6 Performance Criteria for 
Debris Impact 

Performance criteria for tornado and hurricane shelters will build on the 
design criteria in Chapter 5, the existing guidance for residential shelters, and 
the manuals and publications listed in Section 5.1.1. The most recent of these 
documents are the National Performance Criteria for Tornado Shelters (July 
2000), ASCE 7-98, and FEMA 320. Although these documents do not address 
some factors and elements of the design of extreme-wind shelters, they 
provide the basis for the criteria presented in this chapter. 

Chapter 5 of this manual and ASCE 7-98 present the information necessary 
for the computation of wind pressures and the loads imposed by winds on the 
walls, roof, windows, and doors of a shelter area. The walls, ceiling, floor, 
foundation, and all connections joining these elements will be designed to 
resist the pressures and loads calculated from the design wind speed without 
localized element failure and without separating from one another. 

The entire shelter structure must resist failure from wind pressures and debris 
impacts. For the in-residence shelter designs presented in FEMA 320, ceiling 
spans and wall lengths were no greater than 8 feet and the design of the wall 
and ceiling was governed by the criteria specified for resistance to the impacts 
of windborne debris. For larger, community shelters, this broad statement 
cannot be made; the structural elements and the building envelope must be 
designed to resist wind-induced loads as well as impacts from debris. 

6.1 Missile Loads and Successful Test Criteria 
Although there is a substantial body of knowledge on penetration and 
perforation of small, high-speed projectiles, relatively little testing has been 
done on lower-speed missiles such as windborne debris impacting buildings. 
In the design of community shelters or other large shelters, wind loads are 
likely to control the structural design. However, C&C and building envelope 
issues may be governed by missile impact requirements. Nonetheless, after 
the shelter has been designed to withstand wind forces from the design wind 
speed, the proposed wall and roof sections must be tested for impact 
resistance from missiles. Roof and wall sections that have been tested for 
impact from the design missile are presented in Appendix E. A wall or roof 
section that is the same as the wall sections in Appendix E may be used 
without additional testing. 

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6.1.1 Propelled Windborne Debris  Missiles 
The standard missile used for the impact tests discussed in FEMA 320 and 
those specified in FEMAs July 2000 edition of the National Performance 
Criteria for Tornado Shelters has remained unchanged. Although windstorms 
with wind speeds less than 250 mph typically result in lower missile speeds 
(for the same size missile), it is recommended that shelter designs be prepared 
for the missile size and wind speeds indicated in this section. 

The standard missile used to determine impact resistance for all wind 
conditions is defined as follows (based on a representative missile for a 250mph 
windstorm): 

 15-lb wood 2x4 (nominal) member 
 typically 12 feet long 
The missile is assumed to be propelled into wall and roof sections at the 
following missile speeds and to impact the test specimen (or shelter) 90 to 
the surface (see Figures 6-1 and 6-2 for examples of damage caused by this 
missile): 

 100-mph missile speed for horizontally travelling missiles 
 67-mph missile speed for vertically travelling missiles 
The static force equivalent of this dynamic impact is difficult to calculate, and 
a direct conversion to a static load often results in extremely large loads. The 
actual impact force of the missile varies with the material used for the wall or 
roof section and will be a function of the stiffness of the material itself as well 
as the overall stiffness of the wall section in which it is used. Therefore, no 
formula for the determination of impact load is provided in this manual. 

Various wall and roof sections tested at the WERC at TTU performed 
successfully. They are summarized in Chapter 6 and described in detail in 
Appendix E. The designer is referred to Appendix G for a selection of wall 
materials that have successfully passed missile impacts under the criteria 
outlined above. 

6.1.2 Falling Debris 
Falling debris also create structural damage, the magnitude of which is a 
function of the debris size and distance the debris falls. Falling debris 
generally consists of building materials and equipment that have significant 
mass and fall short distances from taller structures nearby. When siting the 
shelter, the designer should consider placing the shelter away from a taller 
building or structure so that if that structure collapses, it will not directly 
impact the shelter. When this cannot be done, the next best alternative would 

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Figure 6-1 

Wood 2x4 launched at 100 mph 
pierced unreinforced masonry 
wall, WERC, Texas Tech 
University. 

Figure 6-2 

Refrigerator pierced by 
windborne missile. 

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Table 6.1 

Windborne Debris (Missiles) 
and Debris Classifications for 
Tornadoes and Hurricanes 

be to site the shelter in such a way that no large structure is within a zone 
around the shelter defined by a plane that is 1:1 (vertical to horizontal) for the 
first 200 feet from the edge of the shelter. 

If it is not possible to site the shelter away from all the falling debris hazards 
at a site, the designer should consider strengthening the roof and wall systems 
of the shelter for the potential dynamic load that may result from these large 
objects impacting the shelter. Minimal guidance concerning the dynamic 
effect of large pieces of debris impacting shelters is available; however, the 
results of some limited testing, and approaches for designing for these loads, 
are discussed later in this chapter as performance criteria. 

6.2 Windborne Debris (Missile) Impacts 
The quantity, size, and force of windborne debris (missiles) generated by 
tornadoes and large hurricanes are unequaled by those of other windstorm 
debris. Missiles are a danger to buildings because the debris can damage the 
structural elements themselves or breach the building envelope. If the missile 
breaches the building envelope, wind may enter the building, resulting in an 
overpressurization of the building that often leads to structural failures. This 
high potential for missiles capable of breaching a buildings exterior supports 
the recommended use of the internal pressure coefficient for partially enclosed 
buildings in the design criteria presented in Section 5.3. In addition, 
windborne debris may kill or injure people who cannot find shelter or refuge 
during a tornado or hurricane. 

