PERFORMANCE CHARACTERISTICS FOR WELDED WIRE SCREEN USED FOR SURFACE CONTROL IN UNDERGROUND 

COAL MINES 

D. R. Dolinar, NIOSH, Pittsburgh, PA 

ABSTRACT 

The fall of small pieces of rock between roof bolts causes over 80 

pct of the ground fall injuries in underground coal mines. Roof screen,

if installed on the bolting cycle, can significantly reduce the number of 

these injuries. As the surface of the roof fails, the screen will have to 

retain the broken rock without failing or excessive deformation. 

Welded wire screen is the most common type of screen material used 

for roof surface control in underground coal mines. 

The load-deformation characteristics and the load capacity of 

welded wire screen were evaluated in the laboratory in a specially built 

test frame. In the frame, the square screen sections were held in place 

by bolts and bearing plates at the four corners while a center load was 

applied to the screen. 

The parameters evaluated included the wire gauge and 

configuration, the bearing plate loads and bolt spacing. The wire 

gauge of the tested screen was 4, 6, 8, 9 and 10. Maximum screen 

capacities for both load and displacement were developed for each of 

the wire gauge sizes along with the screen stiffness. This information 

can be used for screen selection and for the evaluation and 

determination of screen performance. 

DISCLAIMER: The findings and conclusions in this report have not 

been formally disseminated by the National Institute for Occupational 

Safety and Health and should not be construed to represent any 

agency determination or policy. 

INTRODUCTION 

Each year in underground coal mines there are between 400 to 

500 injuries and 1 to 2 fatalities that are caused by the fall of relatively 

small pieces of rock between the roof supports. Essentially, the fall of 

these small pieces of rock cause over 80 pct of the ground fall injuries 

(Compton et al., 2007). In mines where welded wire screen is installed 

on cycle, the number of these types of injuries has been reduced 

significantly (Robertson and Hinshaw, 2001). Because of the potential 

for reducing ground fall injuries, the National Institute for Occupational 

Safety and Health (NIOSH) has an interest in increasing the use of 

welded wire screen. As part of this effort, NIOSH is evaluating the 

performance characteristics of welded wire screen as used in 

underground coal mines by conducting a laboratory testing program at 

the Pittsburgh Research Laboratory (PRL). 

Tests to evaluate the performance characteristics of various types 

of screen or mesh have been conducted in both Canada and Australia. 

The earliest investigation of screen was done in Canada in the early 

1980s (Pakalnis and Ames, 1983). In this study, various gauges of 

welded wire and chain link screen were tested by bolting the screens 

along a rib in an underground mine and conducting a pull test with a 

plate in the center of the screen. The welded screen had wire gauges 

of 4, 6, and 9 with 4 x 4-in wire spacing. The screen was installed and 

tested with the bolts in a diamond pattern with respect to the screen 

wire configuration. These tests established the general loaddisplacement behavior of the screen. Further, a relationship was also 

developed between the wire gauge or diameter and the screen load 

capacity. 

Another later study, also conducted in Canada, evaluated welded 

screen performance on a laboratory test frame (Tannant, 1995). A 

center load was applied to the screen with the screen tested in both 

square and diamond configurations with respect to the bolts. The bolt 

load and bolt spacing were also varied in these tests. The welded 

screen gauges were 4, 6 and 9. While slippage of the screen at the 

plates was noted during these tests, it could be controlled by the bolt 

torque. Peak load capacities were determined for each gauge of wire 

with the load capacity increasing with wire diameter. Screen stiffness 

changed significantly with orientation of the wires with respect to the 

bolting pattern. 

A test frame was also built in Australia to evaluate welded wire 

screen performance and the various parameters that could affect that 

performance (Thompson et al., 1999). Again, a center load was 

applied to the screen. The screen had a wire diameter of 0.22 in (4 

gauge) and a wire spacing of 4 in. This is the most commonly used 

screen in the Australian mining industry. In these studies, various bolt 

spacings were used with the welded screen placed either in a square 

or diamond orientation with respect to the bolts. Bolt loads and bearing 

plate sizes were also varied. The primary conclusion from this study 

was that the stiffness is a function of the bolting pattern and screen 

configuration. Further, slippage of the screen at the bearing plates will 

affect the stiffness. 

In a recent NIOSH study, the performance characteristics of an 8-

gauge welded wire screen were evaluated using a laboratory test 

frame (Dolinar, 2006). The screen was tested in a configuration that 

simulates the installation in U.S. coal mines with the bolts placed in a 

square pattern with respect to the screen. The 8-gauge welded screen 

is the most commonly used screen in the U.S. coal industry. It was 

found in this study that the plate conditions including the bolt load, 

bearing plate size and load surface affect the yield and peak load and 

the stiffness of the welded wire screen. Further, screen performance 

was altered by whether the screen was fixed or was allowed to slip at 

the bearing plates. 

Numerical modeling has also been used to evaluate screen 

performance (Murali et al., 2006). An advantage of the numerical 

modeling is that variables can be examined that are not easy to 

simulate in a laboratory test. In the modeling studies, the effects of 

variable screen sizes and wire gauges were examined. The modeling 

indicated that the displacements will be higher with the smaller screen 

sections that are generally used in the laboratory tests than for the full 

screens that are typically installed underground. Further, these models 

have also shown that most of the load is being carried by the wires that 

are directly under the bearing plates. 

