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Check of prestressed concrete according to EN 1992-1-1

The check of prestressed concrete is performed only according to EN 1992-1-1. There are not any implementation of special check according to EN 1992-2 which is code for design of bridges. There is only possible to used special TDA calculation according that code (see "Construction stage analysis (TDA EN 1992-2)").

Concrete setup

The final concrete setup is synthesis of national dependent values and standard setup independent values.

Code dependent values are possible to see in Project Data > Code > National annexes.

Code independent values are possible to see in Libraries > Concrete setup.

Complete setup is visible in appropriate concrete check filtered according to check. For instance see concrete setup for Allowable stresses of concrete.

Some important settings from concrete setup will be explained in the following chapters

User defined section only

This functionality is suitable for fast performing of concrete checks only in user defined section, where is supposed the most loaded structure and extreme results. Check is performed only in those user defined d section and duration of check is shorter. It is available for all concrete checks and design.

Concrete area weakened by reinforcement bars

When this check box is switch ON, then area of concrete CSS is reduced by bars. It has effects on all concrete 1D checks.

Concrete area weakened by prestressed bars

When this check box is switch ON, then area of concrete CSS is reduced by bars. It has effects on all concrete 1D checks.

Warning and errors

When some check is performed then warning or error can be printed in the table.

All warnings and errors are stored in the concrete setup.

Member check, single check

The modelled structure can be checked by two ways:

• globally using Member check
• detailed using Single check

Member check

Member check is performed from the standard concrete check service. The results are displayed along the selected members. The output table has different output according to extreme:

Global – one extreme result of all checked members

Member – one extreme results of each checked member

Section – extreme results in each section along all checked members

Cross-section – one extreme results of all checked cross-section

Single check

Almost all concrete checks has single check. It is detailed analysis of one cross-section. Action button to go there is following in the bottom of service.

Dialogue of single check provided to user selects:

Extreme of normal forces (N+; N-; Vz+; Vz-...)

Section along the selected member

Appropriate tabsheet with detailed results. Tabsheets are dependent on type of concrete service, but mainly there are following:

Cross-section

Loads

Strain

Stress

Stress/strain diagram

Check in named items – fibres, cuts, joints, parts of CSS

Definition of named items was described in chapter "Library of Named items". Here the using in concrete checks will be explained.  The idea of using named part is to get user friendly results and to increase speed of check. SCIA Engineer supports following named items in the following services.

Named fibres

Check response

Allowable stresses of concrete

Named cuts

Allowable principal stresses

Named joints

Check response – check of shear in construction joint

Design As

Named parts of CSS

Check response

Allowable stresses of concrete

Allowable principal stresses

For instance you can see results of allowable concrete stress for upper fibres

for member check

for single check – only named fibres are available in single check

CSS characteristic, transformed

The service of CSS characteristic is service which can provide to user all necessary information about the CSS. This service is situated in Concrete Advanced > 1D member.

This service performs calculation of geometrical properties of cross-section with including the following:

• the prestressed reinforcement
• the non-prestressed reinforcement
• ducts of tendons
• time

Type of characteristic with two items

• normal (only one check box from concrete, reinforcement and tendons can be switched on)
• transformed

Calculated components - three check boxes

• concrete (for transformed characteristic, this check box will be always switched on)
• reinforcement
• tendons

Subtracting from concrete will be active if the check box Concrete is switched on

• reinforcement
• tendon
• ducts

Characteristic related to with the following items

• to centre of gravity of cross-section
• to centre of gravity of cross-section phase ( new combo Phase of cross-section with the following items:

• all phases,
• by selected combination,
• defined by user

• to selected named fibres (new combo with named fibres will be active)
• to the point (new properties for definition y and z coordinates will be active)

Values

the values for normal characteristic  A, Iy, Iz,ty,tz,Sy,Sz,bw, Wy+, Wy-, Wz+, Wz-, iy, iz

the values for transformed characteristic  Ai, Iyi, Izi,tyi,tzi,Syi,Szi; Wyi+, Wyi-, Wzi+, Wzi-, iyi, izi

The results can be following:

Not calculated internal forces

The frame XZ which is used for the time dependent analysis doesn’t respect effects of torsion in this project. The envelopes of mobile loads for extreme Mx (torsion moment) should be analysed on different project type frame XYZ. The value of Mx should be defined in project Frame XZ as Not calculated internal force.

The user should defined not calculated internal forces in menu Loads > Not calculated internal forces for selected load case.

The dialog for definition of Not calculated internal forces is following. There are several types. We use type Mx.

Distribution should be following; type Polynomic is used for definition only in one section near the second support.

The value of Mx is defined in this dialog.

