Download - 11 Continuous Beams
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Continuous Beams
Sagging Moment Region` Hogging Moment Region
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Continuous Composite Beam System
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Continuous Composite Beam Advantages greater load capacity due to redistribution
of moments greater stiffness and therefore reduce
deflection and vibration.
Disadvantages increase complexity in design susceptible to buckling in the negative
moment region over internal supports. Two forms of buckling may occur: (i) local
buckling of the web and/or bottom flange (ii) lateral torsional buckling.
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Optimum Span/Depth Ratio of Composite Beam
Simply Supported Beam:
L/D = 18 to 22
Continuous Beam
L/D = 25 to 28
L = Span Length
D= Overall depth, including the concrete or
composite slab
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Introduction
Continuous beams may be more economical than simply supported beams.
However, special phenomena may occur which must be taken into account in design, such as: – local buckling of compressed plate elements – lateral-torsional buckling – cracking of concrete due to tensile stresses
These all occur in the hogging moment regions. In the sagging moment regions, design checks are similar
to those of simply supported beams.
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ft f
b f
hs
tension
compression
P.N.A.
Hogging Moment Resistance depends on the reinforcements within the effective width
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Effective Width of the Concrete Flange (1)
The effective width of the concrete flange depends on the distance between the zero-moment points, approximated by Le.
L1 L2 L3
1 10.85eL L=
( )2 1 20.25eL L L= +
3 20.7eL L=4 32eL L=
be1 or be2 = Le/8 < actual width Le = length between points of zero moment
2e1eoeff bbbb ++=
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Effective Width of the Concrete Flange (2)
L1 L2 L3
1 10.85eL L=
( )2 1 20.25eL L L= +
3 20.7eL L=4 32eL L=
At mid-span or an internal support: 2
01
eff eii
b b b=
= + ∑
/ 8ei e ib L b= ≤At an end support:
2
01
eff i eii
b b bβ=
= + ∑( )0.55 0.025 / 1.0i e eiL bβ = + ≤
Defining
b0: distance between the centers of the outstand shear connectors 8
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Hogging Moment Resistance effective width For internal beam, the effective width of concrete slab in the negative (hogging) bending region is given by Le = 0.5L and corresponds to an effective width of L/8, where L is the clear span (rather than L/4 for the positive (sagging) moment in a simply supported beam). This means that the bar reinforcement is concentrated in a relatively narrow width over the internal supports.
beff = L/8
For hogging moment
beff = 0.7L/4
For sagging moment
L1 L2 L3
1 10.85eL L=
( )2 1 20.25eL L L= +
3 20.7eL L=4 32eL L=
if bo = 0, and L=L1= L2
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Example : Effective width of edge beam
A continuous beam of uniform section consists of two spans and a cantilever, as shown in the figure below. Calculate the effective width for the mid-span regions AB and CD, for the support regions BC and DE.
A B C E
N
N’ 0.3 2.0
Section N-N’
6 m 8 m 2 m
D
0.15
Units: m
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For segment AB,
0.85 6 5.1 meL = × =
1 / 8 0.638 m > 0.3 me eb L= =
0.3 2.0
Section N-N’
1 0.3 meb =
2 / 8 0.638 m < 2 me eb L= =
0.15 0.638 0.3 1.088 meffb = + + =
0.15
Example : Effective width of edge beam
11
2
01
eff eii
b b b=
= + ∑
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Example 6.1
Perform similar calculation for the rest of the locations,
AB BC CD DE Le 5.1 3.5 5.6 4
Le/8 0.638 0.438 0.7 0.5 b0 0.15 0.15 0.15 0.15 be1 0.3 0.3 0.3 0.3 β1 - - - - be2 0.638 0.438 0.7 0.5
β2 - - - - beff 1.088 0.888 1.15 0.95
All units: m
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Classification of Composite Cross-Section
EN 1993-1-1 clause 5.5.2: Classification should be according to the less favorable class of elements in compression. A steel component restrained by attaching it to a reinforced concrete element may be placed in a more favorable class. Simply supported composite beams (sagging moment) are almost always in Class 1 or 2, because:
1. the depth of web in compression (if any) is small 2. the connection to the concrete slab prevents local buckling of the steel
flange.
