Effect of creep on composite steel concrete section

PREPAREDBY: ENG. KARIMAN MOSTAFSA
REVISEDBY: ENG. KAMELANWAR KAMEL
EFFECT OF CREEP ON COMPOSITE STEEL-
CONCRETE SECTION

Contents
1. Components of composite steel-concrete beam.
2. Behavior of composite and non-composite girders.
3. Definitions according to ECP.
4. Methods of construction.
5. Design considerations.
6. Calculation of stresses.
7. Creep and shrinkage.
8. Design for creep and shrinkage according to AASHTO LRFD and ECP.
9. Creep and unshored (unpropped) beam.
10. Creep and shored (propped) beam.
11. Creep with respect to causing Loads.
12. Effect of creep on composite beam.
13. Effect of degree of shear interaction on composite action.
14. Factors effecting the long-term stress of steel-concrete composite beam, including load factors and non-load
factors.
15. CSI Bridge model analysis and results.
16. Construction stage using CSI-Bridge.
17. Conclusion
18. References

1. Componentsof compositesteel-concretebeam

2. Behaviorof compositeand non-compositegirders

3. Definitionsaccording to ECP
1.1 Composite member: is a structural member with components of concrete and of
structural or cold-formed steel, interconnected by Shear connection so as to limit the
longitudinal slip between concrete and steel and the separation of one component
from the other.
1.2 Shear connection: is an interconnection between the concrete and steel
components of a composite member that has sufficient strength and stiffness to
enable the two components to be designed as parts of a single structural member.
1.3 Composite Beam: is a composite members subjected mainly to bending.
1.4 Composite Column: is a composite member subjected mainly to compression or
to compression and bending.
1.5 Composite slab: is a slap in which profiled steel sheets are used initially as
permanent shattering and subsequently combine structurally with the hardened
concrete and act as tensile reinforcement in the finished floor.

3. Definitionsaccording to ECP

• Case (I) – Unshored (Unpropped) Beam:
In this case, the steel beam has no intermediate support during casting of
concrete and it Works to support the slab form. Once forms are removed and
concrete has cured, the section will act compositely to resist any additional
dead load and the applied live load.
4. Methodsof construction

• Case (II) - Shored (Propped) Beam:
- In this case, temporary intermediate supports, shoring, are used during
casting of concrete. After curing of concrete, the shores are removed and the
section acts compositely to resist all loads.
- Temporary propping should not be removed until
the concrete has achieved 75% of its design strength.

• Case (II) - Shored (Propped) Beam:
- Shoring system representation.

• Effective width (be) :
In ordinary girder theory the bending stress is assumed constant across the girder
width and is calculated from the bending formula, f=M*y/I.
Since the composite section has a wide top flange, plate theory indicates that the
stress in the concrete slab is not uniform across the girder width. Referring to Fig. 6.5,
the stress is maximum over the steel girder and decreases non-linearly as the distance
from the supporting girder increases.
5. Designconsiderations

ECP defines the portion of the effective width of the concrete slab on each side of
the girder centerline bEL or bER so that 2be shall be taken the least of:
1- (L/4)
2- Spacing between girders from center to center.
3- 12𝑡 𝑠𝑙𝑎𝑏+𝑏𝑓𝑙𝑎𝑛𝑔𝑒
- Where L is the actual span between the supports.
- In case where b1 is different from b2, then the effective width be1 will be different from be2.

- For Continuous Beams:
If the adjacent spans are unequal, the value of be to used in calculating bending
stress and longitudinal shear in the negative moment regions shall be used on the
mean values obtained for each span separately.

7. Creepand shrinkage
• Creep: is increase in strain over time under the sustained constant stress, while
Shrinkage: is decrease in volume with time.
• In steel-concrete composite structures, creep and shrinkage are highly associated
with concrete, and these two inelastic and time-varying strains cause increase in
deformation and redistribution of internal stresses.
• Factors affecting the creep of concrete:
1- The curing condition of the concrete at the time the stresses are applied.
2- The intensity and duration of their effect.
3- The quality of the concrete.
4- The degree of humidity of its surroundings.

