Wallforms

Whilst the majority of atria have only roof glazing, a significant and growing number also have one or two sidewalls of glazing. In multiple-vertical or stacked atria, and in tropical latitudes, sidewall glazing is the only possible way of lighting. Views out of the atrium became possible and important to the design. Such walls may simply be continuations of the building frame and cladding or integrated into the planes used to accommodate wind or seismic bracing. Various forms are possible, including trussed walls supporting framed glass, planar glazed walls, lamella forms, and Vierendeel forms.

The glazed walls can simply be a continuation of the building envelope.

The Pan American Life Assurance building in New Orleans illustrates this form, often used by SOM. The two stacked atria are irregularly formed inside the envelope and this fact is disguised during daylight by the overriding frame and cladding treatment. The advantages are more than visual discipline; a high shade factor is achieved with little differential thermal movement in this hot climate.

The atrium wall can be located to coincide with the main structural bracing, providing a strong visual expression.

Diagonal bracing is an economic way to provide wind and seismic bracing but is usually unwelcome, running across inhabited window walls or, worse still, across interior space. Bracing in an atrium can be positively welcome as an expressive device, emphasizing the non-habitable nature of the volume. Examples include the Hennepin County Courthouse in Minneapolis by John Carl Warneke and the Queens West Centre, Cardiff, where diagonal lattice wall and roof frames are used.

Trussed walls can be used to support framed glass with the minimum of visual intrusion.

Tall glass walls have considerable wind loads to resist, and rolled section framing set against the glass can get very bulky and expensive to span several storeys. Trussed mullions are now available with several curtain wall systems, for spans of three or more storeys; taller walls use trussed mullions, transoms or space structures. The Dallas Hyatt by Welton Beckett has trussed mullions hanging from a bridge of bedrooms across the open side of the atrium.

The Air and Space Museum in Washington, by HOK, uses triangular space trusses linking mullion pairs.

Crystal Cathedral in Garden Grove, California, by Johnson Burgee uses a full space truss for walls and roof in this naturally climate-controlled glass building.

The sidelit atria at Arup Associates' Broadgate building have space trusses for transoms which provide cleaning access platforms and internal sunbreakers. Sunbreakers support is a logical role for trussed sidewalls in hot latitudes, as at the San Antonio Hyatt in Texas, by Ford Powell and Carson, where large angled shades sit on the sloping members.

Planar glazed walls can be suspended from spanning structures above.

The technology for suspended glass assemblies has made considerable advances and can provide frameless glass sidewalls up to five storeys tall. Planar glass can be carried on frames too and this is customary with walls over l5 m high. The Spectrum Building, Denver by McOg Architects utilizes this method, with steel tubes spanning a void and carrying the glass sheets and stiffening fins unobtrusively.

The Financial Times printing works designed by Nicholas Grimshaw uses external masts, projecting arms and tension cables to stiffen the glazed suspenders.

Lamella forms are possible for complex shapes.

The Roy Thompson Concert Hall in Toronto has an unusual form in which the foyer encompasses the circular auditorium block, the public spaces containing several levels of balconies and connecting stairs to the various levels of seating. The designer, Arthur Ericson, has wrapped the public space in a hyperboloid, that is a tapered curving skin similar to the lower part of a cooling tower. The structure is in the form of a lamella frame of steel tubes and the glass panes are 'flat' diamond shapes fixed by adhesive to each location strip. The circular form is used to facilitate cleaning, with rigs running between the head and foot beams.

Vierendeel wall frames are structurally efficient and can be used for complex forms.

Wall skins, as well as roofs, can be framed on Vierendeel principles, the most famous being Felix Samuely's proposed structure to span the shop window facade to Simpsons, Piccadilly (l936). Vierendeel frames are used particularly where the wall forms are highly irregular. They typically extend over a height of at least one storey, forming a deep, open girder capable of span long distances whilst avoiding excessive visual intrusion. An example is the structure devised for the end walls of the atrium at the Royal Bank of Canada, Toronto, by designers WZMH. All one can say is that the effect is totally overpowering and as such is a design curiosity.