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Frequently Asked Questions (FAQ)
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Since null elements (t = 0, t *> 0) have no influence on the moments of inertia or section moduli, but are required for the calculation of shear areas, for example, the shear stresses can also be calculated on these elements.
The display or calculation of the stresses on null elements can be activated in the calculation parameters, as shown in Figure 01. You can select whether the stresses are redistributed to the connected elements or displayed directly. Figure 02 shows shear stresses on an example cross-section.
Stiffeners may only contain the elements with a thickness t > 0 mm. If you select null elements (thickness t = 0 mm), the message shown in Figure 01 appears. It is necessary to replace the null elements by the elements with a thickness t > 0 mm.
The stiffener shown in Figure 2 includes Element 8 and Element 3. However, Element 3 has a thickness t = 0 mm, so the message in Figure 01 is displayed. For Element 3, define a thickness t > 0 mm. As an alternative, you can delete Element 3 and connect Element 8 directly to the panel. This is also shown in the video.
AnswerThe unusual values of shear areas, that is, greater than A, may occur for parametric thin-walled sections, for example, that have an atypical geometry and may not be ideally thin-walled.The shear areas are calculated according to the theory of thin-walled cross-sections in SHAPE‑THIN. Therefore, the defined cross-section should have reasonable dimensions with regard to the width-thickness ratios. The thickness t* of the connecting null element can be used to affect the shear transfer and thus the value of a shear area.
In SHAPE-THIN, you have the option to connect independent cross-section parts by using so-called null elements. Thus, you get a total cross-section and can also use it for the design in RSTAB or RFEM.
It is necessary to assign a thickness t to each element. This value is the actual thickness of the cross-section part for the full transfer of the normal actions. Furthermore, the effective thickness t* controls the shear transfer for "artificial" coupling elements with the thickness of t = 0. These null elements are used to create a shear connection between two elements that do not have a common node. The definition of a shear thickness t* > 0 is required for those null elements that have no contact to normal elements (t > 0) along their entire lengths.
AnswerElements are connected together if you have a common node. Thus, Elements 1 and 2 shown in Figure 01 are connected by Node 2.
If there is no common node available, you can create a shear-rigid connection by using a null element. In the case of the null element, the normal thickness t is equal to zero. However, the shear thickness t* must be greater than zero. Determination of the shear thickness t* is required for the null elements that are not affected by "normal" elements (t > 0) along their entire length. It is recommended to specify the thickness of the thinnest element adjacent to the null element.
A null element can be defined manually or created by using the "Connect node and element" function. Please make sure that the connection is always created by using nodes of the elements and not help points (see Figure 02).
If connecting the elements with cross-sections, the cross-sections have to be previously reduced into the individual elements by using the "Reduce Cross-Section into Elements" feature (Figure 03). This is also demonstrated in the video.
In chapter 7.2.1 of the SHAPE-THIN manual, you can find the formulas for shear areas and Bredtschem torsional moment of inertia. There, the element thickness t * is in the denominator, so the division by zero would be a problem. For "free" null elements, an equivalent thickness must therefore be defined by the user, for example Example, the thickness of one of the adjacent elements.
Select the null element and select "Effective thickness for shear transfer" in the "Edit Element" dialog box. Then, the input field t * becomes accessible. Then, the calculation is possible.
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Wind Simulation & Wind Load Generation
With the stand-alone program RWIND Simulation, wind flows around simple or complex structures can be simulated by means of a digital wind tunnel.
The generated wind loads acting on these objects can be imported to RFEM or RSTAB.
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