• Concrete is subject to two phenomena which alter the strain and therefore the
deflection of the composite beam.
• During casting the wet concrete gradually hardens through the process of
hydration. This chemical reaction releases heat causing moisture evaporation which
in turn causes the material to shrink. As the slab is connected to the steel section
through the shear connectors, the concrete shrinkage forces are transmitted into the
steel section. These forces cause the composite beam to deflect. For small spans
this deflection can be ignored, but for very large spans it may be significant and
must be taken into account.
• Under stress, concrete tends to relax, i.e., to deform plastically under load even
when that load is not close to the ultimate. This phenomenon is known as creep and
is of importance in composite beams. The creep deformation in the concrete gives
rise to additional, time dependent, deflection which must be allowed for in the
analysis of the beam at the service load stage.
7. Creepand shrinkage

• The actual calculation of creep stresses in composite girders is theoretically complex and not
necessary for the design of composite girders.
• Instead, a simple approach has been adopted for design in which a modular ratio appropriate to
the duration of the load is used to compute the corresponding elastic section properties.
• As specified in AASHTO LRFD Article 6.10.1.1.1b
- For transient loads applied to the composite section, the so-called ″short-term″ modular
ratio n is used.
- For permanent loads applied to the composite section, the so-called ″long-term″ modular
ratio of 3n is used.
- The short-term modular ratio is based on the initial tangent modulus, Ec, of the concrete,
while the long-term modular ratio is based on an effective apparent modulus, Ec/k, to account for
the effects of creep.
8.1. Design forcreepand shrinkageaccording to AASHTO LRFD

• As specified in ECP 10.1.4.8:
8.2. Design forcreepand shrinkageaccording to ECP

8.3. Design forcreepand shrinkage
• There are different mathematical methods used to calculate the Bending
Moments, Shear Forces, Resultant forces due to Creep and Shrinkage, Creep
coefficient, Shrinkage strain, Deformation, Curvature and slip amount
depending on several factors:
1- Full or Partial shear interaction between Steel and Concrete.
2- Type of applied load.
4- Simple or Continuous beam.
3- Short- and Long-Term Analysis.
4- Construction Method.
5- Modular Ratio.

9. Creepand unshored (unpropped) beam
• During construction the steel section is loaded
with the dead weight of wet concrete. The steel
section is stressed and deforms.
• The concrete and the connectors remain largely
unstressed, apart from the shrinkage stresses
developed within the hardened concrete.
• It can be seen, in Figure 6.8, that the wet concrete
ponds, i.e. the top surface of the concrete remains level
and the bottom surface deforms to the deflected
shape of the steel section.
• The dead load due to the weight of wet concrete is
a substantial proportion of the total load and the
stresses developed in the section are often high.
• Additional live loads are carried by the composite
section which has almost the same stiffness as that
of the propped beam.
• The stresses present in the unpropped section can
therefore be obtained by summing the wet
concrete stresses and the composite stresses.

10. Creepand shored (propped) beam
• During construction the steel section is supported
on temporary props. It does not have to resist
significant bending moment and is therefore
unstressed and does not deflect.
• Once the concrete hardens the props are
removed. Each of the component parts of the
beam then takes load from the dead weight of the
materials.
• However, at this stage, the beam is acting as a
composite element and its stiffness and resistance
are very much higher than that of the steel section
alone.
• The deformation due to dead loads is, therefore,
small. Any further live loading causes the beam to
deflect.
• The total stresses present in the beam can be
found by summing the stresses due to dead and
live loads.

11. Creepwith respecttocausing loads
• 11.1 Creep due to permanent loads (P)
• Figure 6.2 depicts the strain development in a
concrete cylinder subjected to a constant
compressive force N at time t0the age of
concrete at loading. One can observe that the
additional deformations due to creep can be 2–
3 times greater than the elastic ones.
• Taken into account that the creep coefficient is
in most cases between 2 and 3, one can easily
understand the importance of considering
creep in calculations of stresses and
deformations. Creep due to permanent loads,
(for example, self weights) will be notated with
the letter P.
• t0: the age of concrete at loading.

• 11.2 Creep due to temporarily permanent loads (PT)
• In bridges, there is also an important type of
loading that refers to permanent loads
whose magnitude changes constantly with
time.
• They are not described as permanent because
of their time-dependent magnitude; therefore,
they are called temporarily permanent
actions and are notated with PT.
• These may be stresses due to secondary
internal forces that are developed in statically
indeterminate structures or due to
longitudinal prestressing.

