LOAD CAPACITY AND STIFFNESS CHARACTERISTICS OF SCREEN 

MATERIALS USED FOR SURFACE CONTROL IN UNDERGROUND COAL 

MINES 

Dennis R. Dolinar, Mining Engineer 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA, USA 

ABSTRACT 

Screen material in the form of welded wire mesh and geogrids 

are used in und erground co al mines to preven t the fall of small 

pieces of rock f rom the roof betw een roof bo lts. Furth er, if the 

screen is installed during the production cycle, roof fall injuries can 

be r educed sig nificantly. Therefore, the Nat ional Institut e f or 

Occupational Safety and Health (N IOSH), b ecause of th e safety 

implications, conducted an evaluation of screen mater ials 

commonly us ed in U.S . co al mines to d etermine the ir supp ort 

characteristics and identify the param eters that could affe ct their 

performance wi th res pect to con trolling the f all of rock from th e 

roof surface. 

To evaluate the load and stiffness characteristics of the screen, 

a test frame was designed and installed in the Mine Roof Simulator 

(MRS) at the NIOSH Pittsburgh Research Laboratory (PRL). With 

this test set up, the scr een is bolted at four corners of the frame and 

a center load is applied. The test set up allows for up to 20 in of 

screen d eflection while the load and scr een d efection are 

continuously recorded. 

A series of tests were conducte d to ev aluate the effects of 

various par ameters such as bolt te nsion, the type of load bearing 

surface and th e s ize of b earing plat es on welded s creen 

performance. The most common screen material used in U. S. coal 

mines is an 8-gauge wire welded in a 4-by-4-in spacing or aperture. 

The peak screen load was normally limited by wire breakage. The 

conditions at th e bearing plate in fluence the nature of the w ire 

breakage. However, the screen stiffness was controlled by slippage 

at the bearing plates, and by weld and wire failure. The size of the 

bearing pl ate ha d a s ignificant effect on th e p erformance of th e 

welded s creen. Ther efore, the des ign cap acity of the 8-gau ge 

screen is evaluated based on plat e size. For a 6-b y-6-in bearing 

plate the average peak load is 2,900 lb with a stiffness of 250 lb/in. 

For an 8-b y-8-in bear ing p late, the av erage p eak lo ad is 4,500 lb 

with a stiffness of 430 lb/in. These test results are based on a 4-by4-ft bolt spacing. A geogrid mesh was also tested. 

INTRODUCTION 

Each year in underground U.S. coal mines, ground falls cause 

between 500 to 600 injuries and four to five fatalities. About 8 0 

percent of the injuries and 10 to 15 percent of the fatalities are the 

result small rock pieces falling between roof bolts. When installed 

during the pro duction cycle, the roof scr een can signif icantly 

reduce these types of ground fall injuries (Rob ertson and Hinshaw 

2001). As a result of the safety implications, the National Institute 

for Oc cupational Saf ety and H ealth (NIOSH) has an interest in 

promoting the u se of roof scr een to redu ce the risk of ground f all 

accidents. Therefore, a testing program is b eing conducted at the 

Pittsburgh Res earch L aboratory (PRL) to evaluate th e l oad 

performance characteristics of roof screen. 

Tests to evaluate the perform ance characteristics of various 

types of screen or mesh h ave 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-by-4-in wire spacing. The 

screen was installed and tested with the bolts in a diamond pattern 

with respec t to the scr een w ire conf iguration. Th ese t ests 

established the genera l load-d isplacement b ehavior of the screen. 

Further, a relationship was also developed between the wire g auge 

or diameter and the screen load capacity. 

Another later stud y also c onducted in Can ada, ev aluated 

welded s creen performance on a l aboratory t est fram e (T annant 

1995). A center load was applied to th e s creen with the screen 

tested in bo th square and d iamond 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 slipp age 

of the screen at the plates was n oted during these tests, it could be 

controlled b y the bolt torque. P eak load c apacities we re 

determined for e ach g auge of wir e wi th the load capacity 

increasing wi th wire d iameter. S creen s tiffness ch anged 

significantly wi th orien tation o f the wir es wit h respect to th e 

bolting pattern. 

