Structural Design Project of Super Tall Building Chicago Spire

Introduction

The 2011-2012 Master of Engineering Structural Design Project was the structural design of the
Chicago Spire (Figure 1.1), located on the west side of Lake Shore Drive in Chicago, Illinois. The
project was provided by John Peronto, P.E. and Mary Williams, P.E. of Thornton Tomasetti’s Chicago
office. Cornell University Lecturer Dr. Shideh Shadravan was the project advisor. The project team
consisted of sixteen Master of Engineering students and two undergraduates. The team members
provided a unique assortment of design experience, academic specialty, and cultural background.
This resulted in a realistic, professional experience