In this present NIOSH study, design parameters and capacities 

are developed based on laboratory tests to assist in the evaluation and 

selection of welded wire screen as used in U.S. coal mines. In this 

case, the effects of the wire gauge, bolt spacing and bearing plate 

loads on the screen capacity and displacement are evaluated. Further, 

the effects of alternative wire spacing configurations on screen 

performance are also examined. Numerical modeling of the welded 

wire screen is used to assist in the evaluation and interpretation of the 

results and in establishing some of the design parameters and 

capacities. Ultimately, the type of information obtained in this and 

similar studies will be used to develop design criteria to assist mine 

operates in the selection and use of roof screen in underground coal 

mines. 
TEST FACILITIES AND PROCEDURES 

A test frame was installed in the Mine Roof Simulator (MRS) that 

was designed with the capability of varying the bolt spacing from 4 to 5 

ft with 4 bolts used to attach the screen to the test frame (figure 1). A 

one-foot square load plate with round corners was used to apply a 

center load to the screen. With the MRS, the screen could be 

displaced between 20 and 22 in. The tests were run in displacement 

control with a displacement rate of 2 in/min. This rate is sufficiently 

slow to assure that the screen is loaded statically and not dynamically. 

A typical test would take approximately 10 min. 

Load Cell 

Load Plate 

Bolt Bearing Plate 

Surface Plate 

Figure 1. Test frame set up used to test the welded wire screen. Bolt 

spacing is 48 x 48-in.

The load was measured using a 20,000 lb external load cell with 

an accuracy of ± 20 lb. The screen displacement was monitored using 

the linear variable differential transducer for the MRS control system 

with an accuracy of ± 0.01 in. During the tests, a computer was used 

to record the load and displacement. 

To hold the screen on the test frame, 3⁄4-in diameter bolts were 

used with bearing plates. The bearing plates were flat, grade 4, 6 x 6-in 

sections with a thickness of 3⁄8 in. A 1-1⁄2-in diameter washer was 

installed between the bearing plate and head of the bolt. A steel loadbearing surface that consisted of one foot square plates attached to 

the frame was used under the screen. 

The screen was placed in a square configuration with respect to 

the wire grid and the test frame and bearing plates that attach the 

screen to the test frame (figure 2). This is similar to the typical 

installation in an underground coal mine. With this arrangement, for 

the welded wire screen, load was transferred from the center load plate 

to the bolts through the center wires crossing the load plate then to the 

eight wires that directly connect the bolts and bearing plates. 

WELDED WIRE SCREEN 

Welded wire screen as manufactured consists of parallel series of 

high strength steel wires that are welded by machine into square or 

rectangular grids. In general, the important parameters for the welded 

screen are the cross-sectional area of the wires or gauge size and the 

wire spacing. The wire gauges used in this series of tests were 4, 6, 8, 

9 and 10. Table 1 gives the wire diameter and cross-sectional area 

and the cross-sectional area per foot of screen for each gauge. 

The wire spacing is designated by the longitudinal and transverse

wire spacing. These terms are related to the manufacturing process 

and direction. For the longitudinal direction, the wire is continuously 

fed through the automatic welding machines from individual spools of 

wire. The transverse wires enter one at a time from the side of the 

machine and are individually welded to the longitudinal wires (Wire 

Reinforcement Institute, Inc., 2008). Because of the manufacturing 

process, it is much easier to alter wire spacing or add wires in the 

transverse direction than in the longitudinal direction. A 4 x 4-in wire 

spacing was tested for all 5 gauge sizes. For the 8-gauge screen, a 4 

x 2-in pattern was also tested. In this case, the transverse wires were 

spaced at 2 in. Further, 8-gauge screens were tested with the 4 x 4-in 

spacing, but with two additional transverse wires near each end. This 

resulted in a 2-in wire spacing for a distance of 8 in from the two 

screen ends. The size of the welded screen tested was 60 x 60-in. A 

typical welded wire screen used underground will be 5 ft wide and 12 

to 15 ft long. The weight of each type of screen is given in table 1. 

Bearing Plate

Load Plate

Figure 2. Schematic of screen test configuration with a square bolting 

pattern with respect to the screen. Heavy bold lines indicate the wires 

connecting the bolts and the dashed lines the wires crossing the load 

plate. The heavy arrows indicate the primary load transfer directions 

along the wires from the load plate to the bolts. 

Table 1. Screen, wire, and weld specifications for welded wire screen. 

Unless noted, the values are for a wire spacing of 4 x 4-in. 

Gauge 

4 

6 

8 

8T2

8R3

9 

10 

Diameter 

(in) 

0.226 

0.195 

0.161 

0.161 

0.161 

0.151 

0.133 

Area 

(in2

) 

0.0403 

0.0299 

0.0201 

0.0201 

0.0201 

0.0177 

0.0138 

Area/ft 

(in2) 

0.1209 

0.0896 

0.0603 

0.0684 

0.0905 

0.0530 

0.0398 

Minimum 

weld 

strength, 

(lb) 

1,400 

1,050 

710 

710 

710 

630 

465 

Weight1

(lb) 

21.6 

15.2 

10.7

15.7 

12 

9.3 

7.6 1

This is the weight of a 60 x 60-in screen. 

2

The wire spacing is 4 x 2-in with a 2-in spacing for the transverse 

wires. 

