Composite Columns I
Transcript of Composite Columns I
-
8/10/2019 Composite Columns I
1/21
Composite
Columns
J Y Richard LiewProfessor
Department of Civil Engineering
1
Tel: +65 6516 2154
Fax: +65 6779 1635
Email: [email protected]
2Concrete filled Tubular Column
-
8/10/2019 Composite Columns I
2/21
Applications of CFTsApplications of CFTs
3
Types of composite columns
Encased Partially encased
4Infilled
-
8/10/2019 Composite Columns I
3/21
Can be com lex in fabrication and/or construction,
General comments on
composite columns
General comments on
composite columns
but
Can be very strong - range of capacities for the
same external dimensions. It may be possible to
keep columns externally similar over all storeys of
a building.
5
ost types ave g n erent re res stance
without additional protection.
bc
b
Concrete-encased sectionsConcrete-encased sections
Completely Encased
h
cz
cycy
h
Concrete usually provides
all necessary fire
resistance
6
cz
t
f
tw
z
-
8/10/2019 Composite Columns I
4/21
= bcb
Concrete-encased sectionsConcrete-encased sections
Partially Encased Steel
=
Concrete is poured in 2
stages with section
horizontal.
Needs additional
reinforcement for fire
7
c
t
f
tw
z
.
May need additional fire
protection material.
May need studs or rebars
welded to section for force
transfer.
= bcb
Concrete-encased sectionsConcrete-encased sections
Fabricated Steel Section
= h
b
h
Concrete may be pumped
into voids during
construction.
8z
twt
f
-
8/10/2019 Composite Columns I
5/21
Concrete-filled hollow sections
Concrete-Filled Rectangular
Hollow Section
y
t
h
Concrete may be pumped
into hollow section during
construction.
Confined concrete has
higher strength than in
9
z
.
Needs additional
reinforcement for fire
resistance.
May need additional fire
protection material.
Concrete-filled hollowsectionsConcrete-filled hollowsections
Concrete-Filled Circular
Hollow Section
y t
Concrete may be pumped
into hollow section during
construction.
Confined concrete under
hoop tension has much
higher strength than in
10
z
normal use.
Needs additional
reinforcement for fire
resistance.
May need additional fire
protection material.
-
8/10/2019 Composite Columns I
6/21
sectionssections
Concrete-Filled Circular Hollow
Section encasin an o en section
ty
The internal steel section
can enhance strength to a
very high level.
11
z
Avoiding local buckling -fully encased sections
Avoiding local buckling -fully encased sections
Concrete cover to section (cy) :
> 40mm
> b/6
must be reinforced laterally,
12
cycy b
-
8/10/2019 Composite Columns I
7/21
Avoiding local buckling - partially
encased/concrete filled sections
Avoiding local buckling - partially
encased/concrete filled sections
k.yf/235 where fy.k is characteristic strength of section
t
d
td
b b
13
290t/d 52t/d 44t/b f
tf
Bare steel 802 Bare steel 40 Bare steel 15
S355 Steel
355
Behaviour of Short Composite Column under compression
14
0.00175
40
0.0035
C40 concrete
-
8/10/2019 Composite Columns I
8/21
Local Buckling of Steel
Both concrete and steel attain same strain under
Steel yield first before concrete reaches its peakcompression stress.
The steel must have sufficient ductility toundergo further strain without local buckling.
Therefore it must be at least a compact section.
15
However, concrete prevents the steel plate frombuckling. Therefore d/t ratio of compositesection can be larger than that of the bare steel.
Material Properties of Concretefck/fcu 20/25 25/30 30/37 35/45 40/50 45/55 50/60
ck(N/mm2)
Ecm(N/mm2)
29000 30500 32000 33500 35000 36000 37000
fck = characteristic cylinder strength
16
cu Ecm =Secant modulus of concrete under short term loading.
For light weight concrete the value Ecm is modified by2
2400
-
8/10/2019 Composite Columns I
9/21
Design Methods
BS5400: Part 5: Code of practice for the design of composite
bridges published by BSI in 1979
BS5950: Part1:2000, Code of practice for structural steel design
published by BSI in 2000: . conservative but simple "cased strut"
method
Eurocode 4: Design of composite steel and concrete structures
Part 1.1: General rules and rules for buildings published by CEN,
17
1992
Reading list: Assessment of current methods for the design of
composite columns in buildings by J Y R Liew - IVLE
General and Simplified Design Methodsin EC4General and Simplified Design Methodsin EC4
General MethodGeneral Method
Simplif ied MethodSimplif ied Method
- ,
Can be used for asymmetric sections,
Needs suitable software for numerical calculation.
