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.
Surfaces in building models can be of many different sizes and shapes. All surfaces can be considered in RFEM 6 because the program allows to define different materials and thicknesses as well as surfaces with different stiffness and geometry types. This article focuses on four of these surface types: rotated, trimmed, without thickness, and load transfer.
Spreadsheet programs like MS EXCEL are very popular with engineers because they allow you to simply automatize your calculations and quickly output the results. Therefore, combining MS EXCEL used as a graphical interface with Dlubal's WebService API is an obvious choice. By using the free xlwings library for Python, you can control EXCEL, and read and write values. The functionality is described in the following, using an example.
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.
For the design of concrete surfaces, the rib component of the internal forces can be neglected for the ULS calculation and for the analytical method of the SLS calculation, because this component is already considered in the member design. To do this, select the check box in the "Details" dialog box. If no rib was defined, this function is not available.
DXF layers of ground plans cannot be used directly in FEA programs because only the outer contours of the elements (walls, ceilings, and so on) are available in the drawing. The FEM programs require system axes, but only the outer contours of the elements (walls, ceilings, and so on) are available in the DXF drawing.
The most common causes of unstable models are failing member nonlinearities such as tension members. As the simplest example, there is a frame with supports on the column footing and moment hinges on the column head. This unstable system is stabilized by a cross bracing of tension members. In the case of load combinations with horizontal loads, the system remains stable. However, if it is loaded vertically, both tension members fail and the system becomes unstable, which causes a calculation error. You can avoid such an error by selecting the exceptional handling of failing members under "Calculate" → "Calculation Parameters" → "Global Calculation Parameters".
In the world of construction engineering, the word "imperfections" has a specific meaning. In general, it describes the incompleteness of a structure or the deviation of a structural component from an ideal shape caused by the production.
A calculation break‑off due to an unstable system can have different reasons. On one hand, it can indicate a "real" instability due to overloading of the system; on the other hand, the error message can result from inaccuracies in the model.
If the calculation of a member model according to the second-order analysis is terminated with an error message, this instability is often caused by failed tension members: As soon as compressive forces appear in a tension member during a calculation step, this member is no longer considered in the following iterations. Thus, the model can become unstable.
Designing rigid end plate connections is difficult for four-row connection geometries and multi-axis bending stresses, because there are no official design methods.
Structures are naturally three-dimensional. However, because it was impossible to perform calculations on three-dimensional models easily in the past, the structures were simplified and broken down into planar subsystems. With the increasing performance of computers and related software, it is often possible to do without these simplifications. Digital trends such as Building Information Modeling (BIM) and new options for creating realistic visualized models reinforce this trend. But do 3D models really offer an advantage, or are we just following a trend? The following text presents some arguments for working in 3D models.
Pay particular attention to the connection points of members and surfaces when you deal with mixed systems, because not all internal forces can always be transferred without difficulty at the coupling location.
The design of cold-rolled steel products is defined in EN 1993-1-3. Typical cross-section shapes are channel, C, Z, top hat, and sigma sections. These are cold-rolled steel products made of thin-walled sheet metal that has been cold-formed by roll-forming or bending methods. When designing the ultimate limit states, it is also necessary to ensure that local transverse forces do not lead to compression, crippling of the web, or local buckling in the web of the sections. These effects can be caused by local transverse forces by the flange into the web, as well as by support forces at the supported points. Section 6.1.7 of EN 1993-1-3 specifies in detail how to determine the resistance of the web Rw,Rd under local transverse forces.
When editing elements via the COM interface, selecting elements is often a problem because it cannot be carried out visually via the work window. The selection can be particularly difficult for models that have been created via the program interface and are then to be modified using a separate program. Apart from the exception, when the selection was made previously via RFEM, there are several alternatives for programming.
The story drift of a building provides valuable information about its structural behavior under seismic loads. These could cause large horizontal deformations and even instabilities. Some standards, therefore, call for a check of the story drift in its center of gravity. It indicates, for example, if a second-order analysis (P-Δ effect) is necessary.
