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


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Applications of CFTsApplications of CFTs

3

Types of composite columns

Encased Partially encased

4Infilled


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


4/21

= bcb


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


Fabricated Steel Section

= h

b

h

Concrete may be pumped

into voids during

construction.

8z

twt

f


5/21

Concrete-filled hollow sections

Concrete-Filled Rectangular

Hollow Section

y

t

h


into hollow section during

construction.

Confined concrete has

higher strength than in

9

z

.

Needs additional


resistance.



Concrete-filled hollowsectionsConcrete-filled hollowsections

Concrete-Filled Circular

Hollow Section

y t


into hollow section during

construction.

Confined concrete under

hoop tension has much

higher strength than in

10

z

normal use.

Needs additional


resistance.




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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


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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


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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


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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.


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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.


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Cross-section resistance to axial compression is the sum of the plastic

compression resistances of each of its elements:

Axial Compression - Cross-

section Resistance


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:


section Resistance


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


12/21

More concrete com ressive resistance is


section Resistance


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


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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


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


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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


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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%


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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).


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.


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


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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


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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

Documents

Composite Columns I