Most experts group missiles and debris into three classifications. Table 6.1 lists 
the classifications, presents examples of debris, and describes expected damage. 

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Although large pieces of debris are sometimes found in the aftermath of 
extreme wind events, heavy pieces of debris are not likely to become airborne 
and be carried at high speeds. Therefore, from research in the field after 
tornadoes and hurricanes, as well as the results of research at TTU studying 
windborne debris in various wind fields, the representative missile has been 
selected as a 15-lb wood 2x4 (1214 feet long). 

This is the same missile criterion specified in Chapter 5 of this manual. Wind 
events have been modeled to show that the selected 15-lb missile will have 
different speeds and trajectories, depending on the event. However, to be 
conservative, it is recommended that test criteria for missile impact resistance 
be as stated in this section and Section 6.1.1. 

Comparisons of results from missile impact tests for missiles other than the 
15-lb wood 2x4 traveling at the design missile speed are discussed in Appendix G. 

6.2.1 Debris Potential at Shelter Sites 
Debris impacting buildings during a severe windstorm can originate from 
both the surrounding area and from the building itself and is not limited to the 
representative missile discussed in Section 6.2. During the development of a 
shelter design, the design professional should review the site to assess 
potential missiles and other debris sources in the area. 

In addition to the wood 2x4 member described in the previous section, roof 
coverings are a very common source of windborne debris (missiles) or falling 
debris (ranging from roof gravel or shingles to heavy clay tiles, slate roof 
coverings, and roof pavers). Other sources of debris include roof sheathing 
materials, wall coverings, roof-mounted mechanical equipment, parapets, 
garbage cans, lawn furniture, missiles originating from trees and vegetation in 
the area, and small accessory buildings. Missiles originating from loose 
pavement and road gravel have also been observed in intense windstorms. In 
one area impacted by Hurricane Andrew, mailboxes were filled with rocks 
and asphalt from surrounding roadways. 

As buildings break apart in severe windstorms, the failures progress from the 
exterior building elements inward to the structural members (e.g., trusses, 
masonry units, beams, and columns). The literature on tornadoes and 
hurricanes contains numerous examples of large structural members that have 
been transported by winds for significant distances. Generally, large debris 
such as structural members are transported significant distances by the 
windfield when a portion of exterior sheathing remains connected and 
provides an aerodynamic sail area on which the wind can act. 

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Rooftop mechanical equipment that is kept in place only by gravity 
connections is a source of heavy deformable debris when displaced during 
high-wind events. Furthermore, additional vulnerabilities to missile and wind 
are created when rooftop equipment is displaced from the roof, leaving large 
openings in the roof surface. Cars and trucks are also moved by strong winds. 
Lightweight vehicles can be moved around in parking lots in winds with gust 
speeds approaching 100 mph. Although pieces of debris larger than the test 
missile (15-lb 2x4) are observed, the speed of these missiles is considerably 
less. From post-disaster investigations, the 2x4 test missile appears most 
representative of the high-energy missile most likely to penetrate conventional 
construction. However, a shelter that has been designed to provide punching 
shear resistance from a 15-lb wood 2x4 and the capacity to resist the large 
wind forces associated with an extreme wind event will likely provide 
protection for some level of impact from larger debris items. Additional 
design guidance concerning large falling debris is presented in Section 6.3. 

6.2.2 Induced Loads From the Design Missile and Other Debris 
Determining static design loads from a propelled missile or a piece of free-
falling debris is a complex computation. This computation depends on a 
number of factors, including the following: 

 material that makes up the missile or falling debris 
 material of the wall, door, window, or roof section being impacted 
 stiffness of the individual elements being impacted 
 stiffness of the structural system supporting them 
 angle of impact between the missile and the structure 
Because of the complex nature of missile and debris impacts, this manual 
does not provide design criteria that can be used to calculate the static force of 
a missile impact on any part of the shelter. To determine adequate missile 
impact resistance for a shelter, the designer should use the performance 
criteria presented in this chapter and the results of successful wall, roof, door, 
and window tests that are presented in Appendixes E and F of this manual. 

6.2.3 Impact Resistance of Wood Systems 
Texas Tech University has conducted extensive testing of wall systems that 
use plywood sheathing. The most effective designs, in terms of limiting the 
number of layers of plywood required, incorporate masonry infill of the wall 
cavities or integration of 14-gauge steel panels as the final layer in the system. 
Appendix E shows wall sections that have been tested with the design missile 
without failing (i.e., provide adequate missile impact resistance). Examples 
are shown in Figure 6-3. 

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For conventional light-frame construction, the side of the wall where the 
sheathing or protective material is attached and the method of attachment can 
affect the performance of the wall in resisting damage from the impact of 
windborne debris. The impact of debris on material attached to the outside 
(i.e., harm side) of a wall pushes the material against the wall studs. Material 
attached to the inside of the wall (i.e., safe or shelter side) can be knocked 
loose from the studs if it is not adequately attached to the studs. Similarly, 
material on the harm side would be susceptible to being pulled off the studs 
by wind suction pressures if it were not adequately attached to the studs. 