3

The screen is reinforced with 2 extra wires in the transverse direction 

at each end of the screen. 

For welded screen which is used primarily to reinforce concrete, 

there is no overall strength requirement. However, there are certain 

ASTM requirements regarding both the weld and wire strength. The 

weld capacity is based on the shear strength. The weld strength in lbsforce shall not be less than 35,000 multiplied by the nominal area of 

the wire in square inches when tested with the specified shear test 

(ASTM A-497-99, 2004). Table 1 gives the minimum weld strength for 

each wire gauge. The tensile strength of the wire must exceed 75,000 

psi with a yield strength of 65,000 psi (ASTM A 83-97a, 2004). 

EXPERIMENTAL DESIGN 

In the tests, the parameters that were varied included the wire 

gauge and configuration, the bearing plate load and the bolt spacing. 

The wire gauge and configuration were given in the previous section. 

The bearing plate loads used were 4,500, 8,000, 15,000 and 25,000 

lbs. The bolt spacing was either 48 x 48-in or 56 x 56-in. The bolt 

spacing was limited by the size of the screen that was used for the 

tests. Table 2 shows the test program that was conducted. For each 

series, 3 screens were tested. 
Table 2. Experimental design used for testing and evaluation of the

welded wire screen. Tests are designated by the x symbol. 

Gauge Plate Load,lb 

4,500 8,000 15,000 25,000

Bolt Spacing 48 x 48 in 

4 x x x x

6 x x x x

8 x x x x

9 x x x x

10 x x x x

8T1

x x x

8R2

x x x

Bolt Spacing 56 x 56 in 

8 x

9 x

10 x 1

The wire spacing is 4 x 2-in with the 2-in spacing for the transverse 

wires. 

2

The screen is reinforced with 2 extra wires in the transverse direction 

at each end of the screen. 

TEST RESULTS 

A load-displacement curve for one of the tests is shown in figure 

3. From such a graph, several parameters are obtained relevant to 

screen performance. The peak load is the maximum load just prior to 

a significant drop in load. The design load is the maximum load prior 

to any significant decrease in the stiffness of the screen. In some 

cases, there may even be a higher load than the peak that occurs, but 

beyond the design and peak loads the behavior is not consistent. The 

large drop in load is the result of significant screen slippage at the 

bearing plate or wire breakage. Screen stiffness is determined based 

on the slope of a line from the design load to a point at 20 percent of 

design load (figure 3). The screen stiffness is calculated from the 

following equation: 

 Ks = (Ld-L20)/(Dd-D20) (1)

where Ks = screen stiffness, lb/in, 

 Ld = design load, lb, 

 L20 = load at 20 percent of design load, lb, 

 Dd = displacement at design load, in, and 

 D20 = displacement at 20 percent of design load, in. 

0

500

1000

1500

2000

2500

3000

0 5 10 15 20 25

Displacement, in 

Load, lb

20 pct of Design Load

Design Load Peak Load

Displacement Offset

Figure 3. Load displacement curve for a test on a welded wire screen 

showing key parameters used to evaluate the screen performance.

A displacement offset (Do) is defined as the intersection of the line 

used to calculate the stiffness and the x-axis (figure 3). The offset is 

the amount of deformation that will occur before the screen begins to 

significantly resist the load. 

Table 3 (see Appendix) gives the average peak and design loads 

and the displacements along with the calculated stiffness and offset 

displacements for each series of tests. The standard deviations for 

specific parameters are given in parentheses. 

EVALUATION OF TEST RESULTS 

In evaluating the effects of the bearing plate load, wire gauge and 

configuration, and the bolt spacing, the design load is used and not the 

peak load. This is because there is a significant reduction in stiffness 

after the design load, indicating that the screen performance is being 

dominated by the slippage of the screen or the screen has been 

damaged sufficiently by wire breakage. Past the design load, the 

screen offers on average little resistance to further displacement. In 

several cases where the screen is tested under displacement control, 

there is a nearly zero or even negative screen stiffness beyond the 

design load. Under a dead weight rock load as would occur 

underground, the screen behavior should be plastic or nearly plastic 

after the design load. Once the rock load exceeds the design load, the 

screen could continue to deflect resulting in large displacements. 

Ultimately, the screen displacement would be stopped if a higher load, 

than the design load such as the peak or maximum load is approached 

because of the increase in stiffness or if the screen failed. 

Bearing Plate Load 

The behavior of the screen is in part controlled by whether the

screen will slip or is fixed at the bearing plates (Dolinar, 2006).

Bearing plate loads can control the degree the screen will slip or is

fixed. Figure 4 shows the load-displacement curves for different

bearing plate loads for the 8-gauge screen with a 4 x 4-in wire spacing

and a 48 x 48-in bolt spacing. For the highest plate load, there is less

displacement up to the design load and the sudden drops in load are

due to wire breakage. Both conditions indicate that the screen is fixed

at the bearing plate. For the fixed condition, wire breakage controls

the design and peak load. For the lower plate loads, there is more

displacement and the sudden drops in load are the result of the

slippage of the screen at the bearing plates. The onset of noticeable

slippage for the 4,500 lb plate load is about 600 lb and for the 8,000 lb

plate load about 900 lb. Even before noticeable slippage, there is

more displacement with the lower plate loads than for the fixed

condition, indicating that there is displacement of the wire under the

plates. There is, however, controlled slippage through the design load.

