Development of a Ready-toAssemble Tornado Shelter from 

Cross-Laminated Timber (CLT)

Impact and Wind Pressure Testing

Robert H. Falk

James J. Bridwell

Tom Williamson

Todd Black

United States Department of Agriculture

Forest

Service

Forest Products

Laboratory

General Technical Report

FPL–GTR–266

April

2019
April 2019

Falk, Robert H.; Bridwell, James J.; Williamson, Tom; Black, Todd. 2019. 

Development of a ready-to-assemble tornado shelter from cross-laminated 

timber (CLT): Impact and wind pressure testing. General Technical Report 

FPL-GTR-266. Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory. 29 p.

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Abstract

The development and use of tornado shelters have helped 

reduce loss of human life associated with extreme weather 

events. Currently, the majority of shelters are built from 

either steel or concrete. The development of the crosslaminated timber (CLT) industry in the United States has 

provided an ideal wood product to resist the debris impact 

and high wind forces associated with tornados. This report 

overviews the design and development of a residential

tornado shelter constructed from CLT. This design was 

impact- and wind-pressure-tested according to the core 

requirements of the International Code Council Standard 

for the Design and Construction of Storm Shelters. Results 

indicate that a residential tornado shelter constructed of 

four-ply CLT walls, a three-ply roof, and a four-ply door can 

safely resist the impact of a 15-lb wooden missile traveling 

at 100-mi/h (mph)—the most severe impact test included 

in the ICC/NSSA-500 standard. Lateral load and uplift 

load testing indicated that the four-ply shelter can resist the 

applied pressure caused by a 250-mph wind, as calculated 

from wind load design criteria. More testing would be 

required to verify the performance of a CLT shelter intended 

for commercial or institutional applications.

Keywords: tornado, impact testing, cross-laminated timber, 

safe room

Contents

Introduction..........................................................................1

Wood Tornado Shelter Design and Construction.................1

Performance Criteria for Tornado Safe Shelters..................3

Test Methods and Data Collection.......................................4

Results and Discussion ........................................................6

Conclusions........................................................................11

Literature Cited ..................................................................11

Appendix A—Impact Test Results on a 

Three-Ply Panel..................................................................13

Appendix B—Impact Test Results on a 

Three-Ply CLT Door ..........................................................15

Appendix C—Impact Test Results on a

Four-Ply CLT Shelter (8 ft by 8 ft) ....................................17

Appendix D—Impact Test Results on a 

Four-Ply CLT Shelter (5 ft by 5 ft) ....................................21

Appendix E—Impact Test Results on a 

Four-Ply CLT Door............................................................27

English unit

Conversion 

factor SI unit

inch (in.) 2.54 × 101 millimeter (mm)

foot (ft) 3.048 × 10–1 meter (m)

mile (mi) 1.609347 kilometer (km)

pound, mass (lb) 4.535924 × 10–1 kilogram (kg)

pound per square foot 

(lbf ft–2)

4.788026 × 101 pascal (Pa)

square foot (ft2) 9.2903 ×10–2 square meter (m2)

Nominal lumber size (in.) Standard lumber size (mm)

2 by 4 38 by 89

2 by 8 38 by 184
Introduction

A tornado shelter is a reinforced structure specifically 

designed to meet the criteria of International Code Council 

Standard for the Design and Construction of Storm Shelters 

(ICC/NSSA 2014) and provides protection in extreme 

weather events, including tornadoes and hurricanes. If a 

tornado shelter is designed according to ICC/NSSA-500, 

occupants should have a very high probability of being 

protected from injury or death.

The recent growth of the cross-laminated timber (CLT) 

industry in North America has made available manufactured 

wood panels that are seemingly ideal for tornado shelters. 

The development of a tornado shelter from CLT that is easy 

to ship, ready to assemble, and quick to fabricate on site 

would not only increase the level of safety for the general 

population but also increase market opportunity for these 

wood products.

This study was initiated to determine if a shelter 

constructed from CLT could meet the impact and wind 

pressure requirements of the ICC/NSSA-500 standard for 

a residential tornado shelter. Results of impact and wind 

pressure testing performed to verify the tornado resistance 

of the developed CLT tornado shelter are presented here. 

Two sizes of safe shelters—8- by 8-ft and 5- by 5-ft—were 

constructed of both three-ply and four-ply CLT panels and 

were evaluated. The fact that the shelters evaluated were 

constructed of five full-sized panels, each of which can 

weight over 1000 lb, make these designs best suited for 

building that can accommodate lifting equipment.

