How to Select Between W and WWF Steel Girders Using Cost and Efficiency Metrics For Road Bridges

How to Select Between W and WWF Steel Girders Using Cost and Efficiency Metrics For Road Bridges

 

Selecting the optimal steel girder type is one of the most important decisions in short‑ and medium‑span bridge (SMSB) design for road bridges. In spans between 12 m and 40 m, engineers typically choose between:

• Rolled Wide-Flange (W) sections
• Welded Wide-Flange (WWF) plate girders

 

Both girder types are included in the Manual of Standard Short-Span Steel Bridges prepared by the Ontario Ministry of Transportation. The manual provides span ranges, girder sizes, weights, and performance data that allow engineers to evaluate each option using cost, span-to-depth efficiency, and span-to-weight efficiency.

 

This article explains how to make that choice using engineering metrics that directly affect project cost, fabrication feasibility, and structural performance.

 

1. Understanding the Two Girder Types

Rolled W Sections

These are hot‑rolled shapes produced in steel mills.

Advantages

• Lower fabrication cost
• Shorter procurement time
• Excellent quality control
• Ideal for spans up to ~24 m

Limitations

• Limited available depths
• Less flexibility in flange/web proportions
• May not meet stiffness requirements for longer spans

 

Welded WWF Sections

These are fabricated plate girders assembled from web and flange plates.

Advantages

• Customizable depth and flange sizes
• Higher span-to-depth efficiency
• Better control of stiffness for longer spans
• Ideal for spans > 24 m

Limitations

• Higher fabrication cost
• Longer lead time
• More welding → higher fatigue considerations

 

2. Efficiency Metrics for Girder Selection

Two key metrics determine girder efficiency:

 

2.1 Span-to-Depth Ratio

This ratio indicates how shallow a girder can be for a given span.

• Higher ratio → more efficient girder
• Lower ratio → deeper girder → more steel → higher cost

 

2.2 Span-to-Weight Ratio

This ratio indicates how much span is achieved per unit girder weight.

• Higher ratio → lighter girder → lower cost
• Lower ratio → heavier girder → higher cost

 

3. Span-to-Depth Ratios: W vs WWF

Using the girder depths from the MTO manual:

 

Rolled W Sections (12–24 m spans)

Typical span/depth ratios: 20–24

Examples:

• W610 at 12 m → ratio ≈ 19.7
• W760 at 16–18 m → ratio ≈ 21–23.7
• W1000 at 24 m → ratio ≈ 24

Interpretation: Rolled sections are efficient for short spans but reach depth limits around 24 m.

 

Welded WWF Sections (18–40 m spans)

Typical span/depth ratios: 23–26

Examples:

• WWF700 at 18 m → ratio ≈ 25.7
• WWF900 at 22 m → ratio ≈ 24.4
• WWF1600 at 40 m → ratio ≈ 25

Interpretation: WWF girders outperform rolled sections in span-to-depth efficiency, especially beyond 20–24 m.

 

4. Span-to-Weight Ratios: W vs WWF

Using girder weights from the manual:

 

Rolled W Sections

Span/weight ratios: 0.11–0.13

Examples:

• W610×91 at 12 m → 0.132
• W760×147 at 16 m → 0.109
• W920×201 at 22 m → 0.109

Interpretation: Rolled sections are weight-efficient for short spans but lose efficiency as spans increase.

 

Welded WWF Sections

Span/weight ratios: 0.09–0.13

Examples:

• WWF700×152 at 18 m → 0.118
• WWF900×169 at 22 m → 0.130
• WWF1600×431 at 40 m → 0.092

Interpretation: WWF girders are most weight-efficient in mid-range spans (18–22 m). Efficiency decreases for very long spans due to heavier webs and flanges.

 

5. Cost Comparison: W vs WWF

5.1 Rolled W Sections

• Lowest fabrication cost
• No welding required
• Lower inspection cost
• Limited depth availability → may require more girders or thicker deck for longer spans

5.2 Welded WWF Sections

• Higher fabrication cost (cutting, welding, assembly)
• Higher inspection cost (NDT, fatigue checks)
• Customizable geometry → may reduce total steel tonnage
• Often more economical for spans > 24 m despite higher fabrication cost

 

6. Practical Selection Guidelines

Use Rolled W Sections When:

• Span ≤ 20–24 m
• Rolled depth meets stiffness requirements
• Project requires rapid fabrication
• Budget is tight and rolled shapes are available
• Fatigue demands are moderate

Use Welded WWF Sections When:

• Span ≥ 24 m
• Rolled shapes are unavailable or insufficient
• High stiffness is required (e.g., high truck volume)
• Skew > 25° (WWF allows better control of flange/web proportions)
• Fatigue-sensitive details require optimized geometry

 

7. Decision Matrix (Engineer-Friendly Summary)

Criterion	Rolled W Section	Welded WWF Section
Span Range	12–24 m	18–40 m
Span-to-Depth Efficiency	Moderate (20–24)	High (23–26)
Span-to-Weight Efficiency	High for short spans	High for mid spans
Fabrication Cost	Low	High
Lead Time	Short	Longer
Fatigue Performance	Better (fewer welds)	Requires careful detailing
Customization	None	High
Best Use Case	Short spans, low cost	Medium spans, optimized stiffness

 

8. Final Engineering Recommendation

For spans up to ~20 m:

Choose Rolled W Sections — they are cheaper, lighter, and efficient.

 

For spans between 20–24 m:

Evaluate both options. WWF may offer better stiffness; W may offer lower cost.

 

For spans beyond 24 m:

Choose Welded WWF Sections — they provide superior span-to-depth efficiency and allow custom optimization.