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International Journal of Current Engineering and Technology E-ISSN 2277 – 4106, P-ISSN 2347 – 5161
©2015INPRESSCO®, All Rights Reserved Available at http://inpressco.com/category/ijcet
Research Article
785| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015)
Seismic Response of Cellwise Braced Reinforced Concrete Frames
Kulkarni J. G.†, Kore P. N.†* and Swami P. S.†
†Department of Civil Engineering, Solapur University, Solapur, India
Accepted 22 March 2015, Available online 25 March 2015, Vol.5, No.2 (April 2015)
Abstract
This paper present an elastic seismic response of reinforced concrete frames with reinforced concrete braces in K or
inverted – A or V – braced pattern which are analyzed numerically for twelve storey building with 5-bay structures.
The responses of braced frames of different patterns (bay, level and combinations thereof and particular cell braced)
have been compared with each other and the same also have been compared with unbraced i.e. bare frame. With
this view point all parameters in dimensionless form, to include geometry of frame, axial forces, shear forces, bending
moments, displacements, location of bracing etc. have been used. Results and finding can be directly used to predict
the behavior of real life structures which are concluded from graphs and comprehensive discussions.
Keywords: Bare Frames, Baywise & Levelwise Braced Frames, Concentrically Braced Frames (CBF), Outriggers,
Cellwise Braced Frames.
1. Introduction
1
The paper includes analyzing the effect of
concentrically braced RC frames (CBRF) on the
behavior of the frame and the amount of drift. The
behaviors of these frames with different brace pattern
are evaluated through structural analysis. The
structural analysis was done on structural models
plotted in the STAAD- structural analysis and design
commercial software. The frame shall be designed to
resist the effect of gravity and earthquake loadings for
strength and serviceability without aid of V-braces
first, to ultimately represent a proposal of an
introduction of V-braces having economical structure
with good behavior and a convenient architectural
distribution.
V-braced frames are one type of Concentrically
Braced Frame (CBF) in which the arrangements of
members form a vertical truss system to resist lateral
forces. The main function of bracing system is to resist
lateral forces. It is therefore possible in initial stage of
design to treat the frame and the bracing as two
separate load carrying system.
The inelastic seismic response of X and K braced
concrete frames with intermediate bracing members is
satisfactory (J. P. Desai, A. K. Jain and A. S. Arya, 1987).
In braced frames, not only lateral resistance and
stiffness enhanced, but also energy dissipation amount
increased significantly (Shan – Hau Xu & Di – Tao Niu,
Jan –Feb-2003). A rational design methodology was
adopted to design the braced frame including the
*Corresponding author: Kore P. N.
connections between the brace members and the
concrete frame (M. A. Youssefa, H. Ghaffarzadehb, M.
Nehdi, 23 October 2006).
Nonlinear seismic behaviour of RC Frames with RC
Braces focuses on evaluation of strength, stiffness,
ductility and energy absorption of reinforced concrete
braced frames and comparison with similar moment
resisting frames and frames with shear wall (A.R.
Khaloo and M. Mahdi Mohseni, 2008).
Perhaps optimization is the most challenging class
of problems in structural optimization because of an
infinite number of iterations which are difficult to
classify and quantify. At the same time, such
optimization is of considerable importance because it
leads to significant material savings. Cross-sectional
dimensions are the simplest design variables. Cellwise
braced frame was tried to combine the advantages
especially regarding the economy of tried pattern and
to provide an obstruction free/ hindrance free
openings. This helps the frame discharge the function
more elegantly.
2. Description of study building structures
In order to study the behavior of moment resisting V-
braced frames (bare frames, fully, partially bay and
partially level braced frames, outrigger frames and
cellwise braced frames) 5 bay 12 storey structures (for
350 mm beam depth) are modeled and analyzed
numerically.
In case of bare frames sections of columns are
reduced from top to bottom which is same for every 3
storey (1-3, 4-6, 7-9, and 10-12) in order to achieve an
Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames
786| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015)
economy in bare frames itself. In all cases, span length
and storey elevation are 4 and 3 meters, respectively. A
typical frame of this type is shown in Fig. 1 below (a &
b). The responses of braced frames of different
configurations (baywise, levelwise, partially and
cellwise braced frames) have been compared with bare
frame and the same also have been compared with
each other.
a b
Fig.1 The specific worst loaded member considered for
the analysis of various frames
The following Table 1 shows total number of cases for
Baywise and Levelwise braced Frames tried for
considered structures. In case of levelwise braced
frames it is wise to restrict ourselves up to for higher
level combinations since it will not going to procure
any economy.
Table 1 Number of cases considered for Baywise &
Levelwise braced Frames for 12 storey structures
No.
of Bays
Braced
No. of Bays Braced at a time
On
e
Two Three Four Five Total
5-bay 5 10 10 5 -- 30
No.
of Levels
Braced
No. of Levels Braced at a time
On
e
Two Three Four Five Total
5-bay 12 66 220 -- -- 298
Total No. of Cases Studied 328
2.1 Check digit algorithms
Natural logarithm of reference number ‘N’ as
dimensionless parameter has been used as abscissa
with respect to various considered parameters. The
reference number used here is pure number, which is
uniquely specified for the frame and bracing pattern
tried.
The check digit special algorithms are used for
alpha and/or alphanumeric character fields. Each
character is assigned a numeric equivalent. The
numeric equivalents are weighted and the products are
summed. The total is divided by the modulus to
determine the remainder. The remainder is compared
to a pre-assigned index to determine the check digit.
Here dimensionless parameter (Ra) i.e. ratio of axial
force which is the ratio of value of axial force in
member C1 for all cases of bays braced to axial force in
same member of bare frame as shown in Fig. 1 (a)
above. Same is applicable to ratio of shear force (Rs)
and to ratio bending moment (Rm). The value of
internal forces in worst loaded column segment (C1) of
bare frame as shown in Fig. 1 (a) above is chosen as
reference value for ratio Ra, Rs and Rm.
3. Internal forces
Forces induced viz. axial force, shear force and bending
moment in one particular worst loaded column
segment is considered for this purpose as shown in Fig.
1 above. In order to facilitate the direct comparison
between bare and fully and/or partially braced frames
and cellwise braced farmes the latter have been
analyzed for the same geometry of mutually
perpendicular/orthogonal members for the same
loading combination for which the bare frame yielded
the maximum design force in the members so selected
as shown in Fig. 1 (a & b) above. However, once the
design forces are evaluated for fully and/or partially
braced structures all individual segments are
redesigned and the minimum required cross sections
and steel percentage was calculated. Ultimately cost
comparison is carried out to compute economy.