Development of a Ready-to-Assemble 

Tornado Shelter from Cross-Laminated 

Timber (CLT)

Impact and Wind Pressure Testing

Robert H. Falk, Research Engineer

James J. Bridwell, General Engineer

USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin, USA

Tom Williamson, Timber Engineering Consultant

Timber Engineering, LLC, Vancouver, Washington, USA

Todd Black, Operations Manager

DR Johnson, Riddle, Oregon, USA

Wood Tornado Shelter Design 

and Construction

DR Johnson, a CLT manufacturer in Riddle, Oregon, USA, 

provided panels for testing. Shelters constructed of both 

three-ply (4-1/8-in. thick) and four-ply (5-1/2-in. thick) 

CLT panels were evaluated. Although four-ply panels are 

not as common in the marketplace as three-ply or five-ply 

panels, previous impact testing of wood panels of different 

wood product types suggested that a panel of about 5 to 6 in. 

in thickness should perform well. Three-ply and four-ply 

panels were chosen for testing with the assumption that if 

four-ply worked, certainly five-ply would also resist the 

loads applied.

The laminations in each panel were 1- 3/8 in. thick and 

alternated direction orthogonally. Therefore, four-ply panels 

were constructed with the first and third and second and 

fourth laminations, respectively, parallel, but oriented 90° to 

each other. The panels were manufactured from Douglas Fir 

as a V1 grade.

Two sizes of tornado shelters constructed from each 

thickness of panel were evaluated. The first was a shelter 

8 by 8 ft in plan (exterior dimensions) with a height of 8 ft 

(Fig. 1). This larger shelter, similar in size to that described 

in Falk and Bridwell (2018) is large enough for other uses 

(such as bathroom, utility shelter, sauna) when not needed 

in an emergency. The second shelter evaluated was 5.5 by 

5.5 ft in plan (exterior dimensions) with a height of 7.67 ft 

(Fig. 2). Per FEMA recommendations of 5 ft2 per person, 

the larger shelter will accommodate nine people and the 

smaller shelter five people.
General Technical Report FPL–GTR–266

2

The CLT panels were provided prefabricated and cut to 

size. Once at the Forest Products Laboratory (FPL), the 

panels were connected together with custom-made 14 gauge 

sheet steel angles. These 4- by 4-in. angles were attached 

at each inside shelter corner with 3/8- by 3-in. lag bolts 

(predrilled, 8 in. on center (o.c)). This panel connector 

system allowed the butting of CLT panels without field 

modification (Fig. 3). This connection system allowed for 

rapid construction. Each shelter was assembled by two 

people using only an overhead crane and hand tools in less 

than 4 hours. 

Another important component of a tornado shelter is the 

entry door. Similar to the overall shelter itself, the door 

must be designed and constructed to withstand the wind 

and impact forces produced by a tornado. In this study, the 

impact resistance of an outswing door overlaying a standard 

36- by 80-in. opening was evaluated. This door was similar 

in operation to the door described in Falk and Bridwell 

(2018).

As shown in Figure 4a, the door was hung with three 

5/8-in. gate hinges, similar to those that might be found on 

a livestock gate (293BC and 294BC, National Hardware 

LLC, Pinedale, California, USA). These hinges were chosen 

because of their low cost and adjustability in hanging an 

overlaid door as well as the fact that they had performed 

well when used on the tornado shelter door described in 

Falk and Bridwell (2018). The door was latched from the 

inside using three cane bolts (Fig. 4b) (Product 5000-242, 

Snug Cottage Hardware Inc., Marysville, Michigan, USA). 

The ability of this hardware to withstand the suction forces 

of tornado-force wind pressures was previously evaluated 

by Falk et al. (2018).

Securing the tornado shelter to the foundation is another 

important aspect of tornado shelter design. The hold-down 

connector, the anchors securing the hold-down connector, 

and the concrete foundation each must have adequate 

structural strength to resist the wind forces produced by a 

tornado. The hold-down connector utilized in the CLT safe 

shelters described here is a simple 13-in. long, 2-in. wide, 

1/4-in. thick A36 steel bar bent in an “L” shape to allow 

attachment to a concrete floor (Fig. 5).

Each hold-down is attached to the tornado shelter using 

five 3/8- by3-in. long lag bolts, and each hold-down has 

a 3/4-in.-diameter through hole on the shorter leg for 

anchoring to the foundation. For the 8- by 8-ft shelter, 

12 hold-downs were used, three on each wall. One holddown was placed approximately at the center of each wall 

Figure 1—8- by 8-ft CLT tornado shelter. Figure 2—5- by 5-ft CLT tornado shelter.

Figure 3—Sheet steel angles for connecting CLT panels.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

3

panel and the other two approximately 12 in. from each wall 

corner. On the wall panel with the door, the middle holddown was secured 12 in. in towards the door from the holddown on the opposite side of the panel from the door hinges. 

For the 5.5- by 5.5-ft shelter, eight hold-downs were used, 

two on each wall placed about 12 in. from each wall corner, 

except in the case of the door panel, where they were placed 

6 in. from each wall corner.

