In order to correctly design a downstand beam or a T-beam in RFEM 6 using the Concrete Design add-on, it is essential to determine the flange widths for the rib members. This article describes the input options for a two-span beam and the calculation of the flange dimensions according to EN 1992-1-1.
In accordance with Sect. 6.6.3.1.1 and Clause 10.14.1.2 of ACI 318-19 and CSA A23.3-19, respectively, RFEM effectively takes into consideration concrete member and surface stiffness reduction for various element types. Available selection types include cracked and uncracked walls, flat plates and slabs, beams, and columns. The multiplier factors available within the program are taken directly from Table 6.6.3.1.1(a) and Table 10.14.1.2.
The German Annex to EN 1992‑1‑1, the National Addition NCI to Article 9.2.1.2 (2), recommends to dispose the tension reinforcement in the flange plate of T‑beam cross‑sections on a maximum of one width corresponding to the half of a computed effective flange width beff,i according to Expression (5,7a).
Parameterized entries provide the engineer with an efficiency-increasing tool. This allows entering structural and loading data so that they depend on certain variables. These variables (for example, length, width, live load, and so on) are called parameters.
The classification of cross-sections according to EN 1993-1-1 using Table 5.2 is a simple method for designing the local buckling of cross-section parts. For cross-sections of cross-section class 4, it is then necessary to determine the effective cross-section properties according to EN 1993-1-5 in order to consider the influence of local buckling in the ultimate limit state designs.
Prestressed concrete slabs consist of composite, uniaxially stressed hollow plates with a width of about 1.20 m. These elements are prestressed with pre-tension in a precast concrete plant. The precasting is usually done with slipformers. Due to the lesser self‑weight of the non‑solid slab and the existing prestress, these precast prestressed hollow core slabs show a lower deflection than loosely reinforced slabs made of solid concrete.
In Part 1, the selection of the design criteria for dimensioning the reinforcement for the serviceability limit state design in RF‑CONCRETE Members and CONCRETE was explained. Now, we go into detail for the function "Find economical reinforcement for crack width design".
When defining the effective slab width of T-beams, RFEM provides the predefined widths that are determined as 1/6 and 1/8 of the member length. A more detailed explanation on these two factors is given below.
Eurocode 2 provides two ways to perform a crack width design. On one hand, the crack width design according to 7.3.3 can be performed without direct calculation by means of tables for the limitation of the member spacing and diameter. On the other hand, the crack width wk can be determined directly according to 7.3.4 and compared to a limit value.
Reinforced concrete surface design for slabs, plates, and walls is possible in the RF-CONCRETE Surfaces module according to the ACI 318-19 or the CSA A23.3-19 standard. A common approach for slab design is the use of design strips for determining the average one-way internal forces over the width of the strip. This design strip method essentially takes a two-way slab element and applies a simpler one-way approach to determine the required reinforcement needed along the strip length.
When performing control calculations and comparing the internal forces and the resulting required reinforcement of downstand beams, large differences can occur. Although the same load assumptions and spans are applied, some programs or the manual calculation display very different internal forces compared to the FEA model. The differences already occur in the case of the centric member and without considering the internal forces' components from the possible effective slab widths.
In accordance with Sec. 6.6.3.1.1 and Sec. 10.14.1.2 of ACI 318-14 and CSA A23.3-14, respectively, RFEM effectively takes into consideration concrete member and surface stiffness reduction for various element types. Available selection types include cracked and uncracked walls, flat plates and slabs, beams, and columns. The multiplier factors available within the program are taken directly from Table 6.6.3.1.1(a) and Table 10.14.1.2.
Different methods are available for calculating the deformation in the cracked state. RFEM provides an analytical method according to DIN EN 1992-1-1 7.4.3 and a physical-nonlinear analysis. Both methods have different features and can be more or less suitable depending on the circumstances. This article will give an overview of the two calculation methods.