After the design load, there is a much larger variability in the load. This

significant change in behavior and the drop in screen stiffness is the

result of a weld point or junction slipping out from under a bearing

plate. 

 

0

500

1000

1500

2000

2500

3000

0 5 10 15 20 25

Displacement, in

Load, lb

4,500 lb

8,000 lb

25,000 lb

Yield Model 

Plate Load

Figure 4. Load-displacement curves from selected tests on an 8-

gauge wire screen at bearing plate loads of 4,500, 8,000 and 25,000 

lb. The load-displacement curve from the numerical modeling of an 8-

gauge wire screen that is fixed at the bearing plates is also shown. 

The yield model follows the 25,000 lb bearing plate test (fixed 

condition) fairly closely until the first wire breaks. 

Numerical modeling can assist in evaluating the screen behavior 

(Murali et al., 2006). In this case, FLAC3D was used (Itasca, 2007). 

Pile and beam elements were used to simulate the mesh. The screen 

dimensions were 60 x 60-in with the screen fixed at the location of the 
bearing plates that simulate a 48-in bolt spacing. The steel wire 

properties were as follows: Young’s modulus equal to 29 million psi, 

Poisson’s ratio equal to 0.3, and yield strength equal to 65,000 psi. 

The wire cross-sectional area, the section modulus and the moment of 

inertia were calculated based on the wire gauge. The loaddisplacement curve with yield for a modeled 8-gauge screen is shown 

in figure 4. The yield model tracks the actual test results for the fixed 

condition (25,000 lb plate load) fairly closely until the first wire breaks. 

This again suggests the screen acts as being fixed at the bearing 

plates. Further, the modeled screen begins to yield at about 720 lb of 

load. This is well below the design load of 1,790 lb, though just below 

the load of about 900 lb when the first wire breaks. 

From table 3, at the lower bearing plate loads of 4,500 and 8,000 

lb, the design load increases with the higher plate load for all the wire 

gauges. Essentially, slippage of the screen at the bearing plates 

dominates the screen behavior and limits the design load. The 

increase in design load results from the additional frictional forces 

developed from the higher bearing plate loads. For the 8-, 9-, and 10-

gauge screen, with the increase in plate load to 15,000 and 25,000 lb, 

the design load actually decreases. This is the result of sufficient plate 

load where the screen is either fixed or partially fixed at the bearing 

plate. In this case, the design load will be limited by wire breakage. 

For the 4- and 6-gauge wires, the design load increases through 

15,000 lb. Slippage that is affected by the bearing plate load still 

controls the screen performance. At a bearing plate load of 25,000 lb, 

the load-displacement curves indicate that some slippage is still 

occurring in the screen with these lower gauge wires (figure 5). The 

load-displacement curve from numerical modeling, using the yield 

criteria for a 6-gauge wire screen, is also shown on figure 5. Above 

about 1,200 lb of load there is more displacement from the tested 

screen than for the modeled screen. This again indicates that the 

screen is not completely fixed. However, it is sufficiently fixed that the 

larger load-drops at and after failure are the result of wire breakage 

and not slippage. It requires higher plate loads to fix the larger 

diameter wire screen at the bearing plates. 

0

1000

2000

3000

4000

5000

6000

0 5 10 15 20 25

Displacement, in

Load, lb

6 Gauge Screen

Yield Model

Figure 5. Load displacement curve for a welded wire screen with a 6-

gauge wire and a bearing plate load of 25,000 lb. The loaddisplacement curve from the numerical modeling of a 6-gauge wire 

screen that is fixed at the bearing plates is also shown. Above a load 

of 1,200 lb, there is more displacement from the tested screen 

indicating that the screen is not completely fixed. 

Wire Cross-Sectional Area 

Figure 6 shows the design load versus the cross-sectional area of 

the wire per foot of screen for each wire gauge and for each bearing 

plate load for the 4 x 4-in wire spacing. Regression lines are shown for 

the 4,500-, 8,000-, and 25,000-lb bearing plate loads. At the lower 

bearing plate loads, the design load increases linearly and at the same 

rate with the cross-sectional area. The slope of the lines is just over 

29,000 lb/in2

. At the two highest plate loads, the strength increases 

nearly linearly but at a rate of about 60,000 lb/in2

 and is comparable to 

that determined from a Canadian study of about 65,100 lb/in2

 (Pakalnis 

et al., 1983). At the two highest plate loads, the lighter gauge screens 

are fixed, and thus have a lower strength than the heavier gauge 

screens that are slipping. All the screens at the lower plate loads are 

slipping at the bearing plates. Therefore, the rate of strength increase 

with the cross-sectional area of the wire is dependent on the bearing 

plate conditions and whether the screen is fixed or slips. 

Plate Load-8,000 lb

y = 29440x + 720

R2

 = 0.98

Plate Load-4,500 lb

y = 29140x + 110

R2

 = 0.98

Plate Load-25,000 lb

y = 59550x - 1450

R2

 = 0.91

Plate Load-15,000 lb

y = 62613x - 1439

R2

 = 0.96

0

1000

2000

3000

4000

5000

6000

7000

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Cross-Sectional Area, in2/ft

Design Load, lb

4,500 lb

8,000 lb

15,000 lb

25,000 lb

Plate Load

Figure 6. The effects of the cross-sectional area of the wire on the 

screen capacity using the design load.

