For an office building located at 1900 K Street in the heart of Washington DC's Central Business District, code restrictions limited the design options, but the architects at Cesar Pelli & Associates made the most of what they had. By elaborating on the requirements with steel, glass and slate, they were able to create some interesting design aspects.

Despite the restrictions, the building was successfully designed to attract attention to itself. "We designed an elegant and unique skin for the building, which makes it stand out from its context," said Susana La Porta Drago of Cesar Pelli & Associates. "Several elements allowed us to achieve this: the glassy proportions of the wall, the sheen of the stainless steel and the green of the stone panels. The wall design has an underlying matrix where the horizontal stone bands are balanced by a rhythm of single and double stainless steel vertical bullnoses."

By further enhancing the required elements, the architects were able to create an ornamental, yet efficient, design. "Our massing options for this project were very limited by code restrictions," Drago said. "However, we thought that the required setbacks and the corner condition could be elaborated into some interesting design features. The corner became a very elegant curved piece; it was very important to us to define this corner with curved stone panels held by curved stainless steel mullions. The setbacks expressed the structure of the building by exposing its stainless steel clad columns."

Stone was selected for this project early on because of its durability and its ability to convey a sense of class and quality, according to Drago. "We were interested in green stones, and we looked at several slates," she said. "Kirkstone slate was very hard and had a dense consistency, which made it a good option for a curtainwall application."

Kirkstone Sea Green slate was specified for the project. Formerly known as Westmorland Green slate, this stone has a beautiful dark green sea color, according to the company. It is made from volcanic ash compressed over thousands of years in the English Lake District. Despite being called a slate, this stone is much harder than other slates and is almost comparable to granite for its durability, Kirkstone reports.

The 1900 K Street project incorporated a total of 5,500 square feet of Kirkstone Sea Green honed slate consisting of 5-foot x 2-foot x 11¿inch straight cladding and 5-foot x 2-foot x 3-inch curved cladding. The stone panels had 6,500 linear feet of kerfs at the top and bottom edges, and they were supported with metal angles.

"We designed a unitized curtainwall system, which means that the curtain-wall units are fabricated and assembled at the shop," Drago said. ""The curtainwall fabricator, Antamex, installed the stone panels at their shop in Toronto. This makes the quality control process significantly easier because most of the work can be inspected at the shop before being shipped to the site. The only work that took place at the site was the installation of the finished units."