NISTIR 7615

Gypsum Stairwell Enclosure Wall System 

Tests under Uniform Static Pressure

NISTIR 7615

Gypsum Stairwell Enclosure Wall System 

Tests under Uniform Static Pressure 

H.S. Lew 

Building and Fire Research Laboratory 

July 2009 

U.S. Department of Commerce 

Gary Locke, Secretary 

National Institute of Standards and Technology 

Patrick D. Gallagher, Deputy Director 
The policy of the National Institute of Standards and Technology is to use The International 

System of Units in its published materials. Because this report is intended for the U.S. building 

construction industry which uses inch-pound units, it is more practical and less confusing to use 

inch-pound units rather than metric units within quoted text.
[Type text] 

Abstract

The importance of the structural integrity of the exit stairway enclosures was clearly shown by 

the collapse of the World Trade Center Towers. The investigation report of the National Institute 

of Standards and Technology recommended that the design, functional integrity, and 

survivability of the egress and other life safety systems should be enhanced by considering 

accidental structural loads. Following this recommendation, the Ad Hoc Committee on 

Terrorism Resistant Buildings (TRB) of the International Code Council (ICC) advocated a code 

change proposal on the structural integrity of exit stairway enclosures during the 2007-2008 code 

development cycle of the International Building Code (IBC). The proposal required that for all 

buildings more than 420 ft in height, the exit stairway enclosure wall surface should be capable 

of resisting a uniform static pressure of not less than 2 psi applied perpendicular to the exterior of 

the enclosure. This report presents results of three two-hour fire rated gypsum stairway 

enclosure assemblies, which are commonly used in high-rise construction, subjected to a uniform 

static pressure applied to the surface. 

Key Words: Enclosure; gypsum; stairwell; tests; walls.
Acknowledgements 

The experimental program reported herein was performed as a joint effort of a number of 

individuals within the Structures Group of the Building and Fire Research Laboratory. 

Specifically, the contributions of Frank Davis, who meticulously prepared test specimens and 

assembled the test setup, Dr. Long T. Phan, who supported the data acquisition and evaluation, 

and Dr. John L. Gross, who captured photographic images of the tests, are gratefully 

acknowledged. Comments and suggestions of Mr. Gary Lewis, Chair of the ICC’s Ad Hoc 

Committee on Terrorism Resistant Buildings, are greatly appreciated. 
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Table of Contents 

Abstract........................................................................................................iii 

Acknowledgements...........................................................................................iv 

Table of Contents..............................................................................................v 

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

2. Test Specimens............................................................................................1 

3. Test Setup..................................................................................................2 

4. Test Results................................................................................................2 

5. Observation and Discussion.............................................................................3 

[Type text] 

1. Introduction

The investigation of the collapse of the World Trade Center Towers by the National Institute of 

Standards and Technology (NIST) concluded that the integrity of stairway enclosures in multistory buildings is important in assuring safe evacuation of occupants1

. The investigation report 

recommended that “the design, functional integrity, and survivability of the egress and other life 

safety systems (e.g., stairwell and elevator shafts and active fire protection systems) should be 

enhanced by considering accidental structural loads such as those induced by overpressures (e.g., 

gas explosions), impacts, or major hurricanes and earthquakes, in addition to fire separation 

requirements.” Following this recommendation, the Ad Hoc Committee on Terrorism Resistant 

Buildings Committee (TRB) of the International Code Council (ICC) advocated a code change 

proposal on the structural integrity of exit stairway enclosures during the 2007-2008 code 

development cycle of the International Building Code (IBC). The proposal required that for all 

buildings more than 420 ft in height, exit stairway enclosure wall surfaces, from the top of the 

floor to the underside of the floor above or roof supporting members, should be capable of 

resisting a uniform static pressure of not less than 2 psi applied perpendicular to the exterior of 

the enclosure. At the March 2008 ICC code development hearings, questions were raised 

whether conventional stairway enclosure systems would meet this requirement. 

To examine the ability of conventional light-weight stairway enclosure systems commonly used 

in high-rise construction to meet the proposed 2 psi requirement, NIST’s Building and Fire 

Research Laboratory carried out three tests of gypsum stairway enclosure systems under static 

uniform pressure. 

2. Test Specimens 

A total of three specimens were tested. The test specimens consisted of gypsum shaft wall 

assemblies having a two-hour fire rating. The test wall sections were 72-in wide and 100-in high. 

They were fabricated by NIST using materials purchased from a local building material supplier. 