A test fram e was also built in Australia to evaluate w elded 

wire screen per formance and the various parameters that could 

affect that perfo rmance (Thompson et al., 1999). Again, a center 

load was applied to the screen. The screen had a wire diameter of 

0.22 in (4 gaug e) and a wire spacing of 4 in . This is th e most 

commonly used screen in the Australian mining industry. In these 

studies, var ious bolt spacings were used with th e weld ed scr een 

placed either in a square or diamond orientation with respect to the 
 

bolts. Bolt loads and bearing plate sizes were also varied . The primary con clusions from the s tudy wer e that the st iffness is a 

function of the bolting pattern and screen configuration. Further, slippage of the screen at the bearing plates will affect the stiffness. 

In this NIOSH study, the performance characteristics of an 8-

gauge weld ed s creen is evaluated using a laboratory test fram e. 

The scr een is test ed in a configuration that sim ulates t he installation in U.S. coal mines. The 8-gauge welded screen is the 

most commonly used mesh in the U.S. co al in dustry. However, 

none of th e previous studies have tested this gauge of scr een nor 

has screen manufactured in the U.S. been tested. To compliment 

the evaluation of the 8-gauge welded screen, a geogrid mesh was 

also tested. 

EXPERIMENTAL SET UP AND TEST PROCEDURE 

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

that was design ed wi th the c apability o f v arying th e bo lt spac ing 

from 4 to 5 ft with 4 bolts used to attach the s creen to the frame 

(figure 1). A one-foot-square load plate with rounded corners was used to apply the load to the center of the screen. With the MRS, 

the screen could be displaced up to 20 in. The tests wer e run in displacement control with a displacement rate of 2 in per m in. A typical test would take approximately 10 min. 

Figure 1. Test frame set up used to test the screen material. 

Bolt spacing is 4-by-4-ft. 

The load was m easured us ing a 20,000 lb ext ernal load ce ll 

with an a ccuracy o f ± 20 lb. The s creen displacement was 

monitored using the linear var iable differential transformer for the 

MRS control s ystem with an accuracy of ± 0.0 1 in. During th e 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 and either 6-by-6-in or 8-by-8-in in size 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. Two types of load-bearing surfaces were used for 

the tests, either a steel or wood plate. These plates were one fo ot square and installed under the screen. 

The screen was placed in a square configuration with res pect 

to the test frame and bolts (figur e 2). This is similar to the typical 

installation in an underground coal mine. With this arrangement, 

for the weld ed screen, load wa s trans ferred fro m the cent er lo ad 

plate to th e bolts through the center wires crossing the lo ad plate 

 then to the eight wires that directly connect the bolts and bearing 

plates. The welded screen was sized to allow for a one-square (4 in) 

extension beyond the bolts on all sides. For the geogrid mesh, the overlap beyond the bolts was approximately 6 inches. 

 

Bearing Plate

Load Plate

Bearing Plate 

Load Plate 

Figure 2 . Sch ematic of s creen test configur ation with a squar e 

bolting p attern with respe ct to the scre en. H eavy bo ld l ines 

indicate the wires connecting the bolts and the dashed lines the 

wires cros sing t he load pla te. The h eavy arrows indicat e th e 

primary lo ad tr ansfer direction s along the wir es from the loa d 

plate to the bolts. 

ROOF SCREEN MATERIAL 

An 8-gauge wire weld ed screen with a 4-by-4-in wire spacing 

was used for most of the te sts. The screen wire has a nominal wire 

diameter of 0.162 in . For welded screen which is used to 

reinforcement concrete ther e is no over all str ength r equirement. 

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 pounds-force shall not b e 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). The area of the 8-gauge wire is 0.02 square inches, resulting in a 

minimum shear weld strength of 700 lb. The tensile strength of the 

wire must exceed 75,000 psi ( ASTM A 83-97 a, 2004) . For the 

screens te sted, the wire te nsile strength based on the 

manufacturer’s specifications was about 89,000 psi. 

A geogrid mesh that was made from polypropylene was also tested. The aperture dimensions were 1-in in the machine direction 

(MD) and 1.2-i n in the cross m achine dir ection (XM D) (Tens ar Earth Technologies 2005). The minimum rib thickness was 0.07 in. 

The ultimate ten sile strength was 1,850 lb p er one ft wid th (MD) 

and 2,050 lb per one ft width (XMD). 

