Manual Steel Cold Formed Sections

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Table of contents  

Introduction .............................................................................................................................................. 1 

Material ..................................................................................................................................................... 2 

Cross-sections ......................................................................................................................................... 3 

Section checks ........................................................................................................................................ 8 

Local Transverse Forces ........................................................................................................................ 9 

Local Transverse Forces Setup Values .............................................................................................. 10 

Local Transverse Forces Additional data ........................................................................................... 11 

Purlin design .......................................................................................................................................... 12 

EN design ............................................................................................................................................... 13 

AISI NAS 2007 design ........................................................................................................................... 14 

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Introduction The steel design module 'Steel cold formed design (EC-EN1993-1-3) is an extension for the EC-EN module esasd.01.01 for steel and is focussed to the design of cold formed profiles according to the European Standard EC-EN 1993. Both section checks and stability checks can be performed by analogy with the existing steel check for hot rolled profiles. This module concerns the following parts: - Determination of the initial shape - Calculation of the effective section properties including local and distortional buckling - ULS Design Checks - Special considerations for purlins restrained by sheeting Code reference: EC-EN1993-1-3:2006 General rules- Supplementary rules for cold-formed members and sheeting Cold-formed steel members are made from structural quality sheet steel that are formed into shape either through press-braking blanks sheared from sheets or coils, or more commonly, by roll forming the steel through a series of dies. No heat is required to form the shapes (unlike hot-rolled steel), and thus the name cold-formed steel. Cold-formed steel members and other products are thinner, lighter, easier to produce, and typically cost less than their hot-rolled counterparts because of the weight reduction. A variety of steel thickness is available to meet a wide range of structural and nonstructural applications.

The steel design module "EC-EN1993-1-3:2006" for the design of cold formed steel members, is integrated and implemented into the existing design of steel members within the Eurocode environment and is an extension from this standard steel code check (esasd.01.01) This module concerns the following parts: - Determination of the initial shape - Calculation of the effective section properties including local and distortional buckling - ULS Design Checks - Special considerations for purlins restrained by sheeting Code reference: EC-EN 1993-1-3:2006 General rules- Supplementary rules for cold-formed members and sheeting EC-EN 1993-1-5:2006 (Parts) EN 1993-1-3:2006/AC:2009 (correction sheet)

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Material The Steel Material library is expanded with the steel grades given for this type of section in the code.

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Cross-sections

Supported types

The following cross-section types are supported for generation of the initial shape and the effective cross-section: - Standard profile library cross-sections - Cold formed pair cross-sections - General thin walled sections - General sections with thin walled representation - Thin walled geometric sections - All other sections which support the centerline and do not have roundings With the general cross-sections functionality, it is possible to draw user-defined cross-sections using the integrated drawing tools or by inserting cross-sections via dxf- or dwg-files. The average yield strength is supported as implemented in EN 1993-1-3. Within the design, the steel core thickness (excluding coating) is used instead of the actual thickness of the cross-section. The 'Metallic Coating Thickness' field is presented where the user can input the coating thickness. The 'core Thickness' field shows the final thickness.

Average yield strength

The Average Yield Strength options is available in case Fabrication is set to Cold Formed.

When the combo box Type of forming is set to Roll forming, the k factor behind this item is set to the value of 7. When the combo box is set to Other method the k factor is equal to 5. The average yield strength will be applied in the following resistance calculations: - axial tension

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- axial compression - bending moment - torsional moment - flexural buckling - torsional (-flexural) buckling - purlin design – cross-section resistance

Steel core thickness

The cross-section manager is expanded with the group Steel core thickness. This group is visible only if Fabrication is set to Cold Formed and the cross-section is taken from profile library and has Formcode between 111 and 126 or is a cold formed pair section (2CFUo, 2CFUc, 2CFCo, 2CFCc, 2CFLT).

When the user activates Steel core thickness, Metallic Coating Thickness input box is shown and the user can input the coating thickness. The Thickness input box then shows the final thickness. This value greyed out as it is calculated by the program. This value is calculated as follows:

tcore = tnominal – tmetallic coatings

With : tnominal = thickness read from the library tmetallic coatings = reduction inputted by the user

Remarks: The Metallic coating input field is limited to a minimal value of 0 mm to avoid negative input.

