Add Shear Walls (frame modelling method)

The Add Shear Walls tool provides a method to model the presence of concrete shear walls in a structure without the need to use the Finite Elements. The Add Shear Walls (frame modelling method) is available as part of Masterframe and does not require any additional modules.

The frame modelling method modifies the selected part of the model to introduce a column with the section properties of the wall, this is, the introduced column section has the same cross sectional properties of the wall added. In addition to this column section, the software also introduces Rigid Link members, the purpose of which is to transfer loads applied at the edges of the wall by beams coming into the edge of the walls. The Rigid Link members are rigid in terms of bending and so any loads applied to them are transferred by to the central column as bending, thus modelling both the axial forces and bending moment in the wall.

The frame modelling method shear walls require the edge of the walls to be present to define the outline of the wall. Therefore, when initially creating a model, columns and beams should be used to create a wire frame outline of the shear walls; the vertical members are used to define the width of the wall element while the beams are replaced by the Rigid Link members and are required to fully transfer load from a floor to the wall.

Limitations

The frame modelling method of creating concrete shear walls has some limitations which can affect the applicability of this method. It is important, therefore, to be aware of the limitations of this method prior to amending a model. Where the shear walls to be added fall outside these limitations, alternative methods need to be considered, or the Finite Element module should be used to model the shear walls as FE surfaces.

Long walls:

Modelling a long length of wall as a single concrete shear wall will not be an accurate model of the real wall behaviour in the actual structure, since the rigid link members will be long and so any load applied to them will result in very large in plane moments in the wall, which will not be an accurate representation of the stress distribution of the wall. For long walls, it will be necessary to model the shear wall as a series or shorter wall lengths, with the rigid link members acting in series to better distribute the forces in the wall, in particular the in-plane bending moments. One method to determine if the wall lengths are appropriate is to view the deflected shape of the wire frame model after analysis. If the curvature of the rigid link members is greater than the column, this would suggest the rigid link members are not sufficiently stiff. This would then suggest that a shorter wall length needs to be used. Alternatively, the stiffness of the rigid link members can be increased by increasing the section size of these elements.

Shear deflections:

Generally, in steel and concrete structures, the shear deflection is regarded to be minimal and therefore need not be considered. However, ignoring the shear deflection for a shear wall could lead to an overestimation of its lateral stiffness and so lead to an unrealistic estimation of the lateral stiffness of the structures which would, in turn, lead to deflections of the structure under lateral loading which would be smaller than would be realistically expected. Therefore, when modelling with the frame modelling method for shear walls, it is important to activate the Include Shear Deflection option in the global analysis options. To do this, go to Analysis>Global Analysis Options.

Base Fixity:

In general, shear walls are supported fully along the length of their bottom edge. In effect the wall is fixed rotationally relative to in-plane bending. As such, the column member introduced as part of the frame modelling method will therefore have a fixed base. Amending this to a pinned support will potentially affect the overall stability of the model and underestimate the lateral restraint the wall provides.

Core shape:

When using a series of shear walls to model a shear core, the overall shape of the shear core can have a significant effect on the results. If the core wall is open, say a U shape, the torsional stiffness of the core as modelled by the shear wall is satisfactory. But for closed cores, the frame modelling method tends to an underestimation of the torsional stiffness of the core. If the centroid of the cores coincides with the centroid of the lateral load, such that no overall torsion of the structure is induced this underestimation is not a factor in the resulting behaviour of the model. However, for cores which are offset relative to the centroid of the load, the torsion on the cores can be significant.

Add Shear Wall pane

The Shear Wall pane can be accessed by going to Create>Add Shear wall (frame modelling method). Selecting this will open the Shear Wall panel on the right hand side of the screen. The layout is shown below.

The Shear Wall panel contains the input area which enables the concrete shear wall to be created.

To create a shear wall, the inputs required are:

1.The wall thickness

2.The start node - This is the node at the top of one corner of the wall to be created

3.The end node - This is also the node at the top of the wall, at the opposite corner of the wall to be added. The start and end nodes define the position and length of the wall to be added

4.Lower Level - this defines the level of the bottom of the wall to be created. The level input is in metres.

5.Upper level - this defines the level at the top of the wall.

When a frame modelling method shear wall is added to a model, it introduces a new central column which represents the wall. Any columns at the edges of the wall will be removed as a default. These elements can be retained as dummy members by selecting the Keep Existing Columns as Dummy Members option.

Selecting Start and Finish Nodes

The start and finish nodes can be selected by using the mouse, moving the pointer over the required node and selecting by clicking with the left mouse button. The viewing navigation and filter tools can be used to filter to a specified part of a model to simplify the task of selecting the nodes. Alternatively, the node numbers can be input manually by selecting the input box and typing in the value.

Lower and Upper Levels

Similar to the start and end nodes, the lower and upper levels can be input either graphically or manually by tying in the required values.

Creating the Wall.

With the required wall information added to the inputs, the shear wall can be previewed by clicking on the

Once created, the wall can be removed by clicking on Undo.