92
CHAPTER 5
EFFECT OF FLAT WIDTH TO THICKNESS RATIO ON
THE STRENGTH OF MEDIUM COLUMNS
5.1
GENERAL
In this chapter, the effect of flat width to thickness ratio (w/t) on the
ultimate load carrying capacity of hollow, PCC in-filled and 1% fiber content
SFRC in-filled light gauge steel box medium columns with slenderness ratio
between 92 and 107 have been studied. A total of 18 columns of size 40 mm
× 60 mm and 1.50 m length were prepared with six columns without in fill
(type A), six columns with plain concrete in-fill (type B) and six columns
with SFRC in-fill (type C) as shown in Table 5.1. Each set of six columns
consist of 1.60 mm, 1.80 mm and 2.00 mm as wall thickness of two numbers
each respectively.
5.2
FLAT WIDTH TO THICKNESS RATIO (w/t)
According to Indian Standard code of specification IS 801:1985, it
is the ratio between width of the elements excluding end fillets to the
thickness of the member. The flat width ratio for 40 mm × 60 mm × 1.6mm
column is [60-2(1.60+2.40)] / 1.60 = 32.50. Similarly, this value for 1.80mm
and 2.00mm thickness are 28.30 and 25.00. As per IS 801:1985, flanges of
closed square and rectangular tubes were fully effective ie (w/t)limit is 38.26.
Hence the chosen sections were fully effective. All the test specimens were
tested under axial and eccentric loading up to their ultimate capacity.
93
5.3
TEST SPECIMENS, EQUIPMENT AND PROCEDURE
The test set up was same as prescribed in the Chapter 4 for medium
columns. The specimens were instrumented to measure both longitudinal
strains and deflections. Electrical resistant strain gauges were fixed on the
faces of each steel tube as shown in Figure 5.1 to measure the longitudinal
strains at mid height. The load was applied in small increments of 5KN and
the observations such as longitudinal strains and lateral deflections at mid
height on two sides of the column were measured. The loads corresponding to
local buckling and post buckling (ultimate) stages were recorded for each
specimen.
Figure 5.1 Test set up
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Figure 5.2 Failure mode
5.4
FAILURE MODE AND ULTIMATE LOADS
The failure mode of the tested specimens is shown in Figure 5.2.
All the specimens were loaded to failure and the test specimens behaved in a
relatively ductile manner. The columns with hollow sections, the failure was
due to over all buckling with significant sign of local buckling at the center.
All the specimens filled with concrete have failed at the mid height due to
concrete crushing and steel yielding with out showing any signs of local
buckling of the shells. Hence, the columns were able to sustain more loads
before
failure
due
to
an
over
all
buckling.
The
SFRC
in-filled columns generally show higher ultimate loads than the corresponding
plain concrete in-filled and hollow columns. The experimental strengths and
the theoretical strengths of all the columns were calculated from the design
provisions given in Eurocode4 and listed in Table 5.1. The plain cement
concrete in-filled columns were taking 36%, 51% and 9% more load when
loaded axially and 63%, 57% and 66% more load when loaded eccentrically
95
for the three types of columns respectively compared to the reference hollow
columns. Similarly when SFRC in-filled columns compared with reference
hollow columns, were taking more load by 105%, 71% and 25% when loaded
axially and 168%, 123% and 69% more load when loaded eccentrically for
the three types of columns respectively. From the Table 5.1 it is clear that the
Eurocode4 predict the ultimate strengths conservatively for all types of
hollow columns.