Two types of specimens were constructed: one with 2-1/2 in steel C-H studs (20 ga.) and the 

other with 4 inch steel C-H studs (20 ga.). Figure 1 shows the cross section of C-H studs. The 

studs were spaced at 24 in on center. For both types, two layers of 1/2-in Type-X gypsum panels 

were applied either vertically or horizontally to the C-H studs with screws (See Table 1), and 1-

in “gypsum liner panels” were inserted into the grooves of the C-H studs. Along the exterior 

edges of the panel, the liner panels were fastened to the C-H studs with screws at 12 in on center. 

The assembled gypsum panel was placed between top and bottom steel J-runners (20 ga.). The 

ends of the C-H studs were fastened to both flanges of the J-runners with screws. Both the top 

and bottom J-runners were fastened to 6-inch x 6-inch hardwood members with screws. The 

wood members were in turn bolted to the steel reaction beams. The top reaction beam was 

attached to a concrete reaction wall with 1-1/2 in diameter post-tensioning bolts, and the bottom 

reaction beam was anchored to the laboratory floor. Thus, the specimen acted as a one-way plate 

 

1

 NIST NCSTAR 1, Final Report on the Collapse of the World Trade Center Towers, National Institute of Standards 

and Technology, Gaithersburg, MD, September 2005 
supported at the top and bottom for a span of 100 in. A specimen placed in the test fixture is 

shown in Figure 2. 

 Table 1 Specimen Description 

Specimen Stud Type Panel Orientation 

1 2-1/2 in. C-H Vertical 

2 4 in. C-H Vertical 

3 4 in. C-H Horizontal 

3. Test Setup 

The test section was loaded over the entire surface by an air bag. The air bag was placed between 

the test specimen and the concrete reaction wall. In all tests, the pressure was applied to the 

gypsum panel side. Thus, the gypsum panels were in compression while the liner panels were in 

tension. 

The air pressure in the bag was increased at a rate of 0.15 psi/min until the test panel could no 

longer sustain the pressure increase as indicated by a large increase in deflection at the center of 

the panel. To monitor any horizontal movement of the J runners during test, a dial gage was 

positioned against the flange of the J-runners (see Figure 2). An LVDT2

 (linear variable 

differential transformer) was placed at the geometric center of the test panel to measure the 

displacement response of the test specimen to the applied pressure. 

The air pressure in the bag was measured by a digital transducer3

. The data from the transducer 

and LVDT were recorded by a digital data acquisition system capable of sampling at a rate of 2 

KHz. However, a sampling rate of 5 Hz was used for the tests since they were essentially static 

load tests. The recorded data were saved to the computer disk for data processing. 

4. Test Results 

Specimen 1 (2-1/2 in C-H Stud)

This specimen had the gypsum panels applied in the vertical direction. When buckling of the 

compression side of the stud occurred, a large increase in the lateral deflection at the mid height 

of the panel was noted. The deflection increased rapidly as the pressure in the air bag increased. 

The testing was stopped at about 2.1 psi. The post-test photograph of this specimen is shown in 

Figure 3. It is seen that buckling of the C-H stud and cracking in the gypsum liner panels on the 

tension side developed at the mid height of the test panel. Removal of the gypsum panels 

 

2

 LVDT used has ±0.001 in. resolution. 

3

 The digital transducer has ±0.01 psi resolution. 
[Type text] 

revealed that local buckling of the compression flange developed in all four C-H studs (Similar 

to Figure 11). 

Figure 4 shows a plot of the air-bag pressure vs. the panel center deflection for Specimen 1. It is 

seen that the panel behaved essentially elastically up to about 1.4 psi. Buckling of the 

compression flange developed at this pressure level and the deflection at the center of the panel 

increased rapidly. 

Specimen 2 (4 in. C-H Stud)

This specimen had the gypsum panels applied in the vertical direction. The post-test photograph 

of this specimen is shown in Figure 5, and a close up view of the buckling of the C-H stud is 

shown in Figure 6. The plot of the air-bag pressure vs. the panel center deflection is shown in 

Figure 7. At about 2 psi, the compression flange began to buckle. Buckling of the web also 

appeared at about 2.4 psi, and cracks developed on the tension side of the gypsum liner panels. 

After reaching 3 psi the panel center deflection increased rapidly (see Figure 7). When the 

specimen could no longer sustain the increase in the air bag pressure, the testing was terminated. 

Specimen 3 (4 in. C-H Stud)

This specimen was essentially the same as Specimen 2, except that the gypsum panels were 

applied in the horizontal direction. The post-test photograph of this specimen is shown in Figure 

8, and a close up view of the local buckling of the C-H stud is shown in Figure 9. The plot of the 

air-bag pressure vs. the panel center deflection is shown in Figure 10. The overall behavior of 

Specimen 3 is similar to that of Specimen 2. At about 2 psi, the compression flange of the stud 

began to buckle. At about 2.5 psi the buckling of the web was noted. Similar to Specimen 2, this 

specimen also reached a maximum pressure of about 3 psi when the panel center deflection 

reached about 3 inches. Removal of gypsum liner panels revealed that all studs developed 

buckling (See Figure 11). 

