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By Sara Hart

The options

With the immediate crisis resolved, it was time to pursue a design strategy for the total recladding of the tower and quadrants. Due diligence required that the architects study every material option. Material durability, service life, serviceability, aesthetics, exposure, cost, and relative constructability were the important factors for consideration. They researched granite, limestone, lightweight natural stone panels, architectural precast concrete/cast stone, glass-fiber-reinforced concrete (GFRC), precast-polymer concrete panels, exterior insulation and finish systems (EIFS), aluminum composite panels, and solid aluminum panels.

 

Several features distinguish the new cladding (above) from the old. The nonstructural columns in the original marble-clad envelope (right bottom) were removed in order to open up the corners in the new scheme (right top).

Each material had advantages and disadvantages, and with the right detailing, any of them could have performed well. However, the architects were focused on only one application—the envelope of Richmond City Hall. Therefore, they limited their judgments to the unique conditions presented in this particular case. As narrowly focused as they were, each material was studied carefully and its attributes and shortcomings were documented in the study. For instance, granite has a higher minimum compressive and flexural strength, which resists the effects of hysteresis, making it an excellent choice with regard to durability and long-term performance. Limestone has these qualities, as well, but requires a greater thickness to meet acceptable safety factors. Lightweight stone panels are relatively new to the architectural industry and were created as an alternative to high-cost natural stone. The typical panel is a thin layer of natural stone and epoxied aluminum honeycomb, which provides structural stability.

Precast concrete/cast stone is durable and less expensive than natural stone and can be formed to many different sizes and shapes and colored, or else finished in a wide range of textures. GFRC is similar to precast. The fiberglass reinforcing supplements or replaces steel reinforcing, thereby reducing the dead load by up to 75 percent. Precast-polymer concrete panels use polymer in addition to or instead of Portland cement. The type of polymer used is an important consideration, especially when freeze-thaw conditions are a determining factor. EIFS are a multicomponent exterior wall systems, consisting of insulation board with a base coat reinforced with glass-fiber mesh, an adhesive or mechanical attachment devise, and a finish coat to protect the entire system. Aluminum composite panels are similar to lightweight natural stone panels. Solid aluminum panels are also lightweight and durable and can be detailed for simple installation.