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1) Composite sections without concrete encasement Compression outstand flange unrestrained from buckling follow EN 1993-1-1 Table 5.2 Web follow EN 1993-1-1 Table 5.2
c t
c
t
Classification of Cross-Section under hogging moment (1)
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Cross-section classification (5)
Classification boundaries for webs in pure bending and uniform compression (EC3)
Class Pure bending Uniform compression 1 72ε 33ε 2 83ε 38ε 3 124ε 42ε
yf
2N/mm 235=ε
rolled
welded
d / t t trolled
welded
d / t
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Class Web subject to bending Web subject to compression
Web subject to bending and compression
Stress distribution
(compression positive)
1 d/t ≤ 72 ε d/t ≤ 33 ε when α>0,5: d/t ≤ 396ε/(13α−1)
when α<0,5: d/t ≤ 36ε/α 2 d/t ≤ 83 ε d/t ≤ 38 ε when α>0,5: d/t ≤
456ε/(13α−1)
when α<0,5: d/t ≤ 41,5ε/α
Stress distribution
(compression positive)
3 d/t ≤ 124 ε d/t ≤ 42 ε when ψ>-1: d/t≤42ε/(0,67+0,33ψ)
when ψ ≤-1:
d/t≤ 62ε.(1−ψ).
Classification for web (negative bending)
+
+
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2) Composite sections with concrete encasement
bc
b bc
b
0.8 1.0cbb
≤ ≤
Web encased in concrete can be assumed to be Class 1 or Class 2 provided: the concrete that encases the steel section should be reinforced, mechanically connected to the steel section, and capable of preventing buckling of the web and of any part of the compression flange towards the web (clause 5.5.3(2) of BS EN 1994-1-1).
Classification of encased beam under hogging moment (2)
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Classification of outstand flanges in uniform compression
Class Type Web uncased Web encased (EC3) (EC4)
1 Rolled 9ε 10ε Welded 9ε 9ε 2 Rolled 10ε 15ε Welded 10ε 14ε 3 Rolled 14ε 21ε Welded 14ε 20ε
yf
2N/mm 235=ε
c t
c
t
rolled welded
c / t
Classification of encased beam under hogging moment (3)
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Cross-section classification
Class of the section is defined as class of the element with the less favourable behaviour (e.g.: class 1 web and class 2 flange = class 2 section)
beff
hchp
tw
d 20t εw
20t εw
Exception: if compression flange is at least class 2 and web is class 3, then the section can be considered class 2: – with the same cross-section, if the
web is encased – with an effective web, if the web is
not encased
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2) The minimum area of reinforcement for Class 1 and 2 sections:
Additional requirements on Class 1 and 2 sections if the reinforcement is in tension:
1) Ductility requirement on steel reinforcement ductility class B and C steels
s s cA Aρ≥235
y ctms c
sk
f f kf
ρ δ=
Ac= effective area of the concrete flange fy = nominal yield strength of the structural steel, MPa fctm = mean tensile strength of the concrete (EN 1992-1-1, Table 3.1 for normal
weight concrete or 11.3.1 for lightweight concrete, see next slide) fsk = characteristic yield strength of the reinforcement kc = coefficient accounting for the stress distribution prior to concrete cracking δ: = 1 for Class 2 and 1.1 for Class 1 (more reinforcing steels for Class 1)
Classification of encased beam under hogging moment (4)
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Mechanical properties of concrete
fck (MPa) 25 30 35 40 45 50 55 60
fctm (MPa) 2.6 2.9 3.2 3.5 3.8 4.1 5.2 4.4
Ecm (GPa) 31 33 34 35 36 37 38 39
Extract from Table 3.1 in EN 1992-1-1 (for normal weight concrete)
For light weight concrete, the fctm and Ecm values for a given grade are reduced by the following coefficient:
( )0.4 0.6 / 2200lctm ctmf f ρ= +
ρ refers to the density of lightweight concrete in kg/m3
( )2/ 2200lctm ctmE E ρ=
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Classification of Cross-Section (8)
( )0
1 0.31 / 2c
c
kh z
= ++
hc
N.A. z0
hc: thickness of the concrete flange, excluding ribs and haunches.
z0: distance between the centroids of the uncracked concrete flange and the uncracked composite section, calculated using n0 for short term loading
0 /a cmn E E=
Ea: modulus of elasticity for structural steel 22
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Plastic cross-section resistance
Basic assumptions: – full connection between steel and concrete – steel & reinforcement are full yielded – resistance of concrete in tension is zero
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Moment Resistance beff
Maximum Tensile force in reinforcement
sskss fAF γ= /
/a a y aF A f γ= γs = 1.15 γa = 1.0
Rw = dtfy
d
t
Fs
Fa
Maximum Compressive force on steel section
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Moment Resistance at Hogging Moment region
Two main cases for which formulae are developed – case 1: plastic neutral axis is in the flange of the
steel section – case 2: plastic neutral axis is in the web of the
steel section
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Plastic cross-section resistance Case 1: Fa > Fs ≥ Rw plastic neutral axis is in the steel flange
sskss fAF γ= /
/a a y aF A f γ=
and
Mpl,Rd = 0.5Faha + Fshs - (Fa - Fs)2/(4bffy)
ft f
b f
hs
tension
compression
P.N.A. hs
PNA from top of steel beam
Rw = Fa – 2bftf fy
Fa
Moment about top of the steel flange
2a s
fy f
F Fzf b−
=
(Fa-Fs)0.5Zf
hs = position of rebars from the beam flange
Simplified Moment Mpl,Rd ≈ 0.5Faha + Fshs
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Plastic cross-section resistance Case 2: Fa > Fs < Rw plastic neutral axis is in the web of the steel section
fsk / γs
Fs
P.N.A.