EXPERIMENTAL DESIGN 

153 

In this particular series of tests, a bolt spacing of 4-by-4-ft was 

maintained. All screens were displaced to 20 in. For the welded 

wire screen, the factors that were varied included the load surface 

of the t est fram e, the siz e o f the b earing pl ate, and the tensio n 

applied to the bolt. The load surface was either wood or steel with 

either a 6-by-6-in or 8-by-8-in bearing plate. The amount of torque 

applied to the bolts was either 100, 150 or 2 00 ft-lb . From a 
 

 

 

torque-tension test on the bo lts utilizing a load cell, a conversio n 

factor of 57.5 lb/ft-lb was determined. This result ed in 

approximately 5,750, 8,625, or 11,500 lb of load being applied to 

the bolts and be aring pla tes from the torque . Because th e bol ts, 

nuts, washers and installation methods will be different in a mining 

situation th an those used in th is experiment, the torqu e-tension 

ratio will also be different. The combination of parameters used in 

the tests is shown below: 

8 gauge welded screen 

Series 1. 6-by-6-in bearing plates, steel surface, 5,750 lb of bolt 

tension, 

Series 2. 6-by-6-in bearing plates, steel surface, 8,625 lb of bolt 

tension, 

Series 3. 6-by-6-in b earing pla tes, wood surface, 5 ,750 lb of 

bolt tension 

Series 4. 6-by-6-in b earing pla tes, wood surface, 8 ,625 lb of 

bolt tension, 

Series 5. 6-by-6-in be aring plat es, wood surface, 11,500 lb of 

bolt tension, 

Series 6. 8-by-8-in b earing pla tes, wood surface, 5 ,750 lb of 

bolt tension, 

Series 7. 8-by-8-in b earing pla tes, wood surface, 8 ,625 lb of 

bolt tension, 

Series 8. 8-by-8-in be aring plat es, wood surface, 11,500 lb of 

bolt tension. 

Geogrid Mesh 

Series 9. 6-by-6-in b earing pla tes, wood surface, 8 ,625 lb of 

bolt tension. 

The number of screens tested for each series varied from 3 to 5. 

TEST RESULTS 

A load-displacement curve for one test is show n in figure 3. 

From s uch grap hs s everal m easurements ar e ob tained r elevant t o 

the screen performance. The peak load is the maximum load ju st 

prior to a signif icant drop in load. In some cas es, there may be a 

higher load that occurs after this point but beyond this initial peak 

load, the screen behavior is not consistent. Th e large drop in load 

indicates that the screen had some significant damage such as wire 

breakage. The yield load is identified as the point where there is a 

significant ch ange in beh avior from a gener al elastic screen 

response to inelastic behavior. 

Screen stiffn ess is determined based on th e slope of a line 

from the peak load to a point at 25 percent of peak load (figure 3). 

The screen stiffness is calculated from the following equation: 

Ks = (Lp-L25)/(Dp-D25) 

where K s = screen stiffness, lb/in,

 Lp = peak load, lb, 

L25 = load at 25 percent of peak load, lb, 

Dp = displacement at peak load, in, 

D25= displacement at 25 percent of peak load, in. 

A displacement offset (Do) is determined as the intersection of 

the line used to ca lculate th e stiffness and the x-axis (figu re 3 ). 

The offset is the amount of defo rmation that will occur b efore the 

screen begins to resist the load significantly. 

3600 

3200 

2800 

2400 

LOAD, lb 2000 

1600 

1200 

800 

400 

Peak Load 

Yield Load 

25 pct of Peak Load 

Displacement Offset 

0

0 2 4 6 8 10 12 14 16 18 20 

DISPLACEMENT, in 

Figure 3. Load-displacement curve for a test on a welded 

screen showing key parameters used to ev aluate the s creen 

performance. 

Table 1. Results of screen tests conducted in the MRS. Values are averages for each test series 

with the standard deviations given in parentheses. 

