The Tie that Binds
Blast-Hazard Window Design Cannot Neglect
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.
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.
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
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
Different anchorage designs are required for the different substrates
encountered in commercial or institutional structures—typically concrete
“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.
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
• 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
• 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
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.
Architects' Guide to Glass & Metal
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