Plate Girder Preliminary Sizing Calculator

Inputs

Enter span and loads. Select method and limits. Then calculate.

Span length (m)

Simply supported span.

Uniform load (kN/m)

Enter factored or service, per your method.

Impact factor

Multiply loads for dynamics (default 1.00).

Point load (kN)

Optional concentrated load.

Point load position from left (m)

Only used if point load > 0.

Design method

Affects bending and shear design stresses.

Steel yield strength Fy (MPa)

Common grades: 250, 345, 355 MPa.

Elastic modulus E (MPa)

Typical steel: 200000 MPa.

Depth ratio (L / d)

Typical: 12–20 for plate girders.

Deflection limit denominator

Example: 800 means L/800.

Include estimated self-weight?

Adds a rough kN/m estimate based on depth.

Minimum web thickness (mm)

Practical lower bound for fabrication.

Min flange width (mm)

Clamp range for preliminary selection.

Max flange width (mm)

Clamp range for preliminary selection.

Example Data Table

Scenario	Span (m)	UDL (kN/m)	Point (kN)	Fy (MPa)	Method	Depth ratio	Sample output (d × tw, bf × tf)
Warehouse runway girder	24	35	0	345	ASD	15	~1600 × 6, 450 × 30
Bridge cross-girder	18	22	120 (mid)	355	LRFD	14	~1290 × 6, 390 × 26
Equipment support beam	12	18	60 (at 6 m)	250	ASD	12	~1000 × 6, 300 × 24

Formula Used

Core relationships used for this preliminary estimate.

Max moment (UDL): M = wL²/8 and max shear: V = wL/2 for simply supported spans.
Point load moment (at a): M = P·a(L−a)/L (peak under the load). Reactions give peak support shear.
Required section modulus: Sreq = M/Fb where Fb is allowable/design bending stress.
Preliminary flange area: Af ≈ 2M / (Fb·(d−tf)) using flange couple action.
Web thickness from shear: tw ≈ (V/τallow)/d, where τallow is an allowable/design shear stress.
Approximate inertia: Ix ≈ 2(bf·tf)(d/2)² + tw·d³/12 (preliminary).
Deflection (UDL): δ = 5wL⁴/(384EI); point-load contribution is approximated for quick checking.

How to Use This Calculator

A simple workflow for early-stage sizing.

Enter the span length and your governing uniform load.
Add a point load if needed, and set its position.
Select ASD or LRFD to match your project approach.
Confirm steel grade, depth ratio, and deflection limit.
Click calculate to view preliminary depth, web, and flange sizes.
If utilization is above 1.00, reduce the depth ratio or increase flange limits.
Export CSV or PDF to share assumptions during coordination.

Professional Article

Context and data points to support preliminary sizing work.

When to Use Plate Girders

Plate girders suit spans and loads where rolled shapes become heavy or too deep. In construction they appear in crane runway beams, transfer girders, long bay framing, and bridge-type members where clearance and weight efficiency must be balanced.

Selecting a Practical Depth

A common starting range is L/12 to L/20 for simply supported members. Deeper girders improve stiffness and reduce flange demand, while shallower girders protect headroom but may require wider or thicker flanges. Depth is typically rounded to practical increments during early coordination.

Understanding Bending Demand

For uniform load, peak moment is estimated with M = wL²/8. A point load adds M = P·a(L−a)/L. An impact factor can amplify loads for moving equipment or dynamic effects before section modulus is calculated. The required elastic modulus follows Sreq = M/Fb.

Web Shear and Proportioning

Shear is resisted mainly by the web through Aw = d·tw. The calculator sizes tw so V/Aw stays below an allowable or design shear stress. For larger depths, minimum web thickness is often kept around 6–10 mm to improve handling and welding.

Flange Sizing Strategy

Flanges carry most bending through a tension–compression couple, so flange area governs strength. A practical flange width is often 0.25d to 0.35d, then adjusted to plate availability, splice needs, and connection geometry.

Deflection and Serviceability

Service limits are checked using δ = 5wL⁴/(384EI) for the uniform component plus a conservative point-load term. Limits commonly range from L/360 for general beams to L/800 or tighter where alignment or finishes are sensitive. Meeting serviceability early reduces late-stage rework.

Detailing and Fabrication Notes

Preliminary sizes should be reviewed for weld access, plate thickness availability, and transport. Slender webs can require transverse stiffeners near supports or load points, and bearing regions may control detailing more than global strength.

Moving from Preliminary to Final Design

Use the results to start coordination and budgeting, then complete a full code-based design. Final checks typically include stability, web buckling, flange local buckling, fatigue where relevant, and connection design using project load combinations. Always verify restraint assumptions before issuing drawings.

FAQs

Plain answers without accordions for quick reading.

1) Is this suitable for final structural design?

No. It provides preliminary dimensions for coordination and early checks. Always complete a full code-compliant design, including stability, buckling, stiffeners, bearing, and connection verification.

2) What depth ratio should I start with?

Many projects begin around L/12 to L/20 for simply supported plate girders. Use a deeper girder for stiffness, or a shallower one when headroom is critical and flange capacity can increase.

3) How should I choose the impact factor?

Use 1.00 for static loading. For moving equipment, crane effects, or vibration-sensitive situations, apply a factor consistent with project specifications or the governing standard used by your engineer of record.

4) Why does self-weight matter for long spans?

On longer spans, girder self-weight can be a meaningful portion of the uniform load. Including it improves the realism of early sizing and helps avoid underestimating moment and deflection demand.

5) What if utilization is greater than 1.00?

Increase depth, increase flange width or thickness, raise the flange width limits, or select a higher steel grade if appropriate. Also confirm that loads and units reflect the intended design stage.

6) Does the calculator include lateral-torsional buckling?

Not explicitly. Lateral stability depends on unbraced length, restraint, and load application. Treat results as a starting point and perform a dedicated stability check during detailed design.

7) Can I use it for continuous spans or cantilevers?

It assumes a simply supported member. For continuous beams, cantilevers, or complex framing, moments and shears differ. Use appropriate structural analysis and then size plates using the resulting design actions.