Bolt Spacing 

Two different bolt spacings were evaluated. However, the screen 

size limited the maximum bolt spacing that could be tested to 56 in. As 

a result, a bolt spacing of 60 in could not be evaluated. With an 

increase in bolt spacing to 56 in, the design loads are not significantly 

different from those with a bolt spacing of 48 in, when compared at the 

same bearing plate load of 25,000 lb where the screens are fixed at 

the bearing plates. However, the screen deflection has increased for 

the same screen load with the wider bolt spacing. Figure 7 shows the 

load-displacement curves for an 8-gauge screen for both the 48- and 

56-in bolt spacing. Based on the tests for the load range from 200 lb to 

the design load, the average increase in displacement with the 8-in 

increase in bolt spacing is 1.21 times. 

0

500

1000

1500

2000

2500

3000

3500

0 5 10 15 20 25

Displacement, in

Load, lb

48 x 48-in Spacing

56 x 56-in Spacing

Yield Model

Figure 7. Load-displacement curves for an 8-gauge welded wire 

screen for both a 48 and 56 in bolt spacing for a bearing plate load of 

25, 000 lbs (fixed). The load-displacement curve from the numerical 

modeling of an 8-gauge wire screen with a 56 in bolt spacing that is 

fixed at the bearing plates is also shown. The yield model follows the 

25,000 lb bearing plate test (fixed condition) fairly closely until the first 

wire breaks. 
The load-displacement curve from numerical modeling of the 56-

in bolt spacing is also shown on figure 7. Except for the offset caused 

by a wire failure, the numerical model results closely follow the loaddisplacement curve for the wider bolt spacing. Based on numerical 

modeling, the average increase in displacement for the loads ranging 

from 200 lb to the design load is for the 8-gauge screen 1.22 times and 

for the 10-gauge screen 1.23 times. Again, the numerical model 

results are close to the test results. Therefore, the numerical models 

can be used to estimate the displacements resulting from the more 

standard bolt spacing of 60 x 60-in. 

Because of geometric considerations, the set-up of the modeled 

screen a 60-in bolt spacing could not be developed that would be 

directly comparable to the results with a 48-in bolt spacing. However, 

a model with a 64-in bolt spacing was developed for an 8-gauge 

screen. The increase in displacement for this bolt spacing over the 48-

in bolt spacing is 1.43 times. The amount of increase in displacement 

for a 60-in bolt spacing can then be determined by taking the average 

of the 56-in and 64-in model results. Based on the modeling, the 

amount of displacement with a 60-in bolt spacing should be 1.33 times 

higher than the 48-in bolt spacing. 

Wire Configuration 

Two wire configurations for the 8-gauge screen were tested that 

were different than the 4 x 4-in wire spacing. For both wire 

configurations, there are three transverse wires that cross under the 

bearing plate. This is in contrast to the two wires for the 4 x 4-in wire 

spacing. For the 8-gauge screen with a 4 x 2-in wire spacing with a 

plate load of 8,000 lb, the design load of 3,570 lb is comparable to that 

of the 6-gauge screen of 3,490 lb. Also, both screens have a similar 

stiffness and design load displacement. The weight of a 60 x 60-in 

sheet of the 4 x 2-in 8-gauge screen is 15.7 lb and the weight of the 6-

gauge screen is 15.2 lb. Both screens are made from about the same 

amount of steel. However, at plate loads of 15,000 and 25,000 lb, the 

design load for the 4 x 2-in 8-gauge screen at 3,660 and 2,720 lb are 

much less than that for the 6-gauge screen of 4,830 and 4,880 lb. In 

this case, the 8-gauge screen is fixed at the bearing plates and the 6-

gauge screen is either not fixed or only partially fixed. Because of the 

difference in performance at the higher plate loads, it would be better 

to use a 6-gauge screen with a 4 x 4-in wire spacing rather than an 8-

gauge screen with a 4 x 2-in wire spacing. The only minor advantage 

of using the 4 x 2-in, 8-gauge screen is the retention of very small 

pieces of rock that might fall through the wider wire spacing. 

Figure 8 shows a picture of the 8-gauge screen with the additional 

transverse reinforcement wires. There are two additional wires at each 

end of the screen resulting in a wire spacing of 2 in for a distance of 8 

in. At the lower bearing plate load of 8,000 lb with the additional wires, 

the design load is 2,500 lb and the stiffness is 231 lb/in as compared to 

a design load of 2,430 lb and a stiffness of 258 lb/in with no extra 

wires. At a bearing plate load of 25,000 lb, with the added wires, the 

design load is 2,950 lb with a stiffness of 372 lbs/in as compared to a 

design load of 1,790 lbs with a stiffness of 295 lbs/in with no 

reinforcement. 

At the lower bearing plate load, slippage controls the behavior 

and there is no benefit achieved with the additional wires. When the 

screen is fixed at the bearing plate, the added reinforcing wires 

increased the design load by 65 pct and the stiffness by 26 pct. The 

increase in strength and stiffness may be attributed to having three 

wires under the bearing plates in the transverse direction which 

provides an additional wire to transfer the load to the bearing plates 

and bolts. Because under the fixed condition wire failure controls the 

screen capacity, the strength increase is proportional to the number of 

wires under the bearing plate. Because there is only about 12 pct 

more material by weight and the reinforcing wires are transverse, there 

should be little increase in cost yet potentially significant gains in 

performance with the added wires. However as noted, the benefits are 

only seen when the screen is fixed. 

