The primary software needed for this new type of concrete formwork is computer-aided design (CAD), used in connection with computer-aided manufacturing (CAM). Designers and architects use 3D CAD to produce design files that are converted to CAM files used to guide CNC routers (and other tools) in order to cut formwork pieces with extreme accuracy—far exceeding what’s achievable by hand.
In addition to traditional CAD software, designers use Rhinoceros (Rhino) developed by Robert McNeel & Associates for 3D modeling, in part because it interfaces with other software with such exotic names as Grasshopper, Flamingo, Penguin, Bongo, and Brazil. These combinations enable both enhanced design and precision layout and cutting of form elements. Projects using this technology range from the straight-forward to very complex, requiring the joint efforts of highly trained teams. This is best illustrated by three projects.
The Court Avenue Storm Water Pump Station
Steve Chmelar, vice president of commercial sales for Carroll Distributing and Construction Supply, Ottumwa, Iowa, says this project for the city of Des Moines, Iowa, included a battered exterior wall (15 degree tilt-back from plumb) with a curving horizontal face in a 1068-foot radius. The concrete wall would be faced with stone afterward so a rough finish was acceptable. The solution was to block out the shapes using expanded polystyrene foam blocks. Dale Mullikin, an employee of ACH Foam Technologies, Washington, Iowa, created the foam shapes on hot-wire cutting tables. His tables accept CAM files and cut either 3D or 2D objects, while software guided the hot-wire cutter to produce the needed shapes. The forming contractor set standard form panels plumb then glued the foam inserts against them prior to placing the concrete.
The Pleasant Hill Radius Wall
When complete, the City of Pleasant Hill Welcome Retaining Wall in Pleasant Hill, Iowa, will retain earth and support the stainless steel signage welcoming visitors to the city. The 105-foot-long wall features multiple curves, so the critical forming element was the radius shaped wales. Gates Concrete Forming, Denver, used a CNC router to cut all the 3/4-inch-thick plywood pieces; 8- and 4-foot sections in order to stagger joints. The pieces were shipped to the jobsite where the segments were glued and screwed together to make up 1 1/2-inch-thick wales. Groups of wales were spaced vertically on the forms in sets of two, located close together with a space for wall ties between. The contractor used one assembled wale to lay out the footings for the wall.
The Boston Harbor Park Pavilion
This pavilion is the first structure built on the Rose Kennedy Greenway, the park located over The Big Dig expressway and operated by the National Park Service and the Boston Island Harbor Alliance. Boston-based architecture and planning consultantUtile’s project manager, Chris Genter, says the two buildings were conceived as an outdoor pavilion both to better connect the building to the surrounding park and to reduce operating and maintenance costs.
Outdoor exhibit and visitor spaces are covered by two concrete and steel canopies to provide shade and cover. “The curved shape of the roof is primarily functional,” says Genter. “Water is collected by the curved shapes and pours off the end of the roof onto a splash block creating a rain event. We also wanted to create a sculptural form that engages park users, identifying it as the hull of a ship, or a bird in flight, or a whale. The shape is allusive enough to be all of these things.”
Utile’s group worked out the curved shape and wanted to construct it out of a low-maintenance material with a thin profile at the edge of the roof; concrete was the clear choice. They used Rhino to design the structure and then retained a consultant who specialized in complex geometry—Joe Lamere, a lecturer at the Massachusetts Institute of Technology (MIT). He used parametric modeling to help Utile refine the design shape in order to conform to the bending limits of plywood so forms could be fabricated. For bidding, Genter says, they sent 3D files to subcontractors since two-dimensional plans wouldn’t adequately describe the shape.
Designing complex geometry. Simpson Gumerptz & Heger (SGH), Waltham, Mass., designed the structural steel and reinforced concrete for the project. Matthew Johnson, principal, say one of the many challenges was the unique geometry of the project, requiring consideration of specialized formwork. In addition to structural analysis, SGH researched various formwork systems, settling on off-site fabricated timber ribs.