The concrete foundation that the tornado shelter sits atop 

must have enough thickness, reinforcement, and surface 

area to resist anchor bolt pullout, foundation sliding, 

and shelter overturning. These design considerations are 

addressed in FEMA P-320 (FEMA 2014), FEMA P-361, 

and ICC/NSSA-500.

Performance Criteria for Tornado 

Safe Shelters

The performance criteria for a residential tornado shelter has 

been standardized in ICC/NSSA-500. This standard presents 

occupancy requirements, impact testing, wind pressure 

testing, ventilation, and other performance criteria for these 

structures. This report is limited to impact and wind pressure 

testing of a residential tornado shelter and its components. 

Impact Criteria

According to ICC/NSSA-500, large missile impact testing 

is an accepted way of assessing the strength performance 

of assemblies and materials used in severe weather shelter 

design. The range of tests is given in Table 1. The tornado 

test imparts the most energy and thus can be considered the 

most severe.

In these tests, the walls of the tornado shelter are subjected 

to the impact of a 2×4 lumber stud (actual 1.5 by 3.5 in.) 

weighing between 14.75 and 15.25 lb. Density and moisture 

content of the missiles are required to ensure that a missile 

13 to 14 ft in length satisfies the weight requirement. 

Missiles are also to be selected such that no knots appear 

within 12 in. of the leading edge and the missile has less 

than 0.5 in. of bow or warp when measured at any point 

along its length.

For a tornado test on a vertical wall, the missile must impact 

the target at a velocity of 100 mi/h (mph). Roof panels 

are impacted with the same size of missile, however the 

impact speed is 67 mph (see table 305.1.1 of ICC/NSSA 

(2014)). Target areas of particular interest are wall–wall 

intersections, wall–roof intersections, and areas around the 

Table 1—Missile testing criteriaa

Test Missile

Missile

size

(lb)

Missile

speed

(mph)

Basic hurricane 2 by 4 wood stud 9 34

Hurricane enhanced–A 2 by 4 wood stud 15 50

Hurricane enhanced–B 2 by 4 wood stud 15 60

Tornado 2 by 4 wood stud 15 100

Hurricane shelter 2 by 4 wood stud 9 0.4 × wind 

zone speed

a

NIST/TTU (2006).

Figure 5—Shelter hold-down.

Figure 4—(a) Door hinge, (b) door latch.

(a)

(b)
General Technical Report FPL–GTR–266

4

sides and top of the door. These are typically vulnerable 

regions that rely on connections for performance.

ICC/NSSA-500 also requires that a paper witness screen of 

taut 70-lb kraft paper be erected no more than 5 in. behind 

the interior surface of the wall being tested. Any perforation 

of the screen by debris or a deforming wall is considered a 

failed test.

A successful impact test requires that the wall meet three 

basic criteria: (1) permanent wall deflection less than 

3 in., (2) no perforation of the witness screen, and (3) no 

perforation of the interior surface of the wall by the missile.

Similar tests were performed on a tornado shelter 

constructed from commodity wood products and are 

detailed by Falk et al. (2018).

Wind Pressure Criteria

In addition to requiring resistance against the impact loads 

generated by wind-blown debris, ICC/NSSA-500 requires 

that a tornado shelter must be able to withstand high wind 

forces. Lateral wind pressure tests and wind suction uplift 

tests were performed to verify that the various components 

of the shelter (wall, roof, and door) remain intact under 

wind pressure loads and that the shelter hold-downs are able 

to adequately transfer wind pressure loads to the foundation. 

Design wind pressures are to be calculated using ASCE/

SEI 7-10 (ASCE 2013), section 6, method 2. As indicated 

by Falk and Shrestha (2016), the calculated lateral pressure 

from a 250-mph wind on the face of an 8- by 8-ft tornado 

shelter is 167 lb/ft2. Similarly, the calculated suction (or 

uplift) pressure is 224 lb/ft2.

Test Methods and Data Collection

Impact Testing

The impact and wind pressure tests were conducted at the 

USDA Forest Service, Forest Products Laboratory (FPL), in 

Madison, Wisconsin, USA. For the impact tests, the shelter 

was anchored to the floor of the test laboratory using the 

hold-down anchors described earlier. These anchors were 

secured using 5/8-in.-diameter Grade 5 bolts to W12x120 

steel beams that were clamped to the floor of the test 

laboratory (Fig. 1).

A 36- by 36-in. frame for the paper witness screen was 

constructed from 2×4 studs. After stapling a piece of 70-lb 

kraft paper taut against the studs, the frame was placed on a 

steel stanchion. For each test, the stanchion was placed 5 in. 

behind the interior of the target wall or door. Figure 6 shows 

the witness screen used for the impact tests.

