# Steel Design

These are not the weight calculators. That group answers how heavy a section is and how much consumable a weld needs; this group answers whether the connection holds. In steel structures almost every failure is in a connection or a stability check rather than in the member — members are selected from tables, connections are made on site.

The four tools form one chain. The beam span tool back-calculates the inertia the deflection limit demands, the section properties tool checks whether the chosen profile has it, the column buckling tool asks whether the column fails first, and the fillet weld tool carries the force into the connection. The order matters: size the section before checking the weld, or you end up proving the weld is fine while the plate it sits on is not.

Three numbers are commonly wrong, and all three err on the unsafe side. A leg size is not a throat thickness — a 6 mm leg has a 4.2 mm throat and 43% less capacity. An effective length factor of 0.5 is theoretical; real fixity is hard to achieve. And a steel beam is usually governed by deflection rather than strength, so a section chosen on strength alone will bounce underfoot.

- Category page: https://jianzhujisuan.com/en/c/steel-design
- Data checked: 2026-10-25

## Tools in this group

- [Steel Beam Span Calculator](https://jianzhujisuan.com/en/steel-beam-span): The depth is not looked up in a table — it falls out of the deflection limit.
- [Section Properties Calculator](https://jianzhujisuan.com/en/section-modulus): Inertia governs deflection, section modulus governs strength — they are not interchangeable.
- [Fillet Weld Strength Calculator](https://jianzhujisuan.com/en/fillet-weld): A 6 mm leg is not a 6 mm throat — capacity follows the 4.2 mm throat.
- [Base Plate Design Calculator](https://jianzhujisuan.com/en/base-plate): Thicker is not the answer — check the bearing area first, then the cantilever.
- [Column Buckling Calculator](https://jianzhujisuan.com/en/column-buckling): Half the effective length means four times the Euler load — but the design resistance differs by less, and how much depends on slenderness.

## Questions

### Is weld strength the same as base metal strength?

No, a fillet weld reaches about half of it. The design strength is fu/(sqrt(3)·βw·1.25), so S275 at 430 MPa gives 234 MPa. A full-strength connection needs a full-penetration butt weld; lengthening a fillet does not help because the intensity per millimetre is capped.

### How thick should a base plate be?

As a cantilever: t = m·√(2fp/(0.9Fy)), with m the projection beyond the column and fp the bearing pressure. At 1500 kN on C30 with a 350×350 plate under a 300×300 column that gives 18 mm, so specify 20. When the area is short, enlarge the plate rather than thickening it — halving the pressure cuts thickness by 29%, while extra thickness costs money squared.

### Why can't the Euler load be used as capacity?

Because Euler assumes a perfectly straight member with no initial bow, residual stress or load eccentricity. Real members have all three and carry several times less. HEB200 at 4 m pinned-pinned: 7379 kN by Euler against 1548 kN of real buckling resistance. Euler only tells you whether slenderness is excessive.

### What governs the depth of a steel beam?

Usually deflection, not strength. Steel is strong enough that a strength-sized section is too flexible. At 6 m and 20 kN/m the strength check needs 360 cm³ of section modulus while L/360 needs 9643 cm⁴ of inertia, one or two sizes larger. Inertia goes with depth cubed, so depth pays far better than flange thickness.