In many frame and truss structures, it is no longer sufficient to use a simple member. You often have to consider cross-section weakenings or openings in solid beams. In such cases, you can use the "Surface Model" member type. It can be integrated into the model like any other member and offers all the options of a surface model. The present technical article shows the application of such a member in an existing structural system and describes the integration of member openings.
The AISC 360-16 steel standard requires stability consideration for a structure as a whole and each of its elements. Various methods for this are available, including direct consideration in the analysis, the effective length method, and the direct analysis method. This article will highlight the important requirements from Ch. C [1] and the direct analysis method to be incorporated in a structural steel model along with the application in RFEM 6.
In RFEM 6 it is possible to save selected objects (as well as whole structures) as blocks and reuse them in other models. Three types of blocks can be distinguished: non-parameterized, parameterized, and dynamic blocks (via JavaScript). This article will focus on the first block type (non-parameterized).
Inserting holes in surfaces is very easy due to the large selection of tools. In order to insert holes or drilling in solids, it is necessary to keep in mind that an opening at the beginning and the end of a continuous hole must be created, as well as a surface that separates the hole from the solids.
RFEM 5 allows you to use many different member nonlinearities for designing a model. In the following text, we look at an example of the use of the "slippage" member nonlinearity. The example is a simplified model of a concrete manhole with a square plan view.
When connecting tension-loaded components with bolted connections, the cross-section reduction due to the bolt holes must be taken into account in the ultimate limit state design. This article describes how the design of the tension resistance according to DIN EN 1993‑1‑1 can be performed with the net cross-section area of the tension member in the RF‑/STEEL EC3 add-on module.
In cross‑sections created in SHAPE‑THIN, the openings, such as bolt holes, can be modeled by using the elements with zero thickness. The program provides two options for calculating shear stresses in the area of such null elements.
The simplest way to model a bolt connection in RFEM 5 is to define a node in the center of a hole, then connect it by means of internal members to the surface.
The definition of the non-linear contact problem plays an important role for more detailed investigations of shear/hole bearing connections or their immediate environment. This article uses a solid model to search for comparable and simplified surface models.
SHAPE-THIN allows you to calculate section properties and stresses of any cross-sections. If a flange or a web is weakened by bolt holes, you can consider this by using null elements. The stresses are subsequently recalculated with the reduced cross-section values. In this case, it is necessary to pay a special attention to shear stresses. By default, these are set to zero in the area of the null elements. When recalculating shear stresses with the reduced cross-section values and without further adaptation, it turns out that the integral of the shear stresses is no longer equal to the applied shear force. The following example shows in detail how to calculate the shear stress.
Sometimes, a model in the graphic window is displayed without filling the entire window, or with overly large margins after clicking the [Show Whole Model] button. To set the size of the graphic margins, click "Options" → "Display Margins and Stretch Factors". The value specifies the percentage of the margin relative to the graphic window size.
Downstand beams or T-beams are often used in reinforced concrete structures. In contrast to the previous representation and calculation options where, for example, a downstand beam was considered as a fixed support and the determined support reaction was applied to a separate member structure using a T-beam section, the ultimate structural FEA software like RFEM allow you to consider the structure as a whole and thus achieve a more precise analysis.