Boundary Conditions

• Will you incorporate edge cables (catenary cables), ridge/valley cables, or radial cables?
• Will the membrane be clamped solely to a rigid steel perimeter?
• Will there be posts supported by guyed cables, or will they have moment connections at their base?
• If using a cone-shaped fabric, will the mast be anchored to the ground or suspended by cables (a “flying mast”)?
• If using a mast, will it be tilted or vertical?

Answering these questions will establish clear boundary conditions for your design.

Form Finding

If you primarily need to determine the fabric’s shape, calculate your defined boundary conditions and fabric mesh into suitable form-finding formulas to generate the desired form. It’s critical to avoid “flat” areas in the fabric, as these will struggle to handle loads effectively. Remember, the membrane supports loads by deflecting and transferring tension back to its perimeter. Consequently, the perimeter must be adequately robust to manage these tension loads, often necessitating a compression ring, frame, or tie-backs in areas without a continuous ring or frame. Additionally, ensure that the design avoids horizontal or near-horizontal sections where rain or snow could accumulate and cause ponding.

Static Analysis

The membrane’s statics dictate the overall detailed design, defining edge geometry and load transfer points. As membrane structures deform under loads like wind and snow, connections to the supporting structure, existing buildings, or foundations must accommodate this movement, a comprehensive Finite Element Method (FEM) model needs to be constructed, incorporating material and section properties. The static calculation, based on the form-finding data, involves extensive analysis. This includes accounting for various load cases, the three-dimensional nature of the roof, and the biaxial expansion behavior of the membrane material.
Pre-stress and Dead Load: The initial tension in the membrane and the weight of the structure itself
Live Load: Loads that can change over time, like snow, rain, or occupancy
Wind Load: Wind pressure from various directions, especially if the structure lacks symmetry. The minimal material weight (around 1-1.5 kg/m²) makes membrane structures sensitive to wind loads. While this is offset by the static equilibrium achieved through shaping and pre-stressing, it places unique demands on the supporting structure and foundations regarding load application
Thermal Load: Expansion or contraction due to temperature changes
Seismic Load: Forces resulting from earthquakes, if applicable
Stability Check: Ensuring the structure can withstand various loads without buckling or collapsing. subsoil bearing capacity rarely dictates foundation design. Depending on soil conditions, tensile loads can be managed using heavy-duty foundations or specialized ground anchors.

The potential for water accumulation or ponding on the membrane must also be evaluated. When choosing membrane strength, a substantial safety factor (typically 5) should be applied.

Material Adjustments

Secondary steel: May need to be shop-welded to primary steel to facilitate perimeter clamping.

Cable congestion: Areas with multiple converging cables might require adjustments to accommodate ear plates and cable terminations.

Membrane cut-outs: Corners of the membrane might need to be cut to accommodate structural elements; these cut-outs may necessitate reinforcement or specific edge treatments.

Field penetrations: Although ideally avoided, membrane penetrations might be necessary. These require detailed design with clamping, reinforcement, or other measures to ensure load-bearing capacity, manage deflections, and maintain water-tightness.

The detailed design phase bridges the gap between analysis and construction, ensuring that the envisioned structure can be translated into a practical and buildable reality.

Cutting Patterns 

Warp & Weft: The direction along the fabric roll’s length. This affects strength and usage.

Seam Layout: Determined by warp direction, roll width, aesthetics, fabric usage, and shape. If the calculations can’t flatten a design within an acceptable error margin, more seams may be needed.

Waste Reduction: Patterning should aim to minimize fabric waste, if aesthetically possible. Triangular and trapezoidal patterns near full width nest better than long triangles, hourglass shapes, or rectangular panels over half-width but less than full width.

Fabric Properties: Understand fabric properties before creating templates. Some projects use stretchy fabric with less concern for pre-stress distribution, while others need uniform pre-stress with stiff fabrics.

Biaxial tests determine compensation—the amount templates are cut smaller to achieve desired pre-stress and geometry when stretched during installation. Most patterning techniques allow for compensation input to generate accurate cut lines.

Safety, Security & Maintenance

Tensioned membrane structures experience significantly more deflection than traditional structures. Therefore, careful consideration is necessary when placing architectural or functional elements like lighting, curtain walls, HVAC, and lightning protection to prevent contact and potential puncture of the membrane.

Furthermore, tensioned membranes are susceptible to tearing.  It’s crucial to acknowledge the possibility of damage from foreign objects during the structure’s lifespan. Factors like Safety Factor and a clear Repair Procedure must be incorporated into the design. A fundamental rule is to never rely solely on the membrane to support steel elements. For example, a mast should have additional support from radial or guy cables to handle its load even if the membrane is compromised.

For maintenance, repairs, and other activities, it’s wise to include lifelines or tie-off points in the design. This ensures the safety of construction and service personnel accessing the membrane.

If the membrane surface is too large to manage as a single piece, sectionalizing with clamped field joints may be necessary.