Proposed Tacoma DADU Building

Project Details

This project involves the structural analysis and design of a two-story detached accessory dwelling unit (DADU) in Tacoma, WA. The comprehensive engineering scope encompasses the design of the foundation system, shear walls, load-bearing walls, columns, beams, floor joists, headers, and roof rafters. The primary structure utilizes a wood-framed system featuring Douglas Fir-Larch sawn lumber, high-strength 24F-V4 DF Glulam beams, and engineered 16″ TJI® 230 floor joists to ensure structural integrity across both levels. The design process adhered to rigorous structural standards, including the International Building Code (IBC 2021), ASCE 7-16, ACI 318-19, and the National Design Specification (NDS 2018). The building was engineered to withstand significant environmental loads, specifically a basic wind speed of 97 MPH (Exposure Category C) and high seismic activity requirements for Seismic Design Category D (SDS=1.061). Lateral load resistance is provided by a strategic layout of wood structural panel shear walls integrated with Simpson Strong-Tie WSWH high-strength wood shearwalls and various hold-down components (e.g., HDUE3, HDUE5). The foundation system consists of reinforced concrete wall footings (WF1, WF2) and pad footings (F1) designed for an allowable soil bearing capacity of 2,000 psf. Detailed analysis ensured all members, including a monoslope roof system designed for snow and wind loads, passed utilization checks for strength and serviceability.

Role

Structural Engineer/Designer

Services Provided

The scope of services included the preparation of structural analysis, design calculations, and structural plans for the proposed Detached Accessory Dwelling Unit (DADU), including associated roof decks and garages. Structural analysis considered gravity loads and lateral forces in accordance with applicable Washington State and Tacoma building requirements. The work involved the design of the gravity and lateral force-resisting systems, including foundations, footings, slabs, floor framing, roof framing, roof deck framing, shear walls, holdowns, anchorage systems, and supporting structural elements. Services also included design checks for roof deck framing, guard systems, and their supporting components to verify compliance with applicable strength and serviceability requirements. Structural drawings were prepared to communicate the design and included foundation plans and details, floor, roof, and roof deck framing plans, lateral system plans identifying shear walls, holdowns, and anchorage requirements, as well as structural notes and typical details suitable for permit submittal, coordination, and construction.