8.4 New Construction
would preclude the additional loss of primary structural
members beyond this localized damage zone (i.e., the loss
of additional columns, main girders, etc.). This does not
preclude the additional loss of secondary structural or
Progressive Collapse. Designs that facilitate or are
non-structural elements outside the initial zone of
vulnerable to progressive collapse must be avoided. At a
localized damage, provided the loss of such members is
minimum, all new facilities shall be designed for the loss
acceptable for that performance level and the loss does
of a column for one floor above grade at the building
not precipitate the onset of progressive collapse.
perimeter without progressive collapse. This design and
analysis requirement for progressive collapse is not part
Building Materials. All building materials and types
of a blast analysis. It is intended to ensure adequate
acceptable under the model International Building Code
redundant load paths in the structure should damage
are allowed. However, special consideration should be
occur for whatever reason. Designers may apply static
given to materials which have inherent ductility and
and/or dynamic methods of analysis to meet this
which are better able to respond to load reversals (i.e.,
requirement. Ultimate load capacities may be assumed
cast in place reinforced concrete and steel construction).
in the analyses.
Careful detailing is required for material such as pre-
In recognition that a larger than design explosive (or
stressed concrete, pre-cast concrete, and masonry to
other) event may cause a partial collapse of the structure,
adequately respond to the design loads. The construction
new facilities with a defined threat shall be designed with
type selected must meet all performance criteria of the
a reasonable probability that, if local damage occurs, the
specified Level of Protection.
structure will not collapse or be damaged to an extent
disproportionate to the original cause of the damage.
Exterior Walls
In the event of an internal explosion in an uncontrolled
Design for limited load:
public ground floor area, the design shall prevent pro-
gressive collapse due to the loss of one primary column, or
Design exterior walls for the actual pressures and
the designer shall show that the proposed design precludes
impulses up to a maximum of ___ psi and ___ psi-msec
such a loss. That is, if columns are sized, reinforced, or
(project-specific information to be provided).
protected so that the threat charge will not cause the
The designer should also ensure that the walls are
column to be critically damaged, then progressive collapse
capable of withstanding the dynamic reactions from the
calculations are not required for the internal event. GSA
windows.
encourages the use of highly confined reinforced concrete
Shear walls that are essential to the lateral and vertical
or stiffened steel columns and discourages the use of steel
load bearing system, and that also function as exterior
column splices within these spaces. For design purposes,
walls, shall be considered primary structures. Design
assume there is no additional standoff from the column
exterior shear walls to resist the actual blast loads
beyond what is permitted by the design.
predicted from the threats specified.
Discussion: As an example, if an explosive event causes the
local failure of one column and major collapse within one
structural bay, a design mitigating progressive collapse
243
SECURITY
DESIGN
8.4
New Construction
Revised March 2005 PBS-P100
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