START-PROF can model virtually any real-world restraint using custom restraints with various directions, combinations, and properties. This includes supports with friction, hangers with lateral forces from deflection, vessel and equipment nozzles, and other complex configurations.
Custom restraints combine multiple external restraint types placed at a node with specific orientations to control displacement and rotation. START-PROF uses Linear and Rotational restraints with these properties:
Rigid double-acting
Rigid single-directional
Spring double-acting
Spring single-directional
Bilinear double-acting
Bilinear single-directional
Friction
Rod
Restraints are either linear or rotational. Linear Restraints - restrain pipe cross-section displacement along an axis. Rotational Restraints - restrain pipe cross-section rotation about an axis. Examples are shown in the table below:
Linear restraints restrict node displacement, but not rotation.
Example 1: Fig. 1 shows a piping system with one linear rigid restraint, which prevents vertical displacement but does not restrict rotation.
Fig. 1
Example 2: Fig. 2 shows a piping system with two linear elastic restraints, which flexibly restrict vertical displacement but do not restrict rotation.
Fig. 2
Rotational rigid restraints restrict node rotation about an axis, but do not restrict displacement.
Example 1: Fig. 3 shows a piping system with one rotational rigid restraint, which restricts rotation of the pipe cross-section. When a bending moment is applied from the left, the pipe bends only between the moment application point and the restraint. No bending occurs to the right of the restraint. Displacement in any direction is unrestricted. This type of restraint is called a "floating anchor" - a seal that prevents rotation but allows linear displacement along coordinate axes.
Fig. 3
Example 2: Rotational restraints are rare in practice. They are typically combined with linear restraints. For example, Fig. 4 shows a piping system with one linear rigid restraint, which restricts vertical displacement, and one rotational rigid restraint, which restricts rotation of the pipe cross-section. When a force is applied left of the rotational restraint, the pipe bends only between the force application point and the restraint. No bending occurs to the right of the restraint.
Fig. 4
Example 3: Rotational restraints, like linear restraints, can be elastic - flexibly restricting cross-section rotation. Fig. 5 shows a structure with one linear rigid restraint, which restricts vertical displacement, and one rotational elastic restraint, which flexibly restricts rotation. This configuration is intermediate between those shown in Fig. 1 and Fig. 4.
Fig. 5
Different linear and rotational restraint types are described below.
Linear rigid double-acting restraints restrict node displacement in both directions along the axis (Fig. 6).
Rotational rigid double-acting restraints restrict node rotation in both directions about the axis (Fig. 3).
Fig. 6
Linear rigid single-directional restraints restrict node displacement in one direction only. They disengage during displacement in the opposite direction. For example, sliding supports restrict downward displacement (Fig. 7a), but not upward - the pipe lifts off the support and the restraint disengages (Fig. 7b).
Fig. 7
Linear (Fig. 8) and rotational (Fig. 5) spring restraints are springs that always function as double-acting restraints (never disengage). Characterized by flexibility C.
Fig. 8
A spring that resists displacement in one direction only. When the pipe moves in the opposite direction, it disengages. Functions like a rigid single-directional restraint but with spring flexibility.
Bilinear restraints require two stiffness values K1, K2 (or flexibilities) and a load F at the transition point between the two linear segments.
A bilinear restraint operates only during displacement in one direction. It disengages when the pipe moves in the opposite direction.
Friction restraints are automatically applied to account for support friction.
When the reaction in a friction restraint is less than the friction force limit Ffr, the friction restraint becomes rigid - preventing pipe sliding.
When the reaction exceeds the friction force limit Ffr, the restraint disengages. A friction force equal to Ffr=R∙μ (Fig. 9a) is applied opposite to the node displacement direction. Where R - vertical reaction creating friction, μ - friction factor.
When the friction force direction is known (for example for guide supports), one linear friction restraint in the friction direction is applied. When the direction is unknown, two perpendicular linear friction restraints in the sliding plane FX and FY (Fig. 9b) are applied, for example along and across the pipe axis. In this case, the total friction force Ftr is calculated as the geometric sum of all friction restraint reactions (Fig. 9b).
Fig. 9
Rod restraints are automatically applied to account for the pendulum effect (additional horizontal reactions Fd, occurring when hangers deflect from vertical by angle α). These reactions equal hanger force projections on the horizontal plane, Fd=R∙Sin(α) and tend to restore vertical alignment. For small angles α, Sin(α)=Δ/L, so Fd=R∙Δ/L. Rod length L is user-defined, while displacement Δ is calculated.
Two perpendicular linear rod restraints FX and FY in the horizontal plane are always applied (Fig. 10b). The total horizontal hanger reaction Ffict is calculated as the geometric sum of all rod restraint reactions.
Fig. 10
Each restraint direction is defined as a vector restricting movement (the direction in which displacement is restricted). Vector direction is set along either local or global coordinate axes, or as three angles between reaction vectors and global or local coordinate axes.
Precise restraint reaction direction (forward or reverse) must be set only for single-directional restraints or restraints with different left/right gaps, since these restraints behave differently depending on displacement direction. Otherwise, direction is irrelevant (Fig. 11a,b).
To determine restraint reaction direction, a coordinate system is placed at the node. Angles are set as follows:
The angle between the restraint reaction vector and the coordinate axis is always acute.
