Limit values for shear stresses from torsion in RF-/STEEL EC3
Very small torsional moments in the members to be designed often prevent certain design formats. In order to neglect them and still perform the designs, you can define a limit value in RF-/STEEL EC3 from which torsional shear stresses are taken into account.
Background of the limit value setting
Especially when designing structures in the 3D model, the result is often torsional moments in the members to be designed due to the modeling. However, the design rules in EN 1993-1-1 usually only contain valid interaction conditions and design methods for the typical stresses from compression, bending, or shear force. For example, torsional moments cannot be considered in the classic methods for stability designs such as the equivalent member method or the general method.
In order to still allow for a simple design and to neglect small torsional moments, the user can define a limit value for the shear stresses from torsion in the detail settings of RF-/STEEL EC3. If the acting shear stresses are then lower than the defined limit value, they are neglected in the design. Thus, the stability analysis is still performed or the more favorable interaction condition is still used.
Enter the limit value in the detailed settings
The limit value is entered separately for cross -section designs and stability designs. There is no further warning that the torsional internal forces below this limit are neglected.
There is no normative basis for this limit value and therefore no general specification. From an engineering point of view, however, we decided to use the default setting of a limit value of 5 % in order to neglect small and mostly unintentional torsional moments in the design.
Primary torsion for open sections is negligible
In the case of open cross -sections, it can generally be assumed that acting torsional moments are not transferred by primary torsion, but by secondary torsion (warping torsion). In RFEM and RSTAB, the internal forces are initially only determined with six degrees of freedom without considering warping. For small acting torsional moments, it seems obvious to neglect the torsional stresses in the design. For large torsional moments, however, a calculation should be carried out with seven degrees of freedom in order to also consider the influence of the warping moment and the secondary torsional shear stresses. This is possible, for example, with the RF-/STEEL Warping Torsion or RF-/FE-LTB add-on modules.
Dipl.-Ing. Oliver Metzkes
Product Engineering & Customer Support
Mr. Metzkes is responsible for the development of the add-on modules for steel structures and provides technical support for our customers.
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This technical article deals with the stability analysis of a roof purlin, which is connected without stiffeners by means of a bolt connection on the lower flange to have a minimum manufacturing effort.
SHAPE-THIN determines the effective cross-sections according to EN 1993-1-3 and EN 1993-1-5 for cold-formed sections. You can optionally check the geometric conditions for the applicability of the standard specified in EN 1993‑1‑3, Section 5.2.
The effects of local plate buckling are considered according to the method of reduced widths and the possible buckling of stiffeners (instability) is considered for stiffened sections according to EN 1993-1-3, Section 5.5.
As an option, you can perform an iterative calculation to optimize the effective cross-section.
You can display the effective cross-sections graphically.
Read more about designing cold-formed sections with SHAPE-THIN and RF-/STEEL Cold-Formed Sections in this technical article: Design of a Thin-Walled, Cold-Formed C-Section According to EN 1993-1-3.
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Design of steel members according to Eurocode 3
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Design of steel members according to Eurocode 3