Girder Deflection Calculator

Enter span, material stiffness, section inertia, and a load case. Use consistent units. For a positioned point load, enter a from the left support.

Example data table

Use the example below to verify inputs and understand the output format.

Formula used

The calculator applies standard Euler–Bernoulli beam deflection relationships for common support and load cases. Deflection is reported as a positive downward magnitude.

How to use this calculator

1. Pick the girder support condition that matches your detailing.
2. Select a load case and enter the load magnitude in your chosen units.
3. Enter span L, modulus E, and inertia I for the bending axis.
4. Choose a serviceability criterion (for example L/360).
5. Press Calculate deflection to show results above the form.
6. Use the CSV or PDF buttons to export a compact report.

Girder deflection overview

Girder deflection is a serviceability check that confirms a member will not feel bouncy, crack finishes, or misalign doors and cladding. This calculator evaluates maximum elastic deflection for classic beam cases using Euler–Bernoulli theory, with inputs for span L, modulus E, and second moment of area I. Results are reported in millimetres, alongside an L/ratio limit.

Inputs that drive accuracy

The stiffness term EI controls deflection. For structural steel, E is commonly near 200 GPa, while reinforced concrete can be much lower and may require an effective E to reflect cracking. Use the correct inertia for the bending axis: strong-axis I for major bending, and ensure your units match the selected dropdowns.

Common support and load cases

The form covers simply supported, cantilever, and fixed-fixed conditions with uniform loads and typical point-load scenarios. Uniform load represents distributed dead and live actions (for example slab and finishes), while point loads represent equipment, wheel loads, or concentrated reactions. For a point load at distance a, the tool evaluates the deflection curve and reports the peak location.

Serviceability limits and interpretation

Limits like L/240, L/360, or L/480 are commonly used to control visual sag and vibration sensitivity. The calculator compares the computed maximum deflection to an allowable value of L / (selected ratio). A PASS indicates the calculated deflection is within the chosen criterion; a FAIL suggests increasing section stiffness, reducing span, adding continuity, or revising loads.

Good practice and limitations

Use service-level load combinations, and confirm whether long-term effects (creep, shrinkage, composite action, or connection slip) matter for your project. For irregular load patterns, tapered members, or staged construction, validate results using a full structural model. Export the CSV/PDF to document inputs, assumptions, and compliance for design reviews. It supports consistent checking across teams.

FAQs

1) Which loads should I use for deflection?

Use service-level loads, typically dead plus relevant live load per your standard. Include sustained components when long-term deflection is critical. Avoid ultimate factors unless your code explicitly requires them for serviceability.

2) What does E represent in this calculator?

E is the modulus of elasticity that links stress to strain in the elastic range. It governs stiffness with I as EI. Use a realistic value for your material, or an effective value for cracked concrete members.

3) Which moment of inertia I should I enter?

Enter the second moment of area about the bending axis that resists the applied load. For wide-flange steel beams, this is usually the strong-axis I for gravity bending. For built-up sections, use the calculated composite I.

4) Why is the maximum deflection not always at midspan?

For uniform load on symmetric spans, the maximum is typically at midspan. With an off-centre point load, the peak occurs closer to the loaded region. Cantilevers often peak at the free end.

5) What does L/360 mean?

It is a deflection limit ratio: allowable deflection equals the span divided by 360. For example, a 12 m span gives 12,000/360 = 33.3 mm allowable. Your project may specify different ratios by occupancy and finish sensitivity.

6) Does the calculator include shear deformation or vibration?

No. It uses classic beam bending equations for elastic deflection. Deep beams, short spans, or composite systems may need shear deformation checks. For vibration-sensitive floors, perform a dedicated vibration assessment.

7) How can I reduce deflection if it fails?

Increase stiffness by selecting a deeper section or higher I, shorten the span with intermediate supports, increase continuity, add composite action, or reduce service loads where justified. Re-check with the same support and load assumptions.