The Other 80 Percent: Strategies for Delegating Design of Nonstructural Systems
October 24, 2019
Nonstructural systems, such as cladding, partitions, utility distribution systems, specialty components, and contents, can comprise 80% or more of the cost of a new building. Poor seismic performance of these systems can result in loss of valuable components, costly repairs, extended downtime, blocked egress, and casualties. Even in moderate earthquakes, damage to nonstructural systems can cause injury or loss of life and result in extended closure of buildings that otherwise exhibit little or no structural damage.
Despite their importance to building function, the design of nonstructural systems is often delegated to third-party engineers or Specialty Structural Engineers (SSE) who are not the primary Architect, Structural, Mechanical, Plumbing or Electrical Engineer of Record for the project. Commonly delegated systems include cladding, interior partitions, suspended ceilings, MEP equipment, MEP distribution systems, metal stairs, specialty architectural components, and fire protection systems. The decision to delegate the design of any particular nonstructural system depends on a number of factors including the experience of the design team, the complexity of the system, the project delivery method, and existing relationships between design-build teams and specialty engineers.
The delegated design process begins with identifying which designs to delegate. Ideally, this is determined at the outset of a project when the scope of work for each design professional is being established. Once a system has been identified as a “delegated design”, the design team must describe the requirements for the SSE in sufficient detail to allow for a complete and coordinated design solution. This is typically communicated with drawings and specifications. For cladding and stairs, this may take the form of elevations, sections, details and performance specifications; for suspended utility distribution systems, this may take the form of piping/conduit plans and performance specifications.
The Structural Engineer of Record (SEOR) can assist with ensuring that performance of the delegated designs is consistent with the overarching seismic performance objective of the building. This is particularly critical for buildings designed for damage control and post-earthquake functionality. For example, an economical cladding choice may require structural systems that exhibit seismic drift less than that allowed by the building code. The SEOR can also help translate overarching objectives into specific performance objectives for each nonstructural system by providing design criteria that the SSE can use as the basis for the delegated design.
Given the importance of delegated designs to the overall success of a project, it is essential to track delegated design submittals, assign responsibility for review of each delegated system, and ensure that there is an Engineer of Record responsible for the design of each delegated system. The SEOR should typically serve as one of the reviewers for any nonstructural system designed for seismic demands. As a minimum, the SEOR’s review should confirm that the design is in accordance with the specified criteria and is consistent with the design intent. The SEOR should also verify that the structure is capable of accommodating the loads imposed by the nonstructural system.
Achievement of project performance objectives requires not only appropriate design, but also proper installation. The California Building Code requires “the registered design professional in responsible charge” to prepare a statement of special inspections. However, the engineer responsible for the delegated design does not always provide system-specific inspection requirements. It may therefore fall to the design team to establish testing and inspection requirements to confirm that the nonstructural system is installed in general accordance with the approved plans and design intent.
Although design of nonstructural systems may be delegated, their design is critical to the seismic performance of a building and requires the attention of the entire design team from the outset of a project when design scopes are determined through construction. In addition to establishing appropriate design criteria, clear communication and proper management of delegated designs is needed to achieve nonstructural performance that meets the goals and expectations established for the project.
Maryann Phipps and Sarah Durphy, both Structural Engineers at Estructure, authored a paper on this subject and presented it at the 2019 SEAOC Convention in Lake Tahoe. You can read the full version of the paper here: The Other 80 Percent Strategies for Delegating Design of Nonstructural Systems. For questions or more ideas on best practices for delegated designs, contact Maryann Phipps or Sarah Durphy at (510) 235-3116.