Metal Matters

The Tie that Binds
Blast-Hazard Window Design Cannot Neglect Anchorage
by Rich Walker

As buildings are increasingly designed with considerations given to terrorist attacks or other possible threats, blast-hazard mitigating design of buildings intended for high-risk locations is increasingly required by many government agencies, private building owners, architects and engineering firms.

Because doors, windows and skylights are often the weakest link in any building enclosure’s ability to withstand exterior forces—including those applied by explosions—the entire window system design must be balanced. This recognizes that if any one part of the system fails, then the entire system fails, meaning that the metal frame (typically aluminum or steel), glazing and anchorage must all be able to withstand the design blast loading to the target level of protection.

Estimating a total system response by aggregating the individual engineering analyses of glazing, framing and anchorage typically results in a very conservative design. Physical testing of the complete assembly to the actual design loads is recommended to provide an undisputable validation of the assembly.

Frames
Frame and anchorage design is performed by applying the breaking strength of the window to the frame and the fasteners.

UFC 4-010-01 calls for, in accordance with ASTM F2248, framing members to restrict deflections of edges of the blast-resistant glazing they support to 1/160th of the length of the supported edge at allowable stress levels under the equivalent 3-second design loading.

Anchorage
Anchors are perhaps the most overlooked component of blast-resistant fenestration design, but can be the proverbial weakest link if not specified properly. They must transfer the load experienced by the window and/or door assembly to the building enclosure at designated anchor locations. Being key elements of the load path, all fasteners should be analyzed to assure that the material, material thickness and number of fasteners are adequate for the load.

To determine those loads, the starting point is to ensure the fastening system will withstand non-blast loads dictated by site-specific wind and impact loading requirements. A contributing factor is the weight of the glass, of the type and thickness, as determined by ASTM E1300. Then, as with other components, the blast-resistant connection design load can be determined in accordance with ASTM F2248-09, based on the applicable explosive weight at the actual standoff distance at which the window is sited.

Different anchorage designs are required for the different substrates encountered in commercial or institutional structures—typically concrete or masonry.

“Anchors are perhaps the most overlooked component of blast-resistant fenestration design, but can be the proverbial weakest link if not specified properly.”

Eve Hinman, PE, of Hinman Consulting Engineers Inc. in Arlington, Va., has pointed out that, for reinforced concrete construction designed to resist high pressure loads (as is typical for embassy construction), anchorage of steel window frames is provided by steel studs welded to a steel base plate. For this type of construction, the frame is typically constructed using a steel stop at the interior face and an angle with an exposed face at the exterior face. The frame is attached to the base plate using high-strength fasteners. Coordination is required to ensure that the fastener locations for the steel frame, the steel studs and the rebar cage are properly arranged. For masonry walls, Hinman says metal straps are recommended for anchoring the window into the wall.

Fastener Guidelines
Several industry guidelines provide useful background and references in the selection of fasteners. These include:
• AAMA 2501-06, Voluntary Guideline for Engineering Analysis of Window and Sliding Glass Door Anchorage Systems, which outlines the minimum requirements for structural engineering analysis of window and sliding glass door anchorage systems;

• AAMA TIR-A14-10, Fenestration Anchorage Guidelines, which explains the modes of anchorage system failure: the material being fastened through (bearing or pull-out and/or pullover), the fastener material itself (shear, tension, combined loading) or the material being fastened to (typically metal, wood or masonry) and describes the determination of design load values.

• AAMA TIR-A9-91, Metal Curtainwall Fasteners. When specifying fasteners, the designer, in addition to specifying loaded performance, must specify fastener quality, corrosion resistance and minimum mechanical properties. Specification of these items usually is done by appropriate reference to ASTM or other recognized standards. AAMA TIR-A9 provides Fastener Load Tables for evaluating the loaded performance of various size fasteners and fastener metals.

It is essential that fasteners have adequate protection against corrosion to prevent eventual failures due to moisture from rain and condensation of water vapor. In addition, protection must be provided against galvanic corrosion, which occurs when dissimilar metals are in contact in the presence of moisture.

The important point in blast-resistant fenestration design is to ensure that the capabilities of all components are balanced to eliminate weak points in the interrelated system and that the whole indeed performs as well as or better than indicated by the sum of its parts.

Additional information on the requirements for specifying these types of systems is available though AAMA 510-06 Voluntary Guide Specification for Blast Hazard Mitigation for Fenestration Systems.


Rich Walker is the president and CEO of the American Architectural Manufacturers Association in Schaumburg, Ill.

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