have the expertise to establish a building’s threat level.
When this is the case, it is recommended that a blast consultant be hired in the initial design phases of a project.
A blast consultant also brings sophisticated analysis tools and experience to the project, and can bring adept, costeffective solutions to the table, especially for more complex structures.
Blast threats are delivered to a building in several ways. These can include hand-carried explosives inside the building or on public sidewalks around the building wall, packages delivered to loading docks, and vehicles in public roadways around building sites.
Typically, explosives do not have to be large to be considered a threat to the building. Less than 50 pounds will fit into a briefcase or backpack, and may receive little notice from even the most experienced security guards.
Blast design considerations for the structure
The most effective defense against explosives is to establish a safe perimeter, or blast standoff. Standoff distances delineated with positive barriers and systems that prevent an attacker’s approach is foremost. Many of our projects include personnel access control and package inspection; loading docks removed to remote facilities; and use of vehicular barriers such as bollards, decorative planters, and walls.
At closer distances, as little as 10 feet of additional standoff can substantially reduce the effects of an explosion.
Screening at public entrances is the next level of defense against explosive devices penetrating into the core of a building. Limitations of screening measures include the fact that personnel and package screening typically are situated inside a facility, and rarely can be removed from the building.
Screening penetrates perhaps 10 to 20 feet into public buildings, which then restricts the area to which structural enhancements need be applied.
Next, consider the structural ramifications of an explosive device that has made it into a building, or is placed near an exterior wall. A fundamental consideration for the structural engineer is that design for such threats is based on dynamic response of the structure to specific blast force.
Buildings, however, usually are designed for static loads. Blast consultants frequently give structural designers equivalent static loads, based on the dynamic analysis, or the government agency provides standard load requirements to use for their design.
As an example, a primary building component affected by a blast force is the slab above an interior explosion.
The slab and framing must account for the changes of pressure inside the building. An interior blast will first push the upper-level structure up and then it will rebound, dropping down with gravity accelerations, increasing the loads on the floor system. Shear stresses, moment stresses, and ductility of the framing all are affected by the increased static loads and load reversals.
A medium-level blast-protection load combination for a static load analysis may be 2(DL) + .5(LL) for the rebound case.
Another consideration is the blast effects on an interior or exterior column.
Criteria for prevention of progressive collapse often call for the removal of one exterior column, anywhere along the building perimeter that is accessible to the public, from the grade level up to the first or second framed floor above grade, and the structure above remains standing, though possibly no longer serviceable. This requirement is often without regard to any specific explosive threat. Considering this scenario, the perimeter beams have their lengths increased significantly, with a point load introduced at mid-span. If the bay spacing is 30 feet, the beam spans would be doubled to 60 feet.
Conventional concrete building design permits reinforcing for the beam bottom steel with minimal continuity at the columns, yet in this scenario, insufficient continuity of bottom bars is provided where it now is needed most. The top steel over the adjacent columns also would be woefully undersized and too short for the increased span length. Design models must account for this condition, and solutions include increasing the beam depth, decreasing the column spacing, requiring more bottom reinforcing to be continuous over the columns with laps staggered, and increasing the quantity and length of top bars.
Removal of corner columns would result in a cantilever condition for the perimeter beams, another design consideration. In both cases, if the building load is too great for the beam design at one level, the beams at each floor above can be designed to carry the load of the individual floors for the new span, or to model the beams and columns above as a Vierendeel truss with moment connections.
The requirements for an interior column are similar, and, depending upon the framing system used, the design considerations would be similar.
An alternate solution to designing for the removal of a column is to design it so as not to fail in the first place, if designing for a specific explosive threat.
Methods for this approach include using spiral ties in the concrete column in lieu of square ties, and wrapping the column in a steel-plated jacket or fiber reinforced polymers (FRP). This method increases the section’s resistance to shattering from an explosion.
Masonry construction follows suit.
Portions of wall sections, often the length between two adjacent transverse walls, as defined by the client’s design criteria, are removed, and upper portions are designed to span over these areas. Bearing wall structures need to be designed to withstand the load redistributions should the frontbearing wall be forfeited in the event of a blast. The masonry walls above this removed level are designed as deep beams, sometimes cantilevering, to support the remaining structure. Floor and roof support after a blast are essential considerations.
The building’s glazing is another important element to consider for blast design. The federal blast standard for glazing is defined by designating a particular pressure over a time period so that glass is designed to fail in a controlled, predictable manner. Glass needs to remain in its frame, and the frame likewise needs to be supported to the main structure. If a blast exceeds the specified load, then the goal is for the glass to remain in one piece rather than fly into thousands of shards with the potential to kill and maim many additional victims. Skylights, likewise, need to be designed for this pressure.
Buildings typically are designed for wind loads that are, of course, affected by building height, site, and importance factors. Typical lateral load magnitudes of 20 to 60 pounds per square foot are common for exterior wall components and cladding pressures.
Blasts, however, have the potential to increase these loads by a factor of 10, requiring an equivalent static lateral load of as much as 350 pounds per square foot. This load adds up quickly, and the upshot is that standard clips to standard exterior wall elements no longer are sufficient.
Many window, curtain wall, and skylight manufacturers are familiar with the federal standards for glazing blast resistance and will provide the glazing and frame design, including design of the anchor to the supporting structure.
In our experience, however, although familiar with these requirements, manufacturers have not had the specialized, in-house expertise to perform detailed analysis and design of their products.
Instead, they have relied on expert consultants. Therefore, before selecting suppliers on competitively bid projects, the structural engineer needs to establish the design loads to the structure supporting windows or obtain this information from a blast consultant.
Structural and architectural coordination
A critical factor in designing effective blast-resistant structures is a strong coordination effort between the structural engineer and the architect. See Coordination is key,” for a list of key coordination items and simplified explanations.
Regarding collaboration, it is crucial for the structural engineer to be involved during schematic design.
Preliminary design may yield dimensions of members such as columns, beams, and walls that are larger than standard to counteract the force of explosives. It is very important that architects and other designers take these oversized elements into account during initial design to avoid costly redesigns, overcosts, and potential delays to the project schedule.
A properly designed building will minimize property and human life loss in the event of a terrorist bomb blast, but it will not prevent it completely.
While losses might be sustained, catastrophic failure should be averted, which is the ultimate design goal.
Terrorist threats, both from within and from outside of our country’s borders, are a real and frightening reality in today’s world. Many projects that a structural engineer may encounter will require that specific structural elements and glazing be designed to protect against specific blast conditions and progressive collapse. The structural engineer must determine the threat level required for design, and establish a procedure for the building design.
Finally, the structural engineer not only must account for these blast effects, but also he or she must coordinate these issues proactively with the design architect and other engineering disciplines that may be affected by changes in the building structural elements. Blast-resistant design can be a challenge, but is effective when performed properly.
All three authors are senior structural engineers located in the Alexandria, Va., office of HDR Inc., an Omaha, Neb.- based architectural, engineering, and consulting firm. Michael G. Paczak, P.E., can be reached at email@example.com or 703-518-8500; Warren Duvall, P.E., can be reached at firstname.lastname@example.org; and Judith Cosby, P.E., can be reached at email@example.com.