3.1.10: Modify Element
“Modify Element” is a multi-component which can be applied to shell-, beam- and truss elements. Use the drop-down list at the bottom of the component to select the type.
By default Karamba3D assumes the cross-section of beams to be steel tubes with a diameter of 114mm and a wall-thickness of 4mm. Use the “ModifyElement”-component with “Element Type” set to “Beam” to set the beam properties according to your choice. Fig. 220.127.116.11 shows how this can be done. There are two variants for using the “Modify Element”-component:
- 1.Insert it in front of the “Assemble”-component and let element objects flow through it (see e.g. the modification of beams in fig. 18.104.22.168). By default the “ModifyElement”-component leaves all incoming elements unchanged. Several “ModifyBeam”-components may act consecutively on the same beam.
- 2.Create a stand-alone element-agent that can be fed into the “Elem”-input of the “Assemble”-component. The input-plug “ShellId” or “BeamId” let you select the elements to be modified. Use regular expressions to specify groups of elements.
Fig. 3.1.10: Modification of the default element properties
These element properties can be modified:
When input “Active” is set to false the corresponding beam is excluded from further calculations until “Active” is reset to true. See section 3.1.5 for an alternative way of setting a beam's activation state.
Beams resist normal forces and bending moments. Setting the “Bending”-input of the “ModifyElement”-component to “False” disables the bending stiffness and turns the corresponding beam into a truss. There exist reasons that motivate such a step:
- Connections between beams that reliably transfer bending and normal force are commonly more expensive than those that carry normal force only. The design of connections heavily depends on the kind of material used: rigid bending connections in wood are harder to achieve than in steel. Yet rigid connections add stiffness to a structure and reduce its deflection. Therefore you are always on the safe side if you use truss elements instead of beams.
- For slender beams i.e. beams with small diameter compared to their length the effect of bending stiffness is negligible compared to axial stiffness. Just think of a thin wire that is easy to bend but hard to tear by pulling.
- Abandoning bending stiffness reduces computation time by more than half for each node with only trusses attached.
- Karamba3D bases deflection calculations on the initial, undeformed geometry. Some structures like ropes are form-active. This means that when a rope spans between two points the deformed geometry together with the axial forces in the rope provide for equilibrium. This effect is not taken into account in Karamba3D first order theory (Th.I.) calculations. In such a case only the bending stiffness of the rope (which is very small) keeps it from deflecting indefinitely. One way to circumvent this lies in using a truss instead of a beam-element when doing first order analysis. The second possibility would be to reduce the specific weight of the rope to zero (see further below). The third possibility would be to start from a slightly deformed rope geometry and apply the external loads in small steps where the initial geometry of each step results from the deformed geometry of the previous one (see section 3.5.4).
Trusses only take axial forces. Therefore they do not prevent the nodes they are connected to from rotating. In case that only trusses attach to a node, Karamba3D automatically removes its rotational degrees of freedom. Otherwise the node could freely rotate which is a problem in static calculations. As soon as one beam with bending enabled connects to a node the node has rotational degrees of freedom. Bear this in mind when the “Analysis”-component turns red and reports a kinematic system. Transferring only axial forces means that a truss reduces a node's movability in one direction. A node that is not attached to a support has three translational degrees of freedom. Thus there must be three truss elements that do not lie in one plane for a node to be fixed in space.
“Height” – which in case of circular tubes is equivalent to the outer diameter D – and wall-thickness of a cross-section influence a beams axial and bending stiffness. Karamba3D expects both input values to be given in centimeter. The cross-section area is linear in both diameter and thickness whereas the moment of inertia grows linearly with thickness and depends on
for e.g. full rectangular sections and on
for e.g. I-profiles and box sections. So in case of insufficient bending stiffness it is much more effective to increase a beams height (or diameter) than increasing its wall thickness.
The input-plugs “EcceLoc” and “EcceGlo” serve to set the eccentricity of the beam-axis with respect to the connection line between its endpoints. Both expect a three dimensional vector. “EcceLoc” refers the eccentricity to the local, “EcceGlo” to the global coordinate system. Eccentricities of beams can also be defined via the “Eccentricity on Beam”-component (see section 3.3.7).
Buckling can be turned off for cross section optimization. This lets you simulate pre-tensioned, slender elements without having to really pretension them. The necessary pretension force is roughly the negative value of the largest compressive axial normal force of all load cases.
For doing cross section optimization it is necessary to know a beam’s buckling length. Karamba3D approximates it using the algorithm described in section 3.5.8. For cases of system buckling this approximation does not lie on the safe side. The input-plugs “BklLenY”, “BklLenZ” and “BklLenLT” allow to specify the buckling length of a beam for its local Y- and Z- axis respectively as well as for lateral torsional buckling. When specified, these values override those from the buckling length calculation of Karamba3D. The value “lg” sets the distance of transverse loads from the center of shear of the cross section. It defaults to zero. Positive values mean that the loads point towards the shear center and thus act destabilizing for lateral torsional buckling. The property “lg” influences the beams utilization with respect to lateral torsional buckling according to Eurocode 3.
Axial normal forces influence the stiffness of a beam in second order theory (Th.II) calculations. If compressive they lower, in case of tension they increase its bending stiffness. Think of a guitar string which vibrates at a higher frequency (i.e. is stiffer) under increased tension. In Karamba3D the normal force which impacts stiffness -
- is independent from the normal force which actually causes stresses in the cross section
. This enables one to superimpose second order theory results on the safe side by choosing
as the largest compressive force
of each beam.
Sets a uniform cross section height throughout the shell.
As for beams,
for shells specifies the in-plane normal force which impacts stiffness in case of second order theory calculations. It is a force per unit of length assumed to be of same magnitude in all directions.