Performing serviceability limit state design also includes taking into account the allowable deformation. Calculating the deformation of reinforced concrete components depends on whether or not the observed cross-section cracks under the applied loading. The governing control parameter in RF-CONCRETE Deflect is the distribution coefficient ζ.
When designing reinforced concrete components according to EN 1992‑1‑1 [1], nonlinear methods of determining internal forces for the ultimate and serviceability limit states are possible. In this case, the internal forces and deformations are determined with respect to their nonlinear behaviour. The analysis of stresses and strains in cracked state usually provides the deflections, which clearly exceed the linearly determined values.
RFEM and the RF-CONCRETE add-on modules provide various options for the deformation analysis of a T-beam in the cracked state (state II). This technical article describes the calculation methods (C) and modelling options (M). Both the calculation methods and the modelling options are not limited to T-beams, but will be only explained using an example of this system.
Buckling analysis according to the effective width method or the reduced stress method is based on the determination of the system critical load, hereinafter called LBA (linear buckling analysis). This article explains the analytical calculation of the critical load factor as well as utilization of the finite element method (FEM).
In the case of combined FEM structures (surface and member elements) as well as folded plate structures, it is possible to attribute a beam structure for the design on a member to a fictitious T-beam cross-section, whose geometry depends on the effective width. When using the "Rib" member type in RFEM, the stiffness is represented by a slab component (surface element) and a web component (member element). This approach has some design specifics that are explained in this article.
Based on the technical article about the ultimate limit state design of rail welds, the following explanation refers to the process of fatigue design of rail welds. In particular, this article explains in detail the effects of considering an eccentric wheel load of 1/4 of the rail head width.
Generally, avoiding cracking in concrete structures is neither possible nor necessary. However, cracking must be limited in a way so that the proper use, appearance, and durability of the structure are not affected. Therefore, limiting the crack width does not mean preventing from the crack formation, but restricting the crack width to harmless values.
The eccentric wheel load application of 1/4 of the rail head width has to be considered only for fatigue design from damage class S3 according to DIN EN 1993‑6. An additional input option in detail settings allows you to consider this eccentricity for fatigue design at the ultimate limit state as well. By selecting this option, the design with the eccentric load applied is always considered without regard to the damage class.
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.
In CRANEWAY, the eccentric wheel loading of 1/4 of the rail head width is used for the fatigue design of welds as well as for craneway girder design according to the National Annex of Germany and as of damage class S3.
Platforms can be connected directly to leg members using the new "Leg Member Axis" option. Thus, it is no longer necessary to define the platform width or coupling member.
Prior to the analysis of steel cross‑sections, the cross‑sections are classified according to EN 1993‑1‑1, Sec. 5.5, with respect to their resistance and rotation capacity. Thus, the individual cross-section parts are analyzed and assigned to Classes 1 to 4. The cross-section classes are determined subsequently and usually assigned to the highest class of the cross-section parts. If plastic resistance is to be applied to further design of cross-sections of Class 1 and Class 2, you can analyze the elastic resistance of cross-sections as of Class 3. In the case of cross-sections of Class 4, local buckling already occurs before reaching the elastic moment. In order to take this effect into account, you can use effective widths. This article describes the calculation of the effective cross-section properties in more detail.
In order to estimate the structural behavior of masonry close to reality by using RFEM, it is necessary to select a material and a material model first. Since masonry responds to tension by cracking, you have to select a nonlinear material model. This can be selected if the RF-MAT NL add-on module is available.
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.
Starting with program version X.06, you can set in RF‑CONCRETE Members or CONCRETE whether the crack width analysis should be performed in any case, or only when exceeding the effective tensile strength of concrete.
The SHAPE‑THIN cross‑section properties program provides convenient options for graphically setting the section reinforcements. When modeling flat bar extensions, you should note one rule only: The length of the element must be greater than the width.
In addition to manually entering values, you can enter line loads in the "Member Load" dialog box using the "Multi-Layer Composition" function. This is a library that contains the compositions of several layers for applying loads. You can freely specify the layer structure using the parameters of description, thickness, density, or surface load, and comment for each layer.