What is the section modulus of HSS18X6X3/4?

Sx = 118 inÃÂÃÂ³, Zx = 158 inÃÂÃÂ³ (strong axis). Sy = 59.6 inÃÂÃÂ³, Zy = 70.4 inÃÂÃÂ³ (weak axis).

What steel grade is standard for HSS18X6X3/4?

ASTM A500 Grade C (Fy = 50 ksi) or Grade B (Fy = 46 ksi). Verify with mill certificate.

What is the moment of inertia of HSS18X6X3/4?

Ix = 1060 inÃÂ¢ÃÂÃÂ´ about the strong axis and Iy = 179 inÃÂ¢ÃÂÃÂ´ about the weak axis. The radius of gyration is rx = 5.92 in and ry = 2.43 in.

HSS18X6X3/4 Steel Rectangular HSS — Section Properties

Dimensions

Property	Value	Unit
Height (h)	18	457.2 mm
Width (w)	6	152.4 mm
Wall Thickness (t)	0.75	19.0 mm
Area (A)	30.3 in²	195.5 cmÃÂÃÂ²
Weight	109.9 lb/ft	163.5 kg/m

Elastic Section Properties

Property	Strong Axis (X-X)	Weak Axis (Y-Y)	Unit
Moment of Inertia (I)	1060	179	in⁴
Elastic Section Modulus (S)	118	59.6	in³
Plastic Section Modulus (Z)	158	70.4	in³
Radius of Gyration (r)	5.92	2.43	in

Torsional Properties

Property	Value	Unit
Torsional Constant (J)	530	in⁴
Warping Constant (Cw)	0	in⁶

Section Profile Summary

Closed tubular section with J = 530 in⁴ — torsional stiffness is significantly higher than an open section of equivalent weight. The flat faces accept welded gusset and end-plate connections without stiffeners.

Key Design Checks (AISC 360)

Check	Formula	This Section
Plastic moment	Mp = Zx × Fy	7,900 kip-in
Deflection	Δ = 5wLÃÂ¢ÃÂÃÂ´/(384EIx)	Use Ix = 1,060 in⁴
Torsion	St. Venant = GJ/L	J = 530 in⁴
Column buckling	KL/r ÃÂ¢ÃÂÃÂ Fcr	r_x = 5.92 in

Design Notes

Wall slenderness: b/t limits per AISC 360 Table B4.1a define compact, noncompact, and slender classifications. Round HSS use D/t limits.
HSS connection design: Welded connections to HSS walls must check chord plastification, punching shear, and sidewall yielding per AISC 360 Chapter K. Through-bolting requires reinforcing sleeves or plate washers.
Concrete-filled HSS: For composite column applications, filling the HSS with concrete increases axial capacity and fire resistance. Composite design follows AISC 360 Chapter I.

Verification (AISC 360): All designs using this section must be verified by a licensed Professional Engineer. Before finalizing member selection, check beam-column interaction (P-M), lateral-torsional buckling, serviceability deflections, and all connection limit states. See Engineering Disclaimer.

Worked Example: Column Capacity — hss18x6x3-4

Scenario: Wind column between windows in a curtain wall system Unbraced length L = 13 ft.

Given:

hss18x6x3-4: Ag = 30.3 in²
r_min = 5.92 in (governing radius of gyration)
Steel: ASTM A500 Gr. C, Fy = 50 ksi, E = 29,000 ksi
Effective length factor K = 1.0 (simply supported)

Step 1 — Slenderness ratio:

KL/r = 156 / 5.92 = 26

Step 2 — Determine critical stress Fcr (AISC 360 Chapter E):

Elastic buckling stress: Fe = ÃÂÃÂÃÂÃÂ²E / (KL/r)ÃÂÃÂ² = ÃÂÃÂÃÂÃÂ² × 29,000 / (26)ÃÂÃÂ² = 423 ksi

For HSS with KL/r ÃÂ¢ÃÂÃÂ¤ 4.71ÃÂ¢ÃÂÃÂ(E/Fy) = 4.71ÃÂ¢ÃÂÃÂ(29,000/50) = 113.4:

If KL/r ÃÂ¢ÃÂÃÂ¤ 113.4: Fcr = 0.658^(Fy/Fe) × Fy (inelastic buckling) If KL/r > 113.4: Fcr = 0.877Fe (elastic buckling)

Step 3 — Design compressive strength:

φPn = φ × Fcr × Ag = 0.9 × Fcr × 30.3 ÃÂ¢ÃÂÃÂ 1,364 kips (preliminary)

For a precise capacity accounting for local buckling and the exact KL/r value, use the Column Capacity Calculator with this section's properties.

Related Resources

Design Resources

Section Properties Lookup — Compare with similar sections
Steel Beam Sizes Reference — Standard beam dimensions

Educational reference only. Verify all section properties against the current AISC 360 Manual and mill certificates before design. Results are PRELIMINARY — NOT FOR CONSTRUCTION.