Calculator Inputs
Enter built up section geometry, steel properties, bracing data, demand loads, and reduction factors.
Example Data Table
| Case | Top Flange | Web | Bottom Flange | Fy | Span |
|---|---|---|---|---|---|
| Light built up beam | 200 × 16 mm | 450 × 10 mm | 200 × 16 mm | 250 MPa | 5 m |
| Industrial transfer beam | 300 × 25 mm | 700 × 16 mm | 320 × 28 mm | 345 MPa | 8 m |
| Strengthened existing beam | 250 × 20 mm | 500 × 12 mm | 300 × 22 mm | 300 MPa | 6 m |
Formula Used
Centroid: ȳ = Σ(Ai yi) / ΣAi.
Elastic inertia: Ix = Σ(Ii + Ai di²).
Elastic moment: My = Fy × Smin, where Smin = Ix / farthest fiber distance.
Plastic moment: Mp = Fy × Zx, using the plastic neutral axis and plate areas.
Adjusted nominal strength: Mn = Mp × Rltb × Rlocal × Rnet × Rconnection × Rcomposite.
Design strength: LRFD = φMn. ASD = Mn / Ω.
Demand moment: M = wL² / 8 + Pab / L for a simply supported beam.
Weld shear flow: q = VQ / Ix, estimated near flange to web transfer.
How to Use This Calculator
Start with plate dimensions from drawings or field measurements. Enter flange sizes, web depth, web thickness, and web count. Use millimeters for section sizes.
Add steel yield strength, modulus, bracing length, and Cb value. Keep Cb near 1.0 unless a verified moment diagram supports a higher value.
Enter loads in kN and kN/m. Choose LRFD for factored design. Choose ASD for service load checking. Press the calculation button to compare demand and capacity.
Review section modulus, plastic modulus, utilization, and weld shear flow. Treat failed or marginal results as a signal for redesign, added plates, shorter bracing, or expert review.
Built Up Beam Capacity Guide
Why Moment Capacity Matters
Built up beams often join plates, webs, and flanges. The combined shape may carry large bending moments. Yet each plate must act together. This calculator estimates that shared bending strength.
Moment capacity depends on geometry and steel yield strength. It also depends on bracing, holes, and weld transfer. A deep web can raise stiffness. Wider flanges usually raise section modulus.
Key Design Checks
The calculator finds the neutral axis first. It then computes gross area and inertia. Elastic section modulus is checked at the top and bottom fibers. Plastic section modulus is also estimated from plate areas.
The nominal bending strength comes from yield stress and section modulus. A local slenderness modifier adjusts thin plate behavior. A lateral bracing factor adjusts long unbraced lengths. Net hole loss and weld efficiency can reduce strength.
Demand is estimated for a simply supported beam. Uniform dead load, live load, and point load are included. The tool compares demand against LRFD or ASD capacity. The utilization ratio shows remaining reserve.
Construction Use
Use measured plate sizes from shop drawings. Confirm that all plates are connected along the member. Built up beams need adequate welds, bolts, or fasteners. Without shear transfer, the plates may slip.
Corrosion loss can be entered for older members. This helps review repairs and field strengthening. A conservative loss value is useful during early checks. Final work should use verified measurements.
The weld shear flow result is a guide. It uses vertical shear, first moment, and inertia. Actual connection design may require more checks. Fatigue, end reactions, and eccentric details can matter.
Good Inputs Improve Results
Use consistent millimeter and kN units. Enter realistic unbraced length between lateral restraints. Increase the Cb value only with reliable moment diagrams. Do not inflate factors to pass a weak section.
This tool supports planning, estimating, and preliminary checking. It does not replace sealed structural design. Codes can require special limits for compactness and stability. Always review critical beams with a qualified engineer.
Field Notes
Check bearing seats and stiffeners near supports. Built up members can fail outside midspan. Camber, residual stress, and fabrication tolerance affect behavior. Record assumptions with every result. Clear notes make later review easier. Retain sketches for shop and site discussions later.
FAQs
What is moment capacity for a built up beam?
It is the bending resistance developed by combined plates, webs, and flanges. The value depends on geometry, steel strength, stability, and connection effectiveness between components.
Can this calculator handle unequal flanges?
Yes. Enter separate top and bottom flange widths and thicknesses. The calculator finds the actual centroid and uses the smaller elastic section modulus.
Why is web count included?
Some built up beams use twin webs or added web plates. Web count increases effective web area and stiffness for this preliminary section calculation.
What does the bracing factor represent?
It represents reduced bending strength when the compression flange lacks enough lateral restraint. Longer unbraced lengths usually reduce usable moment capacity.
Should I use LRFD or ASD?
Use LRFD when checking factored loads against resistance factored capacity. Use ASD when checking service loads against allowable bending capacity.
How is plastic section modulus estimated?
The tool divides the section into rectangular plates. It locates the plastic neutral axis and sums area times distance from that axis.
What does net hole loss do?
It reduces the nominal capacity for bolt holes, cope cuts, or similar section losses. Use measured net area checks for final design.
Can corrosion be included?
Yes. Enter a corrosion allowance in millimeters. The tool reduces plate sizes before calculating area, inertia, and bending capacity.
What does weld shear flow mean?
It estimates force transfer per millimeter between the flange and web. It helps judge whether the connection can make plates act together.
Is this enough for permit drawings?
No. It is a preliminary calculator. Permit drawings usually need code checks, load combinations, connection design, and professional engineering review.
Why can utilization exceed one?
A utilization ratio above one means demand exceeds selected capacity. Increase section size, improve bracing, lower loads, or request structural review.