Delegated Design vs. Design Assist for Mass Timber Structures

Mass timber projects often benefit from engineering input beyond the Structural Engineer of Record (SEOR). This input can come in the form of delegated design once much of the design is complete, or design assist, where guidance is provided throughout the project design phase. This expert tip highlights the differences between these approaches and key considerations. 

Delegated Design 

Delegated design involves transferring responsibility for an element or portions of the design from the SEOR to a contracted specialty engineer. The specialty engineer is often brought in at the beginning of the construction phase, but the timing largely depends on the scope of work being delegated. Delegated design is formalized through contracts, ensuring clear accountability and liability.

With mass timber projects, scope items are often broken down into the following categories: 

• Connection design – Connectors are one of the most common delegated design elements in mass timber structures. This is not due to lack of engineering knowledge, but rather to variability in the mass timber manufacturing process. The scope of connection design is often taken on by a timber manufacturer who has in-house design capabilities or relationships with specialty engineering groups. This allows the manufacturer to provide connections that work with their specific manufacturing process and timber elements and provide the most economical solutions. In addition to providing the reactions, it is important to note any fire-resistance rating requirements, as well as the desired connector visual appearance and configuration.  
• Member design – Engineers may opt to delegate the design of framing members such as glulam beams, columns, and mass timber panels. This type of design delegation is less common and requires some level of preliminary design to set expectations in terms of general beam depths and floor plate thickness. This approach can ultimately yield efficient results but could be challenging for publicly-bid projects where the specialty engineer has not been brought in early enough to generate the information needed for accurate contractor bids.   
• Lateral design – Similar to member design, design of the lateral force-resisting system can also be delegated. This is more common when the design team has delegated the entire timber design.  

All three scope items—connection, member, and lateral—can be combined and delegated as a full timber design package.  

With all aspects of delegated design, it is important to remember that the SEOR remains responsible for both code compliance and overall structural integrity. As more design responsibilities are delegated, especially in mass timber projects, the complexity of coordinating with other building systems can increase. Delegating substantial portions of the design, such as member sizing and connection detailing, can create challenges in aligning the mass timber structure with other critical systems like facades and MEPF.  

For example, the thickness and depth of mass timber floor panels may directly influence the exterior wall details, affecting thermal performance, water management, and facade anchorage. Similarly, the sizes and locations of mass timber framing and lateral elements can impact interior layouts, ceiling heights, and the routing of MEPF systems. Without early and thorough coordination, these dependencies can lead to clashes and unforeseen costs late in the design and construction process. By establishing clear communication channels and integrating design efforts early, a design team can minimize conflicts and ensure a cohesive building system. 

Design Assist

A joint paper by the American Institute of Architects (AIA) and American Institute of Steel Construction (AISC), Delegated Design, Design Assist, and Informal Involvement – what does it all mean?, describes design assist as a process where a contractor (or subcontractor) provides information to assist a design professional’s design, typically before pricing for the work has been agreed upon or the work has been awarded. The design professional and contractor typically have separate written contracts with the owner that describe the scope of the contractor’s design assist services and extent to which the design professional can rely on the contractor’s information. This is a common approach with mass timber projects, especially when the design team does not have extensive timber experience. Bringing in a specialist early to help guide some of the decision making can help avoid redesign and schedule slip, and ensure the most economical configuration. As with delegated design, it is the SEOR’s responsibility to set the design parameters and clearly communicate them with the design assist team.  

The Handoff

For both delegated design and design assist arrangements, contract drawings and specifications should detail all performance requirements and clearly define scope. The following should be included in the SEOR’s drawings, and close collaboration on these elements should be established with design assist arrangements:  

• Load maps indicating load conditions for each floor and roof 
• Member layout identifying locations of columns, beams, and mass timber panels, including their span direction 
• Reactions of design forces broken out by load case (both gravity and lateral) that connections must support  
• Deflection limits for critical locations such as building perimeters, and for multi-story projects with sensitive facade systems like brick veneer 
• Fire-resistance ratings for mass timber elements, with clear delineation of exposed vs. protected timber 
• Connection intent, including general geometry of connections to ensure the aesthetics match expectations of the owner and design team 

Other unique project constraints, such as vibration limits, seismic drift compatibility, penetrations, or special inspection requirements should also be clearly conveyed. 

Cost Considerations 

Both delegated design and design assist can require additional design fees to cover the specialty engineer’s scope of work, which can be perceived as a drawback. However, when looking at overall project costs, design fees are typically a small fraction of the total. The additional cost of engaging a mass timber engineering specialist can help streamline the delivery process to avoid inefficient designs, as well as the shop drawing process to keep mass timber production and the project on schedule.  

Ensuring the proper documentation of design criteria and intent, and involving mass timber specialists early in the project, can lead to a more efficient, cost-effective construction phase, while maintaining the SEOR’s oversight of overall structural integrity. 

Additional Resources 

Design Team Requirements for Mass Timber Delegated Design – EQUILIBRIUM 

Timberlab Guide for Full Delegated Mass Timber Design – Timberlab