Consequently, sheathing materials bearing on the framing members should be 
securely attached to the framing members. Tests have shown that sheathing 
attached using an AFG-01 approved wood adhesive and code-approved #8 
screws (not drywall screws) penetrating at least 1-1/2 inches into the framing 
members and spaced not more than 6 inches apart provides sufficient capacity 
to withstand expected wind loads if the sheathing is attached to the exterior 
surfaces of the wall studs. These criteria are also sufficient to keep the 
sheathing attached under impact loads when the sheathing is attached to the 
interior surfaces of the studs. For information about oriented strand board or 
particleboard sheathing, see Appendix G. 

Figure 6-3 

Wall sections constructed of 
plywood and masonry infill 

(a) and plywood and metal 
(b). 
DEFINITION 

AFG-01 is an American 
Plywood Association (APA) 
specification for adhesives for 
field gluing plywood to wood 
framing. 

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6.2.4 Impact Resistance of Sheet Metal 
Various gauges of cold rolled A569 and A570 Grade 33 steel sheets have been 
tested in different configurations (see Figure 6-4 for an illustration of a 
representative wall section). The steel sheets stop the missile by deflecting 
and spreading the impact load to the structure. Testing has shown that if the 
metal is 14 gauge or lighter and is backed by any substrate that prevents 
deflection of the steel, the missile will perforate the steel. If the 14-gauge or 
lighter steel sheets are placed between plywood layers or between plywood 
and studs, the steel does not have the ability to deflect and is perforated by the 
missile. Therefore, on a wood stud wall, a 14-gauge steel sheet can resist 
perforation only when it is used as the last layer on the non-impact face on the 
interior (shelter side) of the wall, as shown in Figure 6-3. 

Figure 6-4 

Uses of expanded metal (a) 
and sheet metal (b) in wall 
sections. 

In laboratory tests at Texas Tech University, 12-gauge or heavier steel sheets 
have never been perforated with the 15-lb wood 2x4 traveling at 100-mph. 
The 12-gauge steel has been mounted directly to studs and mounted over solid 
plywood. Test samples have used the standard stud spacing of 16 inches on 
center (o.c.). Increased spacing between supports affects the permanent 
deformation of the steel sheet. Permanent deformation of 3 inches or more 
after impact is deemed unacceptable. Tests have not been performed to 
determine the maximum support spacing that would control the 3-inch 
permanent deformation limit. 

Designs provided in FEMA 320 include the use of sheet metal in shelter roof 
construction. If sheet metal alone is relied on for missile impact protection, it 
should be 12 gauge or heavier. 

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6.2.5 Impact Resistance of Composite Wall Systems 
Composite wall systems require rigorous testing because there is no adequate 
method to model the complex interactions of materials during impact. Tests 
have shown that impacting a panel next to a support can cause perforation 
while impacting midway between supports results in permanent deformations 
but not perforation. Seams between materials are the weak links in the tested 
systems. The location and length of seams between different materials are 
critical. Currently the best way to determine the missile shielding ability of a 
composite wall system is to build and test a full-scale panel that consists of all 
the materials and structural connections to be used in constructing the panel. 
See Figure 6-5 for an illustration of a representative composite wall section. 

Figure 6-5 
Composite wall section. 
6.2.6 Impact Resistance of Concrete Masonry Units 
Texas Tech research has demonstrated that both 6- and 8-inch-thick concrete 
masonry unit (CMU) walls that are fully grouted with concrete and reinforced 
with #4 reinforcing steel (rebar) in every cell (see Figure 6-6) can withstand 
the impact of a 15-lb 2x4 wood member striking perpendicular to the wall 
with speeds in excess of 100 mph. 

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CHAPTER 6 
Figure 6-6 
Concrete masonry unit (CMU) 
wall sections. 
6.2.7 Impact Resistance of Reinforced Concrete 
Research related to the design of nuclear power facilities has produced a 
relatively large body of information and design guides for predicting the 
response of reinforced concrete walls and roofs to the impact of windborne 
debris. The failure modes have been identified as penetration, threshold 
spalling, spalling, barrier perforation, and complete missile perforation 
(Twisdale and Dunn 1981). From a sheltering standpoint, penetration of the 
missile into, but not through, the wall surface is of no consequence unless it 
creates spalling where concrete is ejected from the inside surface of the wall 
or roof. Spalling occurs when the shock wave produced by the impact creates 
tensile stresses in the concrete on the interior surface that are large enough to 
cause a segment of concrete to burst away from the wall surface. Threshold 
spalling refers to conditions in which spalling is just being initiated and is 
usually characterized by small fragments of concrete being ejected. When 
threshold spalling occurs, a person directly behind the impact point might be 
injured but is not likely to be killed. 

However, as the size of the spalling increases, so does the velocity with which 
it is ejected from the wall or roof surface. When spalling occurs, injury is 
likely for people directly behind the impact point and death is a possibility. In 
barrier perforation, a hole occurs in the wall, but the missile still bounces off 
the wall or becomes stuck in the hole. A plug of concrete about the size of the 
missile is knocked into the room and can injure or kill occupants. Complete 
missile perforation can cause injury or death to people hit by the primary 
missile or wall fragments. Design for missile impact protection with 
reinforced concrete barriers should focus on establishing the minimum wall 
thickness to prevent threshold spalling under the design missile impact. 
Twisdale and Dunn (1981) provide an overview of some of the design 
equations developed for nuclear power plant safety analysis. 