• 11.3 Creep due to imposed deformations (D)
• Imposed deformations in bridges may be due to support settlements.
• These displacements may be sudden or time varying.
• Sudden support settlements are introduced to the intermediate supports of continuous composite bridges
to limit cracking. This is an alternative solution to longitudinal prestressing.
• Time-varying support movements may arise due to soil consolidation.
• In Figure 6.4, the reduction of stresses after an instantaneous induced strain is illustrated.
• The resulting stresses decrease gradually due to creep.

• For the usual concrete dead loads, concrete does not behave as an elastic material.
• Actually, concrete is a plastic material subjected to progressive permanent deformation under
sustained loads (creep).
• Fig. 6.11. illustrates how creep strain changes with time, specifically the gradual increase and
decrease in strain depending on time and loading condition. The process is not fully reversible
and thus creep recovery is not complete. Even after the load on the concrete has been completely
removed, permanent and irreversible creep strain remains, although the elastic strain is
recovered.
• It is known that only permanent loads causing compressive stresses
in concrete produce creep. Moving loads have little effect,
as they do not last long.
12. Effectof creeponcomposite beam

• Suffice it to say, when a composite steel girder is subject to a constant sustained loading, such as
permanent loads applied to the composite section (e.g. barriers, railings, wearing surface, etc.), the
concrete deck stress is not constant.
• As time passes, the concrete creeps. The strain in the steel girder increases and the steel stresses
become larger, while the strains and concomitant stresses in the concrete deck are reduced. The
reduction of stress in the concrete is a function of the relative stiffness of the girder and the
concrete deck.
• Concrete stresses in composite beams are reduced by creep. Therefore the maximum concrete
stress should be determined by neglecting creep.

• A clearer picture on the effects of creep on composite girders is given in Figure 6.6.
• One can see that the deck slab is at time t0 under compression. This is the time that loading ML is
imposed.
• Due to creep, time-dependent cross-sectional forces are developed that redistribute tension from
concrete to steel; thus, concrete stresses become lower and steel stresses higher.
• The cross-sectional properties of the concrete slab are reduced through the long-term modular
ratio 3n.
• In contrast, structural steel keeps its stiffness and as a result, time-dependent redistributions arise.

• The load deflection response of a steel section
alone and of a composite beam, both propped
and unpropped, is shown in Figure 6.10
Fig 6.10 Load deflection response for a steel section
alone and a composite beam propped and unpropped

13. Effectof degreeof shearinteractiononcompositeaction
• If slip is free to occur at the interface
between the steel section and the
concrete slab, each component will act
independently, as shown in Figure 4.
• If slip at the interface is eliminated, or
at least reduced, the slab and the steel
member will act together as a
composite unit.
• The resulting increase in resistance
will depend on the extent to which slip
is prevented.
• It should be noted that Figure 4 refers to the use of
headed stud shear connectors. The degree of interaction
depends mainly on the degree of shear connection used.

• In real construction applications, the number of shear connectors required to achieve full shear
interaction may be so large that it is not practical to accommodate them in a composite beam due
to the associated problems of cost and workability.
• Therefore, the current design provisions generally allow some slip effects in composite structures
as long as these remain within safety limits for serviceability.
• Many engineers focus instead on controlling the amount of slip and the resultant behavior
effectively.

• In a study of the accuracy and reliability of various composite cross-sectional analyses introduced
in the present design codes, Nie and Cai (2003) focused on the design specifications that have
adopted the transformed cross-section method for the analysis of composite beams.
• After presenting the equivalent flexural rigidity concept, including the effective section modulus
and moment of inertia when composite beams are subject to shear slip effects.
• They concluded that including slip effects may result in a reduction in stiffness of up to 17% for
short span beams, which means that predictions should indeed take into account slip effects to
improve the accuracy of calculations.
• Noting that the existing design specifications ignore slip effects in many cases, Nie and Cai pointed out that those AISC
specifications that do take into account slip effects tend to generate conservative predictions for partial composite sections,
in contrast to the fully justifiable predictions for full composite sections in the AISC specifications.