Not all the missiles used in this study satisfied all the ICC/

NSSA-500 criteria. Sourcing 13-ft-long 2×4s that were 

straight enough to physically load into the cannon barrel 

and safely fire was problematic. Although each missile used 

in this study met the temperature storage requirements, 

weighed at least 15.0 lb, and satisfied the knot distance 

requirement, some missiles were slightly shorter than 

required. Overall, tests performed in this study used missiles 

that averaged 12.75 ft in length and 15.25 lb in weight. We 

believe this discrepancy in length is insignificant because 

missile mass is the factor that determines impact force, not 

missile length. The trailing edge of each missile was affixed 

with a Delrin sabot weighing 0.4 lb. 

Impact testing was performed using a modified Mega 

Launcher II (referred to as a “cannon”) built by Spudtech, 

LLC (New London, Minnesota, USA) (Fig. 7). The cannon 

used compressed air to propel each missile. The pressure of 

the compressed air was adjusted to control the speed of the 

missile via an external control apparatus. 

Figure 6—Witness screen erected inside 

the shelter.

Figure 7—Debris impact cannon.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

5

Missile speed was computed with the use of two 

photoelectric sensors (FS2-65, Keyence Corp., Osaka, 

Japan) spaced at 12 in. inside the muzzle of the cannon. 

As the missile was launched past the sensors, signals 

from the sensors were conditioned in a programmable 

counter (P6000A, Newport Corp., Irving, California, USA) 

that output time-of-flight between the sensors. Velocity 

was computed by dividing the sensor spacing by the 

time-of-flight.

Wind Pressure Testing

Panel Test Method

An airbag (custom made by MatJack, Indianapolis Industrial 

Products, Inc., Indianapolis, Indiana, USA) was sandwiched 

between the panel and the floor of the test laboratory to 

simulate a uniformly applied wind load (Fig. 8). Pressurized 

air was delivered to the bag through a regulator (Model 

QBX, Proportion-Air, Inc., McCordsville, Indiana, USA). 

The regulator was controlled through proportional feedback 

using LabVIEW software (National Instruments Corp., 

Austin, Texas, USA). Deflection was measured on the top 

face in the geometric center of the panel using a cableextension position transducer (Celesco PT101, 2 in. fullscale (Celesco Transducer Products, Inc., Chatsworth, 

California, USA)). Deflection data were captured using 

LabVIEW software at a frequency of 1 Hz.

Shelter Lateral Load Test Method

Similar to the impact tests, the full-size shelter was secured 

to the strong floor of the test laboratory through a series of 

steel beams, which in this case were placed adjacent to a 

reinforced concrete strong wall (Fig 9). The same airbag 

described earlier was sandwiched between the tornado 

shelter and the strong wall to simulate a uniformly applied 

wind load to the side of the shelter (Fig. 10). The lateral 

wind pressure load was applied using the same regulator, 

software, and procedure as outlined for the pressure test on 

the panel.

Figure 8—Panel wind pressure test setup.

Figure 9—Tornado shelter tied down to strong floor.

Figure 10—Airbag between tornado 

shelter and strong wall.
General Technical Report FPL–GTR–266

6

Three-Ply Roof Panel

According to ICC-500 (ICE/NSSA 2014), roof panels are 

required to withstand a missile speed of at least 67 mph. 

An 8- by 8-ft roof panel was impact tested twice (P2, P3, 

App. A), once at the geometric center and once near the 

panel edge. The roof panel survived tests with missile 

speeds well in excess of the requirement (84 mph and 

78 mph, respectively).

Three-Ply Door

Next, a three-ply CLT door overlaying a standard 36- by 

80-in. opening was impact tested. The door was hung on an 

8- by 8-ft panel and was tested under the same conditions as 

if it had been hung on a shelter. As indicated in Appendix B, 

the door was impacted near the upper latch (D1), near the 

hinge (D2), and at the geometric center of the door (D3). 

The door passed all three tests with little damage and no 

debris.

Shelter Testing

Three-Ply 5- by 5-ft Shelter

The three-ply panel failure mentioned above suggested that 

an 8- by 8-ft shelter constructed of three-ply panels would 

not perform well, but we speculated that a three-ply shelter 

of smaller size might pass the impact test. Therefore, a 

5- by 5-ft (interior dimension) three-ply shelter was built 

and subjected to impact. Figure 12 shows the results of 

Using the same setup as described by Falk et al. (2018), 

the two upper corners of the shelter on the opposite side 

from airbag were fitted with position transducers (Celesco 

PT101, 2 in. full-scale (Celesco Transducer Products, 

Inc., Chatsworth, California, USA)). Deflection data were 

continuously acquired throughout the loading sequence 

using MTS 793 software at a rate of 1 Hz.

