Determine the basic shape of the structure you want to design i.e. Cone, Barrel Vault, Folded Plate, Hyperbolic Paraboloid (Saddle shape – “Hy-Par” for short), a combination of these, or something different. Remember, for a membrane which has an equal stress field, the shape will be that of a soap film for the given boundaries. If one can’t imagine a soap film taking that natural shape then the membrane also will not take that shape without some outside influence (i.e. supports or inflation pressure).
Determine the boundary conditions. Will there be edge cables (“catenary cables”), ridge or valley cables, radial cables etc.? Will there be only a ‘hard perimeter’ of clamping to steel beams? Will there be posts that are guyed with cables or will they be moment connected at the base? If using cone shaped fabric, will the mast be attached at the ground or will it be suspended by cables (i.e. “flying mast”)? Will it be canted?
Determine which software would be used for the design. There are commercially available software packages available for form finding of fabric based on the shapes and boundary conditions. If one is tasked with member sizing, a more complete Finite Element Method software package may be required.
Design
If only the shape of the fabric is desired, simply plug in the boundary conditions and fabric mesh into an appropriate form finding software package and generate the shape. Be sure to avoid “flat” fabric which would be a concern for resolving (carrying) loads. Remember, the membrane can only resolve loads by deflecting and carrying that load in tension back to the perimeter. The perimeter must be of sufficient size to support the tension loads – often this means having a “compression ring” or frame around the outside or some sort of tie-back when there is not a continuous ring/frame at the perimeter. Also be sure to avoid having areas of horizontal or nearly horizontal membrane in which the membrane would deflect under rain or snow loading and cause a pond to form.
If member sizing or complete Engineering Design is desired, a full FEM model will need to be developed, including material and section properties. Also, the loading needs to be determined. Load cases include but are not limited to: 1. Prestress and Dead Load, Live Load, Wind Load (multiple directions if the structure is not symmetric), Thermal and or Seismic if applicable, and Stability Check. The potential for ponding will also need to be checked during this process. When selecting membrane strength, a significant safety factor must be used (usually 5).
It may be beneficial to produce a physical model to gain a better understanding of the behavior and for verification of the form. The physical model may also give insight as to the constructability of the tensioned membrane structure. Constructability must be considered throughout the design process to ensure that the conceived design can actually be built by some reasonable means.
Detailed Design/Patterning
If the structure is planned to be built, the detailed design needs to be executed. Chances are, the FEM model that was used for analysis is based on workpoint and centerline geometry but as we know, the steel members, connections, clamping, and cable ends have finite sizes and need to be able to be safely assembled on site. This means that there may be secondary steel that gets shop welded to the primary steel in order to accept perimeter clamping. There may be areas where multiple cables come to one point and there isn’t enough room for the ear plates and cable terminations/pins to fit so perhaps an adjustment needs to be made to the analysis model. There may be areas where the membrane corners need to be cut out to fit these items – it needs to be determined if those cut-outs can have a free edge or if they need to be reinforced somehow. Penetrations within the field of the membrane should be avoided whenever possible but sometimes they cannot be avoided. Now is the time to detail those with clamping/reinforcement or whatever might be needed to accommodate the loads, deflections, and water-tightness at those points.
If the student will be patterning the membrane, a software package for that purpose is desired although patterning may be done by other rational methods as well (i.e. physical model). Some items need to be considered for the patterning process:
Warp Orientation – ‘warp’ is the direction along the length of the rolled goods. Orientation is to be considered for strength and/or usage requirements.
Seam Layout - is based on warp direction, width of rolled goods, aesthetics, fabric usage, and fabric shape. With regards to fabric shape, if the patterning software cannot flatten a template to within a quantity of error that is deemed acceptable to the designer then more seams may need to be added.
The amount of waste fabric should also be considered for during the patterning process. If aesthetically acceptable, the patterns should be produced to minimize waste. Triangular and trapezoidal patterns that are nearly full width produce much better nesting results than long triangles, hourglass shaped, or even rectangular panels which are greater than half but less than full width.
The fabric properties need to be well understood before template production. Some projects utilize very stretchy fabric with little or no concern of prestress distribution while other projects may require uniform prestress utilizing very stiff fabrics. In the latter case, biaxial tests should be performed in order to determine fabric compensation. Compensation is the amount in which the template is cut smaller than its intended size so that when installed, it must be stretched in order to achieve the desired prestress value and final geometry. When the biaxial test is run and compensations have been selected, most patterning software allows for input of a compensation value so that the software can generate the appropriate cut line.
Further Considerations
Tensioned Membrane Structures deflect quite a bit more than conventional structures. Because of this, care must be taken when positioning architectural or functional elements such as lighting, curtain walls, HVAC, lightning protection, etc. so that those elements do not come in contact with the membrane and puncture it.
Tensioned membrane is also subject to tearing. One should consider that a foreign object may puncture or tear the membrane in its lifetime. For this reason Safety Factor, Repair Procedure, and other factors must be considered. As a rule, membrane should never be used as the only means of support for steel elements. For example, a mast element should have radial or guy cables that can support its load in a deflected position in the event that the membrane is not there.
For maintenance, repairs, etc. it is a good idea to include provision for Lifelines or Tie-off locations during the design so that Construction or Service personnel can access the membrane safely.
Sectionalizing (clamped field joints) may be required if the membrane surface is too large to handle.
Reference Material
Software – Most large Tensioned Membrane Structures contractors have their own in-house software but more and more commercially available software is being developed. “MPanel” is the name of a form finding and patterning software that can be interfaced with AutoCad.
Design Guide – The “European Design Guide for Tensile Surface Structures” (2004) is a useful design guide published by TensiNet and available through TensiNet Publications
Standard – ASCE 55-10 “Tensile Membrane Structures” is the current design standard.