Peak 25 pct of peak Offset Yield Torque Peak load Stiffness Yield load Test displacement Load Displacement displacement displacement (ft-lb) (lb) (lb/in) (lb) (in) (lb) (in) (in) (in) 

Welded Screen/6-by-6 Plate/Steel Load Surface 

Series 1 100 2780(467) 18.6 695 7.3 3.5(0.4) 186(36) 820(27) 7.7 

Series 2 150 2890(416) 18.2 723 7.4 3.4(1.4) 202(37) 1460(277) 9.3 

Welded Screen/6-by-6 Plate/Wood Load Surface 

Series 3 100 3283(782) 19.2 821 8.2 4.9(0.8) 230(75) 1033(153) 9 

Series 4 150 3563(475) 16.7 884 8.1 5.4(0.2) 313(20) 1838(309) 10.5 

Series 5 200 2100(265) 11.3 525 6.5 4.9(0.2) 325(30) 1800(300) 10.3 

Welded Screen/8-by-8 Plate/Wood Bearing Surface 

Series 6 100 5533(350) 19.7 1383 10.8 7.7 452(36) 683(76) 7.3 

Series 7 150 5467(115) 19 1333 9.5 6.6(0.2) 435(24) 1733(306) 10.4 

Series 8 200 2450(10) 10.8 612 6.1 5.2(.06) 398(59) 2216(404) 10.3 

Geogrid/6-by-6 Plate/Wood Load Surface 

Series 9 150 1308(298) 13.1 327 9.8 8.2 (0.9) 234(41) 1308(298) 13.1 
Table 1 gives the av erage peak and yield loads and 

displacements along wi th th e ca lculated stif fness and offs et 

displacements for each series of tests. The standard deviations for 

specific p arameters ar e given in par entheses. Load-displacement 

curves for specific tests are shown in figures 4 to 6. 

Figure 7. Damage to welded screen in the form of broken 

wires. The broken wires connect the bolts and bearing plates. 

directly connecting the bolts and bearing plates. 

The dam age to each s creen w as als o as sessed b y not ing th e 

number and location of the weld and wire failures. Figure 7 shows 

typical wire breakage to a scr een. Figure 8 shows the average 

number of weld and wire failures per test for each test series. Of a 

total of 104 wire f ailures, 102 occurred along th e eigh t w ires. 

 

30

Wires
 

25 

Welds 

AVERAGE NUMBER OF FAILURES 20 

15

10 

5 

0 

 6x6/Wood 6x6/Wood 6x6/Wood 6x6/Steel 6x6/Steel 8x8/Wood 8x8/Wood 8x8/Wood 

100 ft-lb 150 ft-lb 200 ft-lb 100 ft-lb 150 ft-lb 100 ft-lb 150 ft-lb 200 ft-lb 

Figure 8. Amount and type of damage noted in each test series. 

The solid bars indicate the average number of broken wires and 

the hatched bars the average number of broken welds per test. 

LOAD, lb 1000 

1500 

2000 

2500 

3000 

3500 

4000 

100 ft-lb 

150 ft-lb 

200 ft-lb 

500 

0 

0 2 4 6 8 10 12 14 16 18 20 

DISPLACEMMENT, in 

Figure 4. Load-displacement curves for screens tested with a 6-

by-6-in bearing plate, wood load surface and bolts torques of 100, 

150 and 200 ft-lb. 

 

6000 

5000 

100 ft-lb
 

150 ft-lb
 

200 ft-lb
 

4000 

LOAD, lb 3000 

2000 

1000 

0
 

0 2 4 6 8 10 12 14 16 18 20 

DISPLACEMENT, in
 

Figure 5. Load-displacement curves for screens tested with an 8-

by-8-in bearing plate, wood load surface and bolt torques of 100, 

150 or 200 ft-lb. 

 

 

3500 

3000 

2500 

100 ft-lb 

150 ft-lb 

Geogrid 

LOAD, lb 2000 

1500 

1000 

500
 

0
 

0 2 4 6 8 10 12 14 16 18 20
 

DISPLACEMENT, in
 

Figure 6. Load-displacement curves for screens tested with a steel 

load surface and bolt tor ques of either 100 or 150 ft-lb with a 6-

by-6-in bearing plate. The load-displacement curve for a geogrid 

mesh with a 6- by-6-in b earing plate, a wood lo ad s urface and a 

bolt torque of 150 ft-lb is also shown. 

