Author: Tikeshwar Mahto, Dy. Director of Mines Safety, Bilaspur Region (India). +917898033693 [email protected]ECONOMICAL AND SAFE DESIGN OF ROOF BAR (GIRDER) FOR STRATA CONTROL IN UNDERGROUND MINES TO EXTRACT THICK SEAMS- -A Case Study of Blasting Gallery- 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 1
Transcript
1. Author: Tikeshwar Mahto, Dy. Director of Mines Safety,
Bilaspur Region (India). +917898033693 [email protected]
ECONOMICAL AND SAFE DESIGN OF ROOF BAR (GIRDER) FOR STRATA CONTROL
IN UNDERGROUND MINES TO EXTRACT THICK SEAMS-A Case Study of
Blasting Gallery- 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. 1
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 L1, L2 R1, R2 a M.S. roof bar
concentrated reactive forces (loads) on the roof bar, and 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 A1
B X -ve -ve R2 (ii) S.F.Diagram Mb R1C A A1 R1C B1 B x (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 4
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 c t2 ymax
= d/2 d t1 Neutral line N1-N2 t From the theory of simple bending;
M/I = /y =E/R Where, b= flange width, d= distance between the two
flanges, t1= thickness of flange, t2= thickness of web, t = bending
stress(tensile), c = bending stress (compressive) , N1- N2 =
neutral line M = moment of resistance or bending moment, I = moment
of inertia, y = distance from neutral axis, E = Youngs modulus, and
R = radius of curvature of internal surface of the deformed
beam(roof bar). Here, M/I = /y Or, = M*y/I So ` ` will be maximum
or minimum when ` y` is maximum or minimum. Thus , for y= 0 , = 0
i.e. bending stress at neutral line is zero and bending stresses at
flanges are maximum. Also ymax = d/2 max = (M/I) *ymax , or, max =
Md/2I Thus, maximum bending stress is at flanges of the roof bar,
as shown in the figure above. 5
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*d 3/12 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 6
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) Cross- section of the web[(d-2t1)*t2)] : :
7mm 180mm*7mm 7
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 L3 L4 L5 A L2 B R2 R1 Fig. 7 8
9. Where, AB R1 & R2 L1 & L2 is roof bar are support
resistances offered by OC props are reactive support resistances
transferred to the roof rock, and are concentrated reactive support
resistances offered by roof bar to the roof rock L3 , L 4 & L 5
Drawing shear force and bending moment diagram for modified
supporting system : L1 L3 L4 L5 L2 A B FREE BODY LOAD DIAGRAM (i)
Loaded beam V +ve R1-L1 A A2 A1 A3 -ve B X R2-L2 (ii) S.F.Diagram
Mb W (R1-L1)/2 - L3*C A A1 A2 A3 B X (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 ) 10
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 (t 2), 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 11
12. The strength of web can be expressed mathematically in the
following ways; S t2, and S 1/hn ,so combining these two equations
we get; S t2/hn where `` 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 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. b
Thickness of flange (t1) Design of flange of roof bar includes the
design of flange thickness (t 1) and width of flange (b). Hence,
minimum thickness of flange (t1) = 10mm , and Minimum width of
flange (b) = 200mm. 12
13. Final design sample of roof bar: 200m m 10mm 150mm 10 mm
The author has done only thing in modified design, that the web
thickness (t 2) 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 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
13
14. COMPAISION OF DESIGN PARAMETERS OF DIFFERENT TYPES OF ROOF
BARS Old type of Roof Bars Design Parameters 150mm*150mm
150mm*200mm Web thickness (t2) Modified Roof Bar 200mm*150mm 7mm
7mm 10.5mm 10.5mm 10mm 10mm Width of Flange (b) 150mm 150mm 200mm
200mm Distance between Flanges (d) 150mm 200mm 200mm 150mm
3050.2cm4 3953.53cm4 2146.42cm4 4403mm2 5260mm2 5300mm2 1218mm2
1260mm2 1300mm2 Thickness Flange (t1) 9 10mm Currently using Roof
Bar 200mm*200mm of Moment of 1714.28cm4 Inertia(I) of Roof Bar
Cross- section of 4525mm2 Roof Bar Cross- section of Web 1240mm2
10mm 14
15. 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. Conclusion: The author has given valuable
suggestion regarding supporting system in BG working After applying
the authors 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.
Date-04-10-2010 Signature of author (Tikeshwar Mahto ) Dy. Director
of Mines Safety, Bilaspur Region (India) +917898033693
[email protected] 15