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.
Both the determination of natural vibrations and the response spectrum analysis are always performed on a linear system. If nonlinearities exist in the system, they are linearized and thus not taken into account. They are caused by, for example, tension members, nonlinear supports, or nonlinear hinges. This article shows how you can handle them in a dynamic analysis.
When a concrete slab is set upon the top flange, its effect is like a lateral support (composite construction), preventing problems of torsional buckling stability. If there is a negative distribution of the bending moment, the bottom flange is subjected to compression and the top flange is under tension. If the lateral support given by the stiffness of the web is insufficient, the angle between the bottom flange and the web intersection line is variable in this case so that there is a possibility of distortional buckling for the bottom flange.
For the stability verification of members using the equivalent member method, it is necessary to define effective or lateral-torsional buckling lengths in order to determine a critical load for stability failure. In this article an RFEM 6-specific function is presented, by which you can assign an eccentricity to the nodal supports and thus influence the determination of the critical bending moment considered in the stability analysis.
A new capability within RFEM 6 when designing concrete columns is being able to generate the moment interaction diagram according to the ACI 318-19 [1]. When designing reinforced concrete members, the moment interaction diagram is an essential tool. The moment interaction diagram represents the relationship between the bending moment and axial force at any given point along a reinforced member. Valuable information is shown visually like strength and how the concrete behaves under different loading conditions.
Nodal releases are special objects in RFEM 6 that allow structural decoupling of objects connected to a node. The release is controlled by the release type conditions, which may also have nonlinear properties. This article will show the definition of nodal releases in a practical example.
A standard scenario in timber member construction is the ability to connect smaller members by means of bearing on a larger girder member. Additionally, member end conditions may include a similar situation where the beam is bearing on a support type. In either scenario, the beam must be designed to consider the bearing capacity perpendicular to the grain according to NDS 2018 Sec. 3.10.2 and CSA O86:19 Clauses 6.5.6 and 7.5.9. In general structural design software, it is typically not possible to carry out this full design check, as the bearing area is unknown. However, in the new generation RFEM 6 and Timber Design add-on, the added 'design supports' feature now allows users to comply with the NDS and CSA bearing perpendicular to the grain design checks.
The optimal scenario in which punching shear design according to ACI 318-19 [1] or CSA A23.3:19 [2] should be utilized is when a slab is experiencing a high concentration of loading or reaction forces occurring at one single node. In RFEM 6, the node in which punching shear is an issue is referred to as a punching shear node. The causes of these high concentration of forces can be introduced by a column, concentrated force, or nodal support. Connecting walls can also cause these concentrated loads at wall ends, corners, and ends of line loads and supports.
Given that realistic determination of the soil conditions significantly influences the quality of the structural analysis of buildings, the Geotechnical Analysis add-on is offered in RFEM 6 to determine the soil body to be analyzed.
The way to provide data obtained from field tests in the add-on and use the properties from soil samples to determine the soil massifs of interest was discussed in Knowledge Base article “Creation of the Soil Body from Soil Samples in RFEM 6”. This article, on the other hand, will discuss the procedure to calculate settlements and soil pressures for a reinforced concrete building.
The quality of the structural analysis of buildings is significantly improved when the soil conditions are considered as realistically as possible. In RFEM 6, you can realistically determine the soil body to be analyzed with the help of the Geotechnical Analysis add-on. This add-on can be activated in the model’s Base Data as shown in Image 01.
To perform deflection analysis in the right manner, it is important to “inform” the program about the exact support conditions of the element of interest. The definition of design supports in RFEM 6 will be shown for a reinforced concrete member set.
The punching shear design, in line with EN 1992-1-1, should be performed for slabs with a concentrated load or reaction. The node where the design of punching shear resistance is performed (that is, where there is a punching problem) is called a node of punching shear. The concentrated load at these nodes can be introduced by columns, concentrated force, or nodal supports. The end of the linear load introduction on slabs is also regarded as a concentrated load and therefore, the shear resistance at wall ends, wall corners, and ends or corners of line loads and line supports should be controlled as well.
