Steel · Columns & beam-columns
Compression member: single equal angle calculator
Equal angle strut: area and minor principal radius rv from dimensions, local buckling kf and member capacity (concentric idealisation).
Standard: AS 4100 Cl 6.3 (αb = 0.5) about the minor principal axis
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Leg length | 90 | mm |
| Thickness | 8 | mm |
| Yield stress | 300 | MPa |
| Effective length | 2.2 | m |
| Design compression N* | 100 | kN |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Area (2b − t)t | 1,376 | mm² |
| Centroid from heel | 25.45 | mm |
| Ix about centroid | 1,066,512 | mm⁴ |
| |Ixy| = Σ A x y about centroid | 633,307 | mm⁴ |
| Iv = Ix − |Ixy| | 433,205 | mm⁴ |
| rv | 17.74 | mm |
| Leg slenderness (b − t/2)/t √(fy/250) | 11.78 | |
| kf (λey = 16, one edge supported) | 1.00 | |
| φNs | 371.5 | kN |
| λn = (le/r)√kf √(fy/250) | 135.8 | |
| αa | 13.95 | |
| λ = λn + αa·αb | 142.8 | |
| η | 0.422 | |
| ξ | 0.782 | |
| Slenderness reduction αc | 0.319 | |
| φNc | 118.5 | kN |
- ✓ N* ≤ φNc
- ✓ le/rv ≤ 180 (strut guide)
Method, formulas and assumptions
- Area (2b − t)t = (2*90-8)*8 = 1376 mm²
- Centroid from heel = (90^2+90*8-8^2)/(2*(2*90-8)) = 25.453 mm
- Ix about centroid = (8*(90-25.453)^3+90*25.453^3-(90-8)*(25.453-8)^3)/3 = 1.067e+6 mm⁴
- |Ixy| = Σ A x y about centroid = ABS(90*8*(90/2-25.453)*(8/2-25.453)+(90-8)*8*(8/2-25.453)*((90+8)/2-25.453)) = 6.333e+5 mm⁴
- Iv = Ix − |Ixy| = 1.067e+6-6.333e+5 = 4.332e+5 mm⁴
- rv = SQRT(4.332e+5/1376) = 17.743 mm
- Leg slenderness (b − t/2)/t √(fy/250) = (90-8/2)/8*SQRT(300/250) = 11.776
- kf (λey = 16, one edge supported) = MIN(1,(2*MIN(1,16/11.776)*(90-8/2)+8)*8/1376) = 1
- φNs = 0.9*1*1376*300/1000 = 371.52 kN
- λn = (le/r)√kf √(fy/250) = 2.2*1000/17.743*SQRT(1)*SQRT(300/250) = 135.82
- αa = 2100*(135.82-13.5)/(135.82^2-15.3*135.82+2050) = 13.946
- λ = λn + αa·αb = 135.82+13.946*0.5 = 142.8
- η = MAX(0,0.00326*(142.8-13.5)) = 0.42151
- ξ = ((142.8/90)^2+1+0.42151)/(2*(142.8/90)^2) = 0.78234
- Slenderness reduction αc = MIN(1,0.78234*(1-SQRT(1-(90/(0.78234*142.8))^2))) = 0.31886
- φNc = 0.31886*371.52 = 118.46 kN
- Angles connected through one leg have eccentricity; apply AS 4100 8.4.6 / design-guide rules for final design.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.