5-Story Mid-Rise Office Building

Summary

Apex Lateral Design Group designed the structural system for a proposed five-story office building located in Irvine, California. The main problem addressed was the need to develop a safe, efficient, and compliant code design that could support everyday gravity and seismic lateral forces. Since the building is located in a seismically active region, this project was governed by the seismic design requirements rather than wind.

This project matters because a building's structural system directly affects the safety of life and performance in the long term, for which cost efficiency is imperative to meet those standards. The final design needed to protect future occupants and satisfy the architect's/clients' design goals, ensuring that the building could safely transfer loads from the roof all the way down into the underlying foundation/soil.

The project scope included the design of the gravity framing, lateral-force-resisting, steel members, connections, base plates, and foundation systems. The team also evaluated other design alternatives, such as including composite or non-composite floor beams, X-bracing versus chevron bracing, and continuous or isolated footings.

Technical Approach/Methodology

The project identified the building loads using the California Building Code (CBC) and the American Society of Civil Engineers (ASCE 7-22) specifications. These loads include:

Dead
Live
Wind
Seismic

The gravity system was designed by determining the tributary areas, converting them into area loads, and then selecting the steel member (via American Institute of Steel Construction) that could safely resist bending, shear, and deflection demands.

For floors 2-5, the team used composite beam design, where the steel beams work together alongside the concrete slab through shear studs. This allowed the beams to be smaller and thus, more efficient because the concrete slab contributed additional strength in tandem with the steel. The roof, however, was designed as a non-composite because it did not include a concrete slab on top of the already existing metal deck that exists throughout all the floors.

For the lateral system, the team compared wind and seismic forces and eventually found that the seismic loading controlled the overall design. As a result, a special concentrically braced frame system with X-bracing was chosen because it provided an efficient and straightforward load path for earthquake forces. The seismic forces were then distributed to each floor and then transferred through diaphragms, chords, collectors, braces, columns, and into the underlying foundation.

The team used manual calculations, building code provisions, AISC, ACI 318, and RAM (software) to check, model, and tweak the overall design. RAM was used as a structural analysis and design tool to verify the member sizes that we obtained via manual calculations. It also helped with code compliance, ensuring that all our pre-assigned sizes were up to standards in terms of the seismic standard and steel provisions.

Outcomes

By the end of the project, Apex Lateral produced a complete design for the five-story office building. The final design includes a composite steel layout for floors 2-5 and a non-composite roof system, in addition to the steel gravity beams, girders, gravity/lateral columns, SCBF X-bracing, chord, drags, base plates, connections, and continuous reinforced concrete footings. All of which represent the specific deliverables produced in the report.

The final gravity system includes W-shaped beams and girders that were chosen based on strength, minimal deflection, and efficiency. The lateral system, on the other hand, uses special concentric braced frames with HSS X-braces that increase in size towards the lower floors due to increasing seismic story shear. The foundation system consists of continuous reinforced concrete footings designed to resist axial, overturning, uplift, and shear demands.

Overall, the project produced a safe and efficient design that satisfies the overall gravity, lateral, and foundation design requirements while adhering to the architectural constraints of the building.