3.1.8: Modify Element
Last updated
Last updated
“Modify Element” is a multi-component that 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 114 mm and a wall thickness of 4 mm. Use the “Modify Element” component with “Element Type” set to “Beam” to set the beam properties as needed. Fig. 3.1.8 shows how this can be done. There are two methods for using the “Modify Element” component:
Insert it in front of the “Assemble” component and let element objects flow through it (see, for example, the modification of beams in Fig. 3.1.10.1). By default, the “Modify Element” component leaves all incoming elements unchanged. Several “Modify Beam” components can act consecutively on the same beam.
Create a stand-alone element agent that can be fed into the “Elem” input of the “Assemble” component. The “ShellId” or “BeamId” input plugs let you select the elements to be modified. Use regular expressions to specify groups of elements.
These element properties can be modified:
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 are reasons for this adjustment:
Connections between beams that transfer bending and normal forces are typically more expensive than those that carry normal force only. The design of connections heavily depends on the material used: rigid bending connections in wood are harder to achieve than in steel. However, rigid connections add stiffness to a structure and reduce its deflection. Using truss elements instead of beams is a safer approach.
For slender beams, i.e., beams with a small diameter compared to their length, the effect of bending stiffness is negligible compared to axial stiffness. Consider a thin wire that is easy to bend but hard to tear by pulling.
Eliminating bending stiffness reduces computation time by more than half for each node with only trusses attached.
Trusses only take axial forces and do not prevent nodes from rotating. Karamba3D automatically removes rotational degrees of freedom for nodes connected only to trusses. When a beam with bending enabled connects to a node, the node retains its rotational degrees of freedom. Be mindful of this when the “Analysis” component turns red and reports a kinematic system. Transferring only axial forces means a truss restricts a node's movement in one direction. A node not attached to a support has three translational degrees of freedom, requiring three truss elements not in the same plane to be fixed in space.
“Height” (equivalent to the outer diameter D for circular tubes) and wall thickness of a cross-section influence a beam's axial and bending stiffness. Karamba3D expects both input values in centimeters. The cross-section area is linear in both diameter and thickness, whereas the moment of inertia grows linearly with thickness and depends on D³ for full rectangular sections and on D² for I-profiles and box sections. Therefore, increasing a beam's height (or diameter) is more effective for insufficient bending stiffness than increasing its wall thickness.
Buckling can be turned off for cross-section optimization, simulating pre-tensioned slender elements without actually pretensioning them. The necessary pretension force is roughly the negative value of the largest compressive axial normal force of all load cases.
Sets a uniform cross section height throughout the shell.
When the “Active” input is set to false, the corresponding beam is excluded from further calculations until “Active” is reset to true. See section for an alternative way of setting a beam's activation state.
Karamba3D bases deflection calculations on the initial, undeformed geometry. Some structures, like ropes, are form-active, meaning the deformed geometry and the axial forces in the rope provide equilibrium. This effect is not accounted for in Karamba3D's first order theory (Th.I.) calculations and leads to large doeformations for rope-like beams. Using a truss instead of a beam-element for first order analysis can circumvent this issue. Alternatively, reduce the rope's specific weight to zero or start from a slightly deformed rope geometry and apply external loads in small steps, where each step's initial geometry results from the previous step's deformed geometry (see section ).
The “EcceLoc” and “EcceGlo” input plugs set the eccentricity of the beam axis with respect to the connection line between its endpoints. Both expect a three-dimensional vector. “EcceLoc” refers to the local coordinate system, and “EcceGlo” refers to the global coordinate system. Beam eccentricities can also be defined via the “Eccentricity on Beam” component (see section ).
This input allows defining the orientation of a beam, working similarly to the orientate-beam component (see section ).
For cross-section optimization, it is necessary to know a beam’s buckling length. Karamba3D approximates it using the algorithm described in section . For system buckling cases, this approximation may not be safe. The “BklLenY”, “BklLenZ”, and “BklLenLT” input plugs allow specifying the buckling length of a beam for its local Y- and Z-axes, as well as for lateral torsional buckling. These values override those from Karamba3D's buckling length calculation when specified. The “lg” value sets the distance of transverse loads from the center of shear of the cross section, defaulting to zero. Positive values mean the loads point towards the shear center, acting destabilizing for lateral torsional buckling. The “lg” property influences the beam's utilization concerning lateral torsional buckling according to Eurocode 3.
Axial normal forces influence the stiffness of a beam in second order theory (Th.II) calculations. Compressive forces lower, and tensile forces increase its bending stiffness. For example, a guitar string vibrates at a higher frequency (i.e., is stiffer) under increased tension. In Karamba3D, the normal force impacting stiffness ( ) is independent of the normal force causing stresses in the cross-section (). This allows superimposing second order theory results safely by choosing as the largest compressive force of each beam.
As with 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.