18
,
Geometric imperfections and residual stresses taken into account in
calculation, using Eurocode buckling curves,
Plane sections remain plane.
-
8/10/2019 Composite Columns I
10/21
Limitation of the simplified method
bc
h
c
z
c
y
c
y
h
b
6%
19
c
cz
t
f
t
w
z
5,0 > (depth/width) > 0,2,
Simplified design method Concrete-encased sectionsSimplified design method Concrete-encased sections
Lon itudinal reinforcement area bc> 0,3% of concrete cross-section
area.
hc
cz
cycy
y
Concrete cover :
y-direction: 40 mm < cy < 0,4 bc
z-direction: 40 mm < cz < 0,3 hc
20
cz
z
Only include area of longitudinalreinforcement in calculating
cross-sectional resistance up to
6% of the area of the concrete.
-
8/10/2019 Composite Columns I
11/21
Cross-section resistance to axial compression is the sum of the plastic
compression resistances of each of its elements:
Axial Compression - Cross-
section Resistance
Axial Compression - Cross-
section Resistance
Concrete-encased sections
s
sks
c
ckc
a
y
aRdpl
fA
fA
fAN
85,0..
21
Section
Concrete
Reinforcement
a = 1.0; c = 1.5; s = 1.15 are material factor of safety
Cross-section resistance to axial compression is the sum of the plastic
compression resistances of each of its elements:
Axial Compression - Cross-
section Resistance
Axial Compression - Cross-
section Resistance
s
sks
c
ckc
Ma
y
aRd.pl
fA
fA
fAN
Concrete-filled hollow sections
22
Confinement causes increased
concrete resistance from 0,85fck to fck.
ect on
ConcreteReinforcement
-
8/10/2019 Composite Columns I
12/21
More concrete com ressive resistance is
Axial Compression - Cross-
section Resistance
Axial Compression - Cross-
section Resistance
Concrete-filled circular hollow sectionsd
caused by hoop stress in the steel section.
Only happens when most of the lateral
expansion of concrete is prevented.
t
23
5,0
d0,1NM Sdmax.Sd Maximum bending moment
Used in design if:
Relative slenderness
Axial Compression - Cross-sectionResistanceAxial Compression - Cross-sectionResistance
Concrete-filled circular hollow sections
Plastic com ression resistance is:
If equivalent eccentricity e=Mmax.Sd /NSd
then for 0 d/10 use 10 = 1 and 20 = 0
-
8/10/2019 Composite Columns I
13/21
Effect of concrete confinement
1
25
2
Basic values 10 and 20 to allow for the effect
of triaxial confinement in concrete filled circular
hollow sections
Length Effect
0.0 0.1 0.2 0.3 0.4 0.5
20 4.9 3.22 1.88 0.88 0.22 0.0
0.75 0.80 0.85 0.90 0.95 1.00
26
e.g., if eccentricity e = 0 and
for very short column, 01 = 10 = 0.752 = 20 = 4.9
y yck sk pl.Rd a 1 c 2 s
a c ck s
y
a y c ck s sk
ck
f ff ftN A A 1 A
d f
ft0.75A f 0.67A f 1 4.9 0.87A f
d f
-
8/10/2019 Composite Columns I
14/21
Summary
Concrete-encased sections
Concrete-filled rectangular hollow sections
pl.Rd a y c ck s skN A f 0.67A f 0.87A f
l.Rd a y c ck s sk N A f 0.57A f 0.87A f
27
Concrete-filled circular hollow sections
ypl.Rd a y 1 c ck 2 s sk
ck
ftN A f 0.67A f 1 0.87A fd f
+
2>1+1
High strength and fire resistance
dvantages of CFSTsdvantages of CFSTs
Circular
High stiffness and ductility
Restraint to local buckling by
concrete
Omission of formwork, reducing
construction cost and time
28Square and rectangular
> OR
-
8/10/2019 Composite Columns I
15/21
Questions
Q1 Why concrete filled tube is more efficientthan encased steel column to resist axial
load?
Q2 Why is it important to use compact
section for composite columns?
Q3 What are the key advantages of
29
concrete filled composite columns
compared to encased columns?
