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Designing a Post-Disaster Building Enclosure for Blast, Hurricane, and Floor Resistance: A Case Study

May 15, 2012

66 Perimeter Center East, Suite 600, Atlanta, GA 30346
Phone: 770-379-8500 • Fax: 770-379-8501 • E-mail:
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This case study will discuss the unique challenges and solutions developed for a hospital
project located in New Orleans, LA. The building enclosure was designed to meet hurricane
impact, blast resistance, and flood requirements. The building enclosure materials
included precast panels, insulated metal panels, rainscreen walls, membrane roofing, and
green roofs. The project was designed to remain fully operational through a design flood
event while allowing the first floor of the building to flood. This required an additional layer
of building enclosure within the building to provide an air and vapor barrier between the
first and second floors in order to maintain the separation of interior and exterior conditions
during a flood event. This also required designed environmental separations for stairwells
and elevator shafts, which run through the entire height of the building. The enclosure is
designed to minimize flood damage and allow for the refurbishment of the ground level following
subsidence of floodwaters. The design requirements were met through close collaboration
among the architect, building enclosure consultant, blast design consultant, structural
engineer, and enclosure system manufacturers.
CHRIS NORRIS is a principal with Morrison Hershfield. He specializes in building enclosure
consulting. His experience includes consultation for new construction projects, assessment
of existing buildings, and expert consultation in support of litigation. He has been
involved with projects throughout North America.
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Physical Security
Performance-Based Design
Rainwater Management
The Southern Louisiana Veterans
Association replacement hospital project is
an approximately 30-acre campus in New
Orleans comprising approximately 1.6 million
sq. ft.; it will replace an earlier medical
center that sustained severe flood damage
and was permanently closed following
Hurricane Katrina. The new campus
includes a total of ten interconnected buildings
and two parking garages.
The new campus is designed to remain
in operation during and following a hurricane
event. The facility is required to
remain operational for a ten-day period following
the loss of municipal power, water,
and sewer. The building design also incorporates
physical security requirements,
which require blast-resistant buildingenclosure
The project architect is Studio NOVA, a
joint venture between NBBJ and local firms
Eskew+Dumez+Ripple and Rozas Ward
Architects. Morrison Hershfield was retained
by Studio NOVA to provide building
enclosure consultation services for the project.
The role of Morrison Hershfield on this
project was to integrate the various performance
requirements for the building enclosure
into a building enclosure design that
achieved both the functional performance
requirements and the architect’s aesthetic
design intent.
This paper discusses the approach
taken to establish a building enclosure
design methodology, some of the key enclosure
design criteria, resulting design decisions,
and some of the specific building
enclosure details developed for this project.
The building enclosure design must
take into account the local climate along
with project- and client-specific requirements.
This project is a post-disaster facility
that is required to remain operational
during and following a hurricane. The exterior
enclosure design of the facility is
required to meet the Veterans Association
(VA) physical security requirements for
blast resistance.
During the schematic design phase of
the project, Morrison Hershfield worked to
establish a project-specific matrix of building
enclosure design factors. The following
are key design factors for the building
enclosure of this project:
The New Orleans climate is a humid
subtropical one with generally mild winters;
hot, humid summers; and an average annual
rainfall of 64 in. This climate results in a
predominant inward vapor drive through
the building enclosure assemblies. The hot,
humid exterior air presents a risk for condensation
within the enclosure assemblies
if air leakage and vapor diffusion are not
properly controlled.
New Orleans is subject to tropical
storms and hurricanes with the potential
for windborne debris. The project site is
subject to moderate flooding during a hurricane
event. The design wind speed at the
project site is a 3-second gust wind speed of
130 mph.
Significant consolidation of fill is expected
at the project site. The buildings are supported
by piles. The fill beneath the firstfloor
slabs is expected to settle by a foot or
The campus is subject to the VA physical
security requirements. All buildings on
the campus are subject to either the VA
“Mission Critical” or the VA “Life Safety”
protected requirements, dependent on the
building use and occupancy.