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CHAPTER 6 
It should be noted that the missiles used to develop the analytical models for 
the nuclear industry, which are most nearly suitable for wood structural 
member missiles, are steel pipes and rods. Consequently, the models are 
expected to provide conservative estimates of performance when a softer 
missile, such as a wood structural member, impacts the walls. A summary of 
test results from a number of investigations (Twisdale and Dunn 1981) 
suggests that 6-inch-thick reinforced concrete barriers are needed to stop a 
15-lb wood 2x4 missile impacting at 100 mph without threshold spalling. 
Texas Tech University research indicates that a 6-inch reinforced concrete 
wall (see Figure 6-7, illustrations a and b) provides sufficient protection from 
the 15-lb wood 2x4 missile impacting at 100 mph. Furthermore, reinforced 
concrete walls constructed with insulating concrete forms with a concrete 
section 4 inches thick (see Figure 6-7, illustration c) also provide sufficient 
protection. The Texas Tech University research also shows that a 4-inch-thick 
reinforced concrete roof provides sufficient protection from a 15-lb wood 2x4 
missile impacting at 67 mph (the free-falling missile impact speed 
recommended in this document). 

Figure 6-7 

Reinforced concrete wall 
section (a), reinforced 
concrete waffle wall 
constructed with insulating 
concrete forms (b), and 
reinforced concrete flat 
wall constructed with 
insulating concrete forms (c). 

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6.3 Large Falling Debris 
The design requirements for the wind speed selected from Figure 2-2 and the 
representative missile impact criteria outlined in Section 6.2 provide most 
shelter designs with roof and wall sections capable of withstanding some 
impacts from slow-moving, large (or heavy) falling debris. The residual 
capacity that can be provided in shelter designs was the subject of limited 
large debris impact testing at Clemson University. The purpose of this testing 
was to provide guidance on the residual capacity of roof systems when the 
shelter is located where falling debris may be a hazard. In this testing, two 
types of shelter roofs were subjected to impacts from deformable, semi-
deformable, and non-deformable debris released from heights up to 100 feet 
and allowed to impact the roofs by free-fall. 

Non-deformable debris included barrels filled with concrete weighing 
between 200 and 1,000 lb. Semi-deformable debris included barrels filled 
with sand weighing between 200 and 600 lb, while deformable debris 
included heating/ventilation/and air-conditioning (HVAC) components and 
larger objects weighing from 50 to 2,000 lb. Impact speeds for the falling 
debris were calculated from the drop height of the debris. The speed of the 
objects at impact ranged from approximately 17 to 60 mph. Impacts were 
conducted in the centers of the roof spans and close to the slab supports to 
observe bending, shear, and overall roof system reactions. 

Cast-in-place and pre-cast concrete roof sections were constructed from the 
design plans in Case Studies I and II in Appendixes C and D, respectively. 
The heavily reinforced, cast-in-place concrete roof performed quite well 
during the impact testing. Threshold spalling, light cracking, to no visible 
damage was observed from impacts by deformable missiles, including the 
large 2,000-lb deformable object that impacted the slab at approximately 
60 mph. Impacts from the 1,000-lb concrete barrel did cause spalling of 
concrete from the bottom surface of the roof near the center of the slab that 
would pose a significant hazard to the occupants directly below the point of 
impact. However, significant spalling required relatively high missile drops 
(high impact speeds). 

Spalling of the slab extended into the slab from the bottom surface to the 
middle of the slab during impacts from the 1,000-lb concrete barrel impacting 
at approximately 39 mph. During this heavy spalling, the largest fragments of 
concrete were retained in the roof by the steel reinforcing. Metal decking (22 
gauge) was successfully used as cast-in-place formwork on one of the test 
samples to retain concrete spalls created by the falling debris. The metal 
decking, however, must be connected to reinforcing within the slab or secured 
to the concrete to contain the spalling concrete. 

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The 1,000-lb concrete barrel completely perforated the flange of the double-
tee beam in one drop from 50 feet (impacting at 39 mph) and caused 
significant damage to the stem in a second drop from the same height. Little 
damage occurred when the deformable debris materials (HVAC units, the 
300-lb sand barrels, and a 1,500-lb deformable object) were dropped on the 
double-tee beams. Only light cracking and threshold spalling were observed 
from impacts from these deformable objects. 

Based on the observed behavior of these roof specimens, it is believed that 
roof designs that incorporate a uniform thickness (i.e., flat slab) provide a 
more uniform level of protection from large debris impacts, anywhere on the 
roof, than a waffle slab, ribbed slab, or other designs that incorporate a thin 
slab supported by secondary beams. This approach is the best means of 
protecting shelter occupants from large impacts on shelter roof systems if 
siting the shelter away from potential falling debris sources is not a viable 
solution. Future research may yield information that will result in a more 
refined approach to designing shelters to resist the forces created by large 
falling debris. 

6.4 Doors and Door Frames 
Door failures are typically related to door construction and door hardware. 
Previous research and testing has determined that steel doors with 14-gauge or 
heavier skins prevent perforation by the design missile traveling horizontally 
at 100 mph. Furthermore, such doors in widths up to 3 feet are capable of 
withstanding wind loads associated with wind speeds up to 250 mph when 
they are latched with three hinges and three deadbolts. Because community 
shelters may have doors larger than those previously tested for use in in-home 
safe rooms, testing was performed for doors up to 44 inches wide. Double-
door systems with center mullions and different types of closure hardware 
were also tested. The information presented here and in Appendix F is a 
compilation of the test information available to date. 