The stability checks for the equivalent member design according to EN 1993-1-1, AISC 360, CSA S16, and other international standards require consideration of the design length (that is, the effective length of the members). In RFEM 6, it is possible to determine the effective length manually by assigning nodal supports and effective length factors or, on the other hand, by importing it from the stability analysis. Both options will be demonstrated in this article by determining the effective length of the framed column in Image 1.
Complex structures are assemblies of structural elements with various properties. However, certain elements can have the same properties in terms of supports, nonlinearities, end modifications, hinges, and so on, as well as design (for example, effective lengths, design supports, reinforcement, service classes, section reductions, and so on). In RFEM 6, these elements can be grouped on the basis of their shared properties and thus can be considered together for both modeling and design.
This article describes how a flat slab of a residential building is modeled in RFEM 6 and designed according to Eurocode 2. The plate is 24 cm thick and is supported by 45/45/300 cm columns at distances of 6.75 m in both the X and Y directions (Image 1). The columns are modeled as elastic nodal supports by determining the spring stiffness based on the boundary conditions (Image 2). C35/45 concrete and B 500 S (A) reinforcing steel are selected as the materials for the design.
RFEM 6 includes the Form-Finding add-on to determine the equilibrium shapes of surface models subjected to tension and members subjected to axial forces. Activate this add-on in the model's Base Data and use it to find the geometric position in which the prestress of lightweight structures is in equilibrium with the existing boundary conditions.
In order to create a surface model with failing supports close to reality, an option called "Failure if contact perpendicular to surfaces failed" is available in RFEM 5 for contact solids under "Contact Parallel to Surfaces".
When modeling structural bearing systems, especially hall structures, some substructures of a foundation with no influence on the rising structure are not modeled in RFEM/RSTAB. In the case of hall structures, these are, for example, reinforced concrete floor slabs, strip foundations, and the ties between column foundations.
A member's boundary conditions decisively influence the elastic critical moment for lateral-torsional buckling Mcr. The program uses a planar model with four degrees of freedom for its determination. The corresponding coefficients kz and kw can be defined individually for standard-compliant cross-sections. This allows you to describe the degrees of freedom available at both member ends due to the support conditions.
In RF-/STEEL EC3, sets of members are calculated according to the General Method (EN 1993-1-1, Cl. 6.3.4) together with the stability analysis. To do this, it is necessary to determine the correct support conditions for the equivalent structure with four degrees of freedom. In most 3D models today, you can quickly lose track of the location of a set of members in the system.
In the case of using slow‑curing concrete (usually for thick components), you can reduce the calculated minimum reinforcement by a factor of 0.85 to apply the load due to restraint, according to EN 1992‑1‑1, Section 7.3.2. However, a precondition for reduction is that the characteristic value of the strength development r = fcm2 / fcm28 does not exceed 0.3. Other key requirements for the application of this reinforcement reduction are specified explicitly in the final planning documents.
Supports contributing to a load reduction only under compression or tension can be defined as nonlinear supports in RFEM and RSTAB. It is not always easy for the user to select the correct nonlinearity for "failure under tension" or "failure under compression".
The automatic creation of combinations in RFEM and RSTAB with the "EN 1990 + EN 1991‑3; Cranes" option allows you to design crane runway beams as well as support loads on the rest of the structure.
To simulate a support clearance in a connection between members, you can use the "Diagram" function for member hinges. To use this function, first define the relevant degree of freedom as release. Then, you can select the "Diagram" function from the drop‑down list.
For automatic load case combination in RFEM and RSTAB, you have to enter the possible interaction of load cases. In addition to the simultaneous or alternative occurrence of all load cases of an action, an option for different combination conditions is possible.
In the RF-GLASS add-on module, 3D rendering is implemented to facilitate the definition of the support conditions. This interactive graphical visualization facilitates the input and control of line and nodal supports. However, the schematic display can also be selected, if necessary.
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.
The support of the cross-laminated timber panel deserves special attention. Usually, a cross‑laminated wall is secured against shearing by means of shear connectors and against lifting forces by means of tie rods.