Full interaction composite beams Partial composite beams
Short-term
analysis of the
cross-section
AISC proposes a lower bound elastic moment of
inertia 𝐼𝐿𝐵, for plastic composite sections, and if the
loading condition is determined, one may calculate
the mid-span deflection, as given in Eq.
5𝑊𝐿4
384𝐸 𝑠 𝐼 𝐿𝐵
(4-13)
The total strain distribution is determined by adding
the slip strain to the initial strain
at the steel-concrete interface.
Time-dependent
analysis using
the AEMM*
There is a gradual increase in the compressive stress in
the steel part as the concrete shrinks due to the effects
of creep and shrinkage. At this point, the internal
stresses are redistributed and the gradual change of
force in the steel is countered by an equal and opposite
restraining force on the concrete in order to maintain
equilibrium.
When the restraining forces due to creep and
shrinkage are released from the cross-section, the
strain distribution changes and additional
deformations occur. These released forces also add
to the slip strain if the beam acts as partially
composite beam.
*AEMM:age-adjustedeffectivemodulusmethod.

14. Factorseffecting the long-termstressof steel-concretecomposite
beam, including load factorsand non-load factors:
• 14.1 Effect of Concrete Age to Loading
- It can be seen that additional stress at the top of concrete slab is tensile stress and that at
the bottom of steel beam is compressive stress. And the longer the concrete age to loading is, the
ultimate additional stress will be smaller.

• 14.2- Effect of Longitudinal Reinforcement Ratio in Concrete Slab
the bottom of steel beam is compressive stress. The larger the longitudinal reinforcement ratio is,
the ultimate additional concrete slab stress at the top of mid-span section will be larger, and the
ultimate additional steel beam stress at the bottom of mid-span section will be smaller.

• 14.3- Effect of Concrete Slab Width
- It can be seen that additional stress at the top of concrete slab changes from tensile stress
to compressive stress. The additional stress at the bottom of steel beam is compressive stress and
its value becomes smaller.

• 14.4- Effect of Steel Beam Height
the bottom of steel beam is compressive stress. The larger the ratio R=hs/hc is, the ultimate
additional concrete slab stress will be larger, and the ultimate steel beam stress will be smaller.
Where; R=hs/hc is the ratio of steel
beam height to concrete slab thickness.

• 14.5- Effect of Environmental Yearly Average Relative Humidity
the bottom of steel beam is compressive stress. And the larger the RH is, the ultimate additional
stress will be smaller.

• 14.6- Effect of External Load Value
the bottom of steel beam is compressive stress. The larger the external load value is, the ultimate
additional stress will be larger.

• 14.7- Effect of Concrete Strength
- It can be seen that additional stress at the top of concrete slab is tensile stress and that at the
bottom of steel beam is compressive stress. The higher the concrete strength is, the ultimate
additional stress will be smaller.

15.1 CSI Bridge model analysis
• A CSI Bridge Model was conducted
using GRILLAGE Method as a Non-
Composite sections to study the effect
of short-term and long-term deflection
as creep is considered to be a Time-
dependent deflection.
• Model Description:
- simply supported bridge of 60.0m span length.
- considering no shear interaction.
- End slip is permitted.

15.2. Resultsof CSI
Bridge model analysis
• We notice a rapid increase in values
of both Moment and deflection in
the short-term after concrete
hardening and applying all additional
loads, and a nearly no change in the
long-term as no composite action
occurred between steel and concrete.
0
50
100
150
200
250
0 50 100 150 200 250 300 350 400 450
Deflection(mm)
TIME (days)
Effect of Creep with Time
0
500
1000
1500
2000
2500
3000
0 50 100 150 200 250 300 350 400 450
MomentService(t.m)
TIME (days)
Effect of Creep on Moment with Time

using GRILLAGE Method as a
Composite sections to study the effect
of short-term and long-term deflection
as creep is considered to be a Time-
dependent deflection, and the effect of
shear interaction on the composite
action.
- considering partial to full shear interaction.
- End slip is prevented or reduced.

