Seismic Structural Design for Modularized Industrial Structures

Modular construction for both facility and process skids has become popular due to the benefits of efficiency, speed, safety, quality, economy, and convenience. Modular design applications include process equipment, power delivery data center modules (PDC), pipe racks, and other equipment which can be assembled off-site. Modularization, while efficient, creates challenges for structural engineers, including, but not limited to: design of the support system for the modules, transportation of the modules, erection planning, and the variety of foundation systems and anchorage conditions to be addressed. In this paper, the systems of modularized structures will be discussed. As with buildings, we will consider moment frames, braced frames, shear wall systems, or non-building structures; depending upon the layout and nature of equipment. Tubes, channel, W shape, and angle are frequently used. The module size and weight may be dictated by transportation, lifting plan, or even the sizes of internal tanks when they exist. Not all modularized structures will be installed on a concrete mat foundation directly. For example, a PDC usually requires a crawl space for electrical conduits; a chemical or process module may require containment so as to collect water or chemicals due to accidental release. Therefore, concrete pedestals, tall columns, or more complicated elevated steel structures may be needed to support the modules. At the interface of the module and its foundation, the seismic shear forces may be significantly larger than that of conventional stick-built system because of the additional mass contained within the module due to its self-contained structure. For seismic resistance, the structural system should follow building codes and standards such as ASCE 7-16. For some configurations a combined system must be analyzed, i.e. the combined effects and behavior of the module and its supporting system must be analyzed together. This may pose a challenge as often the module is designed and completed by a vendor prior to the design of the supporting structure and/or foundation. For example, a cantilevered column support is not a foundation, but a supporting structure. As per ASCE 7 Section 15.3, a combined analysis may be required for non-building structures. The requirements ASCE 7 Section 15.3 are as follows: Type 1: the non-building structure is less than 25% of the combined structures. Type 2: the non-building structure is larger or equal to 25% of the combined structures, and where fundamental period, T, of non-building structure is less than 0.06 seconds. For both Type 1 and Type 2, the module (non-building) can be considered as a rigid element per Chapter 13 (Nonstructural Components). Type 3: the non-building structure is larger or equal to 25% of the combined structures, where the fundamental period, T of non-building structure is 0.06 seconds or greater. For Type 3, the non-building structure and supporting structure shall be analyzed combining the structural characteristics of both, and should be modeled together in a combined model. Some modules that have building-like lateral force resisting systems can be designed as building per Chapter 12 to avoid higher seismic loads required for non-buildings as per Chapter 15.