Member Loads

Member loads are effects applied along structural members. They include distributed loads, concentrated forces and moments at a station, and thermal loads.

Types of Member Loads

You can apply three main types of loads to members:

1. Distributed Loads

Loads laid out continuously along part or all of the member's length. Common examples:

• Uniform Distributed Load (UDL) - Constant magnitude, like a floor slab resting on a beam.
• Trapezoidal Load - Magnitude varies linearly from start to end (e.g., wind pressure varying with height).
• Partial Loads - Loads that only apply to a specific segment of the member.

2. Concentrated Loads

A row can contain a point force, a point moment, or both at the same member station. Moments follow the right-hand rule about the selected axes.

3. Member Thermal Loads

Temperature effects applied to the entire member. Three independent components:

• Uniform temperature change (ΔT) - The whole cross-section heats or cools by a constant amount. In a fully restrained member this produces axial force EAαΔT; in an unrestrained member it just elongates.
• Gradient along local-y (∂T/∂y) - A linear temperature variation along the section's local-y axis (vertical for an upright horizontal beam). In the default orientation, a positive through-depth gradient heats the top fibre more than the bottom and bends the member about local z. Common in solar exposure on roofs, bridge decks, and differential thermal between floors.
• Gradient along local-z (∂T/∂z) - The same effect along the member's local-z axis, the other transverse direction. For a default-orientation section this is the through-width gradient.

An optional α override column lets you apply a thermal expansion coefficient that differs from the member's material, useful for stainless or aluminium members embedded in a steel frame, or for matching a textbook example with a non-standard α.

Adding Member Loads

You can add loads via the Loads Dialog or the context menu.

Changes in the Applied Loads dialog are applied to your model automatically as you edit the table. There is no OK or Apply button.

Using the Loads Dialog

1. Click the Loads button (↓ icon) in the ribbon.
2. Select the Member Distributed, Member Concentrated, or Member Thermal tab.
3. Enter data in the spreadsheet grid.

Using Context Menu

1. Select specific members in the 3D view.
2. Right-click and choose Member Distributed Loads, Member Concentrated Loads, or Member Thermal Loads.
3. The dialog will open on the matching tab with the selected member IDs pre-filled.

Distributed Load Properties

When defining distributed loads, you specify:

• Case: The load case ID (e.g., "1" or "DL").
• Member: The ID(s) of the members to load. Supports ranges (e.g., "1-5, 8").
• Axis: The coordinate system for the load direction (Global or Local).
• Units: The unit used for position (% or Length unit like m).
• Start/Finish Position: Where the load begins and ends.
  • For %: Use 0 to 100 (0 = start node, Node A; 100 = end node, Node B).
  • For Length: Use distance from the start node, Node A (e.g., 2.5m).
• Start/Finish Force (X, Y, Z): The magnitude of force per unit length at the start and end positions.

Examples:

• Full Length UDL: Start Pos = 0, Finish Pos = 100 (%), Start Force = -5, Finish Force = -5.
• Triangular Load: Start Force = -10, Finish Force = 0.

Concentrated Load Properties

For point loads on members:

• Position: The location of the point load along the member.
  • For %: 50 is the midpoint.
  • For Length: Distance from the start node, Node A.
• Force (Fx, Fy, Fz): The concentrated force components along the selected Local or Global axes (kN, kip, etc.). Positive values act toward the corresponding positive selected axis.
• Moment (Mx, My, Mz): The concentrated couple in the selected moment unit. For Local axes, Mx is Saint-Venant torsion; My and Mz are bending couples.

Worked concentrated-moment example

For a 6 m beam, choose Local, position 50%, set forces to zero, and enter Mz = 20 kN·m. The solver applies a positive 20 kN·m bending couple at midspan using the right-hand rule about local z. The internal Mz diagram has the corresponding jump at 3 m.

Static moments contribute to reactions, deformations and member forces, but contribute no translational modal or seismic mass. A combined row contributes mass only through its eligible force component.

Member Thermal Load Properties

Each row applies one combined thermal effect to one or more members:

• Case: The load case ID.
• Member: The ID(s) of the members to load. Supports ranges (e.g., "1-5, 8").
• ΔT (°C / °F): Uniform temperature change applied to the entire cross-section. Positive = heating, negative = cooling. In a free member this produces axial elongation α·ΔT·L; in a restrained member it produces axial force.
• ∂T/∂y (°C/m or °F/ft): Linear temperature gradient through the section's geometric depth, along section local y (vertical for an upright horizontal beam). On a sideways member it follows the 90° section roll (stays attached to the section as drawn); on an angle / Z-purlin section (non-zero Alpha) the solver automatically decomposes this geometric-frame gradient onto the principal axes (see the notes below). Positive value means the local +y face is hotter than the local −y face. Drives curvature about the local-z axis.
• ∂T/∂z (°C/m or °F/ft): Linear temperature gradient along the member's local-z axis, the other transverse direction, the through-width gradient in the geometric frame, likewise rolled by sideways and decomposed onto the principal axes for Alpha sections. Positive value means the local +z face is hotter. Drives curvature about the local-y axis.
• α override (1/°C or 1/°F): Optional. Replaces the material's thermal expansion coefficient for this row only. Type a number directly: for AISC steel use 1.17e-5 in metric mode (1/°C) or 6.5e-6 in imperial mode (1/°F). Leave blank to use the material's α.