The building enclosure systems and
materials are required to be nonproprietary.
All systems and materials are specified
through performance criteria, which are
intended to ensure an appropriate level of
performance for the project while allowing
multiple vendors to provide bids for the
building enclosure systems for this project.
A general building enclosure design
methodology was developed based on the
project-specific design factors. The following
outlines the building enclosure design
methodology for this project. Following is
the design methodology used as a guide to
select and detail the building enclosure
Rainwater exposure management is
critical for this project due to the high
annual rainfall, combined with the potential
for wind-driven rains and the fact that the
buildings are required to remain operational
following a hurricane event.
All roof, wall, and glazing assemblies are
required to provide a dual line of defense
against rainwater penetration. All wall and
glazing assemblies are required to prevent
rainwater penetration at a static pressure of
20 psf (per testing in accordance with ASTM
E331), which is approximately 30% of the
maximum positive design pressures as
determined from a wind tunnel study. The
maximum positive design pressures for the
project were 70 psf at the building corners
and 65 psf for the main façade areas,
respectively. AAMA guidelines typically call
for field-testing at 10% of the design wind
pressure or 15% for architectural windows.
Higher static test pressures were selected
for this project because of the potential for
severe wind-driven conditions, combined
with the post-disaster occupancy requirements
for the facility. Verification of rainwater
performance is to be determined by
mock-up testing prior to construction and
quality assurance testing during construction.
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Air Leakage Control
Vapor Diffusion Control
Hurricane Resistance
Blast Resistance
Controlling air leakage is important —
both for energy conservation reasons and
for moisture control reasons. In the humid
New Orleans climate, air infiltration
through the building enclosure could result
in condensation within the assemblies as
hot, humid exterior air cools as it leaks
through the assemblies to the interior.
The buildings are designed with a continuous
air barrier to prevent air infiltration
and exfiltration through the building enclosure
between the interior and exterior. The
components and materials making up the
air barrier vary between assemblies. Care
has been taken to ensure that all air barrier
components are properly integrated at all
An air barrier is also provided within the
buildings between the first and second
floors. This air barrier is provided in order
to prevent migration of moisture and contaminants
in the event that the first floor is
flooded during a hurricane.
Air barrier materials used are required
to have a maximum air leakage rating of
0.004 cfm/sf at 1.57 psf. The building
enclosure design is based on a maximum
allowable air leakage rate of 0.25 cfm/sf at
1.57 psf for the whole building. The air barrier
is required to resist wind loads, stackeffect
pressure, and mechanical pressurization
Continuity of the air barrier is also
important to reduce the risk of unintended
pressure effects between the interconnected
buildings, which could occur due to differing
air leakage rates in adjacent buildings.
A campuswide airflow model was conducted
by RWDI to verify that the air barrier system
and mechanical systems would work properly
together to prevent unintended pressure
effects between buildings.
The wall and roof assemblies are
designed to prevent moisture accumulation
due to vapor diffusion. A vapor retarder is
provided within the ground-floor slab
assembly to prevent vapor diffusion
through the slab-on-grade. A vapor retarder
is also provided between the first and second
floors to prevent moisture migration in
the event that the first floor is flooded during
a hurricane.
The wind loads for the project were
developed through a wind tunnel study con-
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ducted by RWDI in conformance with
ASCE-7-05. A wind tunnel study was elected
for this project rather than relying on
code-calculated wind loads in order to
develop a more thorough understanding of
wind loads on the building, and particularly
to make sure that localized effects were
properly accounted for in the roofing and
cladding design.
Design wind loads for the building
enclosure systems are specified per the project-
specific wind tunnel study with performance-
based criteria for allowable deflections.
