# Frequently Asked Questions (FAQ)

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• ### How can I achieve a pressure contact with a tension failure between a transverse member and a weakly curved membrane surface?

The nonlinear contact between member and membrane can be done by a set of member elements between the member and the surface. This requires that the member lies eccentrically in the plane of the compressive force resulting from the membrane effect and the connection line of the membrane connection. The geometric distance between member and membrane itself has to be aligned to the physical distance between member axis and membrane connection.

To ensure that the coupling runs homogeneously over the entire coupled length, it is necessary to ensure a uniform arrangement with the same number of FE nodes on the member axis and on the projected contact line on the membrane surface. This division and orientation of the FE nodes is achieved by placing the corresponding topology nodes on the member axis and the corresponding line of contact on the surface. The distance of the Topology Node affine to the selected mesh size of the connected Membrane Surface should be selected.

The coupling itself must be designed with a rigid member failing nonlinearly under compressive load between the resulting nodal pairs. In this case, the specified nonlinearity must be implemented with a member nonlinearity "Failure under tension". The connection of the rigid member in the area of the eccentric member is completely compatible (bending-resistant) and must be carried out with a free translational hinge related to the rigid member axis in the y / z axis in the area of the membrane.

Due to the selected nonlinearity and the alignment with the compression force resultant in connection with the free translational hinge, this contact modeling is able to transfer only compressive forces to the connected cross member. In case of a suction load, the coupling components fail and the membrane moves away from the cross member without hindrance.

• ### In my model, I get the error message 414 for a contact solid. It is not possible to perform a calculation.

FAQ 002544 EN Modeling | Structure RFEM

The message says that the lines of the contact surfaces are not parallel. Therefore, it is recommended to check the nodes of the contact surfaces again and to check whether the lines are really exactly parallel to each other.

• ### How can I quickly create a contact solid between two contact surfaces in RFEM?

FAQ 002443 EN Modeling | Structure RFEM

The quickest way to create a contact solid between two planar surfaces is to use the function "Create Solid by Contact." Select both contact surfaces and select the corresponding function in the shortcut menu (see Figure 01).

The program automatically creates a contact solid including all boundary surfaces and openings in the solid. All you have to do is define the contact properties in the settings for the contact solid (see Figure 02).

• ### Can solids be modeled and statically calculated in RFEM?

FAQ 002423 EN Modeling | Structure RFEM

Naturally, volume elements can also be generated with RFEM. These are above all always useful if it is z. B. is particularly shaped machine components or particularly thick components. Even if an image as a surface element is no longer possible, the image by means of volume elements often offers. As a result of the calculation of solids, we obtain deformations, stresses and distortions.

Other areas of application of volume elements include contact problems. Thus, for example, with a contact volume, the contact conditions of complex connections with the contact properties "failure in pressure" or "failure in tension" for the contact perpendicular to the contact surfaces can be taken into account. Parallel to the surfaces are the properties "Failure if contact with surfaces does not work", "Full power transmission", "Rigid friction", "Rigid friction with limitation", "Elastic friction", Elastic friction with limitation "and" Behavior of the elastic volume "to choose from.

Contact properties between two contact surfaces can also be realized via area approvals (see following general descriptions):

There is already an interesting video from Dlubal Info Day from 14. December 2016 with the topic "Dlubal-Infotag online 2016 - Part 4: Modeling and calculation of volume elements in RFEM "on our homepage :):

• ### For wall ends and wall corners, no soil contact stresses are deducted in RF-PUNCH pro, even though the type "Foundation" has been set. It is also nowhere possible to define that the soil contact stresses should be deducted. Why does this happen?

If you perform a punching shear design for a wall corner or a wall end at a floor slab or foundation plate, this entry is not necessary.

Since the punching load is determined at a wall corner or a wall end from the shear forces in the control perimeter when performing the punching shear design, the ratio of the surface load is missing anyway within the control perimeter (up to the load application). Therefore, this entry is also not necessary in the 1.5 dialog box.

If you, however, perform the punching shear design for a single column at a foundation plate with the punching load from the normal force of this column, you can enter the "Deductible Surface Load" at the end of the dialog box "1.5 Nodes of Punching Shear - Node of Punching Shear - Details" which allows to determine ΔVEd.
• ### Is is correct, that you have to enter the allowable soil pressure as the characteristic value in input form 1.1 General Data in FOUNDATION Pro? Is the design value determined automatically?

Yes, this is correct.

The characteristic value of soil pressure is entered in input form 1.1 General Data. See Figure 01.

The design value is:
${\mathrm\sigma}_\mathrm{Rd}\;=\;{\mathrm\sigma}_\mathrm{Rk}\;/\;\;{\mathrm\gamma}_{\mathrm R;\mathrm v}$

The estimated partial factor for spread foundations can be found in the parameters of the national annex you selected for the design. See Figure 02.
• ### I placed two solids on top of each other. There are nodes/lines/surfaces which are used by both solids. Is there a possibility to generate a failure?

FAQ 002280 EN Modeling | Structure

If the contact between two overlaying plates or solids is simulated, you have to model a contact solid between the two plates / solids usually with a thickness from central axis to central axis of both contact surfaces. The material of the contact solid should be the same as for the surfaces in order to be connected. If the surfaces have different materials, we recommend using the material with inferior properties for the contact solid. The following blog entries from our website provide some information about contact solids: Blog entries contact solid. You can also have a look at the following demo video where we show modeling a contact solid: Demo video contact solid

• ### How can I determine the resulting force of a surface support or its contact stress in RFEM?

FAQ 002276 EN Results RFEM

For some time, there has been a helpful function that allows you to display the resultant of the contact stress per surface. Accordingly, these resulting contact stresses are shown in the results tables of the Contact Stresses surfaces (Table 4.21) in columns F, G, and R (see Figure 1).

Alternatively, you could also determine the resultant by hand. If your model has a uniform FE mesh whose elements determine the surface, you can export the determined values of the contact stresses from RFEM and multiply it with the surface area of the finite elements to obtain resulting forces per element. To obtain the entire resulting force of a surface support, you have to sum up the individual resultant elements. As the values of the contact stresses shown in Table 4.21 are displayed for each grid point, it is recommended to adjust the grid of the surface so that a grid point always lies, if possible, in the center of a finite element. The grid of the surface can be adjusted with "Edit Surface" in the "Grid" tab (see Figure 2).
• ### Is it possible to assign a nonlinear material to a contact solid? Which material is most suitable?

FAQ 002272 EN Modeling | Structure RFEM

Nonlinear materials are not allowed for contact solids. It is not possible to make a general statement regarding which material should be used. A contact solid can represent a rubber body or a steel plate. We recommend to select an isotropic material here which corresponds or is similar to the real material in its properties.

• ### What is the effect of contact stresses σz at curved surfaces?

FAQ 001079 EN Calculation RFEM

The contact stresses σz on rotational surfaces act in the direction of the local z-axis of the surface.

Each FE element has its own local coordinate system, which corresponds to the local coordinate system of the surface. If the local z-axis of the surface shows outwards, then it does this at all FE elements and thus along the entire surface.

The direction of acting at curved surfaces can be controlled in the graphic, when selecting the 'FE Mesh → On Surfaces → FE Axis Systems x,y,z' option in the Display navigator.

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