Single or Articulated Member in Stability Analysis of Double Angles
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A previous article describes the design of double angles. It dealt with the analysis performed on a single member.
Now the question is, under which boundary conditions is this actually allowed. EN 1993‑1‑1, Clause 6.4.4, provides design rules for so‑called ‘closely spaced built‑up members’. Double angles with the back‑to‑back arrangement must be always connected by screws or welded tie plates. The maximum spacing between interconnections is defined as 15 times the minimum radius of gyration of one angle. There are no specific values for the space between members, but it should be in the range of usual filler plate thicknesses.
If these limits are exceeded, you have to design an articulated member. In principle, the global internal forces can be determined on a single member. In addition to pre-deformations, you should also consider the shear stiffness. This can be controlled by the value t* in RFEM and RSTAB (see the figure). This value is applied when calculating the shear area and depends on the design of bracing or framing. Formulas for t* are described, for example, in Petersen: Stahlbau, 4th edition, Table 17.1 (t* is indicated as tE here).
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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.
Frequently Asked Questions (FAQ)
- Why do I get a design ratio for the stability analysis according to 18.104.22.168 in the STEEL EC3 add-on module? Why is a * added to Equation (6.36)?
- I design a frame with a taper (docked cross-section). STEEL EC3 classifies the taper in Cross-Section Class 3. Accordingly, the elastic resistances are taken into account, which is very unfavorable. According to the standard, the taper should be categorized in Class 1, and thus the plastic reserve should also be usable.
I design an eccentrically modeled wall beam in RF‑/STEEL Warping Torsion, which is loaded transversely with a distributed load. While the bending moments in the main program result in zero at the member start and member end due to the hinges, the RF‑/STEEL Warping Torsion add-on module displays moments at these locations, which incorrectly reduces the span moment.
How do I get the boundary moments equal to zero at this point?
- I cannot find any reason why no plastic design is performed for a class 1 cross-section. Normally, you have to select the check box for the elastic design.
- How can I design any SHAPE‑THIN cross-section in detail in RFEM or RSTAB?
- Is it possible to deactivate the stability analysis by member within a case in the RF‑/STEEL EC3 add-on module?
- I design a set of members using RF‑/STEEL EC3. In accordance with the manual calculation, the design is fulfilled, but it cannot be performed successfully in the add-on module. Why?
- What is the difference between the RF‑/STEEL and RF‑/STEEL EC3 add-on modules?
- When entering data in the RF‑/STEEL EC3 add-on module, I get the error message "Incorrect location of the intermediate lateral restraint". Why?
- How can I use springs for nodal supports in the design of sets of members in RF‑/STEEL EC3?