1. Author: Tikeshwar Mahto,
Dy. Manager,
RG OCP-II (SCCL)
ECONOMICAL AND SAFE DESIGN OF ROOF BAR (GIRDER) FOR STRATA CONTROL
IN UNDERGROUND MINES TO EXTRACT THICK SEAMS-
-A Case Study of BG- Method
Abstract
Blasting Gallery is a method of working to extract thick seams ( 8m - 15m ) in single
lift. Strata control mechanism is a very critical aspect of BG-method, as the height of
working is more than 10m. Natural support has very important role in overcoming
dynamic load created by the hanging goaf, particularly in case of massive sand stone
roof. Artificial supports are only for resisting separation of immediate roof. Hence,
design of natural support as well as temporary supports are very- very important for
the strata control point of view. In this paper, the author is concentrating on the
temporary supports used in Blasting Gallery Method. The author has critically
diagnosed about the drawbacks and failure of existing supporting system (roof bar)
and also suggested a modification for effective utilization of supports. If, the design
of roof bar as suggested by the author is implemented effectively, a huge amount of
rupees will be saved in purchase of roof bar every year and also an effective support
resistance can be developed for the safe working of BG-method.
CRITICAL STUDY OF FAILURE (OR PREMATURE YIELDING) OF ROOF BAR USED IN
BLASTING GALLERY
The roof bar of I- section used in BG working is loaded with different stresses like
direct stress(compressive stress), shear stress, bending stresses etc. and failure of
which is caused by either any one of these or due to combined effect of these
stresses. The roof bar of I – section is made of two different load bearing
components, web and flanges. Flanges are for bearing bending moment and bending
stresses and web is for resisting shear stress and direct stress (compressive stress).
Case study made by the author reveals that the failure of roof bar is due to bending of
flanges in centre and failure of web at the edges of the roof bar. The above-mentioned
failures of roof bar are due to faulty design & selection of roof bar. The author has
critically diagnosed about it, and has made some modification in design of roof bar,
which is mentioned below.
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2. Failuare of roof bar due to faulty supporting system:
Support assembly being practiced in BG- method is shown here.
Free body load diagram of support assembly of Fig.3 can be drawn in the following
ways for clear representation of diffirent forces acting on support assembly
Where, AB a M.S. roof bar
L1, L2 concentrated reactive forces (loads) on the
roof bar, and
R1, R2 support resistances offered by the O.C Props
(15-20Tons, )
2

3. Drawing shear force and bending moment diagram for the normal suppoeting system:
(i) Loaded roof bar
V
+ve
+ve R1
B1
A B X
A1
-ve
-ve
R2
(ii) S.F.Diagram
Mb
R1C R1C
A x
A1 B1 B
(iii) B.M. Diagram
3

4. We can see from the free body load diagram and fig-3 in which the support
resistances (R1 & R2) offered by the Open Circuit props are directly acting on the steel
roof bar and not on the roof of the gallery, because there is no contact between roof
and roof bar. In this case total support resistance is utilized for bending the roof bar
and not for resisting rock load. When R1 & R2 increases, L1 & L2 also increases which
tries to bend the roof bar.
From, Fig-4, R1 + R2 = L1 + L2
It means total support resistances offered by O.C props are inversely transferred on
the roof bar, which tend to bend the bar. After yielding of roof bar, support resistance
decreases and adverse situations like, bed separation, side spalling, overriding,
props dislodgment etc., are created.
The roof bar assembly in yielded condition is shown in Fig.5
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5. Flexural strength calculation
This is for calculation of bending stresses in flanges of the roof bar due to bending
moment as calculated above.
The section of the roof bar and stresses in flanges of the bar can be drawn in the
following ways;
b
αc
ymax = d/2
t2 d
Neutral line N1-N2
αt
t1
From the theory of simple bending;
M/I = α/y =E/R
Where, b= flange width,
d= distance between the two flanges,
t1= thickness of flange,
t2= thickness of web,
αt = bending stress(tensile),
αc = bending stress (compressive) ,
N1- N2 = neutral line
M = moment of resistance or bending moment,
I = moment of inertia,
y = distance from neutral axis,
E = Young’s modulus, and
R = radius of curvature of internal surface of the deformed beam(roof bar).
Here, M/I = α/y
Or, α = M*y/I
So `α ` will be maximum or minimum when ` y` is maximum or minimum.
Thus , for y= 0 , α = 0 i.e. bending stress at neutral line is zero and bending stresses
at flanges are maximum.
