by Samuel J. Armijos, AIA
One of the beauties of fabric architecture is how few components are required to create a structure: the primary structural support system such as the mast and arch, the fittings and hardware used to tension the membrane, and the membrane itself.
Most fabric structures require a compression member (mast or arch) to form a complex shape. The member usually has welded cleats or struts that enable it to support the membrane, cable, or other structural components. Masts can terminate on the ground or, in the form of a “flying mast,” be supported by a series of cables. Flying masts allow a column-free space in a mast-supported structure.
A base plate forms the connection between the membrane structure and the ground, wall, building, or adjacent structural system. The base plate is usually welded to the bottom of a compression member or connected to the mast with a pin connection assembly.
Boss plates, which are steel plates or rings welded to the main plate, are used for localized strengthening of a bolted connection.
Membrane plates are custom-designed plates used to link a membrane to the structural support system. These plates accept membrane catenary cables and pin connection hardware. Time-consuming to design, they are the key to a successful tension membrane structure.
A bale ring is a compression ring used to support the membrane on a mast-supported structure. The ring is also used to reduce the stress of the membrane at the top and ease in the fabrication and installation of the membrane. Normally, the membrane is clamped to the ring and the entire structure is tensioned at the top by raising the ring. Bale rings vary in shape and size depending on the complexity of the design and the total load to be carried into the support. The ring can be left open for ventilation or covered by a metal, fabric or glass top.
Cables create the edge of tension membrane structures. When located along the perimeter, they are called catenaries; when along the underside of the membrane, they are called the radial or ridge cable; and when over the top of the membrane, valley cables. Catenary cables are the most visible and follow the perimeter stretching from mast to mast. They are installed inside a pocket in the membrane or supported along the edge with cable straps. They usually terminate with a threaded rod or forked clevis, a cast pin-connected fitting that is attached to the membrane plates.
Another method of treating the edge of the membrane is with a Keder or roped edge, a unique construction of heavy-duty coated polyester welded to a core of solid PVC attached to the boundaries of the membrane to provide a strong and flexible edge for extrusions and clamping.
Each perimeter mast requires either a large moment connection or a series of cable tiedowns to withstand the loads. Tiedown cables are generally attached to cleats on the top of a mast and connected to anchors installed in the ground with turnbuckles.
Fabric structure hardware consists mostly of parts made for the yacht, bridge-building, rigging, and mountain-climbing supply industries. Shackles, turnbuckles, and toggles are just a few of the hardware choices available to link the membrane and the primary structural support. Shackles are U-shaped pieces of hardware used in the rigging industry that require a bolted connection with movement in various directions. Turnbuckles and toggles are threaded components used to adjust the length of ropes, wire, rods, and membrane plates.
A fabric clamp is a steel or aluminum profile used to fix the end of a membrane to a curb or join to membranes to each other or to a structural system. Clamp plates of aluminum or steel are normally used to provide a watertight seal along a frame, beam, or adjacent structure. The clamp can be extruded or cast to have a distinct profile.
Hardware comes in a variety of finishes and styles.
I highly recommended you contact a hardware supplier or manufacturer and request a sample.
Till Next time.