Reinforced Concrete Beam Designer
Pick a beam's width, depth, concrete and steel strengths and rebar, and watch the moment capacity and the ductile/brittle reinforcement check update live.
Change the beam width, depth, concrete strength, steel strength and rebar to see the moment capacity and the under/over-reinforced check update live.
a = As·fy / (0.85·fc'·b), Mn = As·fy·(d − a/2), φMn = 0.9·MnAbout the Reinforced Concrete Beam Designer
Free reinforced concrete beam designer. Pick a beam's width, depth, concrete and steel strengths and rebar, and watch the moment capacity and the ductile/brittle reinforcement check update live. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for engineering, the reinforced concrete beam designer runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Pick a beam's width, depth, concrete and steel strengths and rebar, and watch the moment capacity and the ductile/brittle reinforcement check update live. Use it to explore engineering ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Reinforced Concrete Beam Designer
- Use the controls to change Width b (mm), Effective depth d (mm), Concrete strength f'c (MPa), Steel yield strength fy (MPa), Number of bars, and more. The simulation reacts instantly.
- Press "Reset to defaults", "Lab report" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Increase the effective depth and watch the design moment climb faster than adding steel does.
- Add bars until the beam becomes over-reinforced and watch the failure-mode warning appear.
- Try the 180 kN·m ductile-design challenge.
- Try the deliberate over-reinforcement challenge.
Key ideas you can learn
- A singly-reinforced concrete beam's design moment capacity is phi*Mn = 0.9*As*fy*(d - a/2), where a = As*fy/(0.85*f'c*b) is the depth of the rectangular (Whitney) compression stress block.
- The reinforcement ratio rho = As/(b*d) compares how much steel is used to the section size; a maximum ratio rho_max keeps the beam 'under-reinforced' so the steel yields first.
- An under-reinforced beam fails ductile: the steel yields, cracks widen and it deflects visibly before ultimate failure - giving warning. An over-reinforced beam fails brittle: the concrete crushes suddenly with little warning.
- Because moment capacity grows with (d - a/2), increasing the effective depth d is usually a far more efficient way to add capacity than adding more steel, which also raises a and can push the section over-reinforced.
Where this is used in the real world
Structural engineers run exactly this under/over-reinforced moment-capacity check on every reinforced concrete beam in buildings, bridges and parking structures before construction drawings are approved.
Who is this simulation for?
Engineering and technology students, makers, robotics clubs and teachers of design and technology. It gives a hands-on feel for how machines behave before you build a real one.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
Why do building codes require beams to be under-reinforced rather than over-reinforced?
An under-reinforced beam's steel yields before the concrete crushes, which produces visible cracking and deflection that warns of impending failure and allows a ductile, gradual collapse; an over-reinforced beam's concrete crushes first, causing a sudden brittle failure with little or no warning, which codes intentionally design against.
Why does adding more rebar not always increase a beam's usable moment capacity as much as expected?
More steel area As increases the compression block depth a = As*fy/(0.85*f'c*b) as well as the tension force, and once a grows enough to push the reinforcement ratio rho past rho_max, the beam becomes over-reinforced and its failure mode switches from ductile steel yielding to brittle concrete crushing, which codes forbid regardless of the nominal moment number.
Is the Reinforced Concrete Beam Designer free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Reinforced Concrete Beam Designer work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.