Wednesday, 21 February 2018

Customer Project - Stainless steel slide

Structural Engineering MŰÉP Ltd.
George Keller+Co. Ingenieurgesellschaft mbh.
Location Sindelfingen, Germany
Built in 2017
Size Length 10.0 m.; height 7.0 m
Material Stainless steel

A 7-meter high and 10 meters long, stainless steel slide's analysis and design were carried out with ConSteel software.

Support legs are made of 219,1 and 80 mm diameter cold formed tube with 3mm thickness. In general, the wall thickness of the slide is 2.5mm, but around the connection points of the support legs, it is increased to 10.5mm.

Buckling analysis

3D model was created in the structural engineering software ConSteel. The slide was modelled as a surface element, support legs were modelled as beam elements. Connections between the legs and the thin wall were entered with rigid link elements.
The elastic buckling behaviour of the model was analyzed by linear eigenvalue solution.

Local buckling of the slide's wall was checked by the reduced stress method of EuroCode.

Friday, 16 February 2018

This is what happens with the cross sections of your model under the hood - The Unified cross section model

Advanced steel structural analysis softwares prefers the finite element method based on 3D thin-walled beam-column elements. This method requires the analysis-oriented modeling of cross-section problems. 
On the other hand, the integrated design modules based on modern design standards (like Eurocode 3) need the design-oriented approach. 
The design of class 3 shapes is based on the linear elastic properties used in analysis, while the design of class 1 and 2 cross-sections requires the plastic properties. Moreover, the design of class 4 shapes uses the effective cross-section model to take the local buckling into consideration. 

It is concluded that the integration of the analysis and standard design needs an approach that covers both areas.

In ConSteel, a unique model-oriented approach has been developed for the integrated analysis of cross-sections that satisfies the requirements of both the advanced beam-column analysis and the automatic standard design procedure. 

During the integrated analysis-design procedure the different program components require different cross-section properties computed on different basic conditions. The elastic analysis used commonly in the standard design needs the nominal properties that are computed on the basic model. The geometric non-linear analysis requires a refined model to compute the stress-dependent Wagner coefficient. The design of class 4 shapes needs the effective cross-sections. However, all the relevant computational models can be derived from the fundamental model. This fact has emerged the object-oriented approach of the problem. Each procedure can be based on the fundamental geometric model and on the nominal properties related to this model.

When a cross section is defined, ConSteel automatically generates the two parallel fundamental cross sections:
  • General Solid Section GSS: Compute the elastic cross-sectional properties for any kind of elastic cross-sections as accurate as possible
  • Elastic Plate Segment EPS: Able to serve specific cross-sectional properties for the standard design procedures

Papp F, Iványi M, Jármai K.Unified object-oriented definition of thin-walled steel beam-column cross-sections. COMPUTERS & STRUCTURES 79: pp. 839-852. (2001) (Abstract)

Friday, 2 February 2018

Tips & Tricks: Ever wondered how to check the GLOBAL and LOCAL stability behaviour on the same model?

When you want to check the global buckling behaviour of your structure, it is very straightforward, that first, you will perform a buckling analysis. The results of the analysis are the actual stability loss forms of the structure, and the load levels, on which these stability losses will happen (so called elastic critical load factors). The elastic critical load factors are calculated for each load combination, and they are being used at the global checks to determine the slenderness and the reduction factors each member. Reducing the cross section resistances with the reduction factors will result the global stability resistance of the structure.

But what can you do, if you want to get a picture about the local buckling behaviour of a single member of your structure, which is sensitive for local buckling?

The answer is the convert members to plate function of ConSteel.

Here is a nice example how to check local buckling problems:

1. The initial model is a frame, with segmented tapered members, built from welded I sections, with a relatively high web at the corner regions:

2. Checking the global buckling behaviour of the structure, with a buckling analysis

It seems, that the dominant part of the frame is the corner region. It may worth to see this region how it behaves for local buckling.

The critical load level for the buckling shape is 4,25

3. Use the Convert members-to-plates function on the beam at the corner region:

All eccentricities, supports, and member parameters are kept during the transformation. Good to know, that at the end of the converted members, so called rigid bodies are automatically created. The rigid bodies provides the proper load transfer process between the bar members, and the plates:

4. Check the local buckling behaviour of this part, along with the whole structure, by performing a buckling analysis on this model:
The critical load factor for this local buckling mode is 2,73 (!)

5. Perform some modification on the plate, by adding some stiffeners to the region:

6. And finally, perform a buckling analysis again to see the difference in local buckling behaviour after the changes:

The critical load factor for this local buckling mode is 3,52

7. Repeat the iteration until the desired load factor is reached...