Shelter Uplift Test Method

A Miller Series HV2 hydraulic actuator (Miller Fluid Power 

Corporation, Des Plaines, Illinois, USA) was set up inside 

the tornado shelter, and two 78-in.-long W6x15 steel cross 

beams spaced at 30 in. (Fig. 11) were used to apply an uplift 

load to simulate the wind suction forces experienced by 

the CLT tornado shelter roof (three-ply). Deflection of the 

shelter was measured using a position transducer (Celesco 

PT101, 2 in. full-scale) attached to the geometric center on 

the external face of the roof. MTS 793 software controlled 

the actuator during the test and also acquired the load and 

deflection data at a rate of 1 Hz.

Results and Discussion

Impact Testing

Panel/Door Testing

Three-Ply Wall Panel

Before building and testing a full-size shelter, an 8- by 8-ft 

three-ply CLT wall panel was impact tested to determine 

if a three-ply panel could resist the impact loads. The first 

test was performed at the geometric center of the panel (P1, 

App. A). The panel failed the test because a piece of debris 

punched a small hole in the witness screen.

Figure 11—Uplift loading on CLT 

tornado shelter roof.

Figure 12—Single impact test to three-ply 5- by 5-ft 

shelter.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

7

the single impact test that was performed on this shelter. 

As shown in Figures 12 to 14 and Appendix D, the threeply panel deflected more than 3 in. and debris pierced the 

witness screen. The shelter therefore failed the test. This test 

and the three-ply panel test described above suggest that a 

three-ply panel was too thin to resist the most severe impact 

forces dictated by the standard.

Four-Ply 8- by 8-ft Shelter

Because of the failure of the three-ply panel and the threeply 5- by 5-ft shelter, an 8- by 8-ft shelter constructed of 

four-ply CLT was tested. Figures 15 and 16 indicate the 

locations of impact on the full-size shelter.

Although four-ply panels are not as common in the 

marketplace as three-ply or five-ply panels, previous impact 

testing of wood panels of different wood product types 

suggested that a panel of about 5 to 6 in. in thickness should 

perform well (Falk et al. 2015). Four-ply CLT was chosen 

to test with the assumption that if four-ply worked, certainly 

five-ply would also work.

Impact tests R1 to R5 were performed in sequence on the 

same shelter wall. As indicated in Appendix C, impact 

test R1 was intended to evaluate the resistance of the wall 

panel by targeting the geometric center of the panel. Tests 

R2 and R3 were positioned to concentrate forces at the 

wall–wall interface. Impact tests R4 and R5 were positioned 

to evaluate the wall–wall–roof and wall–roof interface, 

respectively. In all cases, the four-ply wall resisted the 

impacts with little or no damage.

Four-Ply 5- by 5-ft Shelter

Because the three-ply 5- by 5-ft shelter did not pass the 

impact test, a four-ply shelter (three-ply roof) of the same 

interior dimensions was constructed and tested. Results are 

tabulated in Appendix D.

Figure 13—Interior damage to threeply 5- by 5-ft shelter.

Figure 14—Tearing of witness 

paper.

Figure 15—Impact locations on wall panel of 

four-ply 8- by 8-ft shelter.

Figure 16—Impact locations on 8- by 8-ft shelter 

around door. (Door not shown for clarity.)
General Technical Report FPL–GTR–266

8

withstand the high wind forces from tornados. ASCE/SEI 

7-10 (ASCE 2013) is used to calculate wind pressure loads.

Panel Tests

Prior to constructing the shelters, wind pressure testing of 

a three-ply CLT panel was performed. As indicated by Falk 

and Shrestha (2016), the calculated wind pressure on an 

8- by 8-ft shelter is 167 lb/ft2 (250-mph wind). An airbag 

(described earlier) was sandwiched between the panel and 

the strong floor to simulate a uniformly applied wind load 

(Fig. 17). The panel was loaded to 1.8× design, or 300 lb/ft2. 

Though the panel deflected over 3 in., there was no damage, 

distress, or permanent deflection (Fig 19). No wind pressure 

tests were performed on a four-ply panel because the threeply panel adequately resisted the factored design loads.

Wall panel—Impact tests S1 to S6 were performed in 

sequence on the same shelter wall. As indicated in Figure 17 

and Appendix D, impact test S1 was intended to evaluate the 

resistance of the wall panel by targeting the geometric center 

of the panel. Tests S2 and S3 were aimed to concentrate 

forces at the wall–wall interface at the mid-height of the 

wall panel. Impact tests S4 to S6 were aimed to evaluate 

the wall–wall–roof and wall–roof interface. In all cases, the 

four-ply wall resisted the impact with little or no damage 

and passed the test.

Door panel—Impact tests S7 to S9 were performed on the 

wall panel containing the door opening. These tests were 

directed near the door edge and indicated if the perimeter 

of the opening could withstand the applied impact forces. 