The angle is positive between the restraint reaction vector and the positive coordinate axis direction (Fig. 11a), and negative between the restraint reaction vector and the negative coordinate axis direction (Fig. 11b). For example, -30° from the Z axis means a 30° angle between the restraint reaction direction and the negative Z axis direction (see examples 3, 4 and 5).
When the reaction direction and coordinate axis are aligned (0° angle), the angle is set to 0° for positive axis direction, and 180° for negative axis direction (see examples 1 and 2).
The sum of squared cosines of all angles must equal 1 (if not, there is an angle error). This condition is automatically checked.
Fig. 11
Examples:
Example no. |
Angles |
Diagram |
Description |
||
|---|---|---|---|---|---|
X |
Y |
Z |
|||
1 |
90 |
90 |
0 |
|
Restraint reaction directed vertically up (along positive Z axis) |
2 |
90 |
90 |
180 |
|
Restraint reaction directed vertically down (along negative Z axis) |
3 |
90 |
60 |
30 |
|
Restraint reaction directed up at 30° to Z axis |
4 |
90 |
60 |
-30 |
|
Restraint reaction directed down at 30° to Z axis |
5 |
-60 |
90 |
30 |
|
Restraint reaction directed up at 30° to Z axis |
Example:
In a sliding support, a single-directional restraint works only for downward displacement (against Z axis) and disengages for upward displacement (along Z axis). This restraint reaction is directed upward (along Z axis). Therefore, to insert this restraint, select the "-Z" option (since the support restricts downward displacement) or enter angles (90,90,0) corresponding to an upward-directed restraint reaction vector.
START-PROF includes standard supports to simplify restraint placement. These supports contain predefined restraint combinations and automatically orient restraint directions based on local element axes. Standard support restraints are described below.
The Xm axis follows the element axis, Zm is perpendicular to the pipe axis upward, Ym is perpendicular to both the pipe axis and Zm axis.
The following symbols represent restraint types and properties:
- rigid linear
double-acting restraint
- rigid linear
single-directional restraint. Arrow
indicates reaction direction.
- rigid rotational
double-acting restraint
- elastic rotational
restraint
- elastic linear
restraint
Friction and rod restraints indicated with arrows.
A fixed anchor restricts linear displacement and rotation in all directions. Two fixed anchor types are shown:
Welded structure with two fixed anchors spaced 2÷4 diameters apart; restrains rotation. Typically implemented as one long support.
Bulkhead design anchor. Pipe passes through a round opening in a concrete wall, with flanges welded on both sides. Common in district heating networks.
Modeled with three linear and three rotational rigid double-acting restraints.
A sliding support rigidly attached to the pipe with a base plate transferring load to supporting structures (modeled as rigid base). Friction occurs at the plate-base interface. The support restricts downward displacement and disengages during upward displacement. Horizontal displacement creates friction force opposite to displacement direction.
Modeled with one vertical single-directional linear rigid restraint and two friction restraints in the horizontal plane.
Uses springs or spring chains to allow thermal expansion and control piping stress through spring pre-compression. Applies one elastic vertical restraint with flexibility C and two rod restraints in the horizontal plane to account for pendulum effect.
Uses springs or spring chains to allow thermal expansion and control piping stress through spring pre-compression. Applies one elastic vertical restraint with flexibility C and two friction restraints in the horizontal plane.
Guide support allows axial displacement. Differs from sliding support by having stops that restrict horizontal displacement perpendicular to pipe axis. Restricts downward displacement and disengages during upward displacement. Axial displacement creates friction force.
Modeled with two linear rigid restraints (vertical single-directional and horizontal double-acting) and one friction restraint along pipe axis.
Similar to single-directional guide support but does not disengage for upward displacement. Can model shell attachments.
Modeled with two linear rigid restraints (vertical double-acting and horizontal double-acting) and one friction restraint along pipe axis.
Vertical rigid rod hanger restricts downward displacement and disengages during upward displacement. When deflected from vertical, horizontal force components develop that tend to restore alignment.
Modeled with one vertical linear single-directional restraint and two rod restraints in the horizontal plane.
Support rigidly attached to pipe with base plate transferring load to supporting structures. Base plate welded to rigid base.
Restricts vertical and horizontal displacement. Rotation resistance is typically negligible since support length is comparable to pipe diameter (≈1 diameter). Rotational restraints are usually omitted.
Applies three double-acting linear rigid restraints.
Constant spring supports are popular alternatives to variable spring supports. The ideal configuration is a rigid rod hanger over a rotating unit with load P. Supporting force P remains constant during vertical displacement. This configuration is impractical due to size. Modern designs use spring-lever mechanisms with spring pre-load adjustments to maintain constant force over specified vertical travel ranges.
This support does not apply restraints. Supporting forces Fx, Fy, Fz are applied directly at the node.
Not all real restraint structures can be modeled using standard START-PROF supports. Often it's necessary to create a custom restraint. Custom restraint examples are shown below:
Supports with long sleeves in concrete walls (more than 2÷4 diameters) restrict rotation. Axial displacement creates metal-on-metal friction.
Model with a custom restraint using two (vertical and horizontal) double-acting linear rigid restraints, plus two rotational rigid restraints restricting rotation perpendicular to pipe axis.
Guide supports can be placed on vertical risers. Pipe passes through a round opening (sleeve) in the ceiling, restricting horizontal displacement. Support brackets welded to the pipe prevent downward displacement.
