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 member's local-y axis (vertical for a horizontal beam). For a section in its default orientation this is the through-depth gradient, top fibre hotter than bottom, bending the member about its local-z axis. 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.

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, 100 = end node).
  • For Length: Use distance from the start node (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 start node.
• 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 along the member's local-y axis: the through-depth gradient (local-y is vertical for a horizontal beam, top fibre hotter than bottom). Enter it in the section's geometric depth direction. 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

Two step-by-step entries to make the first row easy. Both 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. Click OK.

If both ends of the member are fixed against translation, the analysis develops axial compression N = E·A·α·30. 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. Click OK.

A fixed-fixed beam under this gradient develops a constant restraint moment of M = E·I·α·100 along the length; a simply-supported beam camber-bows upward with no internal force.

Example 3: Combined uniform + gradient

Same member with both effects active. Type both numbers in the same row; they superpose linearly:

• 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: offset on the side of the member that's hotter, the local +y face for positive ∂T/∂y, the local +z face for positive ∂T/∂z. Sign-flipped for negative gradients. Lets you read both axis and sign at a glance even with labels off.

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

• Storage: the gradient is stored exactly as you type it: a per-length rate (°C/m). The section depth is applied at analysis time, not baked in at input, so the gradient stays the value you entered even if you later swap the section or toggle sideways. The resulting curvature is κ = α · ∂T/∂y (depth-independent), and the moment is E·I·κ, so changing the section changes the moment through its EI, while your °C/m input is preserved.
• Angle / Z-purlin sections: for sections with non-zero principal-axis rotation, the gradient is automatically decomposed onto the principal axes before the solver applies the fixed-end forces. So a gradient typed in the geometric depth direction produces moments about both principal axes, matching benchmark FEA tools.
• Sideways members: the sideways flag rolls the section 90° about the member axis, and the thermal gradient follows the roll so it stays attached to the section as drawn: ∂T/∂y through the depth, ∂T/∂z through the width, both rotating 90° with the section (the response and the canvas rail roll to match). The per-length °C/m value you typed is preserved; only its direction rotates.
• Sign convention: positive ∂T/∂y means the local +y face is hotter; positive ∂T/∂z means the local +z face is hotter.
• 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. Modal frequencies are unaffected (thermal is a static load, not a stiffness or mass change).

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.

Area Load Generator

Converts a floor/roof pressure (kPa or psf) into distributed loads on supporting beams (tributary widths and panel edge loads), instead of applying UDLs member by member.

How to Open

• Whole model: Loads Dialog → Generate Area Loads. Pick load case, pressure, distribution, and which floor levels to load, then Apply Loads.
• Selection-scoped (partial floors): Select the boundary beams, right-click Add Area Load. Only those members are loaded; levels are inferred and there is no level picker. Use this for one bay, a corridor, or an equipment footprint.

If Add Area Load is disabled, hover for the reason (single beam, columns only, diagonal framing, or no closed panel for two-way). In selection-scoped mode, distribution buttons that the selection cannot support are also greyed out with a tooltip.

Distribution Types

• One-way: Tributary widths on parallel beam lines. You must choose whether X-running or Z-running beams receive the load because deck or reinforcement orientation cannot be inferred safely from geometry. Piecewise UDLs apply only where adjacent lines overlap longitudinally. Needs at least two overlapping parallel support lines (isolated or staggered lines with no common strip are not loaded).
• Two-way: 45° yield-line panels. Short edges get triangular loads, long edges trapezoidal, square panels triangular on all sides. Needs a closed rectangular bay: two X-running and two Z-running boundary beams meeting at the four corners.

Options

• Load direction: X, Y (gravity), or Z. Positive pressure on Y is downward (−Y). Positive X or Z is along +global; enter a negative pressure to flip a lateral direction.
• Replace previous area-generator loads (default on): replaces only earlier area-generator loads on affected members in the same load case; manual and other-generator distributed loads are preserved. Off appends.
• Excluded members (whole-model only): omit beams from the tributary set (widths recalculate). Hidden in selection-scoped mode.
• Crossing members: a warning appears if beams at the same level cross in plan (regions may overlap).

Levels, Eligibility, and Failures

• Levels group nodes within 50 mm in Y; both beam ends must sit in that band.
• Only horizontal members aligned with global X or Z are loaded. Sloped beams, columns, and diagonal braces are ignored (orthogonal X/Z floor grid only).
• Failures explain the geometry (e.g. two-way missing an edge direction or closed corners; one-way isolated lines with indeterminate tributary width)—not a generic “no beams” message.
• After two-way generation, total force is checked against pressure × loaded area (within 1%). One-way on irregular plans skips this check to avoid false warnings.

Tips

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