The required locations of all expansion
joints and all horizontal and vertical movement
joints within each building enclosure
system and between systems are identified
on the architectural drawings. The movement
joints accommodate the anticipated
story drift of the primary building structure
as well as thermal expansion/contraction of
the enclosure systems themselves. The
expansion joints accommodate movements
of the primary building structure at expansion
joint locations. The structural displacements
for the base building structure that
the systems must accommodate are established
by the project structural engineer.
The decision was made to show the movement
joint and expansion joint locations to
ensure that movements of the building
structure will be properly accounted for in a
systemic fashion within the building enclosure
Hurricane resistance of the building
enclosure is a critical aspect to the function
of this facility. As a post-disaster medical
facility, it is critical that the functionality of
the building be maintained. While it is
unrealistic to design the building to sustain
zero damage in a hurricane event, it is crucial
that redundancy be incorporated into
the enclosure design such that the building
will remain watertight and airtight following
a hurricane event.
FEMA 577, “Design Guide for Improving
Hospital Safety in Earthquakes, Floods, and
High Winds,” was used as a resource in
determining hurricane performance
requirements and selecting and detailing
the building enclosure assemblies.
In order to address the potential for
windborne debris during a hurricane, all
building enclosure systems are required to
meet large- and small-missile impact resistance
requirements (ASTM E1886 and
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ASTM E1996 test standards). Based on the
design wind speed and building occupancy,
this project required that “missile E”1 be
used for large missile testing.
Blast hardening of government facilities
has become a key design requirement in the
post-9/11 world. The purpose of blastresistant
design is to limit injury to the
building occupants, to prevent progressive
collapse of the building, and to limit the
damage so as to reduce the impact on facility
operations. The building enclosure is
required to absorb the initial blast impact
and transfer loads back to the primary
building structure.
The specific performance requirements
for the building enclosure are dependent on
the occupancy. For this project, the blast
resistances for exterior enclosure assemblies
are based on the VA physical security
requirements. The responsible party for
blast-resistant design varies among exterior
enclosure systems. In some cases, blast
resistance is achieved by prescriptive
design; in others, it is achieved by performance
Morrison Hershfield worked with the
physical security consultant, Hinman, to
verify that each of the selected building
enclosure assemblies can meet the project
requirements for blast resistance.
The exterior wall materials were selected
by Studio NOVA to achieve the desired
architectural intent for the project.
Morrison Hershfield provided consultation
to develop complete wall assemblies based
on the building enclosure design methodology.
Morrison Hershfield also provided project-
specific details for complex interfaces.
Exterior wall cladding materials include
insulated-foam-core metal panel, precast
architectural concrete sandwich panels,
unitized curtain wall, and composite metal
panel. The following section describes each
of the assemblies and the approach taken
by each to meet the design methodology.
This wall assembly is composed of the
following elements, from exterior to interior:
• Prefinished insulated-foam-core
metal panel installed on clips
• Fluid-applied, vapor-impermeable
air/weather barrier over fiberglass-
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mat gypsum sheathing
• Steel-stud framing
• Interior gypsum board and finishes
In some locations, CMU is used in place
of the steel-stud assembly with fiberglassmat
gypsum sheathing. See Figure 1.
The foam-core panel joints are pressureequalized
and drained to the exterior. The
fluid-applied air/weather/vapor barrier
provides a secondary level of protection
against rainwater penetration. Drainage of
the wall assembly is provided at the base of
the wall and at window heads. The fluidapplied
air/weather barrier was selected
because it provides a waterproof layer, and
the material itself is selfgasketing
at typical fastener
The fluid-applied air
/weather barrier provides
a continuous air barrier
behind the foam-core panels.
A fluid-applied air barrier
was selected because it
provides a monolithic air
/weather barrier that is
fully adhered to the
sheathing. The fully adhered
nature of the air
/weather barrier prevents
stress on the material due
to billowing of the material
under negative pressure,
which can occur with
mechanically fastened sheet air barriers.
The metal skin on the foam-core panels
is vapor-impermeable and provides an exterior
vapor barrier outboard of the insulation.