As indicated in Figure 18 and Appendix D, three tests were 

performed. In test S7, the missile was directed adjacent to 

the door between the door top and the roof panel. Tests S8 

and S9 were directed between the upper and lower door 

hinges, respectively. In all tests, the door panel resisted the 

impact with no significant damage nor debris and passed the 

test.

Four-Ply Door

Although the three-ply door passed the impact test, the 

fact that the three-ply panel and three-ply 5- by 5-ft shelter 

failed prompted testing of a four-ply door. As indicated in 

Appendix E, three tests were performed (DD1 to DD3). 

The four-ply door withstood the impact with no debris, 

excessive deflection, nor damage to the door interior and 

passed the test.

Wind Pressure Tests

In addition to resisting the impact loads generated by 

windblown debris, a tornado shelter must be able to 

Figure 18—Impact locations on 5- by 

5-ft shelter around door. (Door not 

shown for clarity.)

Figure 19—Three-ply panel at 1.8× design load.

Figure 17—Impact locations on 5- by 5-ft 

shelter on wall panel.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

9

Full-Size Shelter Tests

Lateral wind pressure tests and wind suction uplift tests 

were also performed on the constructed four-ply shelter to 

evaluate the ability of the various components of the shelter 

(wall, roof) to remain intact under wind pressure loads and 

to evaluate the ability of the shelter tie downs to adequately 

transfer wind pressure loads to the foundation. The full-size 

shelter was secured to the strong floor of the test laboratory 

through a series of steel beams adjacent to a reinforced 

concrete strong wall (Fig 9). As described earlier, an airbag 

was used to induce the simulated wind load (Fig. 10). 

The lateral wind load pressure derived from ASCE 7 was 

calculated to be 167 lb/ft2 (Falk and Shrestha 2016).

Two tests were performed, the first with the shelter oriented 

such that the door was opposite the airbag and the second 

with the door 90° to the side in contact with airbag. Air 

pressure was incrementally increased in the airbag, and over 

approximately 2 min the load was increased to a maximum 

of 2.3× design, or 395 lb/ft2. Using the same setup and 

taking the same deflection measurements as described 

by Falk et al. (2018), two upper corners of the shelter 

(opposite side from airbag) were fitted with deflection 

measuring gauges (Celesco PT101, 2 in. full-scale (Celesco 

Transducer Products, Inc., Chatsworth, California, USA)), 

and deflection was continuously measured throughout the 

loading sequence using MTS 793 software. For the first 

test, measured deflections for the upper two corners of the 

shelter are designated as S1 and S2. For the second test, the 

deflections of the upper corners are S3 and S4, respectively.

Figures 20 and 21 show deflection as a function of applied 

wind pressure for the two tests. Figure 21 shows the 

deflection of the shelter with the door opening opposite 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 50 100 150 200 250 300 350 400

Lateral displacement, in.

Airbag pressure, lb/ft2

SP1

SP2

Figure 20—Test 1: displacement as a function of wind pressure.

Figure 21—Test 2: displacement as a function of wind pressure.

0 50 100 150 200 250 300 350 400

Airbag pressure, lb/ft2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Lateral displacement, in.

SP3

SP4
General Technical Report FPL–GTR–266

10

the airbag. Note that the shelter deflected symmetrically 

(deflection S1 is about equal to deflection S2) and reached 

a maximum of about 1.0 in. at 2.3× design. For test 2, 

deflection magnitude is greater than for Test 1 (1.5 in. at 

2.3× design). Also, the deflections were slightly different, 

with the deflection of the door opening side greater than the 

side without the door (S4 > S3). This suggests that the door 

opening has an effect on overall stiffness of the shelter. This 

result is consistent with similar tests performed by Falk et 

al. (2018).

In both tests, the shelter walls and roof showed no damage 

or distress at 2.3× design. Some bending of the tie-downs 

and pullout of a lag bolt from the tie-downs on the load 

side did occur (Fig 22). The authors feel that this localized 

distress is not worrisome because significant capacity 

Figure 22—Pullout of lag bolt at 

2.3× design wind load.

remained (four of five bolts still carrying load) at 2.3× the 

design load. Note that ICC/NSSA-500 does not specify 

lateral deflection limits for tornado shelters. 

Roof Uplift

Two roof uplift tests were performed on the four-ply 8- by 

8-ft shelter. During the first test, load was applied at a rate 

of about 240 lb/s such that the total uplift force reached 

1.0× design load (14,300 lb) after one minute. The load 

was released at the same rate. Maximum deflection of the 

geometric center of the roof at 1.0× design load was 1.38 in. 

(Fig 23). Permanent deflection after the load was fully 

released was 0.54 in., a result of bending in the sheet metal 

angles connecting the roof to the walls of the shelter. 

During the second test, uplift force was applied at the 

same rate up until the shelter reached 1.0× design load. 

The test was then manually paused for inspection for 10 s. 

