Boundary conditions for bars

Which internal forces can be transferred (or which rigidity a connection) between the ends of two bars, is determined using this button .
The following dialogue appears:

You see quick buttons (see further) and two columns to define the boundary conditions at the start and end point of bar.

• Start point = point with the smallest X-coordinate
  • If X-coordinates are the same, it’s the point with the smallest Y-coordinate
    • If Y-coordinates are the same, it’s the point with the smallest Z-coordinate
• End point = the other point

This dialog can contain padlocks depending on the properties of the elements you have selected. Read all about the padlocks in this article.

Top part (quick buttons)

The meaning of the quick buttons is as follows:

• : tie rod
  A bar in which only axial tension can be transferred. The use of tie rods always results in an iterative calculation due to their non-linear behaviour.
  The quick buttons for tie rods sometimes give iterations problems in the elastic analysis. The solution is then to model the tie rods using a function instead of using the quick buttons.
  
  A tie rod is not the same as a cable. A cable (= like a clothes line) cannot be modelled in Diamonds (more info).
  
  The counter part of a tie rod is a compression bar.
• : hinged bar
  At both bar ends My’ and Mz’ cannot be transferred. All other forces can be transferred.
• : hinge at the start of the bar
  At the start point of the bar My’ and Mz’ cannot be transferred. All other forces can be transferred.
• : hinge at the end of the bar
  At the end point of the bar My’ and Mz’ cannot be transferred. All other forces can be transferred.
• : clear all hings
  At both bar ends, all internal forces can be transferred

Whenever you select a quickbutton, the relevant options will be turned on/off in the middle of the dialog.

Middle part

Instead of using the quick buttons, you can restrict the various degrees of freedom directly. For each degree of freedom, using the pull-down menu, you can:

• set the degree of freedom fixed.
• enter a spring using a value.
• enter a function (see below).

Custom function

A rigidity diagram presents the displacement (or angular displacement) in relation to the applied force (reaction force, moment, normal force or shear force). The definition of a rigidity diagram is at the end of a bar is similar to the definition for a support.

• Select the option “Function” from the drop down menu.
• Either choose one of the existing functions or define a new one using .
  A new dialogue appears.
• Click .
  
• Name the function.
• For each sign of the internal force, choose if:
  • the internal force cannot be transferred = free
  • the internal force can be transferred = fixed
  • the internal force can be transferred linear = value
  • the internal force can be transferred variable = function
    When you choose this option, keep the following remarks in mind:
    • The function must always go through the origin.
    • Avoid (almost) perfect horizontal or vertical branches in the graph.
      They describe a relation, not a function y= f(x). One x-value can lead to multiple solutions y or multiple x-values lead to the same solution. This causes arithmetic problems.
      Use gently sloping branches instead.
• You can save a defined function in a TXT-file using the button . An existing function (TXT format) can be loaded via .

Example of a custom function on a bar end	Example of a custom function on a support
The diagram shows that the rotation rigidity M/φ is constant as long as the applied moment is less than 45,4kNm. Once the moment is exceeded, the rigidity varies according to M. If the moment exceeds 68,2kNm, a plastic hinge is formed because an additional moment produces an angular deformation infinitely large.	The foundation can’t bear tension and only 100kN compression. The rigidity diagram should lay in the 1st and 3rd quadrant because in the formula F=k∙x the spring constant k cannot take on a negative value. If the function does not have a valid range, a red message will appear below this window.

Bottom part (haunch)

The last option allows you to apply a lower (z’ < 0) or upper (z’ > 0) haunch to each bar end. A haunch is characterized by length and height.
By applying a haunch, you locally increase the profile’s resistance and rigidity. The stresses calculated by Diamonds will decrease because of the increased profile height. The steel verification also takes into consideration the variable cross-section. The haunch also influences the distribution of moments because the variable rigidity is also included in the global rigidity matrix.
The haunch will also be be taken into account when the connection is detailed in PowerConnect. And conversely, these dimensions will be fulfilled when a haunch is added to the connection in PowerConnect.