The displaying of not calculated internal forces in 3D window is following

SLS – concrete checks

Prestress crack check

The cracks of the prestressed members are calculated according to chapter 7.3 from EN 1992-1-1 and check is performed in service Concrete Advanced > Member check > check of prestressed concrete > Crack control. The prestressed structure is with bonded tendons and will be checked for frequent combination according to table 7.1N from EN 1992-1-1. The exposure class is set in chapter "Exposure class" as XD3. The decompression has to be checked in this case.

Allowable concrete stresses

Domain knowledge of allowable concrete checks

The explanation of displayed values is following

The stress before and after anchoring

• In compression

• σ_cc,max – allowable stress in compression before and after anchoring, defined in chapter 5.42 in EN 1992-1-1

• (5.42)
• k₆ = 0,6 (for pretensioned concrete is possible to increase allowable stress on value 0,7 when it is verified that cracks don’t appear)

• σ_c,aa – calculated stress in concrete,

• In tension

• σ_cc,max – allowable concrete stress in tension before and after anchoring; defined in Concrete setup and default is 0 MPa
• σ_c,aa – calculated stress in concrete,

Stress from SLS combination

• Characteristic combination – longitudinal cracks

• In compression

• σ_cc,ch – allowable concrete stress in compression from SLS characteristic combination, only for exposure class XD, XF a XS.

• (7.2.(2))
• k₁ = 0,6

• σ_cq,min – minimal stress in concrete after application self weight all permanent and variable loads

• In tension

• f_ct,eff - allowable concrete stress in tension from SLS characteristic combination,
• The value  f_ct,eff is possible to set in Concrete setup > Allowable stresses:

• f_ctm – the mean axial tensile strength
• f_ctm,fl – the mean flexural tensile strength

• f_ctm,fl = max{(1,6-h/1000)∙f_ctm; f_ctm)}
• h total depth of CSS

• σ_cq,max – maximal stress in concrete after application self weight all permanent and variable loads

• Quasi-permanent combination – great creep

• In compression

• σ_cc,qp – allowable concrete stress in compression from SLS quasi-permanent combination, linear creep may be assumed

• (7.2.(3))
• k₂ = 0,45

• σ_clt,min – minimal stress from long-term load

• In tension

• f_ct,eff,qp  – allowable concrete stress in tension from SLS quasi-permanent combination, The value  f_ct,eff,qp is possible to set in Concrete setup > Allowable stresses; default 0 MPa
• σ_clt,max – maximal stress from long-term load

Other not checked, only drawn

σ_p,inc – increment of stress from selected LC

Some important values needed for calculation is recommended to set before check is performed.

Exposure class

The exposure class of concrete is possible to set in Concrete setup > Design defaults. The check of allowable concrete stresses and crack width depends on this exposure class. The class XD3 is set in this example.

The exposure class for each member is also possible to set by Member data.

The factor for prestressing used in allowable concrete stresses

The upper and lower factors of prestressing force for check of allowable concrete stresses are possible to set in Concrete setup > SLS > Prestressing.

The factors for check of allowable concrete stresses

The factors for the calculation of limit values of concrete stresses from the code EN 1992-1-1 is possible to set in Concrete setup > Allowable stresses.

The check of Allowable concrete stresses

The user has possibility to perform this check in Concrete Advanced > Member check > Check of prestressed concrete > Allowable concrete stresses. The results of the check will be calculated and drawn for selected combination and value. For instance Check value of the SLS characteristic combination in 100 years is following.

The output table with extreme Member is following.

For the selected values σ_cc,ch and σ_cq,min are results following

and output table is following

The concrete stress should be calculated for selected fibre only

Then results for upper fibres are following

The detailed analysis only in one section is possible using button Single check

Check of prestressing reinforcement

Domain knowledge

The explanation of displayed values is following

The stress before anchoring

• σ_p,max – allowable stress in prestressing before anchoring

• (5.41)
• k₁ = 0,8; k₂ =  0,9
• k₃ = 0,95 (σ_p,max = k₃*f_p01,k) if special measurement of prestress force is applied

• σ_p,pa – calculated stress before anchoring,

The stress after anchoring

• σ_p,m0 – allowable stress in prestressing after anchoring,

• (5.43)
• k₁ = 0,75; k₂ =  0,85

• σ_p,aa – calculated stress after anchoring

The crack limitation from SLS characteristic combination during service – cracks and deformation

• σ_pm –  allowable stress in prestressing from SLS characteristic combination

• (7.2(5))
• k₅ = 0,75

• σ_p,LTL – calculated stress after longterm losses
• σ_pq,min – minimal stress in concrete after application self weight all permanent and variable loads in prestressing
• σ_pq,max – maximal stress in concrete after application self weight all permanent and variable loads in prestressing

Others not checked stresses, only drawn

• σ_pq,max – σ_pq,min – the range of stresses in prestressing from maximal and minimal load in selected combination (envelopes of mobile loads)
• σ_p,un – unbalanced stresses, difference of stresses calculated from the strain determined from structural analysis once as elastic stress (Hook’s law) and the second stresses as nonlinear stress-strain relationship
• σ_p,inc – increment of stress from selected LC

The factors for the calculation of limit values of concrete stresses from the code EN 1992-1-1 is possible to set in Concrete setup > Allowable stresses.