f / γy af / γy a
beff
tension
hchp
ha
Fa
Fatw
zw
h /2a
. . (0.5 ) 0.5pl Rd apl Rd s a s s wM M F h h F z− = + + −
2s
ww y
Fzt f
=
sskss /fAF γ=
and
PNA from centriod of steel beam
/a a y aF A f γ=
Moment about the center of the steel beam
2. . (0.5 ) 0.25 / ( )pl Rd apl Rd s a s s w yM M F h h F t f− = + + −
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Analysis of Continuous Beam
In BS EN 1994-1-1, the design of continuous composite beams may be based on two approaches to determine the design bending moments in negative (hogging) and positive (sagging) bending: • Clause 5.4.4 states that linear elastic analysis may be
used for composite beams with all section classifications using maximum permitted moment redistributions
• Clause 5.4.5 states that rigid plastic analysis may be used for Class 1 sections
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Methods of Analysis
•Simplified table of moment coefficients (BS5950:Part 1: 3-1) •Elastic analysis – uncracked section •Elastic analysis – cracked section •Plastic hinge analysis (class 1 section only)
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Simplified table of moment coefficients (BS5950 Part 1: 3-1 Table 2.)
* *
* *
* *
This method accounts for pattern loading, cracking of concrete and yielding of steel. 30
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Examples (BS5950 Part 1: 3-1 Table 2)
0.79wl2/8
0.50wl2/8 2 spans Non reinforced plastic section
0.52wl2/8
0.67wl2/8
0.80wl2/8
3 spans Reinforced compact section
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Some restrictions are placed on this method: uniform section with equal flanges and without
haunches. Steel beam should be the same in each span. The loading should be uniformly distributed. Unfactored imposed load should not exceed 2.5
times the unfactored dead load. No span should be less than 75% of the longest. End span should not exceed 115% of the length of
the adjacent span. There should not be any cantilevers.
Restrictions on the use of Simplified moment coefficients
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Elastic Analysis of Continuous Beam
The elastic analysis method depends on whether the composite cross-section is considered to be uncracked, or cracked in negative bending. For uncracked analysis, the stiffness of the beam is treated as being constant along its length. For cracked analysis, the stiffness of the beam is reduced in the negative (hogging) moment region and hence lower percentage redistributions of moment are permitted in comparison to the uncracked case.
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Cracked and un-cracked analysis
(b) Crack section
EIg
EIn EIg EIg
Uncracked analysis: use EIg through out Cracked analysis: Use EIn near the support and EIg outside the 15% length of the support
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Analysis of Continuous Composite Beam Distribution of bending moment
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Elastic Analysis
Uncracked Section (positive moment) n is the ratio of the elastic moduli of steel to concrete taking
into account the creep of the concrete (may assume n = 13) r is the ratio of the cross-sectional area of the steel section
relative to the concrete section , Aa/(beff hc). Iay is the second moment of area of the steel section
ay
3ceff
2apca
g In12hb
)nr1(4)hh2h(A
I +++
++=
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Ar = area of reinforcement hr = distance of reinforcement from the top of concrete slab
Cracked Section, Negative Moment
2( 2( ))4( )
a r a p c rn ay
a r
A A h h h hI I
A A+ + −
= ++
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hr
ha
hc hp
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Elastic analysis – moment redistributions
Analysis Method for Composite
Section
Section Class to SS EN 1993-1-1 1 2 3 4
Un-cracked Section 40% 30% 20% 10%
Cracked Section 25% 15% 10% 0%
Maximum moment redistributions for elastic global analysis of continuous composite beams (per cent of the initial value of the bending moment to be reduced)
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Resistance against combined bending and shear
Interaction diagram
V Sd
C B
A
V pl.Rd
V pl.Rd0,5
Mf.Rd
_M
Rd
_M
V.Rd
_
Low shear – moment capacity not reduced
Low bending – shear capacity not reduced
High bending and shear – interaction formula
−−⋅−+= −−−−
2
.... 121)(
Rdpl
SdRdfRdRdfRdv V
VMMMM
Moment capacity of flanges only
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Problems for continuous beam design
High shear
Lateral-torsional buckling at hogging moment region Shear connection design
Loss of serviceability due to concrete cracking
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Lateral-torsional buckling
The theory of lateral-torsional buckling of continuous beams over supports is rather complex.
