Enter Beam and Torsion Data
Formula Used
Acp = b × h, and pcp = 2(b + h).Aoh = (b - 2c)(h - 2c), Ao = 0.85Aoh, and ph = 2[(b - 2c) + (h - 2c)].Tcr = 0.33λ√f'c × Acp² / pcp.At/s = Tu / (φ × 2Ao × fyv × cotθ).φTn = φ × 2Ao × At × fyv × cotθ / s.Al = (At/s) × ph × (fyv/fyl) × cot²θ.θtwist = TsL / GJ, using a rectangular torsion constant estimate.All calculations use SI units. Torque is converted internally from kN·m to N·mm.
How to Use This Calculator
- Enter the beam width, depth, effective depth, and stirrup centerline cover.
- Add concrete strength, steel strengths, torsion, shear, and service torque.
- Select a strength factor, density factor, and truss angle.
- Enter the stirrup diameter, number of resisting legs, and spacing.
- Enter the longitudinal torsion steel already planned for the beam.
- Press the calculate button and review the result above the form.
- Use the CSV or PDF button to save the calculated example.
Example Data Table
| Example | b mm | h mm | Tu kN·m | Vu kN | fc MPa | Stirrup | Spacing mm |
|---|---|---|---|---|---|---|---|
| Edge beam | 300 | 600 | 85 | 180 | 35 | 2 legs, 10 mm | 150 |
| Spandrel beam | 350 | 700 | 130 | 260 | 40 | 4 legs, 12 mm | 125 |
| Balcony support | 250 | 500 | 55 | 120 | 30 | 2 legs, 10 mm | 175 |
Concrete Torsion Design Overview
Concrete beams carry torsion when loads act away from their shear center. Edge beams, spandrels, balcony supports, and transfer beams often receive this action. Torsion creates diagonal tension. It also demands closed stirrups and longitudinal corner steel. A practical calculation must check strength, cracking, spacing, and service twist together.
Why Torsion Matters
Ignoring torsion can make a beam unsafe. Cracks may wrap around the section. Stirrups may open if they are not closed. Longitudinal bars may also become insufficient. This calculator uses a thin tube model. It estimates the enclosed area inside the closed stirrup. It then checks the steel needed to resist factored torque.
Input Quality
Accurate inputs matter. Beam width, depth, cover, and stirrup size control the torsion area. Concrete strength affects cracking torque. Steel yield strength affects reinforcement demand. The angle theta adjusts the truss action. Most practical designs use thirty to sixty degrees. A forty five degree value is common for preliminary work.
Strength Review
The calculator compares factored torsion with available design torsion strength. It also calculates required transverse steel per unit spacing. The provided spacing is checked against strength and a maximum spacing rule. A longitudinal torsion steel estimate is shown. That value helps the designer review corner bars and side face steel.
Service Review
Service torque is used for elastic twist. The twist estimate uses a rectangular torsion constant and concrete shear modulus. This check is approximate. Cracked concrete beams are usually less stiff. Use the value as a warning signal. Large twist needs better detailing, deeper sections, or reduced eccentric loading.
Best Practice
Use this page for early design and checking examples. Confirm final work with the project code, load combinations, detailing rules, and licensed engineering review. Torsion design is sensitive to detailing. Closed stirrups must be properly anchored. Longitudinal bars should be placed near corners. Good detailing keeps cracks controlled and improves ductile behavior.
Common Example Workflow
Start with the factored torsion from structural analysis. Use actual beam dimensions, not nominal guesses. Enter stirrup centerline cover carefully. Try the selected stirrup diameter and spacing. Read the utilization ratio first. Then review cracking torque and twist. Finally, adjust spacing or section size until the warning messages become acceptable for your design basis. Document assumptions with each saved result.
FAQs
What is concrete beam torsion?
Concrete beam torsion is twisting caused by eccentric loads, edge loading, frame action, or offset supports. It creates diagonal tension around the beam and usually needs closed stirrups with longitudinal torsion bars.
Which units does the calculator use?
The calculator uses millimeters, megapascals, kilonewtons, meters, and kilonewton meters. It converts torque internally to newton millimeters so the SI formulas remain consistent.
What is Aoh in torsion design?
Aoh is the area enclosed by the centerline of the outer closed stirrup. It represents the effective hollow tube used by the torsion model for reinforced concrete beams.
Why is Ao taken as 0.85Aoh?
Many torsion design procedures reduce the enclosed core area to approximate the effective shear flow area. The calculator uses Ao equals 0.85Aoh for that design estimate.
What does At over s mean?
At over s is transverse torsion reinforcement per unit spacing. A larger torque, smaller core area, or lower steel strength increases the required value.
How should I choose theta?
Theta is the compression strut angle used in the space truss model. Many preliminary checks use 45 degrees. Project rules may allow values from 30 to 60 degrees.
Does the calculator design final reinforcement?
It gives a structured design check and reinforcement estimate. Final detailing must follow the governing code, project drawings, anchorage rules, bar spacing limits, and engineering review.
Why does the result show a cracking torque?
Cracking torque helps compare applied torsion with the estimated torque that may cause diagonal torsion cracking. It is useful for deciding whether torsion is minor or significant.
What causes a spacing warning?
A spacing warning appears when the provided spacing is larger than the calculated strength spacing, the maximum spacing rule, or the selected strength demand.
Why is service twist approximate?
The twist calculation uses elastic concrete stiffness and a rectangular torsion constant. Cracked beams, reinforcement layout, restraint, and load history can change actual twist.
Can I use this for prestressed beams?
Use caution. Prestressed concrete torsion design can include extra effects, different stress limits, and special detailing rules. Treat this tool as a preliminary comparison only.