Critical wind loads on doors and door frames are calculated according to the 
guidance presented in Chapter 5 of this manual and ASCE 7-98 for C&C 
loading. Calculations indicate that the maximum wind load expected on a 
door system (due to external suction wind forces combined with internal 
pressures for a 250-mph design wind) is 250 psf or 1.75 psi. Doors have been 
tested at these pressures through laboratory pressure tests. The doors were 
tested with positive pressure. The doors and frames were mounted as swing-in 
or swing-out doors to simulate either positive or negative pressures acting on 
the door. The doors were tested from both sides with positive pressure 
because the door and frame could not be sealed properly to pull a vacuum on 
the door to simulate negative pressures. Sliding door systems have been tested 
in the same manner. 

DESIGN AND CONSTRUCTION GUIDANCE FOR COMMUNITY SHELTERS

NOTE 

The design pressure for a 250mph 
wind on doors in wall 
corner regions of a community 
shelter is 1.75 psi for components 
and cladding (C&C) 
elements with an area of 21 ft2. 
Locating the door outside the 
corner region reduces the 
design pressure for the door to 
approximately 217 psf or 1.5 psi 
(corner regions are defined as 
the first 3 feet from the corner, 
10 percent of the least wall 
dimension, or 4 percent of the 
wall height). These pressures 
are different from the 1.37-psi 
maximum door pressure used 
for the small, flat-roofed 
shelters in FEMA 320 that were 
assumed to be designed for 
enclosed building conditions 
(as defined in ASCE 7-98). 

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CHAPTER 6 
NOTE 

The weak link of door systems 
when resisting wind pressures 
and debris impact is the door 
hardware. Testing was performed 
on a limited number of 
door and door hardware 
systems that represented off-
the-shelf products to indicate 
their expected performance in 
shelters. Although these 
systems passed the missile 
impact tests, they did not pass 
the maximum wind pressure 
tests. The maximum wind 
pressures on any shelter occur 
at building corners in Wind 
Zone IV. Therefore, any shelter 
door system in Wind Zone IV 
should be protected by an 
alcove or debris barrier until 
further testing can be performed 
or until other door and 
hardware systems are successfully 
tested for the design wind 
pressures. See Appendix F for 
more detailed guidance. 

PERFORMANCE CRITERIA FOR DEBRIS IMPACT 

6.4.1 Door Construction 
Door construction (primarily the exterior skin) has been found to be a limiting 
element in the ability of a door to withstand missile impacts, regardless of the 
direction of door swing (inward or outward). Both steel and wood doors have 
been tested for missile impact resistance. Previous research and testing has 
determined that steel doors with 14-gauge or heavier skins prevent perforation 
by the design missile. Furthermore, such doors in widths up to 3 feet are 
capable of withstanding forces associated with wind speeds up to 250 mph 
when they are latched with three hinges and three deadbolts. At this time, no 
wood door, with or without metal sheathing, has successfully passed either the 
pressure or missile impact tests using the design criteria for 250-mph winds. 

6.4.1.1 Single-Door Systems Less Than 36 Inches Wide 
The following is a list of single-door systems less than 36 inches wide that 
have successfully withstood the missile impact criteria of this manual: 

 
Steel doors with exterior skins of 14 gauge or thicker. These doors can be 
used without modification of the exterior skin. The internal construction of 
the doors should consist of continuous 14-gauge steel channels as the hinge 
and lock rails and 16-gauge channels at the top and bottom. The minimum 
hardware reinforcement should be 12 gauge. The skin should be welded the 
full height of the door. The weld spacing on the lock and hinge rails should 
be a maximum of 5 inches o.c. The skin should be welded to the 16-gauge 
channel at the top and bottom of the door with a maximum weld spacing of 
2-1/2 inches o.c. The door may include fill consisting of polystyrene infill 
or a honeycomb core. Greater strength can be gained through the use of 
doors that have internal 20-gauge steel ribs. 
 
Lighter-skinned steel doors may be used with modification. The 
modification is the addition of a 14-gauge steel sheet to either side of the 
door. The installation of the steel should be with 1/4-inch x 1-1/4-inch self-
tapping screws with hexagon washer heads attached at 6 inches o.c. along 
the perimeter of the sheathing and 12 inches o.c. in the field. The internal 
door construction should meet the specifications listed above. 
 
Site-built sliding doors constructed of two layers of 3/4-inch plywood and 
an 11-gauge steel plate attached to the exterior face of the door with 1/4inch 
x 1-1/4-inch self-tapping screws with hexagon washer heads attached 
at 6 inches o.c. along the perimeter of the sheathing and 12 inches o.c. in 
the field. These doors must be supported by pockets capable of 
transferring loads on the door to the shelter wall. 
6.4.1.2 Single-Door Systems Greater Than 36 Inches Wide 
A pressure test was performed on a single door 3 feet 8 inches wide (44 inches) 
and 7 feet tall. The door was constructed as described in the first bullet of 

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Section 6.4.1.1. The door was installed in a 14-gauge frame constructed 
within an 8-inch reinforced CMU wall and connected to the CMU with steel 
T-anchors spaced at 16 inches o.c.; the void between the frame and the 
masonry wall was grouted solid. The door was connected to the frame with 
five 4-1/2-inch heavyweight hinges. The latching hardware on the door tested 
was the single-lever-operated hardware (described in Section 6.4.3). 