The air/weather barrier is also vaporimpermeable,
which provides a secondary
vapor barrier to protect the moisture-sensitive
wall components behind. Any moisture
that condenses on the exterior surface of
the secondary vapor barrier can drain from
the system.
The steel stud backup wall is designed
to work in conjunction with the foam-core
metal panels to resist hurricane debris
impact. Impact testing of this assembly is
required to demonstrate that the assembly
meets the project requirements. Relevant
prior-test reports or project-specific impact
testing is required.
The steel-stud back-up wall is designed
to work in conjunction with the foam-core
metal panels to provide a blast-resistant
assembly. CMU backup is used in place of
steel stud in locations requiring higher blast
resistance. The steel-stud and CMU walls
are designed by the project structural engineer.
The cladding and attachments are
designed by the manufacturer. The level of
blast resistance required varies and was
determined based on modeling by the blast
consultant for the project.
The composite-metal
panel assembly consists of
composite metal panels
over a fluid-applied air
/weather barrier on cement
board, over corrugated
steel decking, over
steel-stud framing with
sprayed polyurethane
foam insulation on the
interior side of the steel
decking. In some locations,
CMU is used in
place of the corrugated
metal and steel-stud
assembly for improved
blast resistance. See Figure
The composite metal panels are
installed in a rainscreen fashion with a
vented and drained cavity between the composite-
metal panels and the air/weather
The fluid-applied air/weather barrier
provides a continuous air barrier behind
the composite metal panels, similar to the
foam-core panels discussed above.
The vapor-impermeable air/weather
barrier provides resistance to inward vapor
drive. The closed-cell sprayed polyurethane
foam provides resistance to outward vapor
drive, although outward vapor drive is not
significant in the New Orleans climate.
The steel-stud backup wall is designed
to work in conjunction with the foam-core
metal panels to resist hurricane debris
impact resistant. Impact testing of this
assembly is required to demonstrate that
the assembly meets the project requirements.
Relevant prior test reports or projectspecific
impact testing is required.
The corrugated steel decking on steel
studs provides blast resistance for this wall
assembly. CMU is used in locations where
higher blast resistance is required. The steel
stud and CMU walls in this assembly are
designed by the project structural engineer.
The precast panels consist of a ribbed
exterior wythe of architectural precast, an
extruded-polystyrene insulation core, and
an interior structural wythe of precast. The
inner and outer wythes are connected with
nonthermally conductive ties to minimize
thermal bridging through the insulation in
the precast assembly. A PVC insulation cap
is cast into the panel around the panel
perimeter. The panel joints are provided
with a dual-stage weeped sealant joint. See
Figure 3.
The precast panels themselves act as a
mass wall. During periods of rainfall, the
concrete will absorb water, which can then
be released during drying periods. The use
Rainwater Management
Air Leakage Control
Vapor Diffusion Control
Hurricane Impact Resistance
Blast Resistance
Rainwater management
Air Leakage Control
Vapor Diffusion Control
Hurricane Impact Resistance
Blast Resistance
Rainwater Management
Figure 1 – An air/weather barrier is
provided behind the insulated-foamcore
metal panel assembly.
Figure 2 – Composite-metal
panel assembly installed
as a rainscreen over the
air/weather barrier.
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Air Leakage Control
Vapor Diffusion Control
Hurricane Impact
Blast Resistance
Rainwater Management
Air Leakage Control
Vapor Diffusion Control
Hurricane Impact Resistance
Blast Resistance
Figure 3 – Dual-stage weeped sealant
joints used at panel-to-panel joints in
the architectural precast assembly.
of a sandwich panel mitigates the risk of a
hairline crack penetrating through the
entire panel thickness, which could cause
water leakage in a monolithic precast panel.
The dual-stage joints at the precast panels
provide a rainscreen interface between the
panels. The interior joint prevents any
water that penetrates the exterior joint from
penetrating to the exterior, and the weeps
redirect water back to the exterior. Finally,
the insulation caps provide a suitable substrate
to which sealant may adhere, which
mitigates the risk of sealant joint failure due
to the inner joint being placed in contact
with the insulation layer.