In the case of a post-critical failure, a substantial change occurs in the geometry of a structure. After reaching the instability of the equilibrium, a stable, strength position is reached again. The post-critical analysis requires an experimental approach. It is necessary to manually load the structure in increments, step by step.
The following article describes a design using the equivalent member method according to [1] Section 6.3.2, performed on an example of a cross-laminated timber wall susceptible to buckling described in Part 1 of this article series. The buckling analysis will be performed as a compressive stress analysis with reduced compressive strength. For this, the instability factor kc is determined, which depends primarily on the component slenderness and the support type.
As of program version 5.06, you can use the option to adjust the effective concrete tensile strength fct,eff,wk at the time of cracking. At the start of the SLS design, the program checks whether the internal forces can cause cracks in the concrete. For this, the effective concrete tensile strength at the time of cracking is applied. You can adjust the strength via the factor. The calculation details display the adjusted value.
As of RFEM Version 5.06, there is the option in RF‑CONCRETE Surfaces to adjust the effective concrete tensile strength at the time of cracking. At the start of the SLS design, the program checks whether the internal forces can cause cracks in the concrete. For this, the effective concrete tensile strength at the time of cracking is applied. You can adjust the strength via the factor. The calculation details display the adjusted value.
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.
RFEM facilitates modeling by the automatic integration of objects into surfaces. However, it is impossible to integrate the objects automatically in the case of curved surfaces. For manual integration, select the relevant surfaces and click the "Edit Surfaces" option in the shortcut menu; then, in the "Integrated" tab, you can integrate the relevant objects using the "Select" function. This way, you can avoid error messages caused by non‑integrated objects when starting the calculation.
Modern buildings are designed with spaces tailored to personal desires and dreams, expressing individual lifestyles. These requirements often include ceilings - whether in houses, office buildings, or public buildings - that have an enormous span and no support, allowing optimal use of the space below. However, this requires a very high stability level for load‑bearing capacity and serviceability reasons. By extending the size of beam or plate cross-sections, you can increase the stability, but the cost effectiveness decreases because of the additional consumption of material. One common solution for these large spans is to use timber or steel downstand beams.
You can obtain many load combinations, especially when using the automatic generation of combinations. These are automatically combined in a result combination (RC) with the OR operator as an envelope. Then, if you select one RC for design in RF‑/STEEL EC3, it may lead to a very long calculation time because the module calculates all combination options individually by default, then displays the results of the governing combination.
The previous post on this topic describes instabilities that may occur when using tension members. The example shown refers primarily to wall stiffening. Now, instability error messages can also refer to nodes within the range of supports. Truss girders and support trusses are especially susceptible to this. What causes the instability here?
Shoring braces usually obtain the "tension member" type. There are a few specifics to note because in the case of uniform, symmetrical structures and solely vertical loads, an error message often appears as follows: "The model is unstable in node No. 20. Free movement around Y-direction."
You can define nonlinear supports in RFEM and RSTAB. In RFEM, these are represented by nodal, line, and surface supports. Many customers contact us because of nonlinearities that are apparently not acting as desired. For example, there is a failing line support in a model. Since the structure is statically determined as supported, a linear nodal support is usually added. If the nodal support rests at the start or the end of a nonlinearly supported line, there is no clear definition of the degrees of freedom, so the nonlinearity cannot be considered properly. In this case, RFEM displays a warning message.
A modell check allows you to find overlapping members, among other things. However, this targeted selection could cause some minor problems. Therefore, there is a selection window now available, which appears when you click on one of the elements. This appears by clicking on one of the elements. Additional information helps you to select the correct member.
Extensive calculations may result in vast amounts of data. Of course, current hard drives and SSDs are measured in terabytes. Therefore, you expect this to be no problem for current computing technology. This is true, in fact, but as often happens, the devil is in the details.