4. Optimum baywise and levelwise location for
bracing frames
4.1 Optimum baywise location for bracing frames 5 bay
12 storey structures
Fully braced frames when analyzed, exhibit the values
of forces and displacements which are changing with
the variation of number of parameters. However, it was
noticed that the frames underwent a very small lateral
displacement than was permissible. It is but obvious
that, when such frames are partially braced i.e. braced
all along the height in a particular or a combination of
number of bays which is less than total numbers of
bays for the frame, similarly when braced all along the
level or a combination of number of levels concerned,
will produce a larger displacement compared to bare
and fully braced frames but within permissible limit i.e.
0.004H.
Hence it was decided to find out such possibility of
developing a particular pattern for partially braced
frames, which would produce smaller forces for worst
load combinations. It goes without saying that the
bracing pattern tried always satisfied strength as well
as serviceability criterion.
It seen clearly that every graph shows two lines
parallel to the x-axis as shown in Fig. 2 below. If these
are considered as the upper and the lower limits as
may be appropriate to a particular dimensionless
Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames
787| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015)
parameter, one finds that many a times in case of
partially braced frames the dimensionless parameters
observed exceeds the upper limits and/or is less than
the lower limit so considered. From which it concludes
that some of the braced frames may allow reduction in
effect of considered parameters hence; it would be
advantageous to suggest certain working range from
specified/accepted minimum to maximum for various
internal forces concerned within which a number of
bracing patterns/cases produce the force being
considered for different cases. However, a number of
cases may be found common to all of them are the
optimum cases as far as force levels are concerned. The
frames which appear between those acceptable ranges
were further taken into account for the analysis and
design purpose. Often symmetrical cases are taken into
account and tried in case of baywise bracings only.
Table 2 Showing percentage of economy achieved in
optimum baywise and levelwise location of bracings
Case
No.
5 bay 12 storey structures
Percentage of
Saving
a B 3 6.13%
b B 2+ B 4 2.35%
c L 2 + L 5 7.41%
d L 1 6.30%
All members of the frame are designed and saving in
material is found out. Amongst all the so called best
choices there is only one or more than one
choice/choices is made available which reduce the
total cost of structures compared to a bare frames,
when all members are designed to carry the most
critical combination of the largest developed forces.
This bracing pattern/ these bracing patterns is/are the
optimum solution/solutions. It is found that such
bracing pattern is most economical than fully braced
one. Following Fig. 2 shows the variation of internal
forces of 5 bay 12 storey structures and Fig. 3 shows
examples of optimum baywise braced frames for 5 bay
12 storey structures and the economy is tabulated in
Table 2 above.
(a) Variation of Axial Force i.e. Ra of 5 bay 12 storey
structures
(b) Variation of Shear Force i.e. Rs of 5 bay 12 storey
structures
(c) Variation of Bending Moment i.e. Rm of 5 bay 12
storey structures
Fig.2 Variation of internal forces of 5 bay 12 storey
structures for baywise combinations
a b
Fig.3 Optimum bay bracing pattern for 5 bay 12 storey
structures
4.2 Optimum levelwise location for bracing frames 5 bay
12 storey structures
The same is repeated for levelwise bracing for
previously mentioned structures and is represented in
the Fig. 4 below. The graphs show variation of axial
force, shear force and bending moment in member C1
as the bracing pattern changes. Overall, these graphs
show the increment in the values of ratios of Ra as the
bracing level and combinations of floor increases, Fig. 4
Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames
788| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015)
(b) show step variation except the cases which lie
below and above the limit of bare and fully braced
frames respectively, and Fig. 4 (c) show the step
variation except the cases which lie below the limit of
bare frames.
Hence, it seems reasonable to accept certain
working range for axial force; shear force and bending
moment from specified/accepted minimum to
maximum for various internal forces concerned within
which a number of bracing patterns/cases produce the
force being considered for different cases. However, a
number of cases may be found common to all of them.
These are most optimum cases as far as force levels are
concerned. Fig. 5 shows examples of optimum
levelwise braced frames for 5 bay 12 storey structures
and the economy is tabulated in table and the
respective combinations are shown in Table 2 above.
a) Variation of Axial Force i.e. Ra of 5 bay 12
storey structures
b) Variation of Shear Force i.e. Rs of 5 bay 12
storey structures
c) Variation of Bending Moment i.e. Rm of 5 bay
12 storey structures
Fig.4 Variation of internal forces (Ra, Rs, and Rm) of 5
bay 12 storey structures for levelwise combinations
a b
Fig.5 Optimum level bracing pattern for 5 bay 12
storey structures
4.3 Optimally braced frames with outriggers
To begin with and to arrive at the outrigger
orientation, a combination of optimum cases of
baywise bracing and levelwise bracing pattern are
tried. It may be noted that when a number of bays or
levels braced simultaneously are increases then
normally the cost effectiveness is lost, as the number of
members increase which compensate the saving
claimed due to reduction in the internal forces induced
in the structures and the reduced cross sections
commensurate with the same.
a b c
Fig.6 Optimum bracing patterns for 5 bay 12 storey
structures
In order to reduce the material cost, optimum
combinations of baywise and levelwise braced frames
are considered and the section sizes are reduced to get
minimum material cost and yet displacement remain
within permissible limits.
Table 3 Showing percentage of economy achieved in
optimally braced frames with outriggers
Case no. (For 5 bay 12
storey structures)
Percentage of
Saving
B (3) + L (9) 6.00%
B (3) + L (7) 5.72%
B (3) + L (10) 5.70%
Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames
789| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015)
The Fig. 6 (a, b & c) shows the partially braced frame
for the combination of specific bays and levels braced.
The respective combinations and economy is tabulated
as shown in Table 3 below.
4.4 Optimally cellwise braced frames
To make the structure reasonably more flexible and
simultaneously requiring a least number of bracing
elements, braces are provided at suitable,
predetermined, well thought selected locations on the
cells occupied by the so called outriggers. It also
provides the more flexibility for architectural
provisions and offers the space to accommodate the
openings/doors and windows. So it is advantageous to
try cellwise bracing in these floors with symmetrical
cases beyond which the economy is going to decrease.