The concrete of the precast panels
themselves provides an excellent air barrier.
The inner sealant joint between precast
panels is continuous and provides continuity
of the air barrier at
panel-to-panel joints.
The extruded polystyrene
insulation used in
the precast sandwich panel
provides the primary resistance
to vapor diffusion
through the assembly,
although the precast concrete
also has some resistance
to vapor diffusion.
The precast concrete
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panels provide the required hurricane
debris impact. Impact testing of this assembly
is required to demonstrate that the
assembly meets the project requirements.
Relevant prior test reports or project-specific
impact testing is required.
The precast panels and anchors are
required by performance-based specifications
to be engineered to resist blast loads.
The panels are attached to the floor slabs
and not directly to columns, as required by
blast-resistant design practices.
A unitized curtain wall was selected for
this project to provide improved quality control
by reducing the extent of field sealant
work and allowing critical seals to be completed
in a factory environment. See Figure 4.
The unitized curtain wall is specified as
a pressure-equalized drainable system with
field-water penetration resistance of 20 psf
when tested per ASTM E1105. All interfaces
between the curtain wall and the adjacent
assemblies are detailed in a rainscreen
fashion. Dual-stage sealant joints are used
at the curtain wall-to-precast interfaces.
Sealant joints are used for the exterior
weather seal at the metal panels while an
extruded-silicone sheet flashing is used for
the inner seal to the air/weather barrier.
The curtain wall is specified with a maximum
allowable air leakage of 0.06 cfm at
6.24 psf. Continuity of the air barrier with
adjacent assemblies is achieved with
sealant at interfaces between the curtain
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wall to precast interfaces and with extruded
silicone elsewhere.
The glass in the curtain wall system is
vapor-impermeable and prevents inward
vapor diffusion from exterior to interior.
There is limited outward vapor drive. Foilfaced
mineral wool insulation is used at the
spandrel areas to mitigate outward vapor
drive during cold periods of weather.
The curtain wall system is a performance-
based system, and the manufacturer
is required to provide a system that meets
the hurricane debris impact requirements
for the project. Relevant prior test reports or
project-specific impact testing is required.
The curtain wall system is a performance-
based system, and the manufacturer
is required to provide a system that
meets the blast-resistance criteria for the
project. Four-sided structural glazing, along
with laminated glass, will be used to retain
the glass within the frame.
The curtain wall manufacturer is
responsible for engineering analysis of the
curtain wall system and anchors to confirm
that the system will meet the blast resistance
requirements of the project. The system
is anchored to the floor slab or to precast
cladding. The system is not anchored
to any building columns.
While many of the measures used to
achieve blast resistance are similar to those
used for hurricane debris impact resistance,
blast resistance does not guarantee
that the system will provide suitable hurricane
debris impact resistance because the
structural loading and the
system responses are different
for blast pressures and
hurricane debris impact
forces. This project requires
that the curtain wall manufacturer
conduct hurricane
debris impact testing as
well as engineering analysis
for blast resistance.
The roof assembly for
this project consists of the
following elements listed
from exterior to interior:
• Two-ply torch-applied
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Figure 4 – Unitized curtain wall installed into precast panels.
Rainwater management
Air Leakage Control
Vapor Diffusion Control
Rooftop Equipment
roof membrane
• Five-eighths-in. Glasmat
gypsum cover board
• Polyisocyanurate insulation
(minimum 3-in.
• SBS modified-bitumen
• Concrete on steel deck
See Figure 5.
The roof is provided with
two levels of protection against
rainwater penetration. The sec-
Figure 5 – The secondary roof membrane on
concrete deck is also used to provide an air
ondary membrane on the roof
deck will prevent water penetration
into the building in the event of a
breach in the primary two-ply modifiedbitumen
roof membrane by wind-borne
debris. In the event of roof damage due to a
hurricane, it is unlikely that immediate
repairs will be possible. The secondary roof
membrane will allow the building to remain
operational and watertight until roofing
repairs can be completed.