Worked Examples

Three worked examples to make the first row easy. They assume metric units (°C, m) and a load case named DL already created.

Example 1: Whole-member heating of +30°C on members 5 and 6

A summer-load scenario: a steel roof beam fully restrained against axial elongation.

1. Right-click the selected members → Member Thermal Loads (or open the Loads Dialog → Member Thermal tab).
2. In row 1 type: Case = DL, Member = 5, 6, ΔT = 30.
3. Leave ∂T/∂y, ∂T/∂z, and α override blank.
4. The load is applied automatically once the row has a Case, a Member, and a non-zero temperature change or gradient.

If both ends of the member are fixed against translation, the analysis develops axial compression N = −E·A·α·30 under the displayed tension-positive result convention. If one end is a roller in the axial direction, the member elongates freely and reports zero axial force.

Example 2: Top-hot gradient of +40°C across a 400 mm-deep beam

A typical sun-on-roof scenario: the top flange is 40°C hotter than the bottom flange. Type the gradient as °C per metre of section depth (a per-length rate), not the raw 40°C, i.e. divide the through-depth temperature difference by the depth.

1. Open the Member Thermal tab.
2. In row 1 type: Case = DL, Member = 5, ΔT = 0, ∂T/∂y = 100 (= 40 °C ÷ 0.4 m).
3. Leave ∂T/∂z and α override blank.
4. The load is applied automatically once the row has a Case, a Member, and a non-zero temperature change or gradient.

With positive α, an upright fixed-fixed beam develops a restraint moment magnitude |M| = E·Iz·α·100. A simply supported beam cambers upward; a cantilever tip bends downward. These examples assume this gradient acts alone.

Example 3: Combined uniform + gradient

Same member with both effects active. Type both numbers in the same row; their responses superpose in linear analysis:

• Case = DL, Member = 5, ΔT = 30, ∂T/∂y = 100.

If you want the two effects on different load cases (e.g. summer uniform on SUMMER, top-hot gradient on SUN) use two rows with different Case values.

Visual on the Canvas

Thermal loads display as a single coloured rail running alongside the member, plus a label showing the values you typed:

• Red rail: heating (positive ΔT).
• Blue rail: cooling (negative ΔT).
• Orange rail: pure gradient (no uniform ΔT).
• Position of the rail: for gradient loads, toward the hotter section face. With both gradients, it points toward their combined hotter direction. The rail follows the sideways section roll.

Thermal rails are gated by both the master Show Loads toggle and the Show Distributed Loads sub-toggle (since thermal is a distributed-style effect along the member).

Notes and Conventions

• Angle / Z-purlin sections: enter gradients in the section's geometric depth and width directions. The solver handles the principal-axis rotation automatically.
• Sideways members: thermal gradients follow the section's 90° roll, just like Local member loads.
• Sign convention: positive ∂T/∂y heats the section +y face more; positive ∂T/∂z heats the section +z face more. The resulting movement and forces depend on the supports.
• Modal / Buckling / P-Delta: a uniform ΔT in a restrained member produces axial compression that correctly feeds into the geometric stiffness for buckling and P-Delta analyses. The current modal analysis is unprestressed: it omits thermally induced force and geometric stiffness, so thermal loads do not change the reported modal frequencies. This is a solver-scope limitation; thermal prestress can affect the natural frequencies of a real structure.

Coordinate Systems

Global Axis

Directions align with the global X, Y, Z axes of the model. Directions do not change as the member rotates.

• Global Y (-): Gravity loads (downward).
• Global X/Z: Lateral loads like wind (if defined globally).

Local Axis

Directions align with the member's own orientation:

• Local x: Along the member axis (i → j).
• Local y: Perpendicular to the member; for a horizontal beam this is the vertical direction. Loads in this direction produce Vy shear and Mz (major-axis) bending.
• Local z: Perpendicular to the member and to local y; for a horizontal beam this is the lateral direction. Loads in this direction produce Vz shear and My (minor-axis) bending.

Useful for loads applied normal to inclined members, such as wind on a sloped roof rafter.

Sign Convention

Global Y is up (so gravity is −Y); positive Local y/z loads act in the positive local axis directions. Moments follow the right-hand rule about the selected axis. Full global/local axis definitions, sideways major/minor mapping, and result sign conventions are in Sign Conventions.

Generating Loads from Pressures

To turn a floor, roof, or wall pressure into member loads automatically, use Generate Loads in the Loads dialog. See Load Generators.

Tips

• You can copy/paste data to and from Excel directly into the loads grid.