Hence it is decided to do cellwise bracing in two ways.
a) In those structures which offer maximum economy
in levelwise bracing and
b) In those structures which offer maximum economy
in baywise bracing
4.4.1 Cellwise bracing – moving levelwise
For 5 bay 12 storey structures case number 03 i.e.
whole central bay braced structure offer 6.13%
economy as compared to bare frame. Hence it is
decided to provide single cellwise bracing pattern that
is moving levelwise in respective levels to find out such
a cell braced frame which gives more economy as
compared to bare frame as well as optimally (central
bay) braced frame i.e. case number 03. So it is tried to
find out more economical solution by way of the
provision of single cell brace rather than providing
whole central bay bracing.
The hidden advantage behind the provision of
cellwise bracing is that, the speedy construction which
is faster than other bracing patterns utilized
previously. The same logic is applied for the
combinations of cells braced more than one i.e. two
and/or three cells braced at a time and so on.
a) Single Cell Braced at a Time
To get the deep insight about savings to achieve in
cellwise braced frames, it is decided to do cellwise
bracing in 5 Bay 12 storey structures for central bay
braced i.e. case number 03 in the same structure.
Table 4 Showing percentage of Economy achieved in
cellwise braced frames
Case no. (For 5 bay
12 storey structures)
Percentage
of Saving
4 9.29%
7 9.26%
3 9.17%
Above table shows the economy obtained for one cell
braced frame which offer more economy as compared
to bare frame as well as other single cellwise braced
combinations of the same frame. The results are
tabulated in the following Table 4 and the patterns of
bracing which gives an optimum result is also shown in
Figure 7.
a b c
Fig.7 Optimum cellwise bracing patterns for 5 bay 12
storey structures
b) Two Cells Braced at a Time
It is seen from following table 6 that the combination of
1st & 6th floor cell gives 9.28% economy which is
51.39% more than central bay braced throughout, but
comparing to single cell braced it gives slightly less
economy and as we move towards upper floor cellwise
bracing combinations the economy is going to reduce
by 1.72% and 2.15% for 1st & 4th floor cell and 2nd & 5th
floor cell braced combinations respectively as
compared to 1st & 6th floor cellwise bracing
combination of the same structure. The results are
tabulated in the following Table 5 and the patterns of
bracing which gives an optimum result is also shown in
Figure 8.
Table 5 Showing percentage of Economy achieved in
cellwise braced frames
Case no. (For 5 bay
12 storey structures)
Percentage
of Saving
1+6 9.28%
1+4 9.12%
2+5 9.08%
a b c
Fig.8 Optimum cellwise bracing patterns for 5 bay 12
storey structures
Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames
790| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015)
c) Three Cells Braced at a Time
It is seen from following table 7 that the combination of
2nd, 5th and 8th floor cell gives 8.94% economy out of
other combinations of three cells braced, which is
45.84% more than central bay braced throughout, but
comparing to single cell braced it offers less economy
and as we move towards upper floor cellwise bracing
combinations the economy is going to reduce. The
results are tabulated in the following Table 6 and the
patterns of bracing which gives an optimum result is
also shown in Figure 9.
Table 6 Showing percentage of Economy achieved in
cellwise braced frames
Case no. (For 5 bay
12 storey structures)
Percentage
of Saving
2+5+8 8.94%
1+4+6 8.87%
2+4+6 8.44%
a b c
Fig.9 Optimum cellwise bracing patterns for 5 bay 12
storey structures
It is seen that single cell braced frames offer more
economy as compared to bare and other cases of cell
bracing combinations which concludes that, when an
appropriately located single cell is braced makes a
structure more rigid than other cases of combinations
of bracings. Further it is seen that the economy of
cellwise bracing configurations tried for other floors
will reduce the economy gradually hence it is wise to
restrict further combinations.
4.4.2 Cellwise bracing – moving baywise
The levelwise bracing combinations are restricted up
to three levels only since because of the bracing
combinations beyond third floor are not offering more
economy and also such combinations are not feasible
to do and represent them.
When ground floor is braced for 5 bay 12 storey
structures throughout in levelwise such bracing
pattern offers 7.41% economy and hence it is decided
to provide cellwise bracing that is moving baywise in
respective bays to yield more economy. So it is tried to
find out more economy due to the provision of single
cell braced (but placed geometrically similar) rather
than providing whole level braced throughout from the
conclusions which are recommended in above cellwise
bracing combinations i.e. moving levelwise.
The same logic is applied for the combinations of
cells braced more than one i.e. two and/or three cells
braced at a time and so on for respective floors which
offered more economy already.
a) Ground Floor Braced
When cellwise bracing is provided by moving baywise
it is wise to consider symmetrical cases since they
behave symmetrically in case of change in earthquake
directions.
Table 7 Showing percentage of Economy achieved in
cellwise braced frames
Case no. (For 5 bay
12 storey structures)
Percentage
of
Saving
Braced cell 3 8.88%
Braced cell 2+4 8.67%
Braced cell 1+3+5 8.34%
The single cell which is symmetrically placed in the
ground floor is same as in the previous table offering
an economy of 8.88%. Following Table 7shows the
other cases of cell braced at ground floor only seeking
the case of specific cell braced which debt more
economy and the Figure 10 shows the cases tried.
a b c
Fig.10 Optimum bracing pattern for 5-bay 12 storey
structures, Ground floor level braced.
b) Fourth Floor Braced
Now for this case fourth floor is braced cellwise to see
the frame with optimum economy. As discussed
earlier, the cellwise bracing is provided by moving
baywise and it is wise to consider symmetrical cases
Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames
791| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015)
since they behave symmetrically in case of change in
earthquake directions. Following Table 8 shows the
cases of cells braced and their combinations at fourth
floor only with symmetry, offering more economy and
the Figure 11 shows the cases tried.
Table 8 Showing percentage of Economy achieved in
cellwise braced frames
Case no. (For 5 bay
12 storey structures)
Percentage
of
Saving
Braced cell 3 9.29%
Braced cell 2+4 9.05%
Braced cell 1+3+5 9.31%
a b c
Fig.11 Optimum bracing pattern for 5-bay 12 storey
structures, Fourth floor level braced.
c) Sixth Floor Braced
In the same way symmetrical cases of cell bracing were
tried for sixth floor braced to obtain more economy.
Now these cases were tried with symmetry as
mentioned above. The cases which offer more saving
are presented in the following Table 9 and Figure 12
shows the cases tried.
Table 9 Showing percentage of Economy achieved in
cellwise braced frames
Case no. (For 5 bay
12 storey structures)
Percentage
of Saving
Braced cell 3 9.26%
Braced cell 2+4 9.00%
Braced cell 1+3+5 9.00%
Comparing the economy of all the different cellwise
braced frames with respect to different floors overall it
is seen that the economy is more in the case of cellwise
braced frames that is moving levelwise as compared to
cellwise braced frames that is moving levelwise . In
fourth floor braced frame i.e. braced cell 1+3+5 offer
9.31% economy which shows that such configurations
offer more stiffness to the frame as compared to other
cases. Similarly in sixth floor braced frame i.e. Braced
cell 3 offer 9.26% economy which is nearly equal to the
cellwise braced frame.
a b c
Fig.12 Optimum bracing pattern for 5-bay 12 storey
structures, sixth floor level braced.