DISCUSSION OF RESULTS 

The cond itions at the b earing plat es were v aried for th e 

welded wire test series. These conditions determine the yield load 

and the post- yield behavior as w ell as the p eak load. The geogrid 

mesh was tested to prov ide a c omparison to th e 8-gaug e we lded 

screen. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Welded Wire Screen Behavior, Yield, 

and Failure Mechanisms 

For the tests on the welded screen, because the load is applied 

to th e c enter of th e s creen and th e s creen is s quare with th e test 

frame and the bolt pattern, there are no wires connecting directly 

from the applied load to the bolts and bearing plates that hold the 

screen (figure 2). The load transfer occurs primarily from the wires 

that cross over the center load plate to the center portion of the 

eight wires that connect the bolts and bearing plates. Th ese eight 

wires become critical to load transfer. As a result, distortion and 

shearing of the wire squares especially along the line from the bolts 

to the center load plate occurs (f igure 9). The distortion can result 

in weld failure. If there are wires running directly from the applied 

load to the bolts and plates, such as when a diamond bolting pattern 

is used, the failure m odes will be altered (Tannant 1995). Under 

such loading conditions wire breakage is the dominate failure mode. 

Figure 9. Shear distortion of wire squares and slippage of wire 

from under the plates. 

During th e initial load ing, th e scre en shows a soft lo addisplacement re sponse becaus e the wires lay horizon tally w ith 

respect to the load d irection and cannot resist th e load. As the 

screen g eometry chang es during loading , th e wi res becom e m ore 

vertical and the refore can res ist the lo ad m ore effectively. This 

results in an increase in stiffness as more load is applied (figure 3). 

As the yield point is approached, the stiffness may remain the same 

or decrease slightly as a result of approaching failure. However, up 

until yield, th e load-displacement curv e is f airly sm ooth. At the 

yield point, a change in behavior occurs, caused by either slippage 

of the screen under the bearing plates or from the failure of one of 

the eight wir es connecting th e bolts that secur e the scr een to the 

test fram e (f igure 3). This fa ilure or slipp age will resul t in a 

sudden drop in load. However, welded screen, when wires break 

has the capability of transferring the load to other wires or when a 

sudden drop in load occurs from slippage the plate friction and load 

are s ufficient ag ain to al low for the load to bui ld ba ck up again. 

This periodic slippage or wire failure produ ces the saw tooth ed 

behavior that continues to the end of the test. Fu rther, weld failure 

will occur during the post-yield phase that will also cause a sudden 

load drop. However, the larger drops are us ually associated with 

wire break age. It is the br eakage of the wir e t hat will g enerally 

limit the peak or maximum load that the screen develops. Slippage 

and weld breaks produce incr eased def lection o r soften ing of the 

screen’s load response. 

Evaluation of Results 

Factors that were varied in the tests included the bearing plate 

size, bolt torque, the load surface. The modification of the bearing 

plate cond itions alt ered th e s creen p erformance b y changing t he 

yield point, post-failur e behavior and ultimately the peak load and 

stiffness. 

Changing the load surface and bolt torque, affected the degree 

of slippag e. Slippage was th e dominate fo rm of post- yield 

displacement for the steel load surface and for a bolt torque of 100 

ft-lb (figure 6) . Incr easing torq ue and chang ing to a wood lo ad 

surface in creased th e yield loads (Tab le 1). The p eak lo ads d id 

appear to in crease with th e change in th e load surfac e bu t on ly 

slightly w ith ad ditional p late load. True pe ak loads for th e st eel 

load surface especially at a 100 ft-lb of bolt torque were not always 

achieved because of the 20 in displacement limit of the MRS. The 

reduced slippage and therefore displacement with the change to a 

wood load surf ace and in creased bolt torque resulted in high er 

screen stiffn ess. Further , the d egree of damag e through 20 in of 

displacement is als o s ignificantly altered b y the s lippage at the 

plates. The least damaged screens were those with a to rque of 100 

ft-lb for either a steel or wood surface (figur e 8). However some 

slippage still occurred at the highest torque level. 