Roof drains are single-level draining at
the roof surface only and not at the level of
the secondary membrane. The drain penetrations,
however, are sealed to the secondary
membrane at the roof deck. The
decision was made not to provide drainage
at the secondary membrane as this membrane
is intended to provide temporary protection
following a hurricane only.
The selected roof assembly conforms to
the recommendations of FEMA 577, which
makes the following recommendations:
• Modified-bitumen membrane for
improved puncture resistance to
small missiles
• A 5/8-in. protection board under the
• A minimum insulation thickness of
3 in. for areas with wind speeds
between 110 mph and 130 mph.
The secondary roof membrane provides
an air barrier for the roof assembly. This
membrane is integrated with the wall air
barriers at the roof perimeter. Various different
interface details have been developed,
depending on the exterior wall assembly.
The roof membrane itself is vaporimpermeable
and prevents inward vapor
drive into the roof assembly. The secondary
membrane on the roof deck is also vaporimpermeable
and prevents outward vapor
drive from the building into the roof assembly.
The provision of a secondary roof membrane
increases the importance of the insulation’s
being dry at the time of roof installation,
because any moisture in the insulation
will be trapped between the primary
and secondary roof membranes.
The architectural design of the building
eliminates rooftop mechanical equipment,
ducting, and vents, as these items can be
problematic in hurricane-prone regions.
Mechanical equipment is located within the
Central Energy Plant building, at interstitial
mechanical floors within the buildings, and
in mechanical penthouses. Louvers in the
exterior walls are
used for ventilation
to eliminate the need
for vent penetrations
through the roofing.
The roof lightning
protection is to be
fastened along the
inside face of the
parapet coping. The
lightning protection
is to be mechanically
fastened into the
parapet nailer in
accordance with the
recommendations of
FEMA 577.
The project includes a vapor retarder
beneath the ground floor slab to prevent
moisture migration through this slab. The
vapor retarder in this assembly is a 15-mil
sheet membrane sandwiched between the
structural slab and a topping slab. The
vapor retarder was placed above the structural
slab in this assembly since the fill
beneath this slab is anticipated to consolidate
and leave a gap of a foot or more
between the undersides of slab and fill. See
Figure 6.
The project also includes a topically
applied vapor retarder barrier on the second-
floor slab. This vapor retarder prevents
moisture migration from the first to second
floor in the event that the first floor is flooded
during a hurricane.
Successful execution of the project
requires the selection of appropriate wall
and roof assemblies, as well as careful
interface detailing to ensure continuity of
the building envelope design intent between
the assemblies.
Three-dimension peel-away details were
developed for the project to show the
assemblies and connectivity of the air and
weather barriers at interfaces. Figure 7
shows a transition membrane from the secondary
roof membrane to the precast panel
for continuity of the air barrier at the
perimeter. The roofing membrane also
wraps the precast parapet beneath the
metal coping. Figure 8 shows a transition at
a building expansion joint between a low
roof and adjacent precast wall. The use of a
three-dimensional detail allowed confirmation
of air and weather barrier continuity.
Figure 6 – Ground-floor assembly with vapor retarder.
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Many design factors had to be considered
in selecting and detailing the building
enclosure systems for this project. Developing
an appropriate building enclosure
design methodology required coordination
and consultation with several specialty consultants
along with the project architect.
Establishing a building enclosure design
methodology early in the project facilitated
the development of a building enclosure
system that meets both the aesthetic and
functional requirements for the project.
1. “Missile E” is defined as an 8-ft.-long
2×4 piece of lumber weighing
9+/0.25 lb. with an impact speed of
80 f/s.
Figure 7 – Roof-to-parapet interface detail.
Figure 8 – Expansion joint detail.
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