It concludes that as we towards upper level with
increased number of cell bracing combinations will
offer savings which are nearly equal to single cell
braced configuration when the braces are provided at
equal spacing.
4.4.3 Combination if cellwise bracing – moving levelwise
and baywise
Based on findings presented in above discussion it is
observed that cellwise braces when provided by
moving levelwise offer some more economy as
compared to when braces are provided by moving
baywise, particularly when we talk about single cell
braced at a time which offer more economy as
compared to bare and other cases of cellwise bracing
i.e. two cells, three cells etc. braced at time in the
structure. But it also shows that when bracing are
provided at equi-spaced and at appropriate level will
offer more economy as compared to single cell braced
configurations.
Hence to obtain still more economy it is decided to
use same strategy which is used in the case of partially
braced frames i.e. to combine the economy obtained in
baywise braced frames and levelwise braced frames
throughout the bay and level respectively.
The combination comprises the case of cellwise
bracing that is provision is made by moving either
levelwise or baywise. When we consider the economy
of single cell braced, in that, the cell braced by moving
levelwise i.e. case number 04 gives 9.29% economy
and the three cells braced at a time by moving baywise
i.e. braced cell 1+3+5 case gives 9.31% economy which
is more as compared to other combinations of cellwise
bracing.
Hence it is again restricted to do the combinations
for those cases which have more economy than other
combinations of cellwise bracings which are provided
Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames
792| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015)
either by levelwise or baywise for 5 Bay G+11
structure i.e. localizing the view over the cases which
produce more economy than other, such as only bay
braced or level braced throughout, combinations of
them and cellwise braced with combination of them
which is one cell less than the number of cells in either
bay or level throughout.
Table 10 Showing percentage of Economy achieved in
cellwise braced frames
Case no. (For 5 bay
12 storey structures)
Percentage
of Saving
Trial 1 9.04%
Trial 2 8.97%
Trial 3 8.88%
Trial 4 8.63%
Trial 1 Trial 2 Trial 3 Trial 4
Fig.13 Optimum bracing pattern for 5-bay 12 storey
structures with arbitrary bracing pattern
6. Storey drift
The lateral displacements of frames were observed at
common point at specific position in 5 bay 12 storey
structures. The comparison is made between bare, fully
braced, partially braced i.e. optimum bay braced and
level braced, outrigger and cellwise braced frames. It
was checked whether the structure satisfies maximum
permissible relative lateral drift criterion as per
IS: 1893-2002 (Part-I) which is 0.004H.
To insure the serviceability criterion, considering
the lowest point as base of structure and highest point
as top of the structure, maximum relative drift is found
and simultaneously compared with the permissible
one as well as with other analyzed structures such as
fully braced, partially braced and outriggers and
cellwise braced frames. The values are tabulated in
Table 11 and the variation is shown in Fig. 14 below.
For bare frames the lateral drift is well within the
permissible one, but it is increasing as one move
towards the top most point of structure. Also the
lateral displacements for fully braced frames are
getting substantially reduced as compared with bare
frame, but these are uneconomical structures from
saving point of view.
For optimally bay braced frames and for outriggers the
lateral displacements are getting reduced by near
about half and more as compared to the bare frames.
There is no valuable reduction in the lateral
displacement for optimally levelwise braced frames.
Table 11 Showing variation of lateral displacement
along height of structure for 5 Bay 12 storey structure
with 350 mm beam depth
Analyzed Structures of 5 Bay 12 storey structures
for 350 mm beam Depth
Ht.FromBase
(m)
BareFrame
(mm)
FullyBraced
Frame(mm)
OptimumBay
Braced(mm)
OptimumLevel
Braced(mm)
Framewith
Outrigger
(mm)
CellwiseBraced
Frame
(mm)
0 0 0 0 0 0 0
2.44 1.05 0.66 0.74 0.59 0.742 1.00
5.44 4.14 1.48 2.27 1.68 2.21 3.81
8.44 8.72 2.26 4.31 4.17 4.10 7.85
11.44 14.23 3.00 6.62 8.04 6.23 12.47
14.44 21.12 4.13 9.44 13.88 8.76 17.10
17.44 28.95 5.18 12.55 21.2 11.46 21.03
20.44 36.98 6.13 15.78 28.97 14.14 26.21
23.44 45.00 7.14 19.13 36.88 16.71 32.89
26.44 52.32 8.01 22.44 44.15 18.58 39.71
29.44 58.58 8.74 25.61 50.4 19.84 45.76
32.44 63.56 9.33 28.56 55.38 21.16 50.63
35.44 67.1 9.77 31.21 58.92 22.67 54.12
38.44 69.57 10.13 33.63 61.4 24.11 56.54
Fig.14 Variation of lateral displacement along height of
structure for 5 Bay 12 storey, 350 mm beam depth for
L/C 7 i.e. (1.5 D.L. + 1.5 E.Q.)
The graph shows the similar variation of lateral
displacement for optimally cellwise braced frame
which is less than that of bare frame but well within
the permissible limit. In lateral displacement 18.73%
decrement is found in the cellwise braced frame as
compared to bare frame. Similarly for partially braced
frame this amount is getting reduced by nearly half and
more as compared to bare frame. Hence it is proved
that for cellwise braced frame though the drift is larger
than bare frame, but is well within the acceptable limit.
In nutshell, it is advantageous to adopt such optimally
cellwise braced frame to increase the economy.
Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames
793| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015)
Conclusions
1) Fully braced frames are very conservative in so far
as lateral drift is concerned but uneconomical and
at the same time optimally braced one have least
forces induced in the structure and produce
maximum displacement but within prescribed
limit.
2) It is seen that maximum economy is obtained with
less number of bracings are used with proper
position and at the same time lateral drift is within
acceptable range offering more economy as
compared to bare frames.
3) Optimally (cellwise) braced frames are stiff, strong,
and an economical structural system.
References
Shan – Hau Xu & Di – Tao Niu, (Jan–Feb-2003), Seismic
Behavior of R.C. Braced Frames, ACI Structural journal,
No.1, 100.