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The Metallic coating input field is limited to a maximal value of tnominal - 0,1 mm. This ensures that the core thickness will never be lower than 0,1 mm (i.e. if the CSS has thickness 2 mm then the user cannot input a metallic coating thickness of 2,3 mm but is restricted to a maximum of 1,9 mm) In both of the above cases a warning should be given and the respective limit should be set. When the steel core thickness is set it is used everywhere where the cross-section thickness is used: - In the cross-section manager, the gross-section properties as well as effective shapes/properties are influenced. - In the check in all formulas/limits where the thickness is used the core thickness is used.

Initial Shape

The cross-section manager is expanded with the Initial Shape. The group Initial shape with the checkbox Enable is visible only if Fabrication is set to cold formed and in case of a general cross-section the thin-walled representation has been defined. When a cold formed section is selected from the standard library or when it has been imported via the general cross-section utility, the initial shape of the cross-section is automatically calculated. The shape of the cross-section is divided to several parts and visualized. Supported element types are: - I (internal element) - F (fixed element – no reduction is needed) - SO (outstand element (symmetrical)) - UO (outstand element (unsymmetrical)) - RUO (Reinforced Unsymmetrical OutstandReinforcement types) - RI (Reinforced Internal) - DEF (Double Edge Fold). The types are automatically associated during the process. The tab Initial shape is generated on which the user can view the initial shape.

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Button Edit initial shape in the Initial Shape group allows the user to edit the initial shape. Button Run analysis in the Initial Shape group opens the Run analysis dialog where the effective properties of the cross-section are calculated.

Effective shape

For the initial shapes within Scia Engineer, the elements are determined between the roundings. For calculating the effective width however, the notional width is used. This is specified in EN 1993-1-3 art 5.1 and figure 5.1 pp 19. This effective width based on the notional width is recalculated again to determine the effective width of the element in Scia Engineer.

Iterative approach for compression and bending stresses For the definition of the compression and bending stresses, the use of iterations, stiffener iteration and overall cross-section iteration, is not obliged by the code but can be done optionally in the steel setup of Scia Engineer. The effective width of Internal Compression Elements and Outstand Compression Elements is calculated according to EN 1993-1-5 art.4.4. The procedure for determining the effective width/thickness of plane elements with edge stiffeners is given in EN 1993-1-3 art.5.5.3.2 and art.5.5.3.1. The procedure for determining the effective width/thickness of elements with intermediate stiffeners is given in EN 1993-1-3 art.5.5.3.3 and art.5.5.3.1. The Run analysis tool graphically shows the effective sections.

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Section checks

Section Checks

In contrast to EN1993-1-1, for cold formed sections according to EN1993-1-3 there is no classification. The following section checks are executed:

axial tension,

axial compression,

bending moment,

shear force,

torsional moment,

local transverse forces,

combined tension and bending,

combined compression and bending,

combined shear,

axial force and bending moment,

combined bending,

local transverse force. The Local Transverse Forces check is executed automatically in each position where there is a skip-change in the shear force diagram. Using the Local Transverse Forces data the user can activate the 'No Local Transverse Forces' Check to specify that at the location of the add data no check should be executed. Rem.: Special procedures are implemented for formcode 115 (Cold formed Omega) cross-sections and in case of stiffened webs that apply to art.6.1.7.4.

Stability Checks

The following stability checks are executed:

flexural buckling,

torsional and torsional-flexural buckling,

lateral-torsional buckling,

bending and axial compression,

bending and axial tension,

combined bending,

tension check. To determinate the torsional-flexural buckling resistance, the general cubic equation is applied. For determination of the bending and axial compression, EN 1993-1-3 allows the engineer to use two possibilities, so the choice between these two methods is given to the user:

EN 1993-1-1 interaction according to article 6.3.3,

alternative method according to EN 1993-1-3 article 6.2.5(2).

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Local Transverse Forces Effects due to Local Transverse Forces are defined in art. 6.1.7 pp 47. The check is executed ONLY in case of Vz loading. The local transverse force resistance is taken relative to the support, NOT according to the local z-axis. Therefore FEd, should be determined according to the LCS axis system and NOT according to the principal axis system! In case the cross-section has multiple webs, for determining the load condition the maximal web height is used. As opposed to art.6.1.7.2(4), within Scia Engineer the exact inputted bearing length ss is used at all times i.e. the simplification of using the minimal length for both opposing loads is not implemented. The local transverse forces check is, by default, executed using default settings. These default settings are specified in the Steel Setup. If desired the user can overwrite these default settings using Local Transverse Forces additional data.