Also ymax = d/2
αmax = (M/I) *ymax , or, αmax = Md/2I
Thus, maximum bending stress is at flanges of the roof bar, as shown in the figure
above.
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6. Moment of inertia(I) of I – section beam:
First, we will calculate moment of inertia of rectangular section beam of same
dimension.
t1
t2
N1 d N2
b
Moment of inertia of rectangular section = b*d3/12
Where, b= width of section of the rectangular beam,
d = height of section of beam.
t1 = thickness of flange of I- section beam,
t2 = thickness of web of beam.
N1-N2 = neutral line
Now cutting the dotted portion of the rectangular section, as shown in the above
figure for calculating moment of inertia (I) of I- section beam.
Hence, section of cut portion of the rectangular beam will be;
d - 2t1
N1 N2
b-t2/2
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7. So, M.I. of two cut portions about N1- N2 = 2* (b- t2/2)(d-2t1)3/12
= (b-t2)(d- 2t1)3/12
Thus, M. I. of I – section beam (girder) will be;
I = M.I. of rectangular beam – M.I. of cut portions.
Or, I = b*d3/12 – (b- t2)(d-2t1)3/12
Or, I = [ b*d3 – ( b- t2)(d-2t1)3]/12
Moment of resistance( bending moment ) can be taken from bending moment
diagram(B.M.D.), as drawn in previous page.
So, maximum bending moment is at centre of the beam(or roof bar );
Or, M = R*C
Where, M = bending moment
R = support resistance by O.C. props, and
C = mid- distance of cogs from the edge of roof bar.
Thickness of web (t2) : 7mm
Cross- section of the web[(d-2t1)*t2)] : 180mm*7mm
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8. MODIFICATION IN SUPPORTING SYSTEM SUGGESTED BY THE AUTHOR:
The author has done nothing extra, but has made some changes after deep study in
BG method of working. In the changed system of supporting, the wooden lagging are
exactly above the O.C. Props to make direct contact of the O.C. props with the roof of
the galleries. The support capacity or strength of the O.C. props are directly
transferred to the roof of the galleries and not to the roof bar, which eliminates the
chances of bending of roof bar and the O.C. props remain always tightened against
the roof. Also, support resistance increases, which can improve strata condition.
The modified system of support assembly is shown in Fig.6, given below. The support
resistance can further be increased by strengthening roof bars properly at both ends.
Modified supporting system
Free body load diagram of the modified system of supporting is shown below in Fig.7
L1 L2
L3 L4 L5
A B
R2
R1
Fig. 7
8

9. Where,
AB is roof bar
R1 & R2 are support resistances offered by OC props
L1 & L2 are reactive support resistances transferred to the
roof rock, and
L3 , L4 & L5 are concentrated reactive support resistances offered
by roof bar to the roof rock
Drawing shear force and bending moment diagram for modified supporting system:
L1 L2
L3 L4 L5
A B
FREE BODY LOAD DIAGRAM
(i) Loaded beam
V
R1-L1 +ve A2
A3 B
X
A A1 -ve R2-L2
(ii) S.F.Diagram
Mb
W (R1-L1)/2 - L3*C
A X
A1 A2 A3 B
(iii) B.M.Diagram
9

10. Failure of roof bar due to faulty design of roof bar:
The author has studied about the failure of roof bar in the BG- panel, which is only
due to faulty design of roof bar. Roof bar used in early years was of 150mm * 150mm
section. Currently BG- panel is using 200mm*200mm girder of I – section. The
thickness of web is about 7mm. It has become use and throw i.e. after using once; it
is being thrown in scrap, because after failure of web there is no further use in
supporting. It has been observed that, using such type of roof bar is not only wastage
of money, but also creating unsafe conditions and increasing heap of scrap in the
mine.
Mode of failure of roof bar observed by the author:
The I-section roof bar( 200mm* 200mm), which is failing in its web due to faulty
design of roof bar and also due to improper strengthening at its ends. The web failure
observed by the author is shown in figure given below.
Section of the failed roof bar (web failure )
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11. MODIFICATION IN DESIGN OF ROOF BAR SUGGESTED BY THE AUTHOR
Design of web of roof bar:
Design of web is very important for resisting shear stress and compressive stress.
When roof bar is tightened against the roof, the web is under compression. Therefore,
the strength of web should be such that, it can bear a load upto designed capacity of
the O.C. props (about 30t).
For the designing of web, two things are important. One is web thickness (t 2) and
another is its height (h).