For test series 5 and 8 where the bolt torque was increased to 

200 ft-lb with the wood surface, the wire squares directly under the 

bearing plates did not move with respect to the test frame and load 

surface and th ese s quares wer e not distorte d. Esse ntially, the 

screen was fixed under the plates. Slippage did occur to the wires 

leading f rom th ese fixed s quares to th e edge of th e plate. As a 

result of the screen being fixed the peak load dropped significantly 

to an average of 2,275 lb while the average number of broken wires 

was at least double that of any other tests (figur e 8.) Essentially, 

the yield load is controlled and yield behavior is dominated by wire 

breakage wher e the s creen is f ixed (figur es 4 and 5). W ithout 

sufficient slippage at the plates, the system may be to stiff resulting 

in higher load concentr ations in sections of th e wires connecting 

the bolts thus causing early failure of these wires. Th erefore, the 

failure of th e wires will d etermine the yield and peak load and the 

post-yield behavior when the screen is fixed and there is little or no 

slippage to the wire squares directly under the bearing plates. The 

peak loads for all these tes ts are s imilar and may r epresent t he 

lowest screen strength that could be expected. 

At the two lower bolt torque levels where there was sufficient 

screen s lippage at the be aring plates, the larger 8-by-8-in be aring 

plate significantly altered the screen performance over that of the 

smaller plates. For the l arger plate, an average peak load of over 

5,400 lb was achieved about 2,000 lb greater than the average peak 

loads for the 6-by-6-in plate with similar bolt torques. Further, the 

stiffness on average is increased by 40 to 95 percent depending on 

the torqu e. Th e la rger p late p rovides be tter c overage ac ross t he 

wires and therefore the load is distributed more uniformly to more 

wires than the 6-by-6-in bearing plates while the wire lengths are 

shortened thus reducing the disp lacement. Also, the larger plates 

appear to be able to provide better control over the slippage. 

The geogrid mesh tested h as a lower peak load (1,308 lb) and 

stiffness (234 lb/in) than the 8 -gauge welded wire scr een tested 

under similar conditions (series 4). Th e peak load is at th e yield 

load while there is no slippage at the plate. At yield the geogrid 

mesh fails in shear or b y tearing along the edge of th e p lates 

resulting in a sudden drop in load (figure 10). The initial damage is 

sufficient to pr event any substantial in crease in load above the 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

yield load with further displacement. Further tests of other geogrid 

mesh are neces sary to identify thos e that m ay hav e a s imilar 

capacity to the 8-gauge screen and to id entify the factors that can 

affect geogrid mesh performance. 

Figure 10. Geogrid mesh sheared along the bearing plate. 

DESIGN CAPACITY OF 8-GAUGE WELDED SCREEN 

To a la rge extent for t he welded 8-gauge screen, the bearing 

plate conditions control the p erformance of the screen in terms of 

peak load and s tiffness. Exc ept for the ch ange in plate size, such 

factors as th e bolt torque level and load surf ace will probab ly not 

be able to be controlled suffi ciently in a m ine to a llow f or 

consistent results. Therefore, for general design purposes, the peak 

strength and stiffness should be based on an average of all the test 

results for the same plate size. 

For an 8-gauge screen with a 6-b y-6-in bear ing plate for 

design purposes, based on twenty tests from series 1 to 5, the 

average peak load (Lp) is 2,900 lb, the average stiffness (Ks) is 250 

lb/in and the offset displacement (Do) is 4 .4 inches. The standard 

deviation for the peak load is 660 lb, for th e stiffness, 64 lb/in and 

for the offs et di splacement 0.09 in. Th e p eak l oad is the d esign 

capacity of the 8-gauge screen for the 6-by-6-in bearing plate. 

When an 8-by-8-in bearing plate is used the average peak load 

(Lp) is 4 ,500 lb , th e ave rage st iffness (Ks) is 43 0 lb /in and offs et 

displacement (Do) is 6.5 in . The standard dev iation fo r the peak 

load is 1 ,540 l b, fo r the stiff ness, 45 lb /in and for the off set 

displacement, 1.1 in. The average values for the 8-by-8-in bearing 

plates are based on nine tests. 

The total displacement at any point up to the peak load can be 

approximated from the following equation: 

Dt = Do + Ls / Ks 

where D t = total screen displacement at given load, in,

 Do = displacement offset, in, 

Ls = screen load, lb, 

Ks = average stiffness, lb/in. 

The total displacement of the screen can be used for design if this is 

the limiting factor rather than the peak load. 