A. R. Khaloo & M. Mahdi Mohseni, (2008), Nonlinear Seismic
Behavior of RC Frames with R.C. Braces, Asian Journal of
Civil Engineering, Vol. 9, No. 6.
J.P. Desai, A.K. Jain and A.S. Arya, (1987) Seismic response of
R.C. Braced Frames, Computers and Structures Vol. 29, No.
4.
M.A. Youssefa, H. Ghaffarzadehb, M. Nehdi, (23 October
2006), Seismic performance of RC frames with concentric
internal steel bracing, Department of Civil and
Environmental Engineering, Shiraz University, Shiraz, Iran,
71345-1676,.

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Seismic Response of Cellwise Braced Reinforced Concrete Frames

  • 1. International Journal of Current Engineering and Technology E-ISSN 2277 – 4106, P-ISSN 2347 – 5161 ©2015INPRESSCO®, All Rights Reserved Available at http://inpressco.com/category/ijcet Research Article 785| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015) Seismic Response of Cellwise Braced Reinforced Concrete Frames Kulkarni J. G.†, Kore P. N.†* and Swami P. S.† †Department of Civil Engineering, Solapur University, Solapur, India Accepted 22 March 2015, Available online 25 March 2015, Vol.5, No.2 (April 2015) Abstract This paper present an elastic seismic response of reinforced concrete frames with reinforced concrete braces in K or inverted – A or V – braced pattern which are analyzed numerically for twelve storey building with 5-bay structures. The responses of braced frames of different patterns (bay, level and combinations thereof and particular cell braced) have been compared with each other and the same also have been compared with unbraced i.e. bare frame. With this view point all parameters in dimensionless form, to include geometry of frame, axial forces, shear forces, bending moments, displacements, location of bracing etc. have been used. Results and finding can be directly used to predict the behavior of real life structures which are concluded from graphs and comprehensive discussions. Keywords: Bare Frames, Baywise & Levelwise Braced Frames, Concentrically Braced Frames (CBF), Outriggers, Cellwise Braced Frames. 1. Introduction 1 The paper includes analyzing the effect of concentrically braced RC frames (CBRF) on the behavior of the frame and the amount of drift. The behaviors of these frames with different brace pattern are evaluated through structural analysis. The structural analysis was done on structural models plotted in the STAAD- structural analysis and design commercial software. The frame shall be designed to resist the effect of gravity and earthquake loadings for strength and serviceability without aid of V-braces first, to ultimately represent a proposal of an introduction of V-braces having economical structure with good behavior and a convenient architectural distribution. V-braced frames are one type of Concentrically Braced Frame (CBF) in which the arrangements of members form a vertical truss system to resist lateral forces. The main function of bracing system is to resist lateral forces. It is therefore possible in initial stage of design to treat the frame and the bracing as two separate load carrying system. The inelastic seismic response of X and K braced concrete frames with intermediate bracing members is satisfactory (J. P. Desai, A. K. Jain and A. S. Arya, 1987). In braced frames, not only lateral resistance and stiffness enhanced, but also energy dissipation amount increased significantly (Shan – Hau Xu & Di – Tao Niu, Jan –Feb-2003). A rational design methodology was adopted to design the braced frame including the *Corresponding author: Kore P. N. connections between the brace members and the concrete frame (M. A. Youssefa, H. Ghaffarzadehb, M. Nehdi, 23 October 2006). Nonlinear seismic behaviour of RC Frames with RC Braces focuses on evaluation of strength, stiffness, ductility and energy absorption of reinforced concrete braced frames and comparison with similar moment resisting frames and frames with shear wall (A.R. Khaloo and M. Mahdi Mohseni, 2008). Perhaps optimization is the most challenging class of problems in structural optimization because of an infinite number of iterations which are difficult to classify and quantify. At the same time, such optimization is of considerable importance because it leads to significant material savings. Cross-sectional dimensions are the simplest design variables. Cellwise braced frame was tried to combine the advantages especially regarding the economy of tried pattern and to provide an obstruction free/ hindrance free openings. This helps the frame discharge the function more elegantly. 2. Description of study building structures In order to study the behavior of moment resisting V- braced frames (bare frames, fully, partially bay and partially level braced frames, outrigger frames and cellwise braced frames) 5 bay 12 storey structures (for 350 mm beam depth) are modeled and analyzed numerically. In case of bare frames sections of columns are reduced from top to bottom which is same for every 3 storey (1-3, 4-6, 7-9, and 10-12) in order to achieve an
  • 2. Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames 786| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015) economy in bare frames itself. In all cases, span length and storey elevation are 4 and 3 meters, respectively. A typical frame of this type is shown in Fig. 1 below (a & b). The responses of braced frames of different configurations (baywise, levelwise, partially and cellwise braced frames) have been compared with bare frame and the same also have been compared with each other. a b Fig.1 The specific worst loaded member considered for the analysis of various frames The following Table 1 shows total number of cases for Baywise and Levelwise braced Frames tried for considered structures. In case of levelwise braced frames it is wise to restrict ourselves up to for higher level combinations since it will not going to procure any economy. Table 1 Number of cases considered for Baywise & Levelwise braced Frames for 12 storey structures No. of Bays Braced No. of Bays Braced at a time On e Two Three Four Five Total 5-bay 5 10 10 5 -- 30 No. of Levels Braced No. of Levels Braced at a time On e Two Three Four Five Total 5-bay 12 66 220 -- -- 298 Total No. of Cases Studied 328 2.1 Check digit algorithms Natural logarithm of reference number ‘N’ as dimensionless parameter has been used as abscissa with respect to various considered parameters. The reference number used here is pure number, which is uniquely specified for the frame and bracing pattern tried. The check digit special algorithms are used for alpha and/or alphanumeric character fields. Each character is assigned a numeric equivalent. The numeric equivalents are weighted and the products are summed. The total is divided by the modulus to determine the remainder. The remainder is compared to a pre-assigned index to determine the check digit. Here dimensionless parameter (Ra) i.e. ratio of axial force which is the ratio of value of axial force in member C1 for all cases of bays braced to axial force in same member of bare frame as shown in Fig. 1 (a) above. Same is applicable to ratio of shear force (Rs) and to ratio bending moment (Rm). The value of internal forces in worst loaded column segment (C1) of bare frame as shown in Fig. 1 (a) above is chosen as reference value for ratio Ra, Rs and Rm. 3. Internal forces Forces induced viz. axial force, shear force and bending moment in one particular worst loaded column segment is considered for this purpose as shown in Fig. 1 above. In order to facilitate the direct comparison between bare and fully and/or partially braced frames and cellwise braced farmes the latter have been analyzed for the same geometry of mutually perpendicular/orthogonal members for the same loading combination for which the bare frame yielded the maximum design force in the members so selected as shown in Fig. 1 (a & b) above. However, once the design forces are evaluated for fully and/or partially braced structures all individual segments are redesigned and the minimum required cross sections and steel percentage was calculated. Ultimately cost comparison is carried out to compute economy. 