Load areas in Advance Design

Load areas in Advance Design:

1.1 Introduction

Every engineer has to perform the load analysis on their own projects, one of the most time-consuming phases. In fact, for each project the designers has to consider different load case families, for example dead load, live loads, climatic loads.

On the structural analysis software, these loads are applied either directly on linear and planar elements (members, slabs, walls) or on specific virtual planes which could be horizontal, vertical or sloped. In Advance Design, such planes are called Load Areas, and are used for distributing of loads on the supporting elements, following a failure line algorithm or a user-defined span direction. Load Areas may serve for the load calculation on the linear supporting elements without necessarily performing the planar element modeling (i.e., roofs, walls, slabs, etc.).
Using Load Areas, you can also obtain a distribution of loads to the sides of the “membrane” elements, that have no rigidities perpendicular on their plane.

Figure 1: Load Areas applied on supporting elements

1.2 Load area native properties

The main purpose of Load Areas in Advance Design is the load distribution on supporting elements. For example, it can be the distribution of loads from the roof to the purlins. Depending on the needs, the load distribution can be carried out in one direction only or in multiple directions. That’s why inside the property of the load area you can define the span direction:

• x: according to the local x axis direction.
• y: according to the local y axis direction.
• xy: according to local xy axes direction.
• Other: according to a custom span direction.

Figure 2: Load areas span directions

The span direction allows the user to split the planar load assigned to the load area into several linear loads over the supporting element. Let’s see an example with a custom span direction, which allows the definition of an arbitrary way of load distribution. We have a simple case of one rectangular Load Area (1m x 5m) with a 1 KN/m2 uniform planar load, on the contour of which there are linear elements:

Figure 3: Planar load with custom span distribution

In this example, load area has 2 side-bearing with no null coefficients: 0.8 (on side 4) and 1.1 (on side 2), The sum of these coefficients is 0.8 + 1.1 = 1.9.

The relative weight of side 4 is then 0.8 / 1.9 = 0.421, respectively 1.1 / 1.9 = 0.579 on side 2.

This proportion is also displayed in terms of rupture polygons (in this case simple rectangles). The left (black) polygon represents 42% of the load and the right (magenta) polygon represents 58% of the load.

Figure 4: Linear loads get with custom span distribution on analytical mode

2. Load Area as rigid diaphragm

2.1 Global assumption

Another important feature of the load area is the possibility to automate the creation of the rigid diaphragm in case of an FEM analysis with horizontal loads (regardless of the type of loads: user defined, climatic, seismic, etc.).

The starting assumption of this functionality is that you should create load areas in the model, which will act as a “rigid diaphragm”. Then, starting from these already defined panels, Advance Design automatically creates the corresponding rigid diaphragm.

When creating a Load Area, we have available options to activate the “rigid diaphragm” behavior, in the corresponding Advance Design property list:

Figure 5: Rigid diaphragm behavior

If you enable the option “Rigid Diaphragm”, Advance Design (when creating the analysis model) will superpose a membrane (2D FEM planar element) with the material and the thickness defined here above.

You also have the possibility to activate the option “Self-weight auto”. In this case, the self-weight of the diaphragm will be calculated automatically (according to the imposed material and thickness) and applied as a planar load over the Load Area. Also, all the loads applied on the Load Area will be directly transfer to the supporting elements depending on the span direction set by the user (this is the normal behavior of the load area elements).

Figure 6: Rigid diaphragm behavior in Advance Design

3. Climatic Behavior

The Load Area has an important role for the climatic load generator inside Advance Design. The climatic generator works with the Load Area, so the first step to create automatically the wind and snow load consist on apply the Load Area around the structure. The climatic loads are applied over the load area according its Climatic Behavior Type, which can be selected in the load area property list.

Figure 7: Load Area type of structure list used by climatic generator

This parameter allows you to distinguish the procedure for generating the climatic load presented by the standard. For example, wind is generated differently when the vertical area is a building wall or when it is a free-standing wall.

The list of supported types of structures depends on the climatic load standard. The longest list is available for Eurocode 1:

• Building
• Protruding roof
• Parapet wall
• Isolated roof with one slope
• Isolated roof with two slopes
• Outdoor panel
• Lattice structure or scaffolding
• Vertical roof slope (shed)
• Awning
• Inclined wall
• Circular wall
• Free standing wall

The default type is Building, which is used to define loads on a building. The other types are used for special cases of geometry.

Figure 8: Type of structure supported by climatic generator of Advance Design.

4. Modelling Load Areas

In Advance Design is very easy to create Load Areas. There are 2 ways:

• By generating them automatically, on a selection
• By modeling in the graphic area

4.1 Generating Load Areas on Selections

The Load Areas can be automatically generated between two selected linear elements. When you are applying the load area on selected elements, it’s important to consider these recommendations:

• The linear elements should not be placed in different planes.
• They should not have common points other their extremities.
• They cannot be collinear.

To apply the load area on selection it’s enough to select two linear objects, right click on the graphic area and select Load Areas / selection.

Figure 9: Load area on selected elements

4.2 Modelling load area

In Advance Design, Load Areas can be created directly in the graphic area, in a similar way as planar objects (walls or slabs). The position of the load area’s extremities is specified by snapping graphically to other objects, or by entering the coordinates in the command line.

The 1st and 2nd points represent the x axis definition, while the 3rd point defines the plane of the planar element along with points 1 and 2. According to this, the local z axis is perpendicular to the planar element with positive projection on the global Z whenever possible (when the planar element is parallel with the global Z, its local z axis will have a positive projection on global X).

Figure 10: Modelling the load area in the graphic area.

Written by Eng. Alessandro Bordin – Software Technical Specialist II 

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