This door system did not withstand the pressure test and failed before 
reaching the design pressure of 1.75 psi. The door failed when the pressure 
reached 1.19 psi. The door deflected during the pressure test and buckled 
around the latching hardware. After this first test, the door could not be closed 
and secured. Further testing to identify door construction for 44-inch doors is 
required before design guidance may be given for these large, single doors. 

6.4.1.3 Double-Door Systems (With Center Mullion) 
A double-door system (with a fixed center mullion) was tested for resistance 
to damage from wind pressures and missile impact. One door was equipped 
with a panic bar mechanism; the other was equipped with a single-action lever 
mechanism. This configuration was tested twice. The door configuration for 
these tests used two doors arranged in a swing-out configuration (a typical 
requirement for code-compliant egress). Each door was 3 feet wide and 7 feet 
tall and was constructed as described in the first bullet of Section 6.4.1.1. The 
doors were mounted in a 14-gauge steel frame with a 4-3/4-inch-deep frame 
with a 14-gauge center steel mullion. The mullion was bolted to the top of the 
frame and to a 12-gauge steel base plate at the sill with a 3/8-inch bolt at each 
location. The bolts extended from the front to the back of the mullion so as 
not to interfere with the doors when they were closed. The steel base plate 
was connected to the foundation below the sill with a 5/8-inch-diameter 
anchor bolt. The center mullion was reinforced with a T-shape 1/4 inch thick 
and 4 1/2 inches deep. The T-shape was welded to the back side of the mullion 
with 3-inch fillet welds at 9 inches o.c. Finally, the frame was attached to an 
8-inch, fully reinforced, CMU wall with steel T-anchors spaced 16 inches o.c., 
and the void between the frame and the masonry wall was grouted solid. No 
grout was placed in the center mullion. 

The double-door system was tested with pressures associated with the 
250-mph design wind and for the 15-lb design missile. This door configuration 
was tested to a pressure of 1.37 psi, but was not tested to failure. However, 
deflection of the double-door system during the pressure testing damaged one 
of the lock mechanisms (this is discussed further in Section 6.4.3). During the 
missile impact tests, one door withstood the impacts and remained closed, but 
the hardware on that door (the panic bar hardware) was no longer operational. 
The second door (with the single-action lever hardware) was damaged such 
that the door was pushed through the frame, causing a rotation in the center 

CHAPTER 6 
NOTE 

Heavy-gauge steel doors have 
been tested for resistance to 
wind pressures. Testing has 
shown that the weak link in 
available door products is the 
door hardware. At the time this 
manual was published, only 
one door/door hardware system 
resisted the pressures from a 
250-mph wind at leeward wall 
surfaces (away from building 
corners); see Appendix F. Wind 
pressures can be reduced at 
building corners with an alcove 
that protects the door system 
from edge effects. See Section 

6.4.3 for testing of door 
hardware systems 
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PERFORMANCE CRITERIA FOR DEBRIS IMPACT 

Figure 6-8 

The door of the shelter in 
Case Study I (Appendix C) is 
protected by a missile-
resistant barrier. Note: the 
shelter roof extends past the 
shelter wall and connects to 
the top of the missile-
resistant barrier to prevent 
the intrusion of missiles 
traveling vertically. 

mullion. For life-safety considerations, these results meet the missile impact 
criteria since the missile did not enter the shelter area. However, when 
functionality is a requirement (such as in the Dade County Florida impact test 
criteria), this result does not meet the impact requirements. 

Therefore, double-door systems require further testing before a system capable 
of resisting the missile impact tests can be specified. Designers who wish to 
use double-door systems should use an alcove system that prevents direct 
missile impacts on the double-doors (see Figure 6-8) or should test double-
door systems and hardware with heavier construction than those described in 
this section before installing the doors in a shelter in Wind Zone IV. 

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PERFORMANCE CRITERIA FOR DEBRIS IMPACT 

CHAPTER 6 
6.4.2 Door Frames 
Sixteen-gauge steel door frames in either a welded or knockdown style are 
known to be adequate to carry design wind and impact loads on a single door. 
Care must be taken in the installation of the frame so that it works properly 
and does not hinder the rest of the shelter construction. Frames used in stud 
construction must be attached to the MWFRS. This attachment is achieved 
with #8 x 3-inch screws, placed 6 inches o.c., installed through the jamb of the 
frame into the studs that make the rough opening of the door. Frames used in 
masonry construction are connected to the structure with T-anchors. It is 
critical that the T-anchors be bent at the internal edge of the masonry so that 
the tail of the anchor does not interfere with the placement of reinforcing steel 
and pea-gravel concrete. 

