Member orientation (the β angle)

Getting a member's orientation right is the difference between a beam that carries its load and one that is 15 times too flexible. This page explains the convention - it is the single most important modelling gotcha in CivilKit Studio.

Why orientation matters

An I-section (UB / UC / W / IPE ...) is far stiffer about its major axis than its minor axis. A 460UB67.1 spanning 8 m under 15 kN/m sags about 13 mm when its web is vertical (loaded on the strong axis) but about 275 mm if it is lying on its side (loaded on the weak axis) - the same section, ~20x the deflection. The model has to know which way up the section sits.

The β (beta) convention

Each member carries a β angle - a roll about the member's own length axis.

β is in RADIANS in the model JSON

In the model JSON beta is radians. The Studio's inspector shows it in degrees for convenience and converts under the hood. So "β = 90°" in the inspector is beta: 1.5708 in the file.

The key case for a horizontal beam:

β	Web	Gravity (vertical) load resisted by
0°	horizontal (section lying flat)	the minor axis - very flexible
90°	vertical (section standing up)	the major axis - the strong axis

So β = 90° stands a beam web-vertical, so its MAJOR axis resists gravity - which is how a floor beam is actually installed. For a horizontal beam under vertical load, β = 90° is almost always what you want.

Field tooltip

The member inspector's Beta ° field carries this hint inline: "β = 90° stands a beam web-vertical - major axis resists gravity." If a member looks far too flexible after solving, check its β first.

How to set it

1. Select the member.
2. In the inspector, set Beta ° (e.g. 90 for a web-vertical floor beam).
3. Re-solve.

After solving you can confirm the orientation worked: the member's section-properties readout shows Ix (major) and Iy (minor), and the deflection should match the major-axis value, not the minor.

Worked example in the gallery

The gallery ships a Floor beam (strong-axis) sample: a simply supported 8 m 460UB67.1 oriented web-vertical (β = 90°) under a 15 kN/m UDL. Its midspan sag is 13.51 mm - the exact major-axis value 5wL⁴/384EI using Izz - proving the section is bending the strong way. Open it to see a correctly oriented beam.

Vertical members (columns)

For a vertical column the β angle rolls the section in plan, choosing which in-plan direction the major axis faces. Set it so the column's major axis aligns with the frame's strong-bending direction (typically in the plane of the portal).

Local axes recap

A member's local axes follow the right-hand rule:

• Local x runs from end A to end B (along the member).
• Local z is the major bending axis (izz, the strong axis).
• Local y is the minor bending axis (iyy, the weak axis).

beta rolls the y-z pair about local x. See the model schema and sections for how iyy / izz map to the section.