Procedure 4 Large Structures Construction Stage Analysis)
6.3.8 Actions on the Element Ends
6.3.8.1 Element End Displacement
The Load Type “Element end displacement” does not prescribe a global displacement value to the point, where it is assigned, but it induces a displacement difference be- tween the specified element end and the nodal point to which it is connected, i.e. a gap or overlapping distance between the element end and the appropriate node is prescribed.
The global deformation behaviour is calculated as a reaction of the structural system to this prescription.
This load type is for instance typically used for simulating support settlements. In this case, an element end displacement in the vertical direction will be applied on the sup- port element (typically a spring element with one node fixed). The end of the support element will be moved by the specified amount in relation to the node. When the appro- priate node is fixed, the specified movement will act like an absolute displacement of the element end point.
If this Load Type is applied to an element of the superstructure, the resulting deforma- tions will represent an influence line for the appropriate internal force at the regarded point.
The Element end displacements may be specified at the start or at the end of an element, and they may be defined either in the global or in the local coordinate system.
Mind the sign conventions:
The element end displacements are defined as vectors from the element end to the appropriate node in the regarded coordinate system, i.e. the node is moved away from the element end by the specified amount. This convention applies also to rotations, where the node is rotated right hand turning in relation to the original position at the element end.
Note: The global deformations and the internal forces which result from these prescribed defor- mations are dependent upon the various constraint conditions (typically from the supports).
Whenever the DOF’s of the node, to whom the element end displacement is applied, is re- strained, the element end will move in the opposite direction than specified for the node.
Example: This example typically describes a 5mm downward support settlement (dis- placement) of a bridge pier. This settlement is simulated by an element end displacement in the global Y direction applied at the element begin of the sup- port element (e.g. spring element 501). By applying a global element end dis- placement (VGA) Vy or a local element end displacement (VLA) Vx of +0.005m the program will try to move the start node upwards by that
amount. But as the start node is restrained the element begin will be moved downwards by 5mm instead.
VGA End-displacement (global – at the start node) – Prescribed displacements and rotations defined in the global coordinate system applied between the element start and the start node.
VLA End-displacement (local – at the start node) – Prescribed displacements and rotations defined in the local coordinate system applied between the element start and the start node.
LENGTH after Def.
XL
ZL
YL
XG
ZG
YG
VGA Vy = +0.005 or VLA Vx = +0.005
END
BEGIN
SYSTEM DEF.
LENGTH befor Def.
XL
ZL
YL
XG
ZG
YG
VGA VZ = +0.005 END
BEGIN (NODE 0)
SYSTEM DEF.
SPING ELEMENT (SUPPORT)
VGE End-displacement (global – at the end node) – Prescribed displacements and rotations defined in the global coordinate system applied between the ele- ment end and the end node.
VLE End-displacement (local – at the end node) – Prescribed displacements and rotations defined in the local coordinate system applied between the element end and the end node.
6.3.8.2 Element End Displacement without statical effect
DSPLA The input for applying a displacement to the begin of the element is pre- pared with this load type.
DSPLE The input for applying a displacement to the end of the element is prepared with this load type.
These load sets cause no internal forces in the structure (no statical effects).
The load sets are used for incremental launching method and later for nonlinear calcula- tion.
6.3.8.3 Load Type DEMO – Support Removal
DEMO Support removal – simulation of removing a previously active support ele- ment. This will typically but not necessarily be a spring element.
The basic requirements for a correct analysis of this loading are:
' The sum of all the internal forces in the structure resultant from all applicable load cases accumulated from all the previous construction stages must be stored in a special load case. (e.g.: LC 1000)
' The accumulated internal forces of the specified element(s) from this specified load case are now automatically applied to the redefined structure and redistributed on the structure in accordance with the normal rules of statics. This case must be considered as a normal load case and must be combined with all other previous load cases to get the total result.
6.3.8.4 Cable end displacement correction
DISCOR The input for applying a displacement to the ends of a cable is prepared with this load type.
Different from the load types VGA, VLA, ... “Element End Displacement“ with this option a displacement of cable nodes (e.g. for consideration of the fabrication shape) can be defined directly.