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Local Transverse Forces Setup Values The Steel Setup is expanded with a new tab Cold formed. This tab contains group Local Transverse Forces. In this group Bearing length ss can be input. Bearing length ss input field is limited to the minimum of 10 mm, no lower values are allowed (see art.6.1.7.3(3) ).

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Local Transverse Forces Additional data Local Transverse Forces data can be defined in the Stability Check Data branch of the Steel Service. Name Local Transverse Forces Check

If OFF the Local Transverse Forces Check will not be performed.

Loading Conditions The Loading Conditions are default be determined automatically. Using this combo box the user can overwrite this setting.

- Determined automatically - End one-flange (EOF) - Interior one-flange (IOF) - End two-flange (ETF) - Interior two-flange (ITF)

Bearing length ss Default from setup The Bearing length ss is by default read from the Steel Setup. Linked to Cross-section If the Bearing length ss combo is set to Linked to Cross-section the user can select a cross-section from the cross-section manager. The bearing width will then be taken equal to the width of this cross-section. Manual input If the Bearing length ss combo is set to Manual input the user can input the desired value in the input box. In this case the input box is limited to the minimum of 10 mm, no lower values are allowed.

Value See above Web rotation prevented

In case the Web rotation prevented checkbox is activated the formulas given in art.6.1.7.2(4) pp 50 are used instead of Figure 6.7a & 6.7b. See the Theoretical Background manual for more information.

Range By default the data from the Local Transverse Forces Data is only used in the actual position on which the data is inputted. Using the Range field the user can specify that this data is valid over a certain range. This can be used in case two (or more) peaks in the shear force diagram are closely spaced. By default this range will be 0 mm to indicate that the data is valid only for the actual section. This range cannot be negative.

Position x Defines the position of the Local Transverse Forces Additional Data on the beam. This is analogous to definition of all other types of Add Data in Scia Engineer.

Coord. definition See above Repeat See above The Local Transverse Forces Check is executed automatically in each position where there is a skip-change in the shear force diagram. Using the Local Transverse Forces data the user can deactivate the checkbox Local Transverse Forces Check to specify that at the location of the add data no check should be executed. In this case, a warning will be shown on the output instead. This is used for example in case a cleat is preventing web crippling effects as specified in art.6.1.7.1(3). When the Local Transverse Forces Check checkbox is not activated all other properties are hidden except the Range parameter.

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Purlin design If the cross-section meets special requirements stipulated in the code and described in the Theoretical Background manuals for the design of steel cold-formed sections, a special purlin check according to EN 1993-1-3 Chapter 10 is performed. Two specific items regarding purlin design exist in the Steel Setup.

National Annex Setup

Available through: Project Data dialogue > National Annex three-dot button > Design of Steel structures > General: tab Cold formed  

Method for χLT The method for determining the LTB reduction factor for buckling resistance of the free flange is given in art. 10.1.4.2 pp 82. This choice can however be specified by the National Annex and is thus added to the National Annex Setup. Currently, only the method as described in this article will be supported, therefore the combo box will have only one item. In the future, when specific methods according to National Annexes will be implemented, they can be added into this combo box.

Steel Setup

Available through: Libraries > Setup > Steel: tab Cold formed

Limit for large axial force Art. 10.1.4.2(5) specifies a „relatively large axial force‟. Within Scia Engineer this is quantified using a limit value. The Input field allow for only values between 0 and 1. The default value is set to 0,10.

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EN design This manual describes the module for design of steel cold formed sections according to EN1993-1-3:2006 including EN1993-1-3:2006/AC:2009. Full details about the current implementation of the code regulations in the program can be found in a separate Theoretical Background Manual.

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AISI NAS 2007 design Despite the fact, that this manual focuses on the module for design of steel cold formed sections according to EN1993-1-3:2006 including EN1993-1-3:2006/AC:2009, the principles described in it are general and are valid for the module for the design of cold formed sections according to the AISI NAS 2007 code. Full details about the current implementation of the code regulations in the program can be found in a separate Theoretical Background Manual dealing exclusively with the AISI NAS 2007 design.