The Sections mask hides an infinite number of commands for modelling and managing sections of any shape, as well as other interesting settings for verifications - let's discover them together

 

The Sections mask lists the cross-sections of beams or thicknesses of flat elements entered in the model.

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-   To add a planar section, specify the name and the thickness in the Planar section boxes and then press Add planar section;

-   To add a beam section, click the Beam section drop-down menu and choose the type among Rectangular, C-shape, Circular, Pipe, Box, By point options.

 

Click the Assign to selected elements button to assign the selected section to the selected elements.

The built-in beam sections available are:

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Further section shapes are available: Omega and cold-formed non-thin-walled C shapes, that can be set from inside the section properties.

Other custom libraries can be present in this list. For each of them, the following mask will be displayed to allow selection. Several libraries for steel sections are included in the program.

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To see the properties (area and moments of inertia) of the inserted sections select a section and press the Edit properties button. This mask allows also to import a section from DXF file and define a section by points or by composing single figures.

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*    WARNING: it is strongly advised to insert units consistent with the initial choices (see Edit > Set units).

In the Add figures and By Points masks, it is possible to edit the component shapes of the section (for both voids and solids) or import a section from DXF with the Import DXF command. The contours of each figure must be polylines, defined clockwise for solids and anticlockwise for voids.

*    WARNING: the insertion of figures in sections does not take place in absolute co-ordinates, as the centre of gravity is recalculated for each change.

In the Properties mask it is possible to specify the design properties of the section to be used during checking. For sections without a defined type (e.g. point sections, customised), a class is not automatically assigned to the section during the verification for steel or aluminium material. For this, the user is asked to specify the class of the section via the Add Property drop-down menu, which presents the following options:

-        Steel verification adds the following data for verification: SectClass (section class), alphaLT (coeff. flexural-torsional buckling), alphay (coeff. buckling around y-y), alphaz (coeff. buckling around z-z), Jw (warping constant)

-        Aluminium verification adds the following data for verification: SectClass (section class), Jw (warping constant)

-        Composite column adds the following data for analysis: Ec_factor (factor for the elastic modulus in the calculation of the homogenisation coeff., default at 0.5) and Reduction_factor (default at 0.9, inertia reduction factor)

-        Composite beam adds the following data for analysis: Factor_Mpos (factor for positive moment inertia) and Factor_Mneg (factor for negative moment inertia)

-        Cold-formed section adds the following data for the verification of thin sections, which is not performed without these flags: CF_tw (section thickness) and CF_rc (radius of curvature between segments)

-        Fibre section adds the following data for the analysis in OpenSees: fibersX (number of fibres in the z-direction of the section) and fibersY (number of fibres in the y-direction of the section) to specify the fibre division for each direction

-        Skip verification adds the flag skip = 1 to skip the verification of the section

-        From text allows the loading of a set of user-defined flags, which are reported as variables in the verifications. The file must contain one variable per line in the format nameVar=value. This is particularly useful for defining different thicknesses in thin sections, for example for a model in mm and a section consisting of 3 figures (which in the case of a thin section are series of segments):

CF_tw=2

CF_tw_1_6=3

CF_tw_1_8=2.5

CF_tw_1_9=2.5

CF_tw_1_10=2.2

CF_tw_1_11=2.2

CF_tw_1_12=2.2

CF_tw_1_13=2.2

CF_tw_1_14=2.2

CF_tw_1_15=2.5

CF_tw_2_1=8

CF_tw_2_2=2.5

CF_tw_2_3=2.5

CF_tw_2_4=2.5

CF_tw_2_5=2.5

CF_tw_3_1=3

CF_rc=2

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*    WARNING: by changing the sectional properties, the section will be converted in “Custom” type. Such operation is not reversible.

 

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Adding customised section

In some cases, sections of a particular shape are used in everyday design, of which you already have the drawings in CAD. NextFEM Designer allows the addition of user sections, defined by points, to its library by following these steps:

1.         Import the DXF of the section into NextFEM Designer

2.         Set any characteristics for verifications (e.g. the verification set for steel)

3.         Export the section using the Export as DXF command from the section context menu

 

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Together with the DXF file, a TXT file containing the section by points and the data entered for verification will also be generated.

Name;Code;PointsXf1 [mm];PointsYf1 [mm];PointsXe2 [mm];PointsYe2 [mm];PointsXf3 [mm];PointsYf3 [mm];Class;OffsetX [mm];OffsetY [mm];alphaLT;alphay;alphaz;Jw [mm^4]

TestSect;custom;0,20,20,0;0,0,20,20;4,8,8,4;4,4,8,8;0;0;3;0;0;0.76;0.76;0.76;0


Simply change the extension of this file to .NFS and copy it into the program installation folder, subfolder "data" (typically this is C:\Program Files\NextFEM Designer 64bit\data).