4. Optimum baywise and levelwise location for bracing frames 4.1 Optimum baywise location for bracing frames 5 bay 12 storey structures Fully braced frames when analyzed, exhibit the values of forces and displacements which are changing with the variation of number of parameters. However, it was noticed that the frames underwent a very small lateral displacement than was permissible. It is but obvious that, when such frames are partially braced i.e. braced all along the height in a particular or a combination of number of bays which is less than total numbers of bays for the frame, similarly when braced all along the level or a combination of number of levels concerned, will produce a larger displacement compared to bare and fully braced frames but within permissible limit i.e. 0.004H. Hence it was decided to find out such possibility of developing a particular pattern for partially braced frames, which would produce smaller forces for worst load combinations. It goes without saying that the bracing pattern tried always satisfied strength as well as serviceability criterion. It seen clearly that every graph shows two lines parallel to the x-axis as shown in Fig. 2 below. If these are considered as the upper and the lower limits as may be appropriate to a particular dimensionless
  • 3. Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames 787| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015) parameter, one finds that many a times in case of partially braced frames the dimensionless parameters observed exceeds the upper limits and/or is less than the lower limit so considered. From which it concludes that some of the braced frames may allow reduction in effect of considered parameters hence; it would be advantageous to suggest certain working range from specified/accepted minimum to maximum for various internal forces concerned within which a number of bracing patterns/cases produce the force being considered for different cases. However, a number of cases may be found common to all of them are the optimum cases as far as force levels are concerned. The frames which appear between those acceptable ranges were further taken into account for the analysis and design purpose. Often symmetrical cases are taken into account and tried in case of baywise bracings only. Table 2 Showing percentage of economy achieved in optimum baywise and levelwise location of bracings Case No. 5 bay 12 storey structures Percentage of Saving a B 3 6.13% b B 2+ B 4 2.35% c L 2 + L 5 7.41% d L 1 6.30% All members of the frame are designed and saving in material is found out. Amongst all the so called best choices there is only one or more than one choice/choices is made available which reduce the total cost of structures compared to a bare frames, when all members are designed to carry the most critical combination of the largest developed forces. This bracing pattern/ these bracing patterns is/are the optimum solution/solutions. It is found that such bracing pattern is most economical than fully braced one. Following Fig. 2 shows the variation of internal forces of 5 bay 12 storey structures and Fig. 3 shows examples of optimum baywise braced frames for 5 bay 12 storey structures and the economy is tabulated in Table 2 above. (a) Variation of Axial Force i.e. Ra of 5 bay 12 storey structures (b) Variation of Shear Force i.e. Rs of 5 bay 12 storey structures (c) Variation of Bending Moment i.e. Rm of 5 bay 12 storey structures Fig.2 Variation of internal forces of 5 bay 12 storey structures for baywise combinations a b Fig.3 Optimum bay bracing pattern for 5 bay 12 storey structures 4.2 Optimum levelwise location for bracing frames 5 bay 12 storey structures The same is repeated for levelwise bracing for previously mentioned structures and is represented in the Fig. 4 below. The graphs show variation of axial force, shear force and bending moment in member C1 as the bracing pattern changes. Overall, these graphs show the increment in the values of ratios of Ra as the bracing level and combinations of floor increases, Fig. 4
  • 4. Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames 788| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015) (b) show step variation except the cases which lie below and above the limit of bare and fully braced frames respectively, and Fig. 4 (c) show the step variation except the cases which lie below the limit of bare frames. Hence, it seems reasonable to accept certain working range for axial force; shear force and bending moment from specified/accepted minimum to maximum for various internal forces concerned within which a number of bracing patterns/cases produce the force being considered for different cases. However, a number of cases may be found common to all of them. These are most optimum cases as far as force levels are concerned. Fig. 5 shows examples of optimum levelwise braced frames for 5 bay 12 storey structures and the economy is tabulated in table and the respective combinations are shown in Table 2 above. a) Variation of Axial Force i.e. Ra of 5 bay 12 storey structures b) Variation of Shear Force i.e. Rs of 5 bay 12 storey structures c) Variation of Bending Moment i.e. Rm of 5 bay 12 storey structures Fig.4 Variation of internal forces (Ra, Rs, and Rm) of 5 bay 12 storey structures for levelwise combinations a b Fig.5 Optimum level bracing pattern for 5 bay 12 storey structures 4.3 Optimally braced frames with outriggers To begin with and to arrive at the outrigger orientation, a combination of optimum cases of baywise bracing and levelwise bracing pattern are tried. It may be noted that when a number of bays or levels braced simultaneously are increases then normally the cost effectiveness is lost, as the number of members increase which compensate the saving claimed due to reduction in the internal forces induced in the structures and the reduced cross sections commensurate with the same. a b c Fig.6 Optimum bracing patterns for 5 bay 12 storey structures In order to reduce the material cost, optimum combinations of baywise and levelwise braced frames are considered and the section sizes are reduced to get minimum material cost and yet displacement remain within permissible limits. Table 3 Showing percentage of economy achieved in optimally braced frames with outriggers Case no. (For 5 bay 12 storey structures) Percentage of Saving B (3) + L (9) 6.00% B (3) + L (7) 5.72% B (3) + L (10) 5.70%
