Lateral-Torsional Buckling (LTB) is a phenomenon that occurs when a beam or structural member is subjected to bending and the compression flange is not sufficiently supported laterally. This leads to a combination of lateral displacement and twisting. It is a critical consideration in the design of structural elements, especially in slender beams and girders.
By means of result combinations, it is possible to create, among other things, the envelopes for internal forces and deformations. Thus, you can quickly find the maxima and minima for the subsequent design.
The RF-/LIMITS add-on module allows you to compare the ultimate limit state of members, member ends, nodes, nodal supports, and surfaces (RFEM only) by means of a defined ultimate load capacity. Furthermore, you can check nodal displacements and cross-section dimensions. In this example, the column bases of a carport are to be compared with the maximum allowable forces specified by the manufacturer.
To control the lateral displacements of a model, you can use the RF-/LIMITS add‑on module. This add‑on module allows you to, for example, run a serviceability limit state analysis to find horizontal nodal deformations and to set it against a limit value.
As gravity loads act on a structure, lateral displacement occurs. In turn, a secondary overturning moment is generated as the gravity load continues to act on the elements in the laterally displaced position. This effect is also known as "P-Delta (Δ)". Sec. 12.9.1.6 of the ASCE 7-16 Standard and the NBC 2015 Commentary specify when P-Delta effects should be considered during a modal response spectrum analysis.
In the AISC 360 – 14th Ed. C2.2, the direct analysis method requires initial imperfections to be taken into consideration. The important imperfection of recognition is column out-of-plumbness. According to C2.2a, the direct modeling of imperfections is one method to account for the effect of initial imperfections. However, in many situations, the expected displacements may not be known or easily predicted.
For the ultimate limit state design, EN 1998 1, Sections 2.2.2 and 4.4.2.2 [1], requires the calculation considering the second-order theory (P-Δ effect). This effect may be neglected only if the interstory drift sensitivity coefficient θ is less than 0.1. The coefficient θ is defined as follows: $$\mathrm\theta\;=\;\frac{\displaystyle{\mathrm P}_\mathrm{tot}\;\cdot\;{\mathrm d}_\mathrm r }{{\mathrm V}_\mathrm{tot}\;\cdot\;\mathrm h}\;(1)$$ where θ is the interstory drift sensitivity coefficient, Ptot is the total gravity load at and above the story considered in the seismic design situation (see Expression 2), dr is the design interstory drift, evaluated as the difference of the average lateral displacements dS at the top and bottom of the story under consideration; for this, the displacement is determined using the linear design response spectrum with q = 1.0, Vtot is the total seismic story shear determined using the linear design response spectrum, h is the interstory height.
The form-finding process in RF-FORM-FINDING displaces the corner nodes of FE elements of a membrane surface in space until the defined surface stress is in equilibrium with the boundary conditions. This displacement is independent of the element geometry. In the case of elements with four corner nodes, the free displacement may cause spatial drilling in the element plane and thus exceed the validity limits of the calculation; therefore, triangular elements are generally recommended for form‑finding systems. Triangular elements remain independent of the corner node displacement and stay within the calculation limitations.
When modeling structural systems or loads, input errors or faulty objects may occur due to subsequent modifications, displacements, and adjustments in the model.