5. Summary and Discussion 

Under uniformly applied static pressure, all three test panels failed due to local buckling of the 

studs. Exposed two end C-H studs showed that the buckling initiated at the compression flange 

and propagated to the web. The 2-1/2 in stud specimen, Specimen 1, was able to resist a 

maximum pressure of 2 psi. Both 4 in stud specimens were able to resist a maximum pressure of 

3 psi. The results of Specimen 2 and Specimen 3 indicate that the direction of application of the 

gypsum panels has little or no effect on the capacity of the specimen. Buckling developed in the 

web of the 4-in studs at a panel center deflection of the specimen about 3 in. At this point, the 

gypsum liner panels cracked, and both specimens were unable to sustain any further increase in 

pressure. The failure mode of all three tests indicates that the capacity of the panel is limited by 

the local buckling strength of the stud. 

Although the ends of the studs were fastened to the flanges of the J-runners, the specimens 

behaved as simply supported rather than fixed (See the top and bottom of the failed specimens). 
The 20-gauge flanges of the J-runners did not provide sufficient stiffness to create fixed end 

conditions. In all three tests, the lateral movement of the J-runners was less than 0.01 in. 

The span of the test specimens was 8 ft-4in. This span length is substantially less than the height 

of the stairway enclosure of a typical commercial building. Assuming that the height of a typical 

story is about 13 ft, the distance between the top of the floor to the bottom of the floor beam 

above would be about 11 ft – 4 in after subtracting 4-in slab and 16-in steel beam. For an 11 ft-4 

in. high gypsum enclosure wall section failing in the same mode as that of the 8 ft - 4 in high test 

panel (i.e., both are failing in the local buckling of the compression flange of the stud), the 

following equivalent static pressure are determined based on the test results. 

Table 2 Capacity of 11ft - 4 in high gypsum wall panel vs. Equivalent pressure of 8 ft-4 in 

 gypsum wall panel 

 

Capacity of 11 ft-4 in high gypsum 

system wall enclosure (psi) 

Equivalent pressure required for 8 ft-4 in 

high gypsum system wall enclosure (psi) 

2 3.70 

1.5 2.78 

1 1.85 

 

 

For typical commercial buildings having 13-ft story height, the tabulated values suggest that 2-

hour rated stairway enclosure system with 20 ga., 4 in C-H studs could resist a uniform static 

horizontal pressure of 1.5 psi, whereas, the same assembly with 2-1/2 in C-H studs could resist a 

uniform static pressure of 1.0 psi. 

To resist 2 psi of horizontal static pressure on a 11 ft - 4 in high wall system comprised of 

gypsum-stud panels having 2-hour fire rating, a 8 ft - 4 in high wall system comprised of the 

same cross sectional property would require a capacity of 3.7 psi. The required capacity can be 

enhanced either by increasing the thickness of the C-H stud or changing the support condition 

from simple to fixed support at the top and bottom of the panel. 
[Type text] 

2-1/2 in J- 4 in J-Runner 

Runner 

4 in C-H 

Stud 

2 1⁄2 in C-H 

Stud 

Figure 1 Cross Section of C-H Studs 

 and J Runners 
Figure 2 Test Setup
[Type text] 

Buckling of compression 

side of C-H stud Crack in tension surface 

of gypsum liner panel 

 Figure 3 Post-test Photograph of Specimen 1 

 2-1/2 inch C-H Stud 
0

0.5

1

1.5

2

2.5

3

3.5

012345

2-1/2 in C-H Stud

Pressure (psi)

Displacement (inches)

Figure 4 Air-bag pressure vs. panel center 

 deflection plot of Specimen 1 
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Figure 5 Post-test Photograph of Specimen 2 

4 inch C-H Stud
Figure 6 Close-up view of Local Buckling of 

C-H Stud - Specimen 2
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0

0.5

1

1.5

2

2.5

3

3.5

012345

4-in C-H Stud (1)

Pressure (psi)

Displacement (inches)

Figure 7 Air-bag pressure vs. panel center 

 deflection plot of Specimen 2 
Figure 8 Post-test Photograph of Specimen 3 

(4 inch C-H Stud) 
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Figure 9 Close-up view of Local Buckling of 

 C-H Stud – Specimen 3 
0

0.5

1

1.5

2

2.5

3

3.5

012345

4-in C-H Stud (2)

Pressure (Psi)

Displacement (inches)

Figure 10 Air-bag pressure vs. panel center 

 deflection plot of Specimen 3 
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Figure 11 Development of local buckling in all four 

 C-H studs - Specimen 3