Section Diagrams

Section diagrams provide a useful facility to investigate the results from an FE analysis along a line, on a rectangular perimeter or on a circular perimeter. Section diagrams can be particularly useful for investigating the bending moment distribution in slabs when looking for the bending moments in 'column' and 'middle' strips, or for investigating the axial force in walls.

To add a section diagram, select the required section type from the icons. Once the icon clicked, the section name will be automatically filled in, denoting the type of section diagram and the section number.

For a line section diagram, the position of the line is defined by either nodes or by coordinates. Nodes can be selected graphically, using the mouse pointer. When a node is selected, the coordinate information will be automatically filled in. The "Pick basic nodes" option restricts the selected nodes to be those present in the MasterFrame 'wireframe' model. To select nodes which are part of the FE mesh, the "Pick basic nodes" checkbox needs to unchecked. If the "Pick Basic nodes" option is selected, if the node numbering of the model is changed, then the section diagram will move with the nodes; otherwise, the coordinates of the line diagram will be the determining setting-out factor and re-meshing the model will not cause the line diagram to change position.

The rectangular and circular section diagrams work in a similar manner, except they are defined on a central point, which is defined by either a node point or by coordinates. In the case of a rectangular section, the section size is defined by the length and breadth of the section perimeter while a circular section is defined by the radius of the perimeter.

The scale of the section diagram is controlled by using the scale drop down.

The interval drop down controls the number of segments the scale diagram is divided into. Increasing the number of segments can help when viewing the diagram. it also affects the number of numerical results which are displayed.

To display numerical values on section diagrams, select the icon. Where results are shown can be controlled using the

icons. The functions of these icons are as follows:
- displays the distance along the line diagram that the result value occurs
- indicates the distance along the line diagram that zero values occur

- this drop down controls the interval at which results are displayed. The Auto option displays the results at point where the results at maximum hogging and sagging values at also at points where step changes occur. Alternatively, the drop down can be used to show results are interval points. The options are 1-5, where 1 shows results at every selected interval point, while 5 shows results at every 5th interval point.

As well as controlling the scale, the direction the results diagram is displayed in can be controlled using the drop down. This does not affect the results axes, only the direction the diagram is displayed in.

By default, the software will display one line diagram at a time, with the displayed section being selected from the Section drop down. All section diagrams can be displayed at once by clicking on the icon.

Further grouping of section diagrams can be achieved by using the Section Diagram Sets feature. A Section Diagram Set is effectively a grouping of section diagrams that are to be displayed at once. To add a Diagram Set, click on the icon. The diagram set name will increment by one, but this can be deleted and a user-defined name added. Adding new section diagrams will now be in this new Section Diagram set.

Line Diagram can be used in conjunction with the Average Strip feature. This allows for a width over which the results of the line diagram are to be averaged. The software carries out an integration over the strip width to calculate the average value. For edge strips, the software detects the region over which the results lie outside the FE surface and these are excluded ; that is, the averaging process only takes place over the width within the FE surface.

This is an average value and not a total. If you require the total force, you must multiply the value by the active part of the strip width.

📓

Note: An average strip width should only be used over an appropriate strip width. Using large width strips may produce overly low average results values since the sign of the results are included in the averaging process. For Flat Slabs, reference should be made the rules for Flat Slabs in either BS 8110 or BS EN 1992-1-1 or, alternatively, the Concrete Centre publication "How to design concrete flat slabs using Finite Element Analysis".

Mzz (kN) moment

The 'Mzz (kN) moment' force quantity is specific to the section diagram line and is not available in the contour output. It relies on the section line to define the axis and width of the section for converting direct stresses over a section into a total moment.

When a section line is drawn and and the 'Mzz section width' is entered, the direct in-plane forces parallel with the section line are integrated with their lever arms perpendicular to the section line to create the Mzz section moment.

For example if we had an FE wall surface 1000mm wide x 250mm thick, drawing a vertical section line at the centre of the wall with a 1.0m Mzz section width, will produce the in plane moment of the wall section (about the local surface z axis) derived in this case from the Fy direct in-plane forces in the wall.

Fy forces at three different section diagrams over the height of the wall section

Mzz section diagram over the 1m section width

The 'Corelation %' check box allows the engineer to asses the suitability of integrating the direct forces to be represented as a moment, which assumes a linear distribution of direct stress and for plane sections to remain plain. The section diagram line now produces a % value equal to the max value of (F/A + M/Z - FE Stress)/(Max section FE Stress) * 100 over the section width. E.g. 25% means at some point over the section width the F/A+M/Z value is 25% less than the FE direct force value. Hence a small % shows goo correlation, meaning the derived Mzz moment can accurately represent he stress state in the plane of the surface.