God is in the details when it comes to tension structure design
Ludwig Mies van der Rohe got it right when he said “God is in the details.” Another well-worn phrase has it that we must sweat the details and the big things will take care of themselves, so with this article we look at tension structure connection details that have proven reliable over time.
The connections are where all the forces that act on and within a tension structure come together, and where the success of the design is made or lost. Problems that occur in a tension structure are most frequently found at the interface between different systems. Tension fabric structures can be described as a convergence of three such systems: a fabric membrane, the supporting structure that anchors and supports the membrane and the tensioned cables that tie the membrane system to the supporting structure. Within each of these three systems is a number of important, crucial connections.
The key to good design practice with tensile structures is to establish appropriate criteria for the connection assemblies. Keep in mind the following interrelated criteria throughout this paper:
- Performance: function, safety and structural behavior
A strong performance focus is critical to good connection design. Special functional requirements must be carefully articulated:
- Will the connection be subject to extensive movement, out-of-plane movement, vibration, or repeated assembly and disassembly as a temporary structure might be?
- Will the connection be exposed to an extreme environment: extreme hot or cold, a marine environment, high humidity or high industrial pollution levels?
- What amount of rotational freedom is required between joined parts?
Safety issues are largely a function of satisfying code requirements and using appropriate engineering methodologies in a structure’s design. Design load requirements and safety factors must be carefully assessed with respect to the structure’s application.
Sophisticated fabric tension structures are being used with increasing frequency as permanent architectural structures, often in public buildings. Higher magnitude loads often require a complete reworking of familiar connection details that are used in temporary installations. Engineering analysis by a competent firm experienced in tension fabric structure engineering practices provides the loads and stresses that the structure’s connections must be designed to accommodate. Comprehensive engineering analysis is a prerequisite for good connection design. Maintenance is an oft-neglected performance criterion that potentially affects safety, cost and aesthetics. Requirements for maintenance are determined as a direct function of three key issues of connection design: processes, materials and finishes. These will be discussed later in the paper.
Constructability (the means and methods of fabrication, assembly and erection) is certainly an important design aspect that can affect cost and quality. We’ve all heard about the wonderful designs that cannot be built. Moreover, a primary objective of good connection design is predictable quality, schedule and cost regarding the fabrication and installation of the connecting components and assemblies. Designs developed without rigorous evaluation of their fabrication and installation requirements will likely result in unanticipated fabrication and assembly problems that can easily affect cost. Good design practice embodies in its product the spectrum of considerations spanning the building process, from concept design through fabrication, assembly, erection and life cycle maintenance.
Cost is usually regarded as an important consideration, and is often the predominant design driver. Connection assemblies for fabric structures typically represent a significant cost center, particularly in permanent architectural applications, which tend to be hardware-intensive (as opposed to software, i.e., the fabric membrane.) A strong focus on connection design will both reduce and control costs, contributing to maintaining the project’s budget.
Aesthetics becomes a paramount design concern in many architectural applications of fabric structures. Connection detailing represents a prime opportunity for designers to add visual interest and excitement to a fabric design, but aesthetic considerations must start with a rigorous analysis of performance requirements and end with craftsmanship. Much can be done simply with good craftsmanship, but craftsmanship alone will not save a flawed design.
Understand the way in which the forces are moving through the connected components. Tensioned fabric structures are stable due to their doubly curved forms generated by tensile force equilibrium. Therefore, elements and connections must encourage and follow direct load paths. The displacements of tensioned structures produced by external loads are relatively large compared to those of more conventional construction systems. This quality must be kept in mind throughout all stages of tension structure design. Connections should allow for displacement and rotation. Details should be simple, flexible and in scale with the overall structure and material used.
Overall, be certain that you truly understand what your connection design will look like when built. Computer renderings are helpful, but full-scale mock-ups are by far the best format for determining issues of form and proportion, as well as fit and function. Spend as much time as you can afford looking at other fabric structures, drawings and photographs of built connection details, keeping in mind what works visually and what does not. Do this, and you should soon recognize tension structure connections that work well both functionally and visually.