The Black Diamond – Royal Danish Library | Structural Glass Cable Net System by PFEIFER Structures | Schmidt Hammer Lassen Architects

As viewed from the Copenhagen Harbor in Denmark, the stark contrasting façade of the Royal Danish Library, known as the Black Diamond, brings inspiration to observers and occupants. The central multi-story atrium separating the masses of the library is a crystal-like trapezoid structural glass façade. The boundary of primary structure frames this large opening allowing for highly tensioned steel cables that support strut extensions to the point-supported glass. This is the structural glass system of Denmark’s Black Diamond.

What is a Structural Glass System?

A structural glass system is a building system assembly built from glass, secondary structure, and components. What makes it “structural glass” is that there are no frames bounding the edges of the glass. The glass is doing structural work to support itself. This kind of system is fully engineered with finite element analyses. It is the least visually obstructive building system for an opening and can achieve large spans vertically and horizontally by a narrow or even transparent network of structures with minimalist connections to the exterior plane of glass.

What a Structural Glass System is NOT

A structural glass system is not an aluminum or steel framed edge-supported glass façade known by names such as curtain wall, unitized curtain wall, storefront, or window wall. It is not a 4-sided structurally silicone-glazed curtain wall, even though the naming convention is similar.

System Materials

1. Secondary Structure: The first built in place structure is the secondary structure. This can come in the form of AESS (Architecturally Exposed Structural Steel), stainless steel cable, galvanized steel cable, steel rod, laminated glass fin, or engineered glulam timber.
   If the secondary structure is rigid, like AESS, miscellaneous metal, glass fin, or wood, then the structure is placed with consideration for anchoring and movement tolerances. If the secondary structure is tension based, like steel cable or rod, then the structure is anchored, tensioned, and monitored to be within allowable tolerances and stresses.
   The nomenclature can differ for this structural phase of the system installation. It can be referred to as a secondary structure, supplemental structure, backup structure, etc. It is secondary because the primary structure is typically designed by the Engineer of Record and installed by the primary building trades for steel, concrete, or wood. The secondary structure for a structural glass system is engineered, installed, and designed to work with and anchor to the primary structure.
2. Components: These are the spider fittings, post fittings, clamps, cable nodes, and angles that connect the structure with the glass. With a rigid secondary structure, the components are bolted on with the appropriate bushings or offset hardware to ensure the final surfacing is planar. With a tensioned secondary structure, the node clamps attach at the intersection of pre-marked locations on the cables.
3. Surfacing: A typical assembly used in a structural glass system is laminated glass because of its strength and cohesive properties. Many variations of glass can be used: monolithic, insulated, laminated insulated, double laminated insulated, etc. The more glass lites within an assembly, the more opportunities there are to include surface treatments like low-E coatings, frits, etching, etc. Alternatively, a surface does not have to be glass. It can be aluminum plate, metal mesh, decking, cross-laminated timber (CLT), acrylic, or phenolic panels.
4. Sealants: Unlike an aluminum curtain wall that manages air and water performance within the mullion, structural glass systems are essentially a solid air and water barrier. Every joint between glass is sealed as well as around the perimeter of the opening. This maintains continuity of the air, water, and thermal barrier of a system. There are scenarios where the system needs to weep out the bottom condition.

System Attributes

1. Shelter: The art and science of constructing shelters has always strategized ways to minimize effects of harsh elements, while maximizing livability, and building with durable materials. A structural glass system is a solution that touches on many of these divergent needs.
2. Transparency: Imagine enjoying yourself in a mountain cabin and then not being able to see any of your natural surroundings. Imagine a retail space with no inward views to entice people to enter. After having the necessities of a shelter, the next great enhancement of public architecture is the capability of inward and outward views.
3. Biophilia: As builders, we create shelters and enclosures. However, our basic desire to enjoy the outdoors, or biophilia – love of living things, continues despite our need for protection from it. When entering a building, we are at a point of transition and can have a sense of arrival. With minimal architectural obstructions, we can have the sense of never having left the outside.
   When using a building, we are within the envelope’s confines. Staying inside too long can adversely affect our psyche, health, and well-being. Providing extended views from the inside to the outside establishes a desirable atmosphere. Structural glass systems are an engineered solution that minimizes architectural obstructions, extends views more than any other building system, and bridges the connection for love of the outdoors.
4. Architecture: “We shape our buildings; thereafter they shape us,”-Winston Churchill. When the owners, architects, and builders align, inspiration to shape the art and engineering of architecture can take root. Structural glass needs the building program to connect with the design process and then connect with the trades. Building a structural glass system is done with the intent to maximize the intersection of shelter, transparency, biophilia, art, and engineering, all with the intent to build something that has the potential to last for generations.
5. Context: Because the elements of a structural glass system can come in numerous variations, including tints, graphics, shapes, etc., architects can design a structural glass system that reflects the culture of their project’s particular region. Cultural regionalism, commercial identities, or even art for its own sake is all in the contextual palette of structural glass systems.
6. Initial Cost: The initial investment of a structural glass system will be higher per square foot than a conventional curtain wall for small and medium spans. However, openings with long spans can only be achieved by using steel profiles and steel cables due to the strength of the material. So, the size of the opening will dictate the structural system used, and structural glass may be one of the most cost-effective options.
7. Reducing Unit Cost: A larger area of a structural glass system will bring an economy of scale and reduce project overhead to a smaller percentage of the total. Also, designing within efficient modules of glass dimensions to minimize cullet and procuring material supply locally will contribute to the lowering of the unit cost of the system.
8. Returns on Investment: There are returns of appreciable leasing rates, lower vacancies, improved community context, increased civic pride, healthy living space, and contributions to the technology of the built environment, all compounding over time. When a building is sold, higher-quality construction will have a greater tendency to keep its value with a greater opportunity for capital gains.