Table 5.1 Comparison of experimental and theoretical strengths
Specimen
Size
Specimen
Specimen type
label
wbt
(mm)
Light Gauge
Steel
Rectangular
Box Section
Without infill
Type (A)
Plain Concrete
in-filled Light
Gauge Steel
Box Section
Type(B)
Steel Fiber
Reinforced
Concrete infilled
Light Gauge
Steel Box
section
Type (C)
*
Theoretical
SlenderTest
ness EccentriLoads
Loads
Ratio
city
(Eurocode4)
Pulti
(l/r)
(e) mm
Ptheory
(kN)
(kN)
91.67
0
34.52
30.51
91.67
6
23.15
20.00
92.23
0
59.64
40.00
92.23
6
41.75
33.88
92.86
0
84.76
55.00
92.86
6
55.13
37.14
*
106.27
0
47.09
45.00
*
106.27
6
37.67
36.14
*
105.58
0
90.25
88.59
*
105.58
6
73.56
68.04
*
105.05
0
92.21
90.29
*
105.05
6
83.18
78.00
107.49*
0
70.63
62.00
107.49*
6
62.86
50.00
106.76*
0
102.02
92.43
106.76*
6
93.35
80.00
106.16*
0
105.95
98.00
A1
A2
A3
A4
A5
A6
B1
B2
B3
B4
B5
B6
C1
C2
C3
C4
C5
40601.6
40601.6
40601.8
40601.8
40602.0
40602.0
40601.6
40601.6
40601.8
40601.8
40602.0
40602.0
40601.6
40601.6
40601.8
40601.8
40602.0
C6
40602.0 106.16*
6
93.44
81.50
The Slenderness ratio for the in-filled columns are calculated using the equivalent area
method.
96
The effect of flat width to thickness ratio on ultimate load was
shown in Figure 5.3 and the effect of slenderness ratio on the ultimate load
was shown in Figure 5.4. When the thickness of the hollow section increased
the flat width to thickness ratio decreased in turn increasing the ultimate load
carrying capacity of the columns. This showed that the flat width ratio has a
significant effect on the ultimate loads.
100
120
100
80
Loads in
60
kN
40
20
0
90
80
70
60
Hollow
Loads in kN
Pcc
28.3
Hollow
40
PCC infilled
30
SFRC infilled
20
Sfrc
32.5
50
10
0
25
32.5
Flat Width to
Thickness ratio
28.3
25
Flat width to Thickness ratio
i) Axial Load Case
ii) Eccentric Load Case
Figure 5.3 Flat width to thickness ratio vs. Ultimate load
120
100
90
100
80
70
80
Load in kN
60
Hollow
PCC infilled
40
SFRC infilled
20
Loads in kN
60
50
40
30
Hollow
PCC infilled
SFRC infilled
20
10
0
0
91.67
92.23
92.86
Slenderness ratio
i) Axial load case
91.67
92.23
92.86
Slenderness ratio
ii) Eccentric load case
Figure 5.4 Slenderness ratio vs. Ultimate load
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It is also observed that the variations in ultimate loads due to the
variations in thicknesses were uniform in the case of hollow columns unlike
the in-filled columns. The slenderness ratios of the in-filled columns are
calculated using the equivalent area method, hence for the in-filled columns
this value was more than hollow columns.
5.5
LOAD vs. DEFLECTION BEHAVIOUR
The load versus mid height deflections plots for all the 18 columns
are shown in the Figures 5.5 to 5.7. These figures show quite clearly that the
deflections are small during the initial part of the loading and increased
rapidly near the ultimate loads. Furthermore, the hollow columns are
exhibiting higher lateral deflections than the PCC in-filled and SFRC in-filled
columns for the eccentric load cases. The thickness of the shell also has an
impact on the deflections. When the thickness is more, the flat width ratio is
less and the lateral deflections are also less. The columns having higher flat
width to thickness ratios are exhibiting higher lateral deflections even at lower
load levels.