15.2. Resultsof CSI
loads, and a slight change in the long-
term in the vertical direction that can
be neglected.
0
50
100
150
200
250
0 50 100 150 200 250 300 350 400 450
Deflection(mm)
TIME (days)
0
500
1000
1500
2000
2500
3000
0 50 100 150 200 250 300 350 400 450
MomentService(t.m)
TIME (days)

using Staged Construction Method to
study the effect of short-term and long-
term deflection as creep is considered
to be a Time-dependent deflection, and
the effect of shear interaction on the
composite action.
- considering partial to full shear interaction.
- End slip is prevented or reduced.

• - STAGE (1):
In This Stage, Only Steel Beams are placed.
Only Dead Load of steel beams is acting.
15.4 Resultsof CSI Bridge model analysis

• - STAGE (2):
In This Stage, Concrete is poured.
Steel Beams carry their own weight and
own weight of wet concrete.
There is no composite action between
Steel and concrete yet.

• - STAGE (3):
In This Stage, Concrete is hardened but
does not carry loads yet..
Composite action between Steel and
concrete starts.

• - STAGE (4):
In This Stage, Concrete`s age is 28 days but
does not carry loads yet.
concrete starts.

• - STAGE (5):
In This Stage, super-imposed dead loads
(Wearing surface, Barrier, Dead Load of Side walks) are
applied.
The composite section (Steel Beams and
Concrete slab) carry the additional loads.

• - STAGE (6):
In This Stage, super-imposed dead loads (Wearing
surface, Barrier, Dead Load of Side walks) are applied.

• - STAGE (7):
applied.

• - STAGES(8):
These Stage represents the long-term to
show the effect of creep as a time-dependent
- Stage (8) after 1 year.

• - STAGES(9):
- Stage (9) after 3 years.

• - STAGES(10):

15.4 Resultsof CSI
loads, and a slow increase in the long-
term.
• Construction Stage using Bridge
Wizard appears to be the best
solution _between all the previous options_
when it comes to Long-Term
deflection studying for a Steel-
Concrete Composite girder as it give
the closest representation of the
effect of interaction between both
materials.
0
500
1000
1500
2000
2500
3000
3500
0 500 1000 1500 2000 2500 3000
MomentService(t.m)
TIME (days)
0
50
100
150
200
250
0 500 1000 1500 2000 2500 3000
Deflection(mm)
TIME (days)

0
500
1000
1500
2000
2500
3000
3500
0 500 1000 1500 2000 2500 3000
MomentService(t.m)
TIME (days)
60.0m Span
55.0m Span
50.0m Span
40.0m Span
0
50
100
150
200
250
0 500 1000 1500 2000 2500 3000
Deflection(mm)
TIME (days)
60.0m Span
55.0m Span
50.0m Span
40.0m Span
• Effect of Span Length on Creep
and Moment with Time:
- All Spans follows the same previous
results for Short-term and Long-term
moment and deflection values.
- The difference arises form the variation
of sections own weight (Dead Load).
15.4 Resultsof CSI

16. Constructionstageusing CSI-Bridge
• Carrying out a construction stage to trace the long-term
deflection of a Composite Steel-Concrete Girder.
• Using Bridge Wizard
• 1- Define Material:
• Defining Concrete material to be

• 2- Define Material:

• Using Bridge Object Data.
• 2- Define Groups:
a- GIRDER_DIAPH. (Steel Girders)
b- ALL_BUT_SLAB (Steel Girders, Links, Hinges and Bearings)
c- SLAB
d- ALL (Steel Girders, Links, Hinges, Bearings and Slab).

• 2- Define Groups:
a- GIRDER_DIAPH. (Steel Girders)
b- ALL_BUT_SLAB (Steel Girders, Links, Hinges and Bearings)
c- SLAB
d- ALL (Steel Girders, Links, Hinges, Bearings and Slab).

• 3- Define Construction Stage Load case:
There is 2 methods to Define Construction Stage Load case:
a) Directly from bridge “Load Cases”.
b) From “Bridge Modeler Wizard” Construction Scheduler. (We will Use This method)

b) From “Bridge Modeler Wizard”
Construction Scheduler.
- STAGE (1):
In This Stage, Only Steel Beams are placed.
Only Dead Load of steel beams is acting.

- STAGE (2):
In This Stage, Concrete is poured.
There is no composite action between Steel
and concrete yet.

- STAGE (3):
In This Stage, Concrete is hardened but
does not carry loads yet..
concrete starts.