After inspection, loading resumed at the same rate until 

the 1.2× design level was reached (17,200 lb). The test 

was manually paused again for a further 90 s of inspection 

before proceeding at the same load rate to 1.5× design load, 

21,500 lb. The shelter was unloaded over the course of 

60 s at a rate of 360 lb/s. Figure 24 is a plot of uplift force 

measured against roof displacement for the second test.

Including permanent deformation from the first test, the 

maximum deflection of the geometric center of the roof at 

1.5× design load was 2.19 in. Permanent deformation after 

the second test totaled 1.22 in., also including the permanent 

deformation from the first test (Fig. 25). In spite of this 

deflection, the lag bolts continued to resist the applied 

loads and the roof remained attached to the wall panels. No 

deflection criteria are given in the ICC-500 standard and the 

authors feel that the deflection was not excessive.

Figure 23—Plot of shelter displacement as a function of uplift force for 

1× design load.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

Uplift force, lb

Roof displacement, in.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

11

Conclusions

Tornado shelters constructed from CLT panels were 

successfully impact and wind pressure tested. Testing 

indicates that a CLT tornado shelter constructed of fourply walls can safely withstand the most severe impact test 

included in the ICC/NSSA-500 standard. A three-ply roof 

withstood the lower speed impact requirement (67 mph). An 

overlaid four-ply door also safely resisted a 100 mph missile 

impact. Although some slight damage occurred, lateral load 

and uplift load testing indicated that the four-ply shelter 

can resist at least 1.5× the calculated wind pressure for 

a 250-mph wind derived from wind load design criteria 

(ASCE 2013).

All CLT panels tested in this study were pre-fabricated and 

full-size (that is, up to 8- by 8-ft with no joints or splices). 

Results from both the impact and wind pressure tests may 

be different for shelters constructed of panels that are not 

full-size and that contain splices or joints.

Literature Cited

ASCE. 2013. Minimum design loads for buildings and other 

structures (ASCE/SEI 7-10). Philadelphia, PA: American 

Society of Civil Engineers. www.pubs.asce.org

Falk, R.H.; Bridwell, J.J. 2016. Development of a tornado 

safe room door from wood products: door design and impact 

testing. Research Paper FPL–RP–686. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 17 p.

Falk, R.H.; Bridwell, J. 2018. A residential severe weather 

safe room from commodity wood products: design and 

development. General Technical Report FPL–GTR–253. 

Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory.

Falk, R.H.; Shrestha, D. 2016. Wind load pressure testing 

of tornado safe room components from wood. Research 

Paper FPL–RP–688. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory.

Falk, R.H.; Bridwell, J.J.; Hermanson, J.C. 2015. 

Residential tornado safe shelters from commodity wood 

products: wall development and impact testing. Research 

Paper FPL–RP–681. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

15 p.

Falk, R.H.; Bridwell, J.; Senalik, C.A.; Begel, M. 2018. 

A residential severe weather safe room from commodity 

wood products: impact and wind pressure testing. General 

Technical Report FPL–GTR–254. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

ICC/NSSA. 2014. ICC/NSS A-500. Standard for the 

design and construction of storm shelters. Washington, 

Figure 24—Plot of shelter displacement as a function of uplift force for 

1.5× design load.

Figure 25—Deflection due to uplift loading on the 

shelter roof.

Uplift force, lb

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.8

Roof displacement, in.

1.6

0 5,000 10,000 15,000 20,000 25,000
General Technical Report FPL–GTR–266

12

DC: International Code Council/National Storm Shelter 

Association.

NIST/TTU. 2006. A summary report on debris impact 

resistance of building assemblies. Lubbock, TX: Texas 

Tech University. Tallahassee, FL: Florida A&M University. 

Tallahassee, FL: Florida State University. Gainesville, 

FL: University of Florida. https://www.depts.ttu.edu/nwi/

research/DebrisImpact/Reports/TTU_Final_NIST_Report_

numbered.pdf.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

13

Appendix A—Impact Test Results on a Three-Ply Panel

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

P1 3-ply wall 106.7 1.9 1.6 N Sliver of debris perforated 

witness screen

N Impact at geometric 

center of panel

Left to right: Back side of panel, sliver of debris, hole in witness screen.

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

P2 3-ply roof 85.9 1.0 <0.5 N Slight cracking of 

back side of panel

Y Impact at geometric 

center of roof panel; 

target speed 67 mph

Left to right: Impact at front of roof panel, slight cracking at back of panel.
General Technical Report FPL–GTR–266

14

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

P3 3-ply roof 78.0 1.0 0.5 N Slight cracking of 

back side of panel

Y Impact 6 in. from 

edge of roof panel; 

target speed 67 mph

Impact at front of roof panel.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

15

Test 

no.

