Design of a welded truss

Technical Article

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This technical article deals with the component and cross-section designs of a welded truss in the ultimate limit state. Furthermore, the deformation analysis in the serviceability limit state is described.

The model is based on example 1.4 in the technical literature [1] . Figure 01 shows a system sketch with dimensions as well as the representation of the cross -sections used.

Actions and design internal forces

The characteristic actions for this partial structure are summarized in [1] and transferred to the corresponding load cases. A load generation on the 3D model would certainly be more realistic and preferable for this building. The combinations of the ULS and SLS design situations are performed automatically in RFEM or RSTAB.

The internal forces and deformations in the load combinations are calculated according to the first -order analysis. This results in the following design internal forces for the ultimate limit state design (Figure 02).

Cross -section classification

To determine the cross-section class, a first design case is created in RF-/STEEL EC3 for all members and without stability designs. The cross-section class is calculated by x-location for the available internal forces. A graphical display of the resulting cross-section class is possible using the Results navigator (Figure 03). All cross -sections are in the QKL 1, thus a plastic cross -section design according to Section 6.2.9 in [2] is possible.

Ultimate Limit State Designs

The cross-section and stability analyzes for the cross-sections as well as the upper and lower flange are now performed. To design the cross-sections (members 1 to 16), a second design case is created in RF-/STEEL EC3. The stability analysis is performed according to Section 6.3.1 in [2] . The effective length factor for the major and minor axis is set to 0.75 according to Section BB.1.3 (3) B in [2]. Design ratios see Figure 04.

For the design of the bottom flange, a third design case is created in RF-/STEEL EC3. Since there are tensile forces in the bottom flange even under wind suction, only cross -section designs are required. Furthermore, it is not necessary to design with a net cross-section, as the cross-sections are welded on and thus there are no holes in the cross-section. Design ratios see Figure 05.

To design the top chord, a fourth design case is created in RF-/STEEL EC3 and the corresponding set of members is selected. In contrast to [1] , the design is performed according to the general method according to Section 6.3.4 in [2] to better represent the boundary conditions. A forked support is assumed on the supports. No fork supports can be assumed at the connection points of the purlins on the upper flange. A lateral support is applied to the upper flange and a torsional spring, which results from the profile deformation of the HE-B 240. Design ratios see Figure 06.

Serviceability limit state designs

For the design of the top flange, a fifth design case is created in RF-/STEEL EC3. In this case, only the load combinations in the SLS are selected. In [2] , no information about allowable deformations is given. These must be coordinated with the client on a project -specific basis. It is therefore based on the limit value of the prestandard of L/200. Design ratios see Figure 07.

Design connections

The design of the welded connections of the flange nodes as well as the connection of the truss to the restrained external columns is not described in this article. The design of the end plate connection in the bottom flange according to the CIDECT method and using the FEM model is explained in detail in this technical article.

Keywords

Truss Roof structure Stability analysis

Reference

[1]   bauforumstahl e.V.: Beispiele zur Bemessung von Stahltragwerken nach DIN EN 1993 - Eurocode 3. Berlin: Ernst & Sohn, 2011
[2]   Eurocode 3: Design of steel structures - Part 1‑1: General rules and rules for buildings; EN 1993‑1‑1:2010‑12

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