- STAGE (4):
In This Stage, Concrete`s age is 28 days but
does not carry loads yet.
concrete starts.

- STAGE (5):
applied.

- STAGE (6):
In This Stage, super-imposed dead loads (Wearing
surface, Barrier, Dead Load of Side walks) are applied.

- STAGE (7):
applied.

- STAGES(8), (9) and (10):
- Stage (8) after 1 year.

Final Construction Stages

In Bridge load cases we will find that a load
case with the scheduled construction has been
created.

CONCULSION
• Steel-Concrete Composite beam can be erected by two methods of construction:
1- Case (I) – Unshored (Unpropped) construction.
2- Case (II) – Shored (Propped) construction.
• Effective Width (be) is used to make an approximation for a uniform stress distribution
over the girder instead of the non-uniform non-linear distribution
• For calculations of stresses the composite section is transformed to an equivalent section
using the modular ratio n.
• Creep and Shrinkage are inelastic and time-varying strains.
• For Steel-Concrete Composite beam creep and shrinkage are highly associated with
concrete.
• Simple approach depending on modular ratio has been adopted to compute the elastic
section properties instead of the theoretically complex calculations of creep.

CONCULSION
• As specified in AASHTO LRFD Article 6.10.1.1.1b:
1- A ″short-term″ modular ratio n is used for transient loads.
2- A ″long-term″ modular ratio 3n is used for permanent loads.
• As specified in ECP 10.1.4.8:
1-For Case (I) – Unshored (Unpropped) construction and live loads are not prolonged type, Creep effect
may be neglected.
2- For Case (II) – Shored (Propped) construction Creep and Shrinkage must be taken into account, The
conservative approach is to reduce the composite moment of inertia using 2n instead of n and 3n instead of n
for Roadway Bridges.
• Due to creep, time-dependent cross-sectional forces are developed that redistribute tension
from concrete to steel; thus, concrete stresses become lower and steel stresses higher.
• The redistributions are not only time- but loading dependent as well; the magnitude of the
redistribution and the final results depend on the type of loading.

Conclusion
• The strain in the steel girder increases and the steel stresses become larger, while the
strains and concomitant stresses in the concrete deck are reduced.
• Shear Interaction between Steel and Concrete in a Composite section controls whether
they will act together or independently (degree of composite action).
• The resulting increase in resistance will depend on the extent to which slip is prevented.
• a composite beam with partial shear interaction has a higher deflection than a beam
with complete shear interaction.
• There is several factors affecting the long-term stress of steel-concrete composite beam,
including load factors and non-load factors.
• Model analysis shows the effect of Creep with Time on Steel-Concrete Composite
Section.

Refrences
[1] EGYPTIAN CODE OF PRACTICE FOR STEEL CONSTRUCTION AND BRIDGES (ALLOWABLE STRESS
DESIGN - ASD) Code No. (205), Ministerial Decree No 279 – 2001.
[2] EGYPTIAN CODE OF PRACTICE FOR PLANNING, DESIGN & CONSTRUCTION OF BRIDGES AND
ELEVATTED INTERSECTIONS, PART 6, ANALYSIS & DESIGN OF STEEL BRIDGES.
[3] METWALLY ABU-HAMD, STEEL BRIDGES, 2007.
[4] Load and Resistance Factor Design (LRFD) for Highway Bridge Superstructures, REFERENCE MANUAL.
[5] Ioannis Vayas and Aristidis Iliopoulos , Design of Steel-Concrete Composite Bridges to Eurocodes.
[6] Min Ding1,2, Xiugen Jiang1, Zichen Lin3 and Jinsan Ju1,*, Long-term Stress of Simply Supported Steel-
concrete Composite Beams, The Open Construction and Building Technology Journal, 2011, 5, 1-7.
[7] Seunghwan Kim, Creep and Shrinkage Effects on Steel-Concrete Composite Beams.
[8] Erkan SAMHÂL from SSEDTA ( European Steel Computer Aided Learning ), April 2005, Lecture 1.1:
Composite Construction.
[9] R.P. JOHNSON, Composite Structures of Steel and Concrete Beams, slabs, columns, and frames for
buildings, Third Edition.

Effect of creep on composite steel concrete section

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