The last 8 feet from the perimeters of each canopy is cantilevered. This and the architect’s requirement for the concrete to be as thin as possible added to the complexity. Johnson says they specified 3/8-inch-diameter rebar (#3) spaced between 4 and 6 inches on-center in both directions to carry the loads and still have adequate concrete cover on top and bottom.
SGH specified the concrete mix and worked with S&F Concrete during the mock-up phase to get the mix just right.
Designing the formwork. Utile, SGH, and Turner Special Projects, Boston, agreed that CW Keller and Associates, Plaistow, N.H., should be the company to translate the model into form pieces and fabricate the timber formwork. The cutting and bending of the curved steel beams was performed by another set of contractors. Although both the steel and wood were fabricated from the same 3D model and should ideally have mated together perfectly, Genter says some minor field adjustments to the formwork insured a perfect fit.
Shawn Keller, president of CW Keller, says his company started 37 years ago doing custom wood millwork. Seven years ago, they started working with architects in New York and Boston doing 3D modeling. Harbor Park provided them the opportunity to fabricate complex geometric concrete forms and also taught them more about the loads imposed by concrete on formwork.
David Anderson, CW Keller’s senior engineer, led the effort to design all 700 rib form pieces and the 400 sheets of 1/4-inch-thick plywood “skins.” One roof spanned 40x60 feet and the other 40x50 feet. He says the architect wanted curving panel form lines (referred to as iso lines) to show on the ceiling, as well as patterned screw holes joining the skin to the ribs. Anderson used Rhino and Grasshopper to design each form element and maximize yield per sheet of plywood.
Anderson says the ribs were spaced 11 inches on center, each composed of two sections of plywood glued and screwed together. Each rib had flat spots cut into the bottom of the form for shores to rest against. The CNC router cut out each piece, numbered for a location and overlapped each other by 2 feet. No two rib forms were alike.
The skin of the form was constructed with two layers of 1/4-inch plywood, with the ends of the first layer overlapping the second layer. “There wasn’t a single square sheet of plywood and each sheet was cut by a CNC router for one location,” says Anderson. “We also decided to predrill the screw holes for fastening the plywood to the ribs. It ensured screws would be located in the right position in the ribs and provide the right patterned look.” Anderson adds that the software calculates the total cubic inches of material used in the form pieces, therefore indicating the total weight of the form.
Performing the concrete work. Ben DeRuzzo, project manager for S&F Concrete Contractors, Hudson, Mass., says this project was by far the most complex slab his company ever worked on. CW Keller provided the plans and wood formwork pieces and S&F assembled everything onsite. Their first step was to build a form table, carefully adjusted for levelness and flatness. Then they installed shores on the table surface, positioned to rest against the flat spots on the bottom sides of the ribs. The deck forms had to match the profile of the steel beams, fitting tight against the top of the beams, allowing the shear rail and Nelson studs to protrude. DeRuzzo says very little form adjustment was needed—accomplished mostly with sanding machines.
To refine the concrete mix and work out the placement process, S&F cast four mock-up panels before casting the decks. Concrete at the center was 5 inches thick tapering to 3 inches at the perimeter.
The stiff 5000-psi concrete mix was placed at a 0.40 water-cement ratio. Workers took time to eliminate bugholes against the form surface using internal vibrators, then struck off the concrete by hand, and floated and troweled as they went. The surface finish wasn’t critical because a waterproof membrane covered the concrete.
DeRuzzo says the concrete work turned out well and everyone was pleased with the results. Very little patching and repair was needed on the ceiling. The Boston Harbor Island Pavilion was completed in December 2010 and opened to the public in June 2011. So far there are only a very small number of hairline cracks in the concrete.
The technology of numerically controlled formwork is in its infancy. You can expect to see greater use as designers recognize the possiblities and construction shapes become increasingly complex. Contractors will need to learn to deal with this kind of formwork, and forming personnel will have to be more focused on the details.