  • 5. Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames 789| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015) The Fig. 6 (a, b & c) shows the partially braced frame for the combination of specific bays and levels braced. The respective combinations and economy is tabulated as shown in Table 3 below. 4.4 Optimally cellwise braced frames To make the structure reasonably more flexible and simultaneously requiring a least number of bracing elements, braces are provided at suitable, predetermined, well thought selected locations on the cells occupied by the so called outriggers. It also provides the more flexibility for architectural provisions and offers the space to accommodate the openings/doors and windows. So it is advantageous to try cellwise bracing in these floors with symmetrical cases beyond which the economy is going to decrease. Hence it is decided to do cellwise bracing in two ways. a) In those structures which offer maximum economy in levelwise bracing and b) In those structures which offer maximum economy in baywise bracing 4.4.1 Cellwise bracing – moving levelwise For 5 bay 12 storey structures case number 03 i.e. whole central bay braced structure offer 6.13% economy as compared to bare frame. Hence it is decided to provide single cellwise bracing pattern that is moving levelwise in respective levels to find out such a cell braced frame which gives more economy as compared to bare frame as well as optimally (central bay) braced frame i.e. case number 03. So it is tried to find out more economical solution by way of the provision of single cell brace rather than providing whole central bay bracing. The hidden advantage behind the provision of cellwise bracing is that, the speedy construction which is faster than other bracing patterns utilized previously. The same logic is applied for the combinations of cells braced more than one i.e. two and/or three cells braced at a time and so on. a) Single Cell Braced at a Time To get the deep insight about savings to achieve in cellwise braced frames, it is decided to do cellwise bracing in 5 Bay 12 storey structures for central bay braced i.e. case number 03 in the same structure. Table 4 Showing percentage of Economy achieved in cellwise braced frames Case no. (For 5 bay 12 storey structures) Percentage of Saving 4 9.29% 7 9.26% 3 9.17% Above table shows the economy obtained for one cell braced frame which offer more economy as compared to bare frame as well as other single cellwise braced combinations of the same frame. The results are tabulated in the following Table 4 and the patterns of bracing which gives an optimum result is also shown in Figure 7. a b c Fig.7 Optimum cellwise bracing patterns for 5 bay 12 storey structures b) Two Cells Braced at a Time It is seen from following table 6 that the combination of 1st & 6th floor cell gives 9.28% economy which is 51.39% more than central bay braced throughout, but comparing to single cell braced it gives slightly less economy and as we move towards upper floor cellwise bracing combinations the economy is going to reduce by 1.72% and 2.15% for 1st & 4th floor cell and 2nd & 5th floor cell braced combinations respectively as compared to 1st & 6th floor cellwise bracing combination of the same structure. The results are tabulated in the following Table 5 and the patterns of bracing which gives an optimum result is also shown in Figure 8. Table 5 Showing percentage of Economy achieved in cellwise braced frames Case no. (For 5 bay 12 storey structures) Percentage of Saving 1+6 9.28% 1+4 9.12% 2+5 9.08% a b c Fig.8 Optimum cellwise bracing patterns for 5 bay 12 storey structures
  • 6. Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames 790| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015) c) Three Cells Braced at a Time It is seen from following table 7 that the combination of 2nd, 5th and 8th floor cell gives 8.94% economy out of other combinations of three cells braced, which is 45.84% more than central bay braced throughout, but comparing to single cell braced it offers less economy and as we move towards upper floor cellwise bracing combinations the economy is going to reduce. The results are tabulated in the following Table 6 and the patterns of bracing which gives an optimum result is also shown in Figure 9. Table 6 Showing percentage of Economy achieved in cellwise braced frames Case no. (For 5 bay 12 storey structures) Percentage of Saving 2+5+8 8.94% 1+4+6 8.87% 2+4+6 8.44% a b c Fig.9 Optimum cellwise bracing patterns for 5 bay 12 storey structures It is seen that single cell braced frames offer more economy as compared to bare and other cases of cell bracing combinations which concludes that, when an appropriately located single cell is braced makes a structure more rigid than other cases of combinations of bracings. Further it is seen that the economy of cellwise bracing configurations tried for other floors will reduce the economy gradually hence it is wise to restrict further combinations. 4.4.2 Cellwise bracing – moving baywise The levelwise bracing combinations are restricted up to three levels only since because of the bracing combinations beyond third floor are not offering more economy and also such combinations are not feasible to do and represent them. When ground floor is braced for 5 bay 12 storey structures throughout in levelwise such bracing pattern offers 7.41% economy and hence it is decided to provide cellwise bracing that is moving baywise in respective bays to yield more economy. So it is tried to find out more economy due to the provision of single cell braced (but placed geometrically similar) rather than providing whole level braced throughout from the conclusions which are recommended in above cellwise bracing combinations i.e. moving levelwise. The same logic is applied for the combinations of cells braced more than one i.e. two and/or three cells braced at a time and so on for respective floors which offered more economy already. a) Ground Floor Braced When cellwise bracing is provided by moving baywise it is wise to consider symmetrical cases since they behave symmetrically in case of change in earthquake directions. Table 7 Showing percentage of Economy achieved in cellwise braced frames Case no. (For 5 bay 12 storey structures) Percentage of Saving Braced cell 3 8.88% Braced cell 2+4 8.67% Braced cell 1+3+5 8.34% The single cell which is symmetrically placed in the ground floor is same as in the previous table offering an economy of 8.88%. Following Table 7shows the other cases of cell braced at ground floor only seeking the case of specific cell braced which debt more economy and the Figure 10 shows the cases tried. a b c Fig.10 Optimum bracing pattern for 5-bay 12 storey structures, Ground floor level braced. b) Fourth Floor Braced Now for this case fourth floor is braced cellwise to see the frame with optimum economy. As discussed earlier, the cellwise bracing is provided by moving baywise and it is wise to consider symmetrical cases
  • 7. Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames 791| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015) since they behave symmetrically in case of change in earthquake directions. Following Table 8 shows the cases of cells braced and their combinations at fourth floor only with symmetry, offering more economy and the Figure 11 shows the cases tried. Table 8 Showing percentage of Economy achieved in cellwise braced frames Case no. (For 5 bay 12 storey structures) Percentage of Saving Braced cell 3 9.29% Braced cell 2+4 9.05% Braced cell 1+3+5 9.31% a b c Fig.11 Optimum bracing pattern for 5-bay 12 storey structures, Fourth floor level braced. c) Sixth Floor Braced In the same way symmetrical cases of cell bracing were tried for sixth floor braced to obtain more economy. Now these cases were tried with symmetry as mentioned above. The cases which offer more saving are presented in the following Table 9 and Figure 12 shows the cases tried. Table 9 Showing percentage of Economy achieved in cellwise braced frames Case no. (For 5 bay 12 storey structures) Percentage of Saving Braced cell 3 9.26% Braced cell 2+4 9.00% Braced cell 1+3+5 9.00% Comparing the economy of all the different cellwise braced frames with respect to different floors overall it is seen that the economy is more in the case of cellwise braced frames that is moving levelwise as compared to cellwise braced frames that is moving levelwise . In fourth floor braced frame i.e. braced cell 1+3+5 offer 9.31% economy which shows that such configurations offer more stiffness to the frame as compared to other cases. Similarly in sixth floor braced frame i.e. Braced cell 3 offer 9.26% economy which is nearly equal to the cellwise braced frame. a b c Fig.12 Optimum bracing pattern for 5-bay 12 storey structures, sixth floor level braced. It concludes that as we towards upper level with increased number of cell bracing combinations will offer savings which are nearly equal to single cell braced configuration when the braces are provided at equal spacing. 