Frames for large single doors should be constructed of at least 14-gauge steel. 
Frames for double-door systems should be constructed of at least 14-gauge 
steel frames and use a 14-gauge, steel center mullion as described in Section 

6.4.1.3. The double-door system used in the testing secured the mullion to a 
12-gauge steel base plate. The base plate was secured to the concrete below 
the doorsill with a single 5/8-inch diameter bolt. However, displacement and 
twisting of the center mullion (and base plate) occurred during the missile 
impact tests. If two bolts are used instead of one, this frame assembly should 
withstand the impact from the design missile and remain functional without 
loss of shape. 
6.4.3 Door Hardware 
Door hardware was found to be another limiting element in the ability of 
doors to withstand wind and missile impact loads. Although some standard 
door hardware was capable of withstanding wind pressures associated with 
Zones II and III (see Figure 2-2), none of the conventional hardware tested 
during the preparation of FEMA 320 (for wind zone IV on Figure 2-2) was 
capable of carrying design wind loads or withstanding missile impacts when 
the impact occurred near the lock set or door handle mechanism. Hence, 
testing found that steel doors with supplemental latching mechanisms near the 
top and the bottom are required to carry design wind loads and to prevent an 
inward-swinging door from being knocked open with a well-placed missile. 
Three latching mechanisms are required so that, if a debris impact close to 
one destroys it, two latches will be left to carry the wind loads. 

6.4.3.1 Single-Latch Mechanisms 
Previous testing of latching and locking mechanisms consisted of testing an 
individual latch/lock cylinder or a mortised latch with a throw bolt locking 
function. In each case, tests proved that these locks, when used alone (without 
supplemental locks) did not pass the wind pressure and missile impact tests. 

DESIGN AND CONSTRUCTION GUIDANCE FOR COMMUNITY SHELTERS

WARNING 

Maintenance problems have 
been encountered with some 
3-point latching systems 
currently in use. If the door 
system uses a latch that 
engages a floor-mounted 
catch mechanism, proper 
maintenance is required if the 
latch is to function properly. 
Lack of maintenance may lead 
to premature failure of the 
door hardware during a high-
wind event. 

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CHAPTER 6 
NOTE 

All doors tested by FEMA prior 
to January 2000 were equipped 
with latching mechanisms 
composed of three, individually 
activated deadbolt closures. 
Between January and May 
2000, multiple latching 
mechanisms activated by a 
single lever or by a panic bar 
release mechanism were tested. 

PERFORMANCE CRITERIA FOR DEBRIS IMPACT 

Further testing proved that doors with these latching mechanisms and two 
additional mortised, cylindrical dead bolts (with solid 1/2-inch-thick steel 
throw bolts with a 1-inch throw into the door jamb) above and below the 
original latch would meet the requirement of the wind pressure and missile 
impact tests. It is important to note, however, that hollow deadbolts containing 
rod inserts failed the tests. 

However, the use of a door with three individually operated latching 
mechanisms may conflict with code requirements for egress for areas with 
large occupancies. Sections 6.4.3.2 and 6.4.3.3 discuss door hardware 
operated with panic hardware and single-action lever hardware. Additional 
guidance on door and egress requirements is provided in Section 6.4.4. 

6.4.3.2 Latching Mechanisms Operated With Panic Hardware 
An extensive search was performed to locate three-point latching systems 
operated from a single panic bar capable of resisting the wind pressures and 
missile impacts specified in this chapter. A single system was selected and 
tested. This system consists of a panic-bar-activated headbolt, footbolt, and 
mortised deadbolt. The headbolt and footbolt are 5/8-inch stainless steel bolts 
with a 1-inch projection (throw) at the top and bottom and are encased in 
stainless steel channels. Each channel is attached to the door with a mounting 
bracket. The headbolt and footbolt assembly can be mounted inside the door 
or on the exterior of the door; only the externally mounted assembly was 
tested. The mortised lock complies with ANSI/BHMA 115.1 standard mortise 
lock and frame preparation (1-1/4-inch x 8-inch edge mortise opening with 
mounting tabs). All three locking points were operated by a single action on 
the panic bar. 

This hardware was used for the double-door tests discussed in Section 6.4.1.3. 
Each of the doors was fitted with the panic bar hardware and three-point 
latches. This system was tested to 1.37 psi without failure. The system also 
passed the missile impact test, and the door remained closed; however, the 
hardware was not operational after the test. 

6.4.3.3 Latching Mechanisms Operated With Single-Action Lever Hardware 
A three-point latching system operated with a single-action lever was also 
tested for its ability to resist the wind pressures and missile impacts specified 
in this chapter. This system meets ANSI/BHMA A156.13 Operational Grade 1 
and fits a modified ANSI 115.1 door and frame preparation. The mortise case 
is heavy-duty wrought steel with a lever-activated latch and a 1-inch solid bolt 
with a 1-inch throw. Operation of the latch activates two 1-inch x 3/8-inch 
solid hookbolts. One hookbolt is located 1 foot 4 inches above the deadbolt 
and the other is located 1 foot 4 inches below the deadbolt. 

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CHAPTER 6 
This hardware system was used in the large single-door tests and the double-
door tests discussed in Sections 6.4.1.2 and 6.4.1.3, respectively. During the 
pressure test on the 44-inch single door, the deflection of the door resulted in 
the hookbolts (engaged in the frame) pulling out of the door itself. During the 
double-door tests, this hardware was damaged during the pressure test when 
the top hookbolt failed at its connection to the door (securing screws failed in 
shear). During the missile impact tests, the hardware resisted the missile 
impacts until a missile shot caused the center mullion to rotate, releasing the 
throw from the mullion. Further testing is required to determine whether the 
hardware or door can be modified to stabilize the hookbolts and prevent failure. 