So, if we increase the web thickness (t2), the strength of web will increase and, if we
increase the height of web (h), the strength of web will decrease.
h
t2
Section of web of the roof bar
The strength of web can be expressed mathematically in the following ways;
S α t2, and
S α 1/hn ,so combining these two equations we get;
S α t2/hn where `α` is proportionality constant.
Or, S =K*t2/hn where
S = strength of web,
K = proportionality constant,
t2 = web thickness, and
h = height of web.
P = Load on web (value of P
Varies in between 10t and 30t)
n= exponent to `h`
P (Load on Web)
t2
h
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12. P
Here, S should be greater than P, and for this the web shall be strengthened as shown
in figure below.
The author hase observed that the value of web thickness (t 2 ) should not be less than
10mm and distance between two flanges (d) not more than 150mm.
Therefore, minimum thickness of web (t2) = 10mm, and
Maximum height of web (h) = d – 2*t1
= (150- 2*10) mm
= 130mm
Design of flanges of roof bar:
As the author has compared the loading parameters of old roof bars and new roof
bar, the bending stresses are less in new type of the bar, which is because of its
larger width of flange.
Thickness of
flange (t1)
b
Design of flange of roof bar includes the design of flange thickness (t1) and width of
flange (b).
Hence, minimum thickness of flange (t1) = 10mm , and
Minimum width of flange (b) = 200mm.
Final design sample of roof bar:
200m
10mm 150mm
10 mm
The author has done only thing in modified design, that the web thickness (t2) has
been increased from 7mm to 10mm and distance between two flanges has been
decreased from 200mm to 150mm.
Proper Strengthening of Roof Bar:
Strengthening of roof bar is very important and essential for the strata control
point of view. Strata load is transferred vertically on the O.C. props through the roof
bar at both ends. Capacity of the O.C. prop is 40tons; therefore roof bar should be
capable to bear the load coming on the O.C. props. For this the roof bar is to be
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13. strengthened properly, otherwise the roof bar will yield prematurely at the ends and
the support assembly will be ineffective.
The scheme of proper strengthening of roof bar is shown in the figure given below:
Section of Roof Bar Longitudinal view of the Roof Bar
Section of the strengthened roof bar longitudinal view of the
Strengthened roof bar
Flange of the bar Web of the girder Edge of the web strengthened
with pieces of C- channel
(2``× 4`` or 3``× 6``)
Plan view of the longitudinal section of the strengthened roof bar
COMPAISION OF DESIGN PARAMETERS OF DIFFERENT TYPES OF ROOF BARS
Old type of Roof Bars Currently using Modified Roof
Design Roof Bar Bar
Parameters 150mm*150mm 150mm*200mm 200mm*200mm 200mm*150mm
Web thickness 9 – 10mm 7mm 7mm 10mm
(t2)
Thickness of 10.5mm 10.5mm 10mm 10mm
Flange (t1)
Width of 150mm 150mm 200mm 200mm
Flange (b)
Distance between 150mm 200mm 200mm 150mm
Flanges (d)
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14. Moment of 1714.28cm4 3050.2cm4 3953.53cm4 2146.42cm4
Inertia(I) of Roof
Bar
Cross- section of 4525mm2 4403mm2 5260mm2 5300mm2
Roof Bar
Cross- section of 1240mm2 1218mm2 1260mm2 1300mm2
Web
Advantages of the modified supporting system and modified design of roof bar:
It eliminates the bending of roof bar, which can be re –utilized ;
Strengthened roof bar can bear a minimum of 30t (compressive) load;
fully utilization of strength of OC props, because props are tightened against the roof
and not to the roof bar ;
support resistance offered by OC props are improved tremendously after modification
in supporting system and design of roof bar. Hence less chances of bed separation ;
rock load will be resisted by the OC props and not by the roof bar, hence abutment
pressure at side will be less which will minimize side spalling ;
props will be tightly intact with roof, therefore no chances of props dislodgment by
hitting side spalled boulders ;
overriding of pillars and stooks will be reduced ;
It will provide safe working conditions for men, machinery and property.
It will be very- very economical and purposeful;
Saving of wastage of money in purchasing roof bar every year.
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15. Conclusion:
The author has given valuable suggestion regarding supporting system in BG
working After applying the author’s suggestion, support resistance has improved in
BG working. The improvement in support resistance has decreased the chances of
layer
separation and over riding of pillars. It is very economical and purposeful. It can save
about Rs. 50 Lacs per annum on purchase of roof bar.
Declaration:
The above observations and comments are of author and not necessarily to the
organization.
Signature of author
(Tikeshwar Mahto )
Date-04-10-2010 Dy. Manager,
RG OC=II
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