In a FEA-driven design that owes more to aerospace engineering than carpentry, SigmaDek Ltd. recently developed a residential deck system that replaces lumber with an all-aluminum substructure consisting of die-cast and extruded components. The new deck system, the first of its kind, is the latest piece of evidence that die casting continues to push into new and unconventional applications.
SigmaDek’s patent-pending deck system makes use of extrusions for the ledger board, beams, joists, and railings. Thirty-one die-cast components, produced by Dynacast, serve as the connectors that hold the extrusions together, creating a unified structure. The system is “deck board neutral,” meaning that the contractor can use any common wood, plastic, or composite decking material.
While aluminum is not a traditional structural material for decks, it turned out to be a smart choice for this application because it allowed SigmaDek’s engineering team to create a structure that is maintenance free, easy to install, and consistent in its structural performance. As SigmaDek president Gary Acinapura points out, deck cost and safety are heavily dependent on the skill level of the contractor with site-built carpentry decks. Some contractors can build a safe, strong deck for a reasonable cost. Some cannot, as shown by the deck failures that periodically make the local news.
SigmaDek, by contrast, does not depend on the skill of the contractor. Thanks to the design freedom available from manufactured aluminum components, the deck system has a wide range of user-friendly, fool-proof assembly features. These include integrated bubble and laser levels to align the substructure. The aluminum components also incorporate numerous locking features that allow components to be joined only one way -- the correct way.
For all its advantages in this application, the all-aluminum substructure posed some difficult engineering challenges related to the strength and manufacturability of SigmaDek’s die-cast components.
Strength was a challenge that had to be addressed early in the design process -- though not because there was any doubt that aluminum components could carry the necessary loads. SigmaDek’s VP of engineering, Brian Boettger, explains that the SigmaDek system represents an entirely new use for aluminum components, so the International Building Code had no provisions for this type of structure. As a result, the deck had to be engineered with a safety factor that’s roughly 2.5 times greater than the building code would specify for a wooden deck structure, driving up design stresses substantially. To take one example, the connector between joist and stair stringer has to withstand localized stresses in excess of 15,000 psi, far above what it’s likely to see in use.
Stresses of that magnitude fall well within the capabilities of die-cast aluminum. Yet SigmaDek engineers wanted to make sure strength didn’t come at the expense of economical casting or feature integration. Thicker wall sections or ribs, for example, might make it easier to meet structural requirements, but they can also add cost to the casting process. So SigmaDek's engineers decided to favor thin-wall structures whenever feasible, adding box-like structures and ribs where needed. Many of SigmaDek's parts have localized wall thicknesses down to 1mm, about half what a less carefully designed part would have required to meet the load requirements.
Walls this thin are not necessarily a problem for Dynacast die casting processes, which can handle wall sections down to 0.5 mm. But the thin-wall parts did require extra optimization from both structural and design-for-manufacturability (DFM) standpoints. For the optimization, SigmaDek collaborated with a product design firm, Precicad Ltd., which performed extensive FEA work on individual components and the structure as a whole. For the DFM work, SigmaDek turned to Dynacast engineers, who suggested dozens of design modifications to improve the manufacturability of the deck system's thin-wall, complex parts. These changes ranged from the elimination of undercuts and the addition of draft angles to the refinement of gates, runners, and overflow design.