The user has possibility to perform this check in Concrete Advanced > Member check > Check of prestressed concrete > Check of prestressing reinforcement. The results of the check will be calculated and drawn for selected combination and value. For instance Check value of the SLS characteristic combination in 100 years is following.

The output table with extreme Member is following.

ULS – concrete checks

Prestress check response

The CSS response on acting combination is calculated in this check. The check is in Concrete Advanced > Member check > Check of prestressed concrete > Check response. The results of the check will be calculated and drawn for selected combination and value. There are following values for selection

• eps_cc – the strain in concrete under compression
• eps_sc – the strain in nonprestressed reinforcement under compression
• eps_st– the strain in nonprestressed reinforcement under tension
• eps_tt – the strain in prestressed reinforcement under tension
• Vzu – the shear resistance
• Tu – the torsional resistance
• Vrdi – the resistance of shear in construction joint

The results are calculated and drawn for F17-EN-ULS(STR) - check value.

The output table with extreme Member is following.

The strain in concrete under compression for F17-EN-ULS(STR) – eps_cc.

The strain in tendons under tension for F17-EN-ULS(STR) – eps_tt.

Prestress check diagram

The resistance of CSS acting by combination of moment and normal force is calculated using interaction diagram in this check. The check is in Concrete Advanced > Member check > Check of prestressed concrete > Check capacity. The default setup options are following:

The recommended type of interaction diagram is following:

• When compression is dominant then NuMu is suitable because when structure is deformed by compression then moment is also increased
• When tension is dominant then Nu is suitable
• When pure bending is dominant then Mu is suitable

The results of the check will be calculated and drawn for selected combination and value. There are following values for selection

• Nu – capacity in axis x (axial capacity)
• Myu – moment capacity in direction y
• Mzu – moment capacity in direction z
• Vzu – shear capacity
• Tu – torsional capacity

The moment capacity in direction y for F17-EN-ULS(STR) – Myu

The axial capacity for F17-EN-ULS(STR) – Nu

The output table for F17-EN-ULS(STR)

The results aim single checks are following:

Vertical section of interaction diagram

Horizontal section of interaction diagram

3D interaction diagram

Shear check

The shear check is performed in Check response and check capacity as value Vzu in property. At the beginning the existing of cracks in ULS is calculated:

• When the CSS is cracked then calculation is done according to chapter 12.6.3
• When the CSS is uncracked and without nonprestressed reinforcement then calculation is done according to chapter 12.6.3

The parameters for the calculation of shear check is possible to set in Concrete setup > ULS > Shear.

The shear check for F17-EN-ULS(STR) – Vzu.

The output table with extreme Member is following.

Torsion check

The torsion check is performed in Check response and check capacity as value Tu in property.  The calculation is performed according to chapter 6.4. The check of torsion is required if check box in Concrete setup > General is turned ON.

The shear check for F17-EN-ULS(STR) – Tu.

The torsion is checked only if stirrups are defined on the beams, when the stirrups are not defined check of allowable principal stresses is required

Allowable principal stresses

The check of allowable principal stresses in tension in ULS is performed for verification that CSS is uncracked CSS under acting load. This check is available in Concrete Advanced > Member check > Check of prestressed concrete > Allowable principal stresses. When the check of tensile stress is required only in compression zone then check box Concrete setup > ULS > Shear has to be checked. The principal stresses are calculated in predefined cuts by user (see "Named cuts").

The service of allowable principal stresses has the property dialog with following values

• Sigma_1 – principal stresses 1 for plane state of stresses in 3D
• Sigma_2 max – principal stresses 2 for plane state of stresses in 3D
• Sigma_3 max – principal stresses 3 for plane state of stresses in 3D
• Sigma_x_max – maximal stress in concrete in axis X
• Sigma_y_max – maximal stress in concrete in axis Y
• Sigma_z_max – maximal stress in concrete in axis Z
• Tau_xy_max – maximal shear stress in concrete in plane XY
• Tau_xz_max – maximal shear stress in concrete in plane XZ

The check value for F17-EN-ULS(STR) is following.

There is possibility to checked only selected named cuts or part of CSS in this service.

The user has the possibility to see detailed results in selected section along the beam using the button Single check. The stress σ_x for F17-EN-ULS(STR) is following.

The stress τ_xz for F17-EN-ULS(STR) is following

The principal stress σ₁ for F17-EN-ULS(STR) is following

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