DESIGN CAPACITIES OF WELDED WIRE SCREEN 

The welded wire screen performance and capacity is dependent 

on the frictional conditions and the load at the bearing plates as well as 

any stress concentrations that may develop in any screen wires under 

the bearing plate. In an underground mining situation, these 

parameters will probably be highly variable and it will be assumed that 

there will be different degrees of slippage and fixity of the screen at the 

bearing plates. Therefore, to develop an estimate of in situ screen 

performance for design purposes based on the laboratory tests, the 

results from all the bearing plate load conditions will be averaged for a 

given wire gauge. The design loads will be used as the measure of the 

screen performance and capacity. 

Figure 8. Wire configuration for 8-gauge screen with two additional 

reinforcing wires at each end of the screen. There are three 

transverse wires under the bearing plate. 

Table 4 (see Appendix) gives the average design loads and 

displacements as well as the screen stiffness and displacement and 

load offsets for the different gauges of wire for a screen with a 4 x 4 in 

wire spacing. The displacements for the 60 x 60-in bolt spacing are 

1.33 time that of the 48 x 48-in bolt spacing, the increase in 

displacement expected with the wider bolt spacing based on the 

numerical models with confirmation by the laboratory tests. Further, 

the results are based on using a 6 x 6-in bearing plate. Other studies 

have shown that a larger bearing plate can significantly increase the 

screen capacity (Dolinar, 2006). 

The design loads and displacements are the suggested maximum 

values for the screen capacity as used underground and for screen 

selection. The design loads and displacements set practical limits for 

screen performance. Beyond the design loads, there is a significant 

change in behavior caused by wire breakage or slippage of the screen 

at the bearing plates. This results in excessive displacement of the 

screen. This does not mean that the welded wire screen cannot 

maintain higher loads than the design loads and still function in 

retaining material. The peak loads are on average about 10 pct higher 

than the design loads. Even higher screen loads were observed 

beyond the peak loads that are on average about 20 pct higher than 

the design loads. However, even though the screen may still function 

by retaining the rock material, there will be excessive displacement 

and screen damage at these higher loads. Ultimately, excessive 

screen displacement can become an operational issue. It should be 

noted that underground performance of the screen could be different 

than the laboratory test results. 

The following equations can then be used to determine the 

average screen load or displacement from the values given in table 4. 

Based on the displacement, the screen load can be calculated from 

equation 2. 

 Ls = Ks x Ds + Lo (2) 

where Ls = screen load, lb, 

 Ds = screen displacement, in, 

 Ks = screen stiffness, lb/in, and 

 Lo = offset load, lb. 
The load offset from table 4 is the intercept of the linear loaddisplacement curves with the load axis (figure 9). The offset load is 

negative. Based on the screen load, the displacement can be 

calculated from equation 3. 

 Ds = (Ls

 - Lo)/Ks. (3) 

With the offset load being negative the equations can only be 

used to calculate the screen loads and displacements beyond the 

offset displacement. In using the equations, it is assumed that it takes 

no load to reach the offset displacement. 

Figure 9 shows the resulting linear load-displacement curves 

based on table 4 and the above equations for both bolt spacings. 

Essentially, these load-displacement curves and the values given in 

table 4 can be used to estimate the screen capacity and performance 

for the different gauge screen for both the 48 x 48-in and 60 x 60-in 

square bolt patterns. This information is for screen evaluation and 

selection as well as design requirements. 

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 5 10 15 20 25

Displacement, in 

Load, lb

4 Gauge

6 Gauge

8 Gauge

9 Gauge

10 Gauge 

A. 48 x 48-in Bolt Spacing

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 5 10 15 20 25

Displacement, in

Load, lb

B. 60 x 60-in Bolt Spacing

Figure 9. Linear load-displacement curves that can be used in the 

evaluation and selection of welded wire screen for a bolt spacing of A) 

48 x 48-in and B) 60 x 60-in. These curves are based on the average 

design load and displacement.

With increasing wire diameter, the design load and in general the 

stiffness increase. For the most common screen used in U.S. coal

mines, the 8-gauge wire screen with a 4 x 4-in wire spacing, the design 

load is just over 2,000 lb at a displacement of 12.6 in for the 48-in bolt 

spacing. In general, the 8- and 9-gauge screens are fairly close in 

performance characteristics with the thinner wire screen obviously 

having less strength and stiffness. The large difference in strength and 

stiffness occurs between screens with 6- and 8-gauge wires, where the 

capacity of the 6-gauge screen is approximately twice that of the lighter 

8-gauge screen. 

CONCLUSIONS 

Laboratory tests were conducted to develop load-displacement 

characteristics that could be used in the evaluation of the performance 

of welded wire screen. In these tests, the wire size and configuration, 

bearing plate loads and bolt spacing was varied. Screen capacities 

are based on the design load and displacement. The design load and 

displacement is the point on the load-displacement curve where there 

is a significant reduction in screen stiffness. 

Bearing plate loads determined whether the screen slipped or 

was fixed. In general, a fixed screen will have a lower design load and 

displacement. When a screen is fixed, the capacity will be limited by 

wire breakage, and if allowed to slip, the capacity will be controlled by 

friction and the bearing plate load. Therefore, the bearing plate loads 

affect the screen capacity and performance. 

