EngineeringMechanical EngineeringPart 11 of 15
Hot Rolled Steel Section Selection
Machine frames are made from the same sections as buildings, but sized by a different logic. Deflection and mounting flatness often govern long before stress does.
- Grades and strengths
- Section families
- Section modulus method
- Self-weight iteration
Executive summary
Hot rolled sections divide into merchant bar — rounds, squares and flats — and structural shapes: universal beams, universal columns, parallel flange channels, and equal and unequal angles. Together they cover almost every machine base, support frame, walkway and guard structure a mechanical designer will detail.
Selection for bending is governed by the section modulus. The method is short, but it contains one step that is easy to omit and always changes the answer at the margin: the beam's own self-weight is part of the load it carries.
Grades and strengths
| Grade | Minimum yield strength | Minimum tensile strength | Comment |
|---|---|---|---|
| 250 | 250 MPa | 410 MPa | The traditional general-purpose grade. |
| 300PLUS | 300 MPa | 440 MPa | Widely supplied as the standard structural grade in Australia. |
| 350 | 350 MPa | 480 MPa | Where higher strength justifies the cost or the weight saving matters. |
Hot rolled sections carry a commercial surface finish and dimensional tolerance. Hot rolled rounds must not be used as rotating shafts in machinery — use bright steel. Similarly, mounting faces on a hot rolled frame need machining or shimming; the rolled surface is not a datum.
Section families and where each belongs
Rounds, squares, flats
Brackets, spacers, pins, gussets, ties and stiffeners. The everyday material of the fabrication shop.
Deep, efficient in bending
Deep webs with relatively narrow flanges. The default for spanning members carrying transverse load.
Balanced in both axes
Roughly square proportions, giving comparable radius of gyration about both axes. Suited to compression members and to beams needing lateral stiffness.
Parallel flange
Frame rails, edge members and skid bases where one flat face aids fixing and access to the inside of the web is needed.
Equal and unequal
Bracing, cleats, framing, ladder and platform members, and support for cladding and grating.
A universal beam is more efficient in bending for the same mass; a universal column resists buckling and lateral-torsional effects better. For a machine base being lifted, transported and shimmed on an uneven floor, the column section's torsional and lateral behaviour is frequently the better engineering even where the beam is lighter.
Beam selection by section modulus
- M
- maximum bending moment, Nmm
- Z
- elastic section modulus about the bending axis, mm3
- fb
- bending stress, MPa
- w
- uniformly distributed load, N/mm
- L
- span, mm
- Establish loads and spanIgnoring self-weight for the first pass.
- Calculate the maximum bending momentFrom the support and loading arrangement.
- Set the allowable bending stressYield strength of the chosen grade divided by the design factor.
- Calculate the required section modulusBending moment divided by allowable stress.
- Select the lightest section that exceeds itFrom the section tables for the chosen family.
- Add self-weight and recalculateThe section's mass per metre becomes part of the distributed load.
- Confirm the section still passesIf it does not, step up and repeat — the heavier section adds a little more self-weight in turn.
- Check deflection and stabilityDeflection limit, lateral restraint, web crippling at supports and local bearing at point loads.
Worked example
A horizontal beam simply supported at each end spans 5 m and carries a uniformly distributed load of 5 kN/m. A grade 250 universal beam is required with a design factor of 2 on yield.
The self-weight iteration
Two candidate sections sit close to the requirement. One has a section modulus of about 123 × 103 mm3 — marginally below the requirement before self-weight is even considered. The next section up offers about 139 × 103 mm3 at a mass of roughly 18 kg/m.
Adding that self-weight, the distributed load becomes 5.178 kN/m, the reactions rise to 12.945 kN and the maximum moment increases to about 16.18 kNm. The required section modulus rises to 129 × 103 mm3. The heavier section, at 139 × 103, still passes; the lighter one never did.
Self-weight added four per cent to the bending moment in this example — enough to eliminate the marginal candidate. On longer spans and lighter imposed loads the proportion is far higher, and on a long walkway or conveyor gantry the beam's own mass can dominate the design entirely.
Beyond bending stress
What else governs
- Deflection limits, which frequently govern machine frames before stress does.
- Lateral-torsional buckling of unrestrained compression flanges.
- Web crippling and bearing at supports and under concentrated loads.
- Compression member slenderness for columns and struts.
- Vibration and natural frequency where rotating machinery is mounted.
What to specify
- Section designation and grade, together.
- Orientation and bending axis on the drawing.
- Machined or shimmed mounting faces where flatness matters.
- Weld preparation and connection detail at every joint.
- Surface treatment appropriate to the environment.
The method above is a mechanical designer's tool for machine frames, supports and secondary steelwork. Building structures, crane runways, lifting beams and anything supporting personnel are governed by structural codes and must be designed and certified accordingly.
Selection checklist
- Steel grade selected and stated with the section designation.
- Section family chosen for the actual loading, not by habit.
- Maximum bending moment derived from the correct support and loading case.
- Design factor applied to yield, and stated.
- Self-weight added and the selection re-verified.
- Deflection checked against a stated limit.
- Lateral restraint of the compression flange confirmed.
- Web crippling and bearing checked at supports and point loads.
- Hot rolled rounds excluded from rotating shaft applications.
- Mounting faces specified as machined or shimmed where alignment matters.
- Structural code compliance confirmed where the structure is not purely mechanical.