120
120
Axail loading
A1-Hollow
B1-Plain
C1-1.0%SFRC
80
60
Eccentric Loading
A2-Hollow
B2-Plain
C2-1.0%SFRC
100
Load ( kN )
Load ( kN )
100
C1
80
60
C2
40
40
B1
20
B2
20
A1
A2
0
0
0
4
8
Lateral Deflection ( mm )
12
0
4
8
12
Lateral Deflection ( mm )
Figure 5.5 Load vs. Lateral deflection of columns with w/t ratio of 32.50
98
120
120
Axial loading
A3-Hollow
B3-Plain
C3-1.0%SFRC
C3
Load ( kN )
80
B
60
A3
40
100
Load ( kN )
100
80
60
40
20
20
0
0
0
4
8
Eccentric Loading
A4-Hollow
B4-Plain
C4-1.0%SFRC
C4
B4
A4
0
12
4
8
12
Lateral Deflection ( mm )
Lateral Deflection ( mm )
Figure 5.6 Load vs. Lateral deflection of columns with w/t ratio of 28.30
Axial loading
A5-Hollow
B5-Plain
C5-1.0%SFRC
120
100
80
C5
60
Load ( kN )
Load ( kN )
100
B5
A5
40
80
0
0
12
B6
40
20
4
8
Lateral Deflection ( mm )
C6
60
20
0
Eccentric Loading
A6-Hollow
B6-Plain
C6-1.0%SFRC
120
A6
0
4
8
Lateral Deflection ( mm )
12
Figure 5.7 Load vs. Lateral deflection of columns with w/t ratio of 25.00
5.6
LOAD vs. MICRO STRAIN BEHAVIOUR
Figures 5.8 to 5.10 show that, the variations of strains with the
loads for all the 18 specimens. Initially, all the in-filled columns showed
uniform load-strain relationship and the hollow columns exhibited large
amounts of strain in the initial stages of loading. The plots clearly indicate
99
that the SFRC in-filled columns show better ductility performance when
compared to the PCC in-filled columns. Thus, the addition of the steel fibers
to the concrete improves its ductility.
120
Axial loading
A1-Hollow
B1-Plain
C1-1.0%SFRC
Load ( kN )
100
80
60
C
40
B1
A1
20
Eccentric Loading
A2-Hollow
B2-Plain
C2-1.0%SFRC
100
Load ( kN )
120
80
60
C2
40
B2
A2
20
0
0
0
0
500
1000 1500 2000 2500
500 1000 1500 2000 2500
Microstrain ( mm/mm )
Microstrain ( mm/mm )
Figure 5.8 Load vs. Micro strain of columns with w/t ratio of 32.50
Eccentric Loading
Load ( kN )
100
80
60
A3
B
40
20
A4-Hollow
B4-Plain
C4-1.0%SFRC
120
100
Load ( kN )
Axial loading
A3-Hollow
B3-Plain
C3-1.0%SFRC
C3
120
80
C4
60
B4
40
20
0
0
500 1000 1500 2000 2500
Microstrain ( mm/mm )
A4
0
0
500 1000 1500 2000 2500
Microstrain ( mm/mm )
Figure 5.9 Load vs. Micro strain of columns with w/t ratio of 28.30
100
Eccentric Loading
Axial loading
A5-Hollow
B5-Plain
C5-1.0%SFRC
Load ( kN )
100
80
C5
60
B5
A5
40
A6-Hollow
B6-Plain
C6-1.0%SFRC
120
100
Load ( kN )
120
80
C6
60
B6
40
20
20
A6
0
0
0
500 1000 1500 2000 2500
Microstrain ( mm/mm )
0
500 1000 1500 2000 2500
Microstrain ( mm/mm )
Figure 5.10 Load vs. Micro strain of columns with w/t ratio of 25.00
5.7
DISCUSSION OF RESULTS
Compared to the reference hollow columns, the PCC in-filled
columns take around 1.50 times and 1.55 times more ultimate loads
respectively, when the loads act axially and eccentrically. Compared to the
reference hollow columns, the SFRC in-filled columns take 1.75 times and
2.25 times more ultimate loads when the loads act axially and eccentrically.
The PCC in-filled columns are showing better load carrying capacity and
lower ductility compared to hollow columns for both axial and eccentric
loads. SFRC in-filled columns have better load carrying capacity and ductility
than the PCC in-filled columns. The flat width ratio has a significant effect on
the ultimate load carrying capacity. Columns with lower flat width ratios have
higher ultimate load carrying capacities. Axially loaded columns show better
strength and ductility than the corresponding eccentrically loaded columns.