Bolt spacing primarily affects the amount of screen displacement 

that occurs. Changing the bolt spacing from 48 to 60-in increased the 

screen displacement by 33 pct. Essentially, the wider bolt spacing 

reduced the screen stiffness thus resulting in more displacement for 

the same load. The wider bolt spacing does not appear to alter the 

screen load capacity. 

For the different wire gauges, there is a linear relationship 

between wire cross-sectional area per foot and the screen load 

capacity. However, the rate of increase will vary depending on 

whether the screens are fixed or slip at the bearing plates. When the 

screens slip, the load capacity increases at a rate of about 29,000 

lb/in2

. At the highest bearing plate loads where the lighter gauge 

screens are fixed and the heavier gauge screens slip, the rate of 

increase is nearly 60,000 lb/in2

. 

An 8-gauge welded wire screen with a 4 x 2-in wire spacing 

weighs slightly more than a 6-gauge screen with a 4 x 4-in wire 

spacing. However, depending on the bearing plate load, the 8-gauge 

screen will either have an equal or lower load capacity than the 6-

gauge screen. Therefore in most situations, the 6-gauge screen 

should be used rather than the 8-gauge screen with a 4 x 2-in wire 

spacing. 

With the addition of reinforcing wires at each end of a welded 

screen, improved performance only occurred when the screen was 

fixed. In this case, the design load and stiffness were significantly 

increased. This was the result of a third wire under the bearing plate. 

No increase in the load capacity occurred when the screen was 

allowed to slip. However, if some additional support may be required, 

consideration should be given to adding the reinforcing wire, because 

there is potential for significant improvement in screen performance 

with only a minimal increase in cost. 

Design loads and displacements were established for the welded 

wire screen for each wire gauge and for two bolt spacings. The design 

loads and displacements are the suggested maximum values for 

screen capacity. The screen stiffness and offset displacement were 

also developed for each wire gauge. This information can be used for 

screen selection and in the evaluation of the underground screen 

performance. 

ACKNOWLEGEMENTS 

The author thanks NUCOR Wire Products Pennsylvania Inc, New 

Salem, PA, for providing the welded wire screen used in this study. 

Company personnel also gave NIOSH researchers a tour of their 

welded wire screen manufacturing facilities that provided useful 

information in understanding the issues regarding the design and 

manufacturing of the welded wire screen. 

REFERENCES 

ASTM A-497-99, (2004), "Standard Specification for Steel Welded 

Wire Fabric, Deformed, for Concrete Reinforcement," pp. 2452-

2460. 

ASTM A 823-97a, (2004), "Standard Specification for Steel Wire, Plain, 

for Concrete Reinforcement, pp. 121-124. 
Compton, C.S., S. Gallagher, G.M. Molinda, C. Mark and G. Wilson, 

(2007), "Roof Screening for Underground Coal Mines: Recent 

Developments," Proceedings of the 26th International Conference 

on Ground Control in Mining, Morgantown, WV, pp. 270-276. 

Dolinar, D.R., (2006), "Load Capacity and Stiffness Characteristics of 

Screen Materials Used for Surface Control in Underground Coal 

Mines," Proceedings, 25th International Conference on Ground 

Control in Mining, Morgantown, WV, pp. 152-158. 

Itasca Consulting Group, (2005), FLAC3D. Fast Lagrangian Analysis of

Continua in 3-Dimensions, Version 3.0, Minneapolis, MN. 

Murali, G.M., J.A. Rusnak, and J.W. Honse, (2006), "Behavior of 

Welded Wire Mesh Used for Skin Control in Underground Coal

Mines," Proceeding of the 25th International Conference on 

Ground Control in Mining, Morgantown, WV, pp. 142-151. 

Pakalnis, V. and D. Ames, (1983), "Load Tests on Mine Screening. 

Underground Support Systems," Canadian Institute of Mining, 

Metallurgy and Petroleum, Special Vol. 35, pp. 79-83. 

Robertson, S.B. and G. Hinshaw, (2001), "Roof Screening: Best 

Practices and Roof Bolting Machines," Proceedings of the 21st 

International Conference on Ground Control in Mining, 

Morgantown, WV, pp. 189-194. 

Tannant, D.D., (1995), "Load Capacity and Stiffness of Welded-Wire 

Mesh," Proceedings of the 48th Canadian Geotechnical 

Conference, Vancouver, Canada, pp. 729-736. 

Thompson, A.G., C.R. Windsor and G.W. Cadby, (1999), 

"Performance Assessment of Mesh for Ground Control 

Applications," Proceedings of the International Symposium on 

Ground Support, Kalgoorlie, Australia, pp. 119-130. 

Wire Reinforcement Institute, Inc., (2008), "Structural Welded Wire 

Reinforcement," Manual of Standard Practice, Hartford, CT, 36 

pp. 
APPENDIX 

Table 3. Results of the tests conducted on the welded wire screen. 

parentheses. 

