ST3X8.6 Steel ST-shape — Section Properties
Dimensions
| Property | Value | Unit |
|---|---|---|
| Depth (d) | 3 | 76.2 mm |
| Flange Width (bf) | 3.57 | 9.07 cm |
| Flange Thickness (tf) | 0.359 | 9.1 mm |
| Web Thickness (tw) | 0.465 | 11.8 mm |
| Area (A) | 2.53 in² | 16.3 cmÃÂò |
| Weight | 8.6 lb/ft | 12.8 kg/m |
Elastic Section Properties
| Property | Strong Axis (X-X) | Weak Axis (Y-Y) | Unit |
|---|---|---|---|
| Moment of Inertia (I) | 2.12 | 1.14 | in⁴ |
| Elastic Section Modulus (S) | 1.02 | 0.642 | in³ |
| Plastic Section Modulus (Z) | 1.85 | 1.17 | in³ |
| Radius of Gyration (r) | 0.915 | 0.673 | in |
Torsional Properties
| Property | Value | Unit |
|---|---|---|
| Torsional Constant (J) | 0.181 | in⁴ |
| Warping Constant (Cw) | 0.077 | in⁶ |
Section Profile Summary
The centroid of a tee is closer to the flange, creating an inherent eccentricity in tension connections. For truss chords and bracing, account for this eccentricity in the connection design — the induced bending moment can be significant for slender members.
At 3" deep — one of the smallest standard sections available. Used for light-duty lintels, bracing, handrail posts, and miscellaneous structural iron.
Key Design Checks (AISC 360)
| Check | Formula | This Section |
|---|---|---|
| Plastic moment | Mp = Zx × Fy | 92.5 kip-in |
| Deflection | Δ = 5wLâÃÂô/(384EIx) | Use Ix = 2.12 in⁴ |
| Torsion | St. Venant = GJ/L | J = 0.181 in⁴ |
| Column buckling | KL/r âÃÂàFcr | r_x = 0.915 in |
Design Notes
- Stem in tension for beams: Orient the tee so the flange is in compression (stem down for gravity loads). Per AISC 360 Section F9, the flexural capacity is significantly higher with the stem in tension.
- Flexural-torsional buckling for columns: Single-axis symmetry means the governing buckling mode may be flexural-torsional rather than pure flexural per AISC 360 Eq. E4-4.
- Connection eccentricity: The centroid is closer to the flange — tension connections must account for the induced bending when the work line differs from the centroidal axis.
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: Tension Capacity — st3x8_6
Scenario: st3x8_6 used as a bottom chord member in a roof truss. The member carries axial tension only under the governing load combination.
Given:
- st3x8_6: gross area Ag = 2.53 in²
- Steel: Fy = 50 ksi, Fu = 65 ksi
- Connection: bolted through the flange with 4 bolts per line (standard holes)
Step 1 — Gross yield (AISC 360 Eq. D2-1):
φTn_yield = 0.9 × Ag × Fy = 0.9 × 2.53 × 50 = 114 kips
Step 2 — Net section fracture:
For bolted connections, deduct bolt holes from Ag: An = Ag − n × dh × tf
The effective net area Ae = U × An depends on the shear lag factor U per AISC 360 Table D3.1.
Check φTn_fracture = 0.75 × Ae × Fu per AISC 360 Eq. D2-2.
Step 3 — Stem-in-tension note:
For WT/ST sections loaded in axial tension, the eccentricity between the centroid and the connection plane introduces a bending moment that should be considered in the connection design. The stem is more flexible than the flange — verify the weld or bolt group can accommodate the induced prying action.
Related Resources
- Steel Beam Capacity Calculator
- Column Capacity Calculator
- Beam Deflection Calculator
- Section Properties Calculator
- Steel Grades Reference
- Section Comparison Tool
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.