For the peak scr een loads developed above, there is no safety 

factor. A margin of safety can b e built into the design capacity by 

reducing the peak load by the standard deviation. Further, how the 

screen is installed with respect to the bolting pattern will affect the 

stiffness and d isplacement. With thes e tes ts, the s creen was 

installed wi th a squar e bo lting pat tern. There are no wir es 

connecting d irectly from the applied center load to th e bo lts 

(figure 2). If a diamond bolting pattern were used, there would be 

wires running directly from the center load to the bolts and p lates. 

As a result, scr een stiffness would be much high er with a d iamond 

bolting pat tern (Tannant 1995) . Howev er, th e squar e bo lting 

pattern wi th a c enter load represents a model for scr een 

performance that should be app licable to U.S. coal mines. The 

stiffness and offsets are also based on a 4-b y-4-ft bolting pattern. 

These par ameters will ch ange w ith a 5-b y-5-ft pattern. Anoth er 

limitation of these test results and design capacities for both plate 

sizes is that they are based on a center load and not a distributed 

load on the screen. 

CONCLUSIONS 

For this stud y, the t est fra me and set up were designed to 

evaluate s creen perform ance with regard to how the scre en is 

installed in U.S. coal mines. As a result, the screen was placed in a 

square configuration with respect to the test frame and bolt pattern. 

A cent er load was the appl ied to the scre en with a one-foo tdiameter plate. With this arrangement, no wires connected directly 

from the bolts to the center load plate. 

For welded wir e s creen, pl ate conditions in cluding the bol t 

torque, be aring plate s ize and l oad s urface will aff ect th e yield, 

peak load, and the stiffness. S lippage of th e screen at the bearing 

plate, which is controlled by bearing plate conditions including bolt 

torque, wi ll red uce the st iffness b y allowing more displa cement 

after yield. Ho wever, without sli ppage the peak load can be at or 

near the yield load because of wire breakage and the peak load will 

be less than if some slippage occurred. Fixing the screen under the 

bearing pla tes at th e corners alters the s creen behav ior. W ire 

breakage als o re duced the s creen s tiffness af ter yield. The m ost 

effective plate condition for enhancing screen performance was the 

use of a l arger bearing pl ate. The larg er be aring pl ate in creased 

both the screen peak load and stiffness significantly. 

The onset of yield is norm ally initiated by either slippage or 

wire br eakage with wire f ailure usual ly limiting the pe ak loa d. 

Depending on the test, extensive weld failur e occurred . Th ese 

failures in general did not appear to initiate yield but reduced the 

screen stiffness by increasing the displacement after yield. 

Except for the bearing plate size other plate conditions such as 

the bolt torqu e and load surfac e wi ll be dif ficult to contro l in a 

mine. Therefore, th e welded s creen p erformance cr iteria wer e 

developed based on plate size on ly and a 4-by-4-ft bolting pattern. 

For the 8-gaug e screen, with a 6-by-6-in b earing plate, the peak 

load is 2 ,900 lb with a stiffness of 250 lb /in. Using an 8-b y-8-in 

bearing plate results in a the peak load of 4,500 lb with a stiffness 

of 430 lb/in. 

REFERENCES 

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

Wire Fabric, Deformed, for C oncrete Reinforcement, pp. 

2452-2460. 
 

 

 

ASTM A 83-97 a (2004). Stan dard Specif ication for Steel Wire, 

Plain, for Concrete Reinforcement. 

Pakalnis, V. and Ames, D. (198 3). Load tests on mine screening. 

Underground Support S ystems. Canadian Institute of Mining , 

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

Robertson, S.B . and Hinshaw , G. (2001). Ro of Screen ing: B est 

Practices and Roof Bolting M achines. Proceedings, 21st 

International C onference on Ground Control in Mining , 

Morgantown, WV, Aug. 6-8, pp. 189-194. 

Tannant, D .D. ( 1995). Load C apacity and Stif fness of WeldedWire M esh. P roceedings, 48 th C anadian Geotechnical 

Conference Vancouver, pp. 729-736. 

Tensar Earth Technologies (2005). Product Specifications, Biaxial 

Goegrid Bx 1500. 

Thompson, A.G., Windsor, C.R. a nd C adby, G. W. ( 1999). 

Performance Assessment of Mesh for Ground Control 

Applications. Proceedings, of the International Symposium on 

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