Station Place: Security & Exchange Commission Headquarters
Washington, D.C.

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Owner

Second Street Holdings / Louis Dreyfus Property Group

Architect

Kevin Roche John Dinkeloo and Associates

GC

Tompkins Builders

Completion

2004

Program

Lobby wall: 90 feet high and 60 feet wide                   Skylight: 55 feet long and 60 feet wide

Building Type

Office

Technology Type

Blast-Resistant Facades, Cable Nets, Structural Glass Facades

Facade

Double curve anticlastic cable-net made of 20 mm cables clamped via stainless steel cast nodes; supported by the perimeter concrete structure and an intermediate steel delta truss

Glass

Face glass: 1 1⁄4 inch total thickness consisting of: 5 ft by 5 ft panel size, insulating glass consists of 1⁄4 inch heat-strengthened clear glass with low-emissivity coating on the second surface, shading coefficient of 0.43, winter U-value of 0.48 and summer U-value of 0.55, 1⁄2 inch air space and silver spacer bar, inner panel consists of 1⁄2 inch laminated glass consisting of 3⁄16 inch clear heat-strengthened glass, 1⁄16 inch polyvinyl butyl and 1⁄4 inch clear heat strengthened glass; provided by Viracon

Skylight: 15⁄16 inch total thickness consisting of: 1⁄4 inch clear tempered glass with low-e coating on the second surface, a shading coefficient of 0.33, a winter U-value of 0.29 and a summer U-value of 0.29; ceramic frit on the second surface, with Viracon Pattern 5005 and frit color V912-LF (white); 1⁄2 inch air space and silver spacer bar; 1⁄2 inch laminated glass consisting of 1⁄4 inch clear tempered glass, 1⁄16 inch PVB and 1⁄4 inch clear tempered glass on inner panels

Description

The first double-curved cable-net enclosure to incorporate both wall and skylight applications

Glass-clad cable-net structures are fast evolving in the United States as one of the dominant forms of highly transparent facade technologies. This new home to the SEC was one of the first examples of structural glass facades in the United States, and was the first double-curved cable-net enclosure to incorporate both wall and skylight applications.

The lobby area of Station Place is enclosed with a cable-net supported 60-by-90-foot glass wall and a 60-by-60-foot skylight. The combined surface area is approximately 9,000 square feet. The structure is composed from 28-millimeter stainless steel cables and clamp fittings (or nodes). A 60-foot-long double curved triangular truss spans the two concrete super columns at the top of the wall and provides support at the intersection of the wall and skylight. The truss also acts as a load transfer and stabilizing element for the adjacent building towers. The wall net comprises 15 rows of horizontal cables and 12 rows of vertical cables, while the skylight net comprises 12 longitudinal and 10 transverse cables. The vertical cables of the net wall align with the longitudinal cables of the skylight. The vertical and horizontal cables are clamped at their intersections with custom stainless steel node assemblies, which in turn receive the hardware by which the glass is fixed to the net. The slight radius the wall structure follows in plan provides the curvature in the horizontal direction. Opposing curvature in the vertical direction is provided by embedded cable connections within the concrete super columns. The opposing curvatures give the cable-net its saddle shaped surface and stability.

Enclos developed an innovative unitized glass-framing system that is bolted directly to a modified cable-net node assembly. The system avoids the premium cost associated with point-fixed glass systems and allows for competitive domestic glass supply. The anticlastic geometry resulted in a major mitigation in the deflection of the cable-nets, which however results in a warped surface that cannot be easily clad with planar glass. Double-curved glass is expensive and impractical for insulated glass, and cold bends have limitation on the glass size and the preload warpage. To minimize this distortion, Enclos optimized the cable-net geometry while maintaining enough curvature to control the skylight and wall deflections. The result is a hybrid geometry extracted from the surface of a torus. The remaining warp is then concealed in the interstitial space of the thin aluminum frame.

In order to achieve the proper shape in the double-curved nets, the clamps must be accurately positioned on the net, and the tensioning of the net must be accomplished with all cables, vertical and horizontal, simultaneously. This requires rigorous methodology frequently involving sophisticated hydraulic jacking gear. Enclos utilizes special survey techniques to map the position of each node. Compensating adjustments in the tensioning of the net can then be computed and implemented. The trick with successful cable-net structures is in the tension: determining appropriate theoretical cable pre-tensions with respect to boundary conditions to yield the most efficient shape of the net.

The installation sequence at Station Place was the following:

1 | Assembly of the net in the factory, including attaching cables and nodes in the horizontal position to allow compensation for final tensioning.

2 | Pretensioning of the net using perimeter with hydraulic jacks attached to a temporary space frame (stiffness similar to the actual structure).

3 | Adjust the nodes to their final position using accurate laser measurements and clamp with the required torque values.

4 | Wrap the cables and nodes in plastic covering, de-tension the net, and roll the net around a spool for transportation.

5 | Transport the spool to the site.

6 | Erect the supporting structure, including the delta truss and assembly of the perimeter jacking system at support locations.

7 | Drape the net and attach to the perimeter jacking system and delta truss.

8 | Tension the net to its final position, utilizing all of the jacks simultaneously. Check node locations.

9 | Install the glass.

In practice, cable-net structures are remarkably resilient and forgiving — they are designed to move. This structure type can deform many times the deflection criteria of conventional steel or aluminum structures without permanent deformation or failure. Deflections in the flat nets can equal 2 feet under wind load in a 100 foot span. Contrary to what many might suspect, this grants the walls the flexibility to withstand the extraordinary loadings resulting from seismic events or bomb blasts.

As with all emergent building technology, cable-nets number among the highest priced facades in the marketplace, due largely to development costs. However, the systems are relatively material-efficient and very simple, and market pricing has already begun to drop rapidly. In efficiently designed structures with the dissemination of assembly and installation know-how, cable-net technology is becoming more competitive in both price and application.

 

 

 

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