FABRIC IS ONLY AS GOOD AS THE STRUCTURE UNDER IT
When designing fabric structures it is critical to take into consideration the strength and design of the building as a whole. At Calhoun we apply our unique 3D Nonlinear Finite Element Analysis to engineer our fabric structures site-specific to suit our customers’ unique locations.
Reputable fabric structures companies will work with their network of sales teams or dealers to obtain site photos and site topography with the orientation and location of the marked building. This allows the fabric structure’s engineers to begin the process of evaluating the site conditions.
The site evaluation ensures that the appropriate parameters are used for the analysis of the structure. The configuration of the building is thereby validated, or changes are made to ensure the structure has the requisite reliability for the site.
A fabric structures company should use external, third-party engineers to review all structures to avoid company bias and ensure the strict objectivity of the design work. Watch our video to learn more about Calhoun’s unique site-specific review that no other fabric structure manufacturer in the market employs.
OUR PROPRIETARY ENGINEERING PROCESS
Through our proprietary engineering process we model the true behavior of fabric relative to environmental factors using our unique 3D Nonlinear Finite Element analysis. Our method produces the most suitable fabric structures to meet your requirements, resulting in a quicker return on your investment and the longest-standing structure in the industry.
During the course of each and every site-specific analysis, a complete 3D nonlinear analysis providing accurate load paths to, and forces at, foundation-resisting elements is completed. Due to the fact that the fabric structures rely primarily on tension only bracing for stability, the pre-tension in the cables and initial inelastic stretch is critical to the performance of the structure. The pre-tension requirements and procedure for initial pre-tension and proof loading are determined through the nonlinear analysis.
CALHOUN'S ENGINEERING METHODS ENSURE STRUCTURAL INTEGRITY
A comprehensive 3D nonlinear analysis is essential to provide guidance in the design of highly reliable and economic foundation systems. In situations where an even higher degree of economy is desired, the foundation wall supported on soil springs may be modeled explicitly in conjunction with the fabric structures such that the interaction between the two can be appropriately addressed wherein the resulting concrete stresses may be used to determine the concrete wall thickness and spacing for reinforcement. This frequently leads to as much as a 40% increase or more in foundation economy with a considerable improvement in overall reliability of the system given the fabric structures and foundation interactions including bearing pressures beneath the wall footings are appropriately addressed in a comprehensive analysis.
Overall, the primary objective of the 3D nonlinear analysis is to provide clear prescriptive recommendations for the structural integrity and limitations of the structure under various code specified load conditions.
DETAILED WIND AND SEISMIC ANALYSIS
In general, seismic load effects have been dismissed for consideration in design referencing the argument that the structure is light and flexible. The fact of the matter is that for structures more than 2-3X as long as they are wide in areas of low snow load and high seismicity, seismic design does control in the longitudinal direction. Moreover, in areas of modest seismicity and roof snow loads over 30 psf, the seismic load effects control in both longitudinal and transverse directions. Lastly, for structures supported on tall concrete walls, the weight of the wall itself nearly guarantees that seismic effects will control in one or both of the transverse and longitudinal directions even in areas of low seismicity. For each and every site specific analysis, a comprehensive seismic analysis is completed which includes the modal response behavior of the building and the building’s actual fundamental period. This analysis is also used in the wind design to ensure that the period of the structure will not substantially affect the behavior of the structure during a design wind event. In situations where the natural frequency of the building is a design consideration for along wind response, the provisions of ASCE 7-22.214.171.124 are used to calculate the resonant response coefficients and dependent gust-effect factors.
FABRIC STRESSES AND SCALED DISPLACEMENT
During a nonlinear analysis, the load is discretized and applied in steps. The fabric cladding is modeled using a combination of proprietary shell and cable elements. These elements have been adapted from rigorous nonlinear analysis models created in using high end aerospace finite element software. The calibration of the rapid analysis elements utilizes physical testing where possible and when necessary. Because the behavior of these structures rely substantially on tension in the warp and weft directions of the membrane the pre-tension in the cladding is also important to the long term performance, and must be determined using a comprehensive analytical approach and detailed hand calculations.
During the course of each and every site-specific analysis, a buckling analysis is completed at minimum for each of the controlling load combinations. This analysis goes to ensure that the assumptions for the effective length estimates used to identify in the mass checks the critical members for detailed checks are suitable for the actual site-specific conditions and whether or not second order effects are a consideration in the design.
An elastic buckling analysis determines the multiplier by which the current load effects must be multiplied to initiate buckling. This type of analysis is especially important for large displacement structures with tension only elements such as these because intermediate values of loads may generate instabilities (e.g. slack cables or fabric) that would otherwise go unnoticed in a static analysis and could lead to failures well below the design load if not properly addressed.
Consideration for this type of failure is mandatory under the ASCE 55-10-126.96.36.199. The systemic 3D buckling analysis ensures that these intermediate instabilities are addressed in the design.