Designing Fabric Structures and Sails

Key steps in tension fabric structure design:

  • Determining parameters and objectives of the structure
  • Shapes and Forms
  • Components of a structure
  • Engineering

 

Parameters and Objectives

The design development phase of a tension membrane structure project usually revolves around a collaborative effort often involving client, architect, engineer, head contractors and fabricator.


The first step is to determine the boundaries of the tensioned structure. Boundaries include walls, beams, trees, fences, columns and cables heights, sub strata services and composition.What does the structure need to achieve eg high visibility, weather protection, shade at certain times of the year, protction from a hot afternoon sun, privacy screening?


The structural connection points are either  walls, beams or columns. In 99% of situations shade sails edges form a curved edge or ”a catenary curve“ between connection points. Cable or webbing attached at the edges will  carry loads to the major structural points. Catenaries are usually curved inward from 7 to 12% of the total length of the span. Once the parameters have been determined, the next step is to develop a design. This usually involves a computer aided design.

Designing shade for the home


Designing for the home can be straightforward when the rules are followed. Basic drawings, measurements, parameters, fabric selection and costs can be  determined quite readily. However regardless of the size of the project there are stringent design, fabrication and installation guidelines. Short cuts and cost cutting can lead to problems.  It is often with smaller structures where the client is looking to the installer for their guidance and expertise that compliance, quality of work, training and skills are particularly important.  

Design factors to consider on domestic shade structures are site access, seasonal sun movement, council requirements, types of materials and location of underground services. A clear communication of the design to the client can then be offered.

While not all Abacus Shade Structure projects are complex, all projects are engineered, carry warranties, are performed by trained personnel who adhere to statutory requirements.

Ultimately however the client decides the preferred contractor based on accumulating the information and answers to compare contractors, product and prices and make an informed decision.

Computer Design and Cutting


At Abacus Shade Structures, shade structures are designed and shaped using sophisticated computer technology.

Computer design software creates 3D modelling viewable from different angles on a screen, facilitate fabrication and installation. Full engineering and wrk shop drawing can be prepared showing dimensions, sizes of structural members, the size and detail of fittings such as cabling required.

Computer patterning allows designers to determine stress loads, fabric quantities, optimal fabric usage and laser cutting.  The designer has flexibility to review and modify the shape.

The increasing complexity of tension membrane structures has meant that design software and CNT computer cutting/ plotting machines indispensable to the tensile structure designer.

Shapes and Form

An understanding of different shapes and form is essential to the design of tension fabric membrane structures. Steel frame configuration and the curvature of the fabric spanning between the frame determines the basic form.

The most common forms of tension membrane structures include:

  • Mast supported system ie Claws, Umbrellas
  • Point or frame supported system ie Abacus Span Shade system
  • Repeat patterned system 
  • Gull wing and Barrel vault designs
  • Cantilevered arch supported system
  • Simple saddle hypar system  ie fabric stressed between posts
  •  

Mast-supported systems, similar to tent structures are made of one or several peaks supported by central poles. A compression ring may connect the fabric to the central support of the mast and facilitates erection and stressing of the fabric. Circus tents, Umbrellas and Inverted Umbrellas fit into this category.
 

Point-supported systems produce a clear span with no central mast. An exterior frame or a series of peripheral masts supports the fabric peaks. This design system is used where an irregular space exists or a free form structure is sought.

The Parallel Edge or Repeat Pattern System offer clear span structures. The fabric is supported along the perimeter by parallel masts with the fabric alternating between high and low points.
 

Cantilevered arch-supported systems, gull wings and Barrel Vault structures. These designs  avoid interior supports in addition to introducing curved compression members as the main supporting element. For lateral stability in cantilevered arch-supported systems, cross arches or diagonal bracing may be used.
To avoid interior supports as well as massive footings, a Span Shade Frame supported system may be appropriate. Fabric is attached to a structural frame. The primary structural components carry the majority of forces within the system, so that the fabric is used purely as a cladding. The Abacus Span Shade structure and knuckle system has made frame supported systems modular and straightforward for installers.

Finally, the simple saddle system or hypar is a four or more point structure formed when the fabric is stretched tautly between a set of alternating high and low points. Much of the appeal of hypars is in their changing 3D appearance from any viewing angle.

The Components Fabric Tension Structure

Membrane Fabric


The membrane forms the enclosure or skin of the structure and can be fabricated a number of ways. It can be sewn, glued, electronically welded and/or heat-sealed. There are a number of seam styles, overlaps and reinforcements that differ from project to project. The most common form of fabrication used is high frequency overlap welding.

The different material properties i.e. strength, thickness, elasticity, weight, etc., make material selection critical. A need for the membrane to be highly abrasion-resistant, low maintenance and "vandal proof" also influences the choice of suitable materials.

Specialised Hardware


Tension Structure hardware consists mostly of parts made for yacht racing, bridge building and rigging. Shackles, turnbuckles, terminal end swages, wire rope grips, are just a few of the hardware choices available. Tensioned structures commonly consist of steel cabled hems with threaded end fittings. Hardware comes in 316 and 304 grade stainless steel, galvanized and custom finishes.

Tensioned Structure details have changed over the past 30 years from heavy bridge building components to state of the art yacht racing technology as slick, shiny stainless steel replaced heavy industrial looking materials. Although the “less is more” approach is the most desirable, the structural loads and requirements needed for a tensioned membrane structure dictate the size and shape of most details.

Aluminium Tracks


Tracks are normally used to provide a watertight seal along a gutter, beam or adjacent building. The material most often used is painted or unpainted aluminium. The track is extruded into a distinct profile and grading.

Posts and Framework


At the core of a tensioned membrane structure is the support frame. This is generally the heaviest and often, the most costly component of the structure. A significant amount of the steelwork may hidden in deep concrete footings. The amount of steel used is dependant upon loads imposed, the design and the membrane used and must factored into the engineering for the project.

Other Production Considerations


Various production processes eg welding, forging, casting, etc. have their own advantages and disadvantages that dictate the design of structural components in a membrane structure.
Transport and Site Access: The ability to transport components to the job site and then unload and position components into place must be considered.
Load Conditions: Testing for water run off needs to be considered. Membrane Structures need to be pitched. The collection of water on flat fabric areas, known as "ponding", must be avoided. A shade structure should have a minimum 1.25:1 fall ratio.

Engineering

Engineering documents include the design and detailing of structural components; the selection and specification of fabric material; a detail and schedule of cables and hardware; and cutting patterns for fabrication. Concept drawings are engineered, and structural calculations are based on these. Upon approval, shop drawings are produced for steelwork and footings and finally fabric patterning completed. Final fabric selection is derived from analysis of the loads and stresses placed on the structure. Fabrics vary in weight, tear strength and shade protection offered. The Engineer may monitor installation during construction and certify the structure and upon completion.

  • Abacus Shade Structures
  • Level 14, Lumley House,
  • 309 Kent Street,
  • Sydney NSW 2000
  • Australia: 02 9831 1218
  • Int: 61 2 9831 1218
  • [email protected]
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