In reality, lateral-torsional buckling is affected by: – beam distortion / lateral deflection of compressed flange – torsional rigidity of section
In design, two types of simplified approach may be
followed: – simplified calculation of lateral-torsional buckling resistance
according to analogy to steel beams (EC3 approach) – application of certain detailing rules that prevent lateral-torsional
buckling
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Lateral-torsional buckling EC3 approach
−− χ= RdLTRdb MM . −
−
=λcr
plLT M
MEC3 LT buckling
curves
In this approach, the elastic critical moment is determined using the so-called “inverted U-frame model”. The use of this model is subject to certain conditions. This model is not discussed here in detail
No lateral-torsional buckling if the lateral-torsional buckling slenderness ratio < 0.4,
0.4LTλ ≤
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Lateral Torsional Buckling at Hogging Moment Region
4.0LT <λ
≤ 0,4.
No reduction in capacity due to lateral torsional buckling
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Prevention of lateral-torsional buckling by bracing
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Member Steel grade S235
Steel grade S275
Steel grade S355
Steel grade S420 or S460
IPE/UB 600 550 400 270
HE/UC 800 700 650 500
Maximum depth h (mm) of uncased steel member to avoid lateral-torsional buckling checks (EC 4 Table 6.1 )
Lateral torsional buckling can also be prevented by limiting the depth of the steel
member at the hogging moment region
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Shear connection design
Basic rules • Connectors should be ductile • Plastic design of shear connection is possible even if global analysis
is elastic, provided that the end cross-sections of the critical length to be designed are at least Class 2
• In hogging moment regions, use of full shear connection is recommended
• In sagging moment regions, partial shear connection may be applied
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Shear Connections in Negative Moment Regions
PRd = design capacity of a shear connector in negative moment regions considering concrete cracking
Total no. of shear connectors = Np + Nn needed between the point of maximum moment and each adjacent support
Nn = Fs/PRd
= tension resistance of reinforcement
s sks
s
A fFγ
=
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0.851 min ; 1.5 1.15
eff c ck s skp n a y
Rd
b h f A fN N A fP
+ = ⋅ +
0.851 min ; 1.5
eff c ckp a y
Rd
b h fN A f
P
= ⋅
11.15
s skn
Rd
A fNP
= ⋅
Fa Fc
Fs
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Deflection
Deflection is affected by Pattern loading Cracking of concrete Yielding of rebars
But yielding of rebars and cracking of concrete have less influence on deflections in services than they do on analyses for ultimate limit states. Simplified method is developed for uniform beam in which deflection is estimated based on uniformly distributed load that the hogging end moments M1 and M2 reduce the md-span moment deflection from δo to δc 48
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Serviceability deflection
δc = δo{1-0.6(M1+M2)/Mo} δo = Deflection of a simply supported beam Mo = maximum sagging moment in the beam when it is simply supported
Deflection of a continuous beam (simplified)
M1 and M2 are moments after redistribution for pattern loads, etc.
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Influence of Pattern loading
To account for pattern loading as shown in the figure, reduce the uncracked moments at the internal supports by 40%
M1 and M2 are moments after redistribution for pattern loads, etc.
50
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Serviceability – Cracking of concrete
In continuous beams, concrete cracking is mainly due to
tensile stresses in the hogging moment regions
This cracking is prevented by limiting bar spacing or bar diameters in the reinforcement
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Serviceability – Cracking of concrete
Limiting bar spacing (for high bond bars only) to avoid cracking over supports
stress in reinforcementσs, N/mm2
maximum bar spacingfor wk = 0,4 mm
maximum bar spacingfor wk = 0,3 mm
maximum bar spacingfor wk = 0,2 mm
160200240280320360
300300250250150100
30025020015010050
20015010050––
this stress is calculated considering tension stiffening
sss σ∆+σ=σ 0sst
ctms
fρα
=σ∆4.0
…with...
unless using a more precise method:
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sss σ∆+σ=σ 0
σso = tensile stress in the reinforcement
0.4 ctms
st s
fσα ρ
∆ =
fctm is the mean tensile strength of concrete; ρs is the “reinforcement ratio” expressed as αst = As / Act Act is the area of concrete flange in tension within the effective width As is the total area of reinforcement within the area Act αst is the ratio
aa IAAI
where A and I are the area and second moment of area of the composite section neglecting concrete in tension and any sheeting, and Aa and Ia are the same properties for the bare steel section.
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Conclusions
Continuous beams offer advantages over simply supported beams, but special phenomena need particular attention during design in the hogging moment regions
In the case of both elastic and plastic design, cross-section classification and resistance calculation are key issues
Lateral-torsional buckling at the hogging moment regions must be prevented by appropriate detailing or by direct check
In shear connection design, hogging moment regions require full shear connection
In the hogging moment regions, the serviceability limit state of cracking of concrete may be relevant
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Example
Homework 7: Continuous Composite Beams