Case Study: The Stolma Bridge, Norway (1991–1992)

Background

The Stolma Bridge in Norway is one of the pioneering examples of lightweight aggregate concrete (LWAC) used in long-span bridge construction. With a 301 m main span, it held the world record for the longest cantilever box-girder bridge at the time of completion (1991–1992).

The key engineering challenge was how to span a deep fjord efficiently. Building heavy piers in the middle of deep waters would have been prohibitively expensive. By reducing the self-weight of the bridge superstructure, engineers could move the piers into shallower waters, thereby minimizing costly foundation works. LWAC provided the material solution.

Role of Lightweight Concrete

The Stolma Bridge utilized LECA lightweight aggregates (expanded clay, 4–12 mm) in the concrete mix. With a 28-day cube density of ~1,940 kg/m³, this concrete was about 20% lighter than normal weight concrete (~2,400 kg/m³), while still delivering high strength:

• Mean cube strength: 70.4 MPa

• Characteristic cube strength: 64.1 MPa

 

This combination of lightness + high strength enabled:

• Longer spans with reduced bending moments.

• Smaller foundations, since the piers could be located in shallow water.

• Lower reinforcement demands, due to reduced self-weight stresses.

• Cost efficiency, with foundation savings outweighing the added cost of lightweight aggregates.

Mix Design (as used on Stolma Bridge)

• Cement (CEM I 52.5): 420 kg/m³

• Silica fume: 35 kg/m³

• Natural sand: 700 kg/m³

• LECA 800 (4–12 mm): 600 kg/m³

• Water: 208 kg/m³

• Additives: Stabiliser 12 kg/m³, Retarder 1 kg/m³

• 28-day density: ~1,940 kg/m³

• 28-day compressive strength: ~64–70 MPa

Calculations and Structural Impacts

1. Self-Weight Reduction

Assume the main box-girder section volume = 20,000 m³ of concrete (illustrative for a 301 m mid-span cantilever).

• NWC weight: $20,000 \times 2,400 = 48,000 \ t$

• LWAC weight: $20,000 \times 1,940 = 38,800 \ t$

• Weight saved: $48,000 - 38,800 = 9,200 \ t$ (≈ 19% lighter)

2. Bending Moment Reduction

For a simply supported span, bending moment scales with w × L² / 8. Reducing uniform load $w$ by 19% directly reduces bending moments by 19%.

• Example: If the original moment = 1000 MN·m, LWAC reduces it to 810 MN·m.

• This lowers rebar and prestressing steel requirements significantly.

3. Foundation Savings

By cutting ~9,200 t from the superstructure:

• Piers could be relocated from deep fjord sections (~80 m water depth) into shallower waters (~30 m).

• Foundation costs drop drastically: fewer piles, less seabed work, shorter shafts.

• Historical estimates suggest >20% cost saving in foundation works compared to a normal weight design.

Outcomes and Benefits

Structural Efficiency

• ~9,200 t lighter structure.

• 19% reduction in bending moment → smaller sections and less steel.

Foundation Optimization

• Piers placed in shallower waters, cutting expensive deepwater piling.

• Overall project savings outweighed the higher price of LWAC.

Durability

• LWAC mix achieved >64 MPa characteristic strength with low permeability due to silica fume, enhancing durability in marine environments.

Engineering Legacy

• Stolma proved LWAC’s viability in large-span infrastructure.

• Paved the way for other record-setting lightweight concrete bridges in Norway (Nordhordland Bridge, Heidrun TLP, etc.).

How LiGrA Could Enhance the Case

If LiGrA (Lightweight Green Aggregates) had been available in 1991, the Stolma Bridge could have achieved even greater impact:

• Lower Density – LiGrA structural concrete at 1,600–1,700 kg/m³ would cut an extra 20% (~7,600 t) beyond the LWAC used.

• Seismic/Wind Load Relief – Lighter deck further reduces dynamic loads in storms.

• Carbon & ESG Benefits – LiGrA is made from recycled coal ash, bauxite residue, and glass, reducing virgin resource extraction and landfill use.

• Lifecycle Gains – LiGrA’s encapsulation technology improves durability against chloride ingress and freeze-thaw cycles, vital in Norway’s coastal climate.

 

Calculation with LiGrA (ρ = 1,650 kg/m³)

• LiGrA weight: $20,000 \times 1,650 = 33,000 \ t$

• Compared to NWC (48,000 t): 15,000 t saved (≈ 31% lighter)

• Extra 5,800 t beyond LWAC → more efficient spans, smaller piers, further cost savings.

Conclusion

The Stolma Bridge demonstrated the transformative power of lightweight concrete in large-span bridge design. By cutting nearly 9,200 tons from the structure, engineers could relocate piers into shallow waters, drastically reducing foundation costs while achieving record-breaking spans.

If built today with LiGrA, Stolma would be 15,000 tons lighter, with greater sustainability credentials through waste recycling and carbon neutrality. It would stand not just as an engineering achievement but as a model of green infrastructure for the circular economy era.

Key takeaway: “Stolma Bridge showed how lightweight concrete saves weight and money. LiGrA today would go further—lighter spans, smaller piers, and a truly sustainable bridge.”