Hollow Structural Sections (HSS) — square and rectangular steel tube — appear in columns, beams, trailer frames, gates, railings, equipment frames, and machine bases. Unlike pipe, HSS is sized by actual outer dimensions: the size you specify is the size you get. This guide covers every standard size and wall, explains which ASTM standard to specify, and addresses the corner radius detail that catches fabricators off guard.
Standards: A500, A500-C, and A1085
Three ASTM standards govern structural HSS. Which one you specify matters for design calculations and dimensional expectations.
ASTM A500 Grade B
The default HSS standard. Most structural HSS currently in service is A500-B.
| Property | Value |
|---|---|
| Yield strength (Fy) | 46 ksi |
| Tensile strength (Fu) | 58 ksi |
| Wall thickness tolerance | −10% of nominal |
The 10% under-tolerance is the critical limitation. If you specify HSS 4×4×1/4 (0.250" wall), the actual wall can be as thin as 0.225". AISC requires using 0.93 × t_nominal in all section property calculations to account for this.
ASTM A500 Grade C
Raises minimum yield to 50 ksi (vs 46 ksi). Wall tolerance unchanged at −10%.
| Property | Value |
|---|---|
| Yield strength (Fy) | 50 ksi |
| Tensile strength (Fu) | 62 ksi |
Increasingly specified for weight-sensitive structural applications where the extra 4 ksi saves one wall thickness step.
ASTM A1085
The newer standard (2013) that fixes the wall tolerance problem.
| Property | Value |
|---|---|
| Yield strength (Fy) | 50 ksi |
| Tensile strength (Fu) | 65 ksi |
| Wall thickness tolerance | −3.5% of nominal |
| Charpy toughness | Required (A500 has none) |
With A1085, AISC allows the full nominal thickness in calculations — no 0.93 correction factor. This matters when wall thickness governs a column or compression member. Availability is more limited than A500.
Notation
Format: HSS [A] × [B] × [t]
- Square:
HSS 4×4×1/4— 4" per side, 1/4" wall - Rectangular:
HSS 6×3×3/16— 6" long side × 3" short side × 3/16" wall
By AISC convention, the larger dimension always comes first for rectangular sections.
Square HSS — Standard Sizes and Available Walls
| Size | Available Wall Thicknesses (in) |
|---|---|
| 1 × 1 | 0.083, 0.120 |
| 1-1/2 × 1-1/2 | 0.083, 0.120, 0.188 |
| 2 × 2 | 0.083, 0.120, 0.188, 0.250 |
| 2-1/2 × 2-1/2 | 0.083, 0.120, 0.188, 0.250 |
| 3 × 3 | 0.083, 0.120, 0.188, 0.250, 0.313 |
| 4 × 4 | 0.083, 0.120, 0.188, 0.250, 0.313, 0.375, 0.500 |
| 5 × 5 | 0.120, 0.188, 0.250, 0.313, 0.375, 0.500, 0.625 |
| 6 × 6 | 0.188, 0.250, 0.313, 0.375, 0.500, 0.625 |
| 8 × 8 | 0.188, 0.250, 0.313, 0.375, 0.500, 0.625 |
| 10 × 10 | 0.250, 0.313, 0.375, 0.500, 0.625 |
| 12 × 12 | 0.250, 0.313, 0.375, 0.500, 0.625 |
Rectangular HSS — Standard Sizes and Available Walls
| Size | Available Wall Thicknesses (in) |
|---|---|
| 2 × 1 | 0.083, 0.120, 0.188 |
| 3 × 2 | 0.083, 0.120, 0.188, 0.250 |
| 4 × 2 | 0.083, 0.120, 0.188, 0.250 |
| 4 × 3 | 0.083, 0.120, 0.188, 0.250, 0.313 |
| 5 × 3 | 0.083, 0.120, 0.188, 0.250, 0.313 |
| 6 × 3 | 0.120, 0.188, 0.250, 0.313 |
| 6 × 4 | 0.120, 0.188, 0.250, 0.313, 0.375 |
| 8 × 4 | 0.188, 0.250, 0.313, 0.375 |
| 8 × 6 | 0.188, 0.250, 0.313, 0.375, 0.500 |
| 10 × 6 | 0.188, 0.250, 0.313, 0.375, 0.500 |
| 10 × 8 | 0.250, 0.313, 0.375, 0.500 |
| 12 × 6 | 0.188, 0.250, 0.313, 0.375, 0.500 |
| 12 × 8 | 0.250, 0.313, 0.375, 0.500 |
Wall Thickness Selection
| Wall | Typical Application |
|---|---|
| 1/8" (0.125") | Handrails, light gates, decorative frames |
| 3/16" (0.1875") | Light trailer tongue boxes, secondary structural members |
| 1/4" (0.250") | Most common — trailer frames, equipment frames, moderate structural columns |
| 5/16" (0.3125") | Heavier frames, crane jibs |
| 3/8" (0.375") | Heavy structural columns, skid bases under significant load |
| 1/2"+ | Extreme loads — verify a W-shape isn't more efficient at this point |
Corner Radius: The Detail Fabricators Miss
HSS is roll-formed from flat strip, which produces rounded outer corners — not sharp 90° corners. The outer corner radius is approximately 1.5× the wall thickness for A500-B.
| Wall | Approx. Outer Corner Radius |
|---|---|
| 1/8" | ~3/16" |
| 3/16" | ~5/16" |
| 1/4" | ~3/8" |
| 3/8" | ~9/16" |
Why this matters: When a flat gusset plate is designed to fit inside an HSS corner, the corner radius creates a gap. A plate with a 90° notch will rock on the corner rather than sitting flush. The correct detail is to notch the plate with a radius larger than the HSS corner radius, or design the connection to contact only the flat faces and weld out any gap.
Deflection Reference
Approximate midspan deflection limit (L/360) for simply supported square HSS beams under 100 lb/ft uniform distributed load:
| Section | Wall | Ix (in⁴) | Approx. span limit |
|---|---|---|---|
| HSS 3×3 | 1/4" | 3.58 | ~6 ft |
| HSS 4×4 | 1/4" | 9.59 | ~8 ft |
| HSS 4×4 | 3/8" | 13.4 | ~9 ft |
| HSS 6×6 | 1/4" | 36.1 | ~12 ft |
| HSS 6×6 | 3/8" | 51.5 | ~14 ft |
| HSS 8×8 | 1/4" | 97.4 | ~17 ft |
These are preliminary sizing estimates. Structural design must be verified by a qualified engineer per applicable building codes.