4.4.3 Combination if cellwise bracing – moving levelwise and baywise Based on findings presented in above discussion it is observed that cellwise braces when provided by moving levelwise offer some more economy as compared to when braces are provided by moving baywise, particularly when we talk about single cell braced at a time which offer more economy as compared to bare and other cases of cellwise bracing i.e. two cells, three cells etc. braced at time in the structure. But it also shows that when bracing are provided at equi-spaced and at appropriate level will offer more economy as compared to single cell braced configurations. Hence to obtain still more economy it is decided to use same strategy which is used in the case of partially braced frames i.e. to combine the economy obtained in baywise braced frames and levelwise braced frames throughout the bay and level respectively. The combination comprises the case of cellwise bracing that is provision is made by moving either levelwise or baywise. When we consider the economy of single cell braced, in that, the cell braced by moving levelwise i.e. case number 04 gives 9.29% economy and the three cells braced at a time by moving baywise i.e. braced cell 1+3+5 case gives 9.31% economy which is more as compared to other combinations of cellwise bracing. Hence it is again restricted to do the combinations for those cases which have more economy than other combinations of cellwise bracings which are provided
  • 8. Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames 792| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015) either by levelwise or baywise for 5 Bay G+11 structure i.e. localizing the view over the cases which produce more economy than other, such as only bay braced or level braced throughout, combinations of them and cellwise braced with combination of them which is one cell less than the number of cells in either bay or level throughout. Table 10 Showing percentage of Economy achieved in cellwise braced frames Case no. (For 5 bay 12 storey structures) Percentage of Saving Trial 1 9.04% Trial 2 8.97% Trial 3 8.88% Trial 4 8.63% Trial 1 Trial 2 Trial 3 Trial 4 Fig.13 Optimum bracing pattern for 5-bay 12 storey structures with arbitrary bracing pattern 6. Storey drift The lateral displacements of frames were observed at common point at specific position in 5 bay 12 storey structures. The comparison is made between bare, fully braced, partially braced i.e. optimum bay braced and level braced, outrigger and cellwise braced frames. It was checked whether the structure satisfies maximum permissible relative lateral drift criterion as per IS: 1893-2002 (Part-I) which is 0.004H. To insure the serviceability criterion, considering the lowest point as base of structure and highest point as top of the structure, maximum relative drift is found and simultaneously compared with the permissible one as well as with other analyzed structures such as fully braced, partially braced and outriggers and cellwise braced frames. The values are tabulated in Table 11 and the variation is shown in Fig. 14 below. For bare frames the lateral drift is well within the permissible one, but it is increasing as one move towards the top most point of structure. Also the lateral displacements for fully braced frames are getting substantially reduced as compared with bare frame, but these are uneconomical structures from saving point of view. For optimally bay braced frames and for outriggers the lateral displacements are getting reduced by near about half and more as compared to the bare frames. There is no valuable reduction in the lateral displacement for optimally levelwise braced frames. Table 11 Showing variation of lateral displacement along height of structure for 5 Bay 12 storey structure with 350 mm beam depth Analyzed Structures of 5 Bay 12 storey structures for 350 mm beam Depth Ht.FromBase (m) BareFrame (mm) FullyBraced Frame(mm) OptimumBay Braced(mm) OptimumLevel Braced(mm) Framewith Outrigger (mm) CellwiseBraced Frame (mm) 0 0 0 0 0 0 0 2.44 1.05 0.66 0.74 0.59 0.742 1.00 5.44 4.14 1.48 2.27 1.68 2.21 3.81 8.44 8.72 2.26 4.31 4.17 4.10 7.85 11.44 14.23 3.00 6.62 8.04 6.23 12.47 14.44 21.12 4.13 9.44 13.88 8.76 17.10 17.44 28.95 5.18 12.55 21.2 11.46 21.03 20.44 36.98 6.13 15.78 28.97 14.14 26.21 23.44 45.00 7.14 19.13 36.88 16.71 32.89 26.44 52.32 8.01 22.44 44.15 18.58 39.71 29.44 58.58 8.74 25.61 50.4 19.84 45.76 32.44 63.56 9.33 28.56 55.38 21.16 50.63 35.44 67.1 9.77 31.21 58.92 22.67 54.12 38.44 69.57 10.13 33.63 61.4 24.11 56.54 Fig.14 Variation of lateral displacement along height of structure for 5 Bay 12 storey, 350 mm beam depth for L/C 7 i.e. (1.5 D.L. + 1.5 E.Q.) The graph shows the similar variation of lateral displacement for optimally cellwise braced frame which is less than that of bare frame but well within the permissible limit. In lateral displacement 18.73% decrement is found in the cellwise braced frame as compared to bare frame. Similarly for partially braced frame this amount is getting reduced by nearly half and more as compared to bare frame. Hence it is proved that for cellwise braced frame though the drift is larger than bare frame, but is well within the acceptable limit. In nutshell, it is advantageous to adopt such optimally cellwise braced frame to increase the economy.
  • 9. Kulkarni J. G. et al Seismic Response of Cellwise Braced Reinforced Concrete Frames 793| International Journal of Current Engineering and Technology, Vol.5, No.2 (April 2015) Conclusions 1) Fully braced frames are very conservative in so far as lateral drift is concerned but uneconomical and at the same time optimally braced one have least forces induced in the structure and produce maximum displacement but within prescribed limit. 2) It is seen that maximum economy is obtained with less number of bracings are used with proper position and at the same time lateral drift is within acceptable range offering more economy as compared to bare frames. 3) Optimally (cellwise) braced frames are stiff, strong, and an economical structural system. References Shan – Hau Xu & Di – Tao Niu, (Jan–Feb-2003), Seismic Behavior of R.C. Braced Frames, ACI Structural journal, No.1, 100. A. R. Khaloo & M. Mahdi Mohseni, (2008), Nonlinear Seismic Behavior of RC Frames with R.C. Braces, Asian Journal of Civil Engineering, Vol. 9, No. 6. J.P. Desai, A.K. Jain and A.S. Arya, (1987) Seismic response of R.C. Braced Frames, Computers and Structures Vol. 29, No. 4. M.A. Youssefa, H. Ghaffarzadehb, M. Nehdi, (23 October 2006), Seismic performance of RC frames with concentric internal steel bracing, Department of Civil and Environmental Engineering, Shiraz University, Shiraz, Iran, 71345-1676,.