After restarting the program, the new section will be available for all future use in the beam section selection menu.

 

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Beams with Euler-Bernoulli or Timoshenko behaviour

The behaviour of an Euler-Bernoulli or Timoshenko beam element is forced differently for the default solver and for OpenSees:

- for the default solver, the shear areas in the section data must be set to zero

- for the OpenSees solver, simply set the noShearK element property to 1:

 

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Composite sections

The support for composite sections includes:

-          Beams with slab like in figure (a) and

-          Columns with coincident centres of gravity (b).

 

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A composite section is automatically detected when:

-          Two or more filled figures, one of them associated to a steel material and one to a concrete material;

-          Properties of the composite section are defined inside the mask Section properties / Properties in order to distinguish a beam or a column section.

 

             WARNING: The effective width of the section Beff must be evaluate by the user as per current code of practice.

The stiffness of beam elements having such sections is evaluated as follows. With subscript s, steel properties are described, while the ones related to concrete have the subscript c.

 

For beams, the moment of inertia for positive bending around the elastic neutral axis na is calculated as:

The moment of inertia of the section for negative bending around the plastic neutral axis is equal to the inertia given by the steel section, as the concrete part is considered as cracked.

As a whole, bending stiffness is estimated as per EC4 and as a function of the factors Mpos and Mneg which can be changed in the “Properties” input mask (figure a).

 

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Hence, the resulting bending stiffness is:

The remaining properties for the beam section are all calculated by dividing the concrete contribution by n.

 

For composite columns, once the factors Ec and R have been specified in the “Properties” mask (figure b), the inertial properties of the section are calculated as follows:

The remaining properties are all calculated by dividing the concrete contribution by n.

 

Strength calculator

By the Strength calculator  button it is possible to define the rebar in a RC section and to evaluate its flexural strength.

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In Define material properties the base material is specified.

The box Rebar position can be used to specify the rebar coordinates (eventually of rectangular shape by checking the option Rect. rebar and by specifying the base dimension of the plate in Rect. base). The rebar coordinates are input on the base of the section local coordinate system, by specifying the diameter and the associated material (that can be set or added by the command “…”). The available materials should be customized by modifying or adding *.nfm files in data/design subfolder in the software installation directory.

In order to insert strands for prestressed RC sections, it is possible to input the initial strand stress by activating the Strand option and by inputting the stress in consistent dimensions.

To obtain the resistant values of the section, specify the Design forces in the required units. By selecting the Elastic option, the response of the section is computed with elastic materials. By selecting Plastic, elastic-perfectly plastic laws are used for material behaviour. In the Results box, the results of the performed computation are reported, linked to the overlying graphs.

The window allows also resistant domains calculations reduced by thermal action. The software will compute the strengths parameters of the section at the selected instant of time and thermal case, on the base of strengths reductions laws for mechanical and resistant features of base material and rebar, if present.

The available options in this mask are:

-          Concrete non-linear behaviour allows to choose between parabola-rectangle and bilinear material behaviour for the concrete material. If a simple rectangular or circular section are present, the Confined concrete option is available in this menu, allowing to consider the confined part of the section inside stirrups. In this case, data in Stirrups tab are required. Spirals are not supported.

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-           Allow concrete tensile strength can be enabled to activate the contribution of tensile strength of the base material;

-          Bilinear hardening ratio allows to specify the ratio between the plastic branch of rebar and their elastic modulus. In case of Steel or Aluminium base material, it specifies the hardening ratio to be used.

-          Steel class section sets the class of the section in case of Steel or Aluminium base material, i.e. if the section can be calculated in plastic field or not.

-          Confidence Factor (C.F.) sets the confidence factor for strength of base material and rebar.

-          n factor  specifies the value of the homogenization factor (modular ratio between steel and concrete). The default is 15.

Stirrups tab helps to define the stirrups for the current section and to obtain a resisting strength for both direction of the section.

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Export Results contains options to:
- plot the tension in the reinforcement bars,
- to perform the calculation with a triangular fiber section
- to change the accuracy of the calculation (recommended value 15, to be increased in the case of very thin parts)
- delete the calculation cache, which is useful when changing reinforcement materials or bars to prevent the program from returning the same results as before when these materials were changed
- export images of the analysed section or points of the resisting domain
- export the section to DXF with the reinforcement
- display the domain in 3D.


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Beam elements allows the import of reinforcements from individual elements, temporarily overwriting those of the section. The Get forces button allows the stresses for the desired abscissa taken from the calculated model.

 

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