Door panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

D1 3-ply 103.4 2.0 <0.5 N None to back side of door Y Shot to upper door 

latch; latch operable 

after impact

Front of door penetration.

Test 

no.

Door panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

D2 3-ply 104.4 0.8 <0.5 N None to back side of door Y Shot to hinge; 

door operable 

after impact

Front of door penetration.

Appendix B—Impact Test Results on a Three-Ply CLT Door
General Technical Report FPL–GTR–266

16

Test 

no.

Door panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

D3 3-ply 103.2 1.6 <0.5 N Slight cracking of 

wood behind impact

Y Shot to geometric 

center of door 

halfway between 

door latches; door 

operable after 

impact

Left to right: Front of door penetration, slight cracking of back of door.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

17

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

R1 4-ply 110.2 1.8 <0.5 N NA Y Impact at geometric 

center of panel

Left to right: Front of panel penetration, close-up of panel penetration.

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

R2 4-ply 107.8 1.5 <0.5 N NA Y Impact at wall–wall 

connection 

(weak side)

Front of panel penetration.

Appendix C—Impact Test Results on a Four-Ply CLT Shelter (8 ft by 8 ft)
General Technical Report FPL–GTR–266

18

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

R3 4-ply 102.5 2.5 <0.5 N NA Y Impact at wall–wall 

connection 

(strong side)

Front of panel penetration.

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

R4 4-ply 105.1 1.5 <0.5 N NA Y Impact at roof–wall 

intersection at top 

center of shelter

Front of panel penetration.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

19

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

R5 4-ply 102.0 1.5 <0.5 N NA Y Impact at roof–wall 

intersection at top 

left of shelter

Photo not available

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

R6 4-ply 114.0 0.8 <0.5 N NA Y Impact at center 

of wall panel 

above door

Front of panel penetration.
General Technical Report FPL–GTR–266

20

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

R7 4-ply 106.4 1.4 <0.5 N NA Y Impact on panel at 

door edge between 

upper hinges

Front of panel penetration.

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

R8 4-ply 102.2 0.5 <0.5 N NA Y Impact on panel at 

door edge between 

door latches

Front of panel penetration.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

21

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

S1 4-ply 103.9 1.5 <0.5 N NA Y Impact to center of 

wall panel

Front of panel penetration.

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

S2 4-ply 101.6 1.0 <0.5 N NA Y Impact at strong 

side of wall–wall 

connection 

Front of panel penetration.

Appendix D—Impact Test Results on a Four-Ply CLT Shelter (5 ft by 5 ft)
General Technical Report FPL–GTR–266

22

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

S3 4-ply 101.1 1.1 <0.5 N NA Y Impact at weak 

side of wall–wall 

connection; slight 

bending of sheet 

metal angle

Left to right: Front of panel penetration, slight bending of steel angle.

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

S4 4-ply 103.4 1.5 <0.5 N NA Y Impact at strong 

side wall–roof 

intersection at top 

of wall

Right impact in photo.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

23

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

S5 4-ply 103.3 1.5 <0.5 N NA Y Impact at wall–roof 

panel intersection at 

center of wall panel

Middle impact in photo.

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

S6 4-ply 103.2 1.5 <0.5 N NA Y Impact at weak 

side of wall–roof 

intersection at top 

of shelter

Left impact in photo.
General Technical Report FPL–GTR–266

24

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

S7 4-ply 99.7 0.8 <0.5 N NA Y Impact at weak side 

wall panel next 

to door opening; 

speed was slightly 

below 100 mph, but 

test was deemed 

acceptable

Front of panel penetration.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

25

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

S8 4-ply 103.4 1.5 <0.5 N NA Y Impact at strong side 

wall panel next to 

door opening near 

top hinge

Front of panel penetration.
General Technical Report FPL–GTR–266

26

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

S9 4-ply 102.4 1.3 <0.5 N NA Y Impact at weak side 

wall panel next 

to door opening 

opposite middle 

hinge

Lower impact next to door in photo.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

27

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

DD1 4-ply 102.7 1.5 <0.5 N NA Y Impact at upper 

center of door

Front of door penetration.

Appendix E—Impact Test Results on a Four-Ply CLT Door
General Technical Report FPL–GTR–266

28

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

DD2 4-ply Not 

recorded

1.3 <0.5 N NA Y Impact near 

upper hinge

Front of door penetration.
Development of a Ready-to-Assemble Tornado Shelter from Cross-Laminated Timber (CLT): Impact and Wind Pressure Testing

29

Test 

no.

Panel 

construction

Missile

speed

(mph)

Front

penetration

(in.)

Permanent

deflection

(in.)

Panel/door

perforated

(Y/N) Observed damage

Passed

test

(Y/N) Additional notes

DD3 4-ply 102.5 1.5 <0.5 N NA Y Impact at geometric 

center of door

Front of door penetration.