6.4.4 Doors and Egress Requirements 
All doors must have sufficient points of connection to their frame to resist design 
wind pressure and impact loads. Each door should be attached to its frame 
with six points of connection (three connections on the hinge side and three 
connections on the latch side). Model building codes and life safety codes 
often include strict requirements for securing doors in public areas (areas with 
assembly classifications). This guidance often requires panic bar hardware, 
single-release mechanisms, or other hardware requirements. For example, the 
IBC and the NFPA life safety code require panic bar hardware on doors for 
assembly occupancies of 100 persons or more. The design professional will 
need to establish what door hardware is required and what hardware is permitted. 

Furthermore, most codes will not permit primary or supplemental locking 
mechanisms that require more than one action to achieve egress, such as dead 
bolts, to be placed on the door of any area with an assembly occupancy 
classification, even if the intended use would only be during an extreme-wind 
event. This restriction is also common for school occupancy classifications. 

These door hardware requirements affect not only shelter areas, but also 
rooms and areas adjacent the shelter. For example in a recent project in North 
Carolina, a school design was modified to create a shelter area in the main 
hallway. Structurally, this was not a problem; the walls and roof systems were 
designed to meet the wind pressure and missile impact criteria presented in 
this manual. The doors at the ends of the hallway also were easily designed to 
meet these criteria. However, the doors leading from the classrooms to the 
hallway were designed as rapid-closing solid doors without panic hardware in 
order to meet the wind pressure and missile impact criteria. This configuration 
was not a problem when the students were in the hallway that functioned as a 
shelter, but it was a violation of the code for the normal use of the classrooms. 
The designer was able to meet the door and door hardware requirements of 
the code for the classrooms by installing an additional door in each classroom 
that did not lead to the shelter area, thereby providing egress that met the 
requirements of the code. 

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CHAPTER 6 
PERFORMANCE CRITERIA FOR DEBRIS IMPACT 

NOTE 

No window or glazing system 
tested for resistance to missile 
impact has met the missile 
impact criteria recommended 
in this manual. 

Another option for protecting doors from missile impacts and meeting the 
criteria of this manual is to provide missile-resistant barriers. The shelter 
designs presented in Appendixes C and D of this manual use alcoves to protect 
doors from missile impacts. A protective missile-resistant barrier and roof 
system should be designed to meet the design wind speed and missile impact 
criteria for the shelter and maintain the egress width provided by the door 
itself. If this is done, the missile impact criteria for the door and code egress 
requirements for the door are satisfied. Although the wind pressures at the 
door should be reduced by the presence of the alcove, significant research to 
quantify the reduction has not been performed. Therefore, the door should be 
designed to resist wind pressures from the design wind. See Figure 6-8 for an 
example of an alcove used to protect a door assembly from missile impact. 

Finally, the size and number of shelter doors should be determined in 
accordance with applicable fire safety and building codes. If the community 
or governing body where the shelter is to be located has not adopted current 
fire safety or model building codes, the requirements of the most recent 
edition of a model fire safety and model building code should be used. 

6.5 Windows 
Natural lighting is not required in small residential shelters; therefore, little 
testing has been performed to determine the ability of windows to withstand 
the debris impacts and wind pressures currently prescribed. However, for nonresidential 
construction, some occupancy classifications require natural 
lighting. Furthermore, design professionals attempting to create aesthetically 
pleasing buildings are often requested to include windows and glazing in 
building designs. Glazing units can be easily designed to resist high-wind 
pressures and are routinely installed in high-rise buildings. However, the 
controlling factor in extreme-wind events, such as tornadoes and hurricanes, is 
protection of the glazing from missile perforation (the passing of the missile 
through the window section and into a building or shelter area). 

Polycarbonate sheets in thicknesses of 3/8 inch or greater have proven capable 
of preventing missile perforation. However, this material is highly elastic and 
extremely difficult to attach to a supporting window frame. When these 
systems were impacted with the representative missile, the deflections 
observed were large, but were not measured. 

For this manual, window test sections included Glass Clad Polycarbonate 
(2-ply 3/16-inch PC with 2-ply 1/8-inch heat-strengthened glass) and four-
layer and five-layer laminated glass (3/8-inch annealed glass and 0.090 PVB 
laminate). Test sheets were 4 feet x 4 feet and were dry-mounted on neoprene 
in a heavy steel frame with bolted stops. All glazing units were impact-tested 
with the representative missile, a 15-lb wood 2x4 traveling at 100 mph. 

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CHAPTER 6 
Summarizing the test results, the impact of the test missile produced glass 
shards, which were propelled great distances and at speeds considered 
dangerous to shelter occupants. Although shielding systems can contain glass 
spall, their reliability is believed to degrade over time. Further testing of the 
previously impacted specimen caused the glass unit to pull away from the frame. 

Testing indicates that glass windows in any configuration are undesirable for 
use in tornado shelters. The thickness and weight of the glass systems 
required to resist penetration and control glass spall, coupled with the 
associated expense of these systems, make them impractical for inclusion in 
shelter designs. 

It is therefore recommended that glazing units subject to debris impacts not be 
included in shelters until products are proven to meet the design criteria. 
Should the shelter design require windows, the designer should have a test 
performed consistent with the impact criteria. The test should be performed 
on the window system with the type and size of glass specified in the design 
and mounted in the actual frame as specified in the design. A PASS on the 
test must agree with the following: 1) the missile must not perforate the 
glazing, 2) the glazing must remain attached to the glazing frame, and 3) glass 
fragments or shards must remain within the glazing unit. It is important to 
note that glass block is also not acceptable. Glass block, set in beds of 
unreinforced lime-rich mortar, offers little missile protection. 

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