Prestress

Prestress is a general description covering prestress loads, initial strains and initial lengths. In all cases the result is a prestress condition (set of forces and/or moments) in the element.

Definition

Name

The name is only used as a convenient way of identifying a load. Optional.

Beam List

This specifies a list of beam, bar, tie, strut or cable elements to load using any one of the forms detailed in “Lists”. The list may be a single item.

In sculpt: The beam list is set to the beam and bar elements in the current selection set. The beam list cannot be edited in the dialog.

Load Case

The load case in which the load applies. The load case gives a way of grouping load effects together.

Type

The user has a choice of the type of loading applied to the elements.

• Prestress Force – This is the force in the element with undeflected nodes, before the structure is allowed to relax into its deflected shape. The resultant force will not necessarily be the same, since the attached structure’s stiffness and applied loading also affect the position of the nodes. This option is equivalent to the Initial Strain and Lack of Fit options: The three prestress types are related by the relevant element’s stiffness and length alone. A positive prestress force is equivalent to a negative Initial Strain or lack of Fit, and will result in a tension in the element for normal structures.
• Initial Strain – This allows the natural length of an element to be longer (positive) or shorter (negative) than the distance between its undeflected node positions. The difference in length is the initial strain multiplied by the node-to-node distance. The eventual length of the element depends on the stiffness of the attached structure, and the other applied loading.
• Lack-of-Fit – Similar to Initial Strain, with the two being related by the element’s undeflected node-to-node length.
• Tendon Prestress – Concrete elements are commonly prestressed using tendons or wires. This option allows the simulation of a tendon by the application of equal and opposite forces to the ends of the element. Since the tendon’s stiffness is much lower than the beam’s, the tendon’s elastic properties do not need to be modelled, and the tendon force is assumed to remain constant over the elastic range of beam deflections. The tendon can be offset from the element’s centroid to induce a moment as well as an axial force. A positive tendon prestress force causes compression in the element. The element forces output from the model are those for the “concrete” section alone: e.g. an unloaded cantilever element that had a prestress applied would show a negative axial force despite having no load applied to the tip.

Prestress Force & Tendon Prestress

The magnitude of the prestressing load. This applies only to the prestress force and tendon prestress load types. A positive force implies a tensile prestress force or a tensile tendon prestress.

Prestress Offsets

The offsets in local (y, z) directions of the tendon prestress. This allows for the introduction of prestressing moments in addition to prestressing force. This applies only to the tendon prestress load type and can only be applied to beam elements. Note that the prestress offset cannot be used in the static non-linear solver.

Initial Strain

The initial strain in the element used to define a lack-of-fit. The value specified should be the factor of the element length by which the element is too long or too short. A positive value means the element is too long compared with the nodal distance and will induce compression in the element, a negative value will introduce tension in the element.

Lack of Fit

The length by which the element is too long or too short. A positive value means the element is too long compared with the nodal distance and will induce compression in the element, a negative value will introduce tension in the element.