Case Study: Bank of America Tower, Charlotte, N.C. (1991)

 

Background

Completed in 1991, the Bank of America Corporate Center in Charlotte, North Carolina, stands as a landmark of modern skyscraper construction. At 871 feet tall with 60 stories, it is the tallest building in North Carolina and an enduring symbol of Charlotte’s rise as a financial hub. Like many supertall buildings of its era, its design faced critical engineering challenges: how to manage immense structural loads while optimizing cost, material efficiency, and usable space.

One of the most innovative solutions adopted was the use of lightweight aggregate concrete (LWAC). At the time, LWAC was gaining traction in high-rise construction as a way to reduce structural dead loads without compromising strength. For the Bank of America Tower, LWAC enabled engineers to design a slimmer, more efficient structure that minimized foundation size and reinforcement steel requirements, while also allowing larger spans and flexible floor plans.

Role of Lightweight Concrete

The tower utilized high-strength lightweight concrete in its upper floors and structural elements. With densities of around 1750–1900 kg/m³ compared to normal weight concrete’s 2400 kg/m³, LWAC reduced the total dead load by thousands of tons. This reduction translated directly into:

• Smaller foundations, saving on excavation and material costs.

• Reduced reinforcement steel, since lighter structures generate lower internal forces.

• Increased rentable floor area, due to fewer columns and thinner slabs.

• Improved seismic performance, as lighter structures experience lower inertial forces during earthquakes.

The use of LWAC also helped balance the tower’s aesthetic ambitions—with its iconic crown design—against practical structural constraints.

Calculations and Structural Impacts

1. Floor Volume

• Floor plate area: 1,700 m²

• Slab thickness: 0.20 m

• Floors: 60

• Concrete volume: $1,700 \times 0.20 \times 60 = 20,400 \ \text{m}^3$

2. Mass Comparison

• NWC mass: $20,400 \times 2,400 = 48,960 \ \text{t}$

• LWAC mass: $20,400 \times 1,890 = 38,556 \ \text{t}$

• Reduction: $48,960 - 38,556 = 10,404 \ \text{t}$ (~21% lighter)

3. Seismic Effect (assume $C_s = 0.08$)

• Base shear reduction: $0.08 \times 10,404 \times 9.81 \approx 8,200 \ \text{kN}$

4. Reinforcement Savings

• Baseline rebar (slabs + PT beams): ~1,800 t (illustrative)

• LWAC steel reduction ~10.5% → ~189 t avoided

• Cost savings (at SGD 2,250/t): ~SGD 425,000

5. Foundation Savings

• Mass reduction equivalent to ~4 deep foundation shafts (~25 MN each)

• Savings realized through fewer shafts, shorter sockets, or thinner mats

How LiGrA Could Enhance the Case

 

 

While the Bank of America Tower was a pioneering LWAC project in 1991, advancements in sustainable materials such as LiGrA (Lightweight Green Aggregates) could further improve such developments today.

Mass & Seismic Benefits with LiGrA:

• LiGrA density: ~1,600 kg/m³

• LiGrA mass (floor system): $20,400 \times 1,600 = 32,640 \ \text{t}$

• Reduction vs NWC: $48,960 - 32,640 = 16,320 \ \text{t}$ (~33% lighter)

• Extra reduction vs LWAC: 5,916 t

• Base shear drop (Cs=0.08): ~12,808 kN vs NWC (~4,643 kN more than LWAC)

• Steel savings: up to 10.5% (~189 t) vs NWC; ~68 t more than LWAC

Foundation Advantage with LiGrA:

• Equivalent to ~6.4 shafts avoided vs NWC; ~2.3 more shafts saved than LWAC

Environmental & ESG Benefits:

• Sustainability – LiGrA is made from recycled waste such as bauxite residue, coal ash, and glass, reducing landfill use and virgin resource extraction.

• Carbon Reduction – Lighter structures reduce both embodied and operational carbon, aligning with today’s net-zero targets.

• Durability – Improved resistance to permeability, freeze-thaw cycles, and carbonation extends service life and reduces maintenance.

• ESG Compliance – By turning waste into resource, LiGrA supports circular economy principles.

If LiGrA had been available in 1991, the Bank of America Tower could have achieved not just structural efficiency but also greater sustainability credentials, anticipating the modern demand for green skyscrapers.

Outcomes and Benefits

The use of lightweight concrete in the Bank of America Tower resulted in:

• Structural efficiency – significant reductions in foundation size and reinforcement steel.

• Architectural flexibility – larger open spans and adaptable floor layouts.

• Economic impact – lower overall construction costs compared to NWC alternatives.

• Pioneering influence – demonstrated the viability of LWAC in supertall buildings, paving the way for its adoption in other landmark projects worldwide.

With LiGrA overlay (today):

• ~16,320 t lighter than NWC (extra 5,916 t beyond LWAC).

• ~12,800 kN reduction in base shear vs NWC (extra 4,600 kN beyond LWAC).

• ~189 t of steel avoided (≈ SGD 425k saved).

• Equivalent to ~6 foundation shafts avoided.

• Plus circular economy and carbon neutrality advantages.

 

BoA Tower – NWC vs 1991 LWAC vs Modern LiGrA (Summary Table)

Material	Density (kg/m³)	Floor System Mass (t)	Weight Saved vs NWC (t)	Base Shear Reduction vs NWC (kN)	Steel Saved vs NWC (t)	Foundation Shafts Avoided (≈25 MN each)
Normal Weight Concrete (NWC)	2400	48960.0	0.0	0.0	0	0.0
1991 LWAC (as built)	1890	38556.0	10404.0	8165.0	121	4.1
Modern LiGrA (Class 1.6)	1600	32640.0	16320.0	12808.0	189	6.4

Conclusion

The Bank of America Tower in Charlotte stands as a testament to the engineering foresight of its era, showcasing how lightweight concrete enabled taller, more efficient buildings. With today’s sustainability imperatives, the integration of LiGrA would make such skyscrapers even more impactful—lighter, greener, and more durable.

This case demonstrates the evolution of high-rise construction materials: from strength and efficiency in the 1990s to sustainability and circular economy solutions today. LiGrA represents the natural continuation of this journey, ensuring that future skyscrapers are not only icons of height but also of environmental stewardship.

Key takeaway: “1991 LWAC shed ~10,000 t off the tower. LiGrA today could shed ~16,000 t, cut extra seismic force, reduce more steel and foundation demand, and recycle waste into a truly green skyscraper.”