Back to Basics
The Rory Meyers Children’s Garden in the Dallas Arboretum is designed to creatively engage children in activities that connect them to nature and spark their interest in natural sciences. The Texas Skywalk within the Children’s Garden presented the design team with the chance to revisit some of the basics and advantages of designing with steel. Fashioned in the shape of a tendril, the 200-foot Texas skywalk is an exterior treetop walkway that traverses a sloped site and terminates at an elevator tower with sweeping views of White Rock Lake. The design goal was to emulate forms and rhythms found in nature, and to allow the exposed steel to educate the garden’s users about its material and structural properties.
Form, Materials, Specifications
The skywalk is composed of two types of steel each particularly suited for its use: Tnemec painted steel for the structure and stainless steel for the guardrail posts and infill.
Steel was selected for the skywalk structural system to maximize span, reduce structural weight and foundation costs, and minimize structural depth of spanning members. Tnemec system was selected over Kynar and galvanizing because of ease of long-term maintenance and ability of the contractor to paint erected steel on-site after a large number of field welding operations.
Twelve-inch steel pipe spanning 30-foot bays are supported by Y-shaped columns of built-up steel plate. Each column has a branch-like triangular column capital that illustrates the transfer of forces from the horizontal members to the vertical columns while alluding to tree forms. Beyond its spanning capability, the 12-inch-diameter main horizontal members of the skywalk eliminate flat surfaces where water and condensation accumulate, and their rolling flexibility accommodates varying radii of the skywalk plan to yield a form that is aesthetically harmonious in the natural setting. “T” sections welded to the top of the pipes were selected as the continuous members to support the Ipe skywalk deck to simplify welding and to avoid flat surfaces for accumulation of dirt and moisture. Painted steel outriggers welded to the steel pipes support the stainless steel toeboard above and are shaped to minimize the apparent depth of structure.
While stainless steel would have been hard to procure and excessively costly for the primary structural members, it is ideal for smaller tactile members such as railing posts and infill, where maintenance of a coating system would have been a challenge. The mesh and the guardrail posts were specified as 316 grade stainless steel due to its greater resistance to corrosion in the exterior setting than 304 grade stainless.
The transition between the heavier mild carbon steel primary structure and the stainless rail structure is limited to welded connection between the stainless steel toeboard and the outrigger using appropriately matched welding electrodes for the two materials. Stainless steel mesh strung on cables for the railing infill was chosen due to its lightness, transparency and woven structure reminiscent of natural forms. Initial designs considered individual repetitive rail panels with mesh infill stretched between railposts. However, to simplify installation, minimize material, and increase a sense of transparency and lightness, the final design has single rail posts with mesh shrink-wrapped around continuous cable threaded through holes in the rail posts. The cable-tensioned sections span up to 100 feet using turnbuckles.
Stainless was selected for the railposts over painted steel for its aesthetic character, protection from cable and mesh abrasion, and capacity to allow bolted connections that don’t require touch up. The posts were laser cut from prefinished 1/2-inch-thick stainless steel sheet, with the cable holes integral to the rail shape. Paired rail posts are used at points approximately 30 feet on-center-to conceal electrical conduit laced from transformers housed at the underside of the skywalk, to feed the continuous LED lighting housed within the Ipe handrails.
Mind the Curves!
Radial dimensioning guided the layout of numerous curved forms throughout the garden, and for actual layout over the vast site, it was critical to carry the radial dimensioning system up to three decimal points. The curved skywalk plan influenced the weight of the steel pipe beams-the sidewall thickness needed to be increased slightly from the required structural thickness to optimize factory rolling.
Finally, never underestimate the importance of mock-ups-especially for non-orthogonal construction. When a section of the railing was mocked up, cable tensioning between the cable holds pulled the mesh infill panel outward from the toekick, creating a gap greater than the code maximum 4-inch opening. After studying the complex effects of convex and concave curves on the tensioned cables, the construction manager, fabricators and design team worked together to tweak and reposition the cable hold position on the standardized rail posts, simplify the cable hardware and cable trajectory so that the tensioned mesh would remain within 2 inches of the toeboard (the size of a toddler shoe), thus providing extra safety to young visitors.
Steel fabrication was the only system able to meet all the requirements of this complex and varied project while respecting this unique site and conforming with the available budget.
Texas Skywalk, Rory Meyers Children’s Garden, Dallas
Owner: Dallas Arboretum
Architect: Dattner Architects, New York City
Structural engineer: Datum Engineers, Dallas
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Beth Greenberg, AIA, is a principal at Dattner Architects, New York City.For more information, visit www.dattner.com.