Values are averages for each test series with standard deviations given in 

Wire 

gauge 

Plate load 

(103

 lb) 

Design 20 pct of design load Displacement 

offset (in) Stiffness (lb/in) 

Peak 

Load (lb) Displacement 

(in) Load (lb) Displacement (in) Load (lb) Displacement (in) 

Bolt Spacing 48 x 48 in 

4 4.5 3,540 (290) 20.1 (1.0) 710 5.3 1.6 (0.3) 192 (13) 3,610 20.7 

4 8.0 4,090 (150) 17.0 (1.0) 820 5.4 2.6 (0.2) 291 (20) 4,846 18.6 

4 15.0 5,830 (160) 17.2 (0.1) 1,170 6.2 3.4 (0.2) 419 (23) 6,590 19.3 

4 25.0 5,290 (320) 15.0 (0.7) 1,060 5.6 3.2 (0.3) 448 (43) 5,290 15 

6 4.5 2,920 (470) 17.3 (1.7) 580 5.9 3.0 (0.2) 202 (13) 3,140 19.5 

6 8.0 3,490 (390) 16.0 (0.7) 700 6.0 3.6 (0.3) 284 (22) 4,320 19.0 

6 15.0 4,830 (390) 15.6 (0.4) 990 6.7 4.4 (0.2) 432 (32) 4,990 16.1 

6 25.0 4,880 (950) 13.7 (1.6) 980 6.1 4.2 (0.3) 514(39) 4,950 14.1 

8 4.5 1,780 (280) 15.0 (1.6) 360 6.1 3.8 (0.1) 158 (4) 2,100 19.4 

8 8.0 2,430 (250) 14.0 (0.6) 490 6.4 4.5 (0.3) 258 (22) 2,640 14.6 

8 15.0 2,080 (420) 11.3 (1.4) 420 5.8 4.4 (0.2) 314 (11) 2,380 13.6 

8 25.0 1,790 (120) 10.0 (0.1) 360 5.1 3.9 (0.1) 295 (5) 2,020 11.2 

8R1

8.0 2,500 (160) 14.8 (0.6) 500 6.1 4.0 (0.1) 231 (7) 2,500 14.8 

8R1

15.0 2,560 (210) 13.2 (1.2) 510 6.0 4.2 (0.1) 284 (18) 2,630 13.6 

8R1 25.0 2,950 (470) 12.3 (1.4) 590 6.0 4.4 (0.1) 372 (16) 3,120 13.1 

8T2 8.0 3,570 (470) 16.0 (2.1) 710 6.3 3.9 (0.1) 296 (12) 4,820 20.7 

8T2

15.0 3,660 (20) 13.4 (0.2) 730 6.0 4.2 (0.1) 397 (15) 4,240 17.8 

8T2

25.0 2,720 (170) 10.4 (0.1) 540 5.0 3.6 (0.1) 405 (25) 2,720 10.4 

9 4.5 1,590 (260) 15.0 (0.9) 320 6.8 4.8 (0.8) 154 (20) 2,040 19.1 

9 8.0 2,290 (250) 14.0 (1.3) 460 6.8 5.0 (0.3) 252 (14) 2,450 14.9 

9 15.0 1,630 (130) 10.6 (0.4) 330 5.8 4.6 (0.2) 271 (27) 2,130 14.0 

9 25.0 1,750 (280) 11.4 (1.1) 350 5.6 4.1 (0.1) 241 (9) 1,890 13.3 

10 4.5 1,310 (200) 13.5 (1.1) 260 6.3 4.5 (0.1) 145 (15) 1,760 17.6 

10 8.0 1,810 (330) 12.0 (1.0) 360 6.7 5.4 (0.3) 275 (28) 2,020 13.4 

10 15.0 1,200 (320) 10.7 (1.4) 240 5.8 4.5 (0.3) 187 (23) 1,300 11.6 

10 25.0 1,120 (70) 10.1 (1.2) 220 5.3 4.2 (0.2) 201 (29) 1,170 11.0 

Bolt Spacing 56 x 56 in 

8 25.0 1,620 (30) 11.6 (0.4) 330 6.3 5.1 (0.1) 252 (18) 1,740 12.9 

9 25.0 1,710 (280) 12.8 (0.7) 380 7.1 5.4 (0.2) 232 (21) 1,970 14.3 

10 25.0 1,210 (100) 11.1 (0.5) 240 6.6 5.7 (0.1) 224 (14) 1,210 11.1 1

The screen is reinforced with 2 extra wires in the transverse direction at each end of the screen. 2

The wire spacing is 4 x 2-in with the 2-in spacing for the transverse wires. 

Table 4. 

screen. 

Design loads, displacements and performance characteristics for various wire gauges used for the evaluation and selection of welded wire

Wire 

gauge 

Design Stiffness (lb/in) Offset Load (lb) Offset 

Displacement (in)

20 pct of Design Load 

Load (lb) Displacement (in) Load (lb) Displacement (in) 

48 x 48-in Bolt Spacing 

4 4,690 17.3 320 -860 2.7 940 5.6 

6 4,030 15.7 340 -1,270 3.8 810 6.2 

8 2,020 12.6 240 -1,000 4.0 400 5.9 

9 1,820 12.7 220 -1,040 4.7 360 6.3 

10 1,360 11.6 200 -920 4.7 270 6.0 

60 x 60-in Bolt Spacing 

4 4,690 23.0 240 -860 3.6 940 7.5 

6 4,030 20.9 260 -1,270 5.0 810 8.4 

8 2,020 16.8 180 -1,000 5.3 400 7.8 

9 1,820 16.9 170 -1,040 6.2 360 8.4 

10 1,360 15.4 150 -920 6.2 270 8.0