Column buckling resistance
Rdplsd NN ,
0.11
22
but
22.012
1
L
30
is the imperfection factor which allows for different levels of imperfections in the columns
= 0.21 for buckling curve a
= 0.34 for buckling curve b
= 0.49 for buckling curve c
-
8/10/2019 Composite Columns I
16/21
Buckling resistance of a composite column -
Strength reduction factor
Buckling resistance of a composite column -
Strength reduction factor
Buckling reducedfrom critical by 1
][
12/122
])2,0(1[5,02
in which
1,0
Rd.plRd.b N/N
Perfect critical
loads
Plastic resistance
Buckling curves for composite columns:
Impf. Column Type
(a) 0,21 L/300 Concrete-filled sections,
reinf < 3%, no steel
section.
31
Relative Slenderness
0 1,0
, ncase -sect ons n
major axis buckling,
Concrete-filled sections,
3%
-
8/10/2019 Composite Columns I
17/21
Non-dimensional Slenderness
= (Npl.Rk /N
sksckcyaRkpl fAfAfAN 85,0..Npl,R = Cross section compression resistance
without material factor of safety
(Npl.Rk is Npl.Rd calculated using a = c = y = 1,0)Characteristic strength
Ncr= Elastic critical load calculated based
on effective stiffness (EI)e
33=
Elastic critical load of a compositecolumn for short term loadingElastic critical load of a compositecolumn for short term loading
Elastic critical load2
e2
cr
)EI(N
Ecm secant modulus of
concrete
c Partial safety factor
for concrete stiffness
(=1,35) to account
for concrete crackin
fl
ssc
c
cmaae IEI
E8,0IE)EI(
For short-term loading
Effective stiffness
0.6
34
under moment
0,8 Reduction factor for
cracking
Lfl is effective buckling length of column
(may be taken as system length for rigid frame).
-
8/10/2019 Composite Columns I
18/21
Elastic critical load of a composite
column for long term loading
Elastic critical load of a composite
column for long term loading
Elastic critical load2
e2
cr
)EI(N
fl
ssccaae IEIE8,0IE)EI(
For long-term loading
Effective stiffness
c cmG.Sd
t
Sd
1E E
N1
N
NG.Sd is permanent
part of the axial
35Lfl is buckling length of column (may be taken as system length for rigid frame).
is EC2 creep
coefficient = 0.5t
design load NSd
See next slide
For slender column under long term load, creep and shrinkage will cause a
reduction in flexural stiffness.
No need to consider if e > 2d and is smaller than the following limit:
Effect of Long Term Load
Section Types Nonsway column Sway Column
Concrete encased 0.8 0.5
Concrete filled 0.8(1-) 0.5(1-)
36
a y
a pl .Rd
A f
N
where is the relative contribution of the
steel section to overall axial plastic
resistance.
-
8/10/2019 Composite Columns I
19/21
Buckling Resistance - EC 4
Design Procedure
Determine
e
Plastic resistance Npl,Rk and elastic criticalbuckling load Ncr
Non-dimensional slenderness ratiopl.Rk
cr
N
N
37
Buckling resistance x Npl,Rd
Check NSd Npl.Rd
HomeworkQ1 Determine the cross-section compression resistance
(Length = 0) of the CHS columns without infilled concrete.
Design a smaller infilled concrete section that can resist
the same axial load as the pure steel CHS section.
(a) CHS 219.1 x 6.3 S355Unfil led = 1460 kNFil led (40/50 concrete) = 2280 kN ( + 56% )
or CHS 168.3 x 6.3 S355J + 40/50 Conc.
(b) CHS 406.4 x 8.0 S355
38
Unfil led = 3550 kNFil led (40/50 concrete) = 7000 kN ( + 100% )
or CHS 273 x 6.3 S355 + 40/50 Conc
Q2 Repeat the above examples with column length = 5m
-
8/10/2019 Composite Columns I
20/21
Q3 (a) Design a UC steel column S355 of 4m length to resist a
factored compression force of 2800kN acting at the centroid of
the cross section.
(b) Redesign it using a fully encased UC section as shown below.
39
UC S355 steel
Column C80
00
6000 6000 6000 6000 6000
3000 3000A
B
1 2 3 4 5 6
AQ4
80
00
C
Column A
A3350
3350
3350
A B C
2
4
3
40
Fig. Q4b Section View of a 25-Storey continuous frame
3750
8000 8000
Columns to
be
designed
G
1
-
8/10/2019 Composite Columns I
21/21
Q5 Simple Construction
41
6m
12m
Column A
Q6
Determine the cross section axial capacity
CHS 219 x 6.3 S355
CHS 168 x 6.3 S355
42
C35/45 Concrete