User Guide

RISA-3D User’s Guide
Version 5.0
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Click the first down arrow in the X Translation field and you will see all the
options available for the boundary conditions:
Reaction - The “Reaction” code specifies full restraint for the indicated direction.
No movement will be allowed in the indicated direction for this joint and a
“reaction” will be calculated and reported.
Fixed - The “Fixed” code also specifies full restraint for the joint in the indicated
direction however no reaction is calculated. The “Fixed” condition actually
removes the degree of freedom from the solution. If you aren't interested in
reactions using the “Fixed” code will result in a slightly smaller model.
Spring - This option models a spring attached to the joint in the indicated
direction and must be accompanied by a spring stiffness. The units for the spring
stiffness depend upon whether the spring is translational or rotational and are
displayed next to the edit box.
For example, if a spring of stiffness 100 Kips per Inch is desired, you would
choose Spring from the drop down list and enter “100” in the adjacent box.
Compression Spring - This models a Compression-Only spring and must be
accompanied by a spring stiffness. This type of spring will only resist
compression (negative displacement); if a tension force (positive displacement) is
applied to it, it will release and provide no resistance.
Tension Spring - This models a Tension Only spring and must be accompanied
by a spring stiffness. This type of spring will only resist tension (positive
displacements); if a compression force (negative displacement) is applied to it, it
will release and provide no resistance.
Now, we want to specify pin supports at the base of our columns, so we want X, Y
and Z translations restrained, but we want the three rotations unrestrained. This is
a standard boundary condition that RISA-3D has built in.
Click the Pinned button and the conditions are filled in for you.
The boxes to the right labeled “Use?” must be checked for a modification to be
applied. This way if you later decide to change a condition for just one of the
parameters, you do not have to set all six to match the desired condition.
The section at the bottom of the box asks “What happens when apply is
pressed?”. We have not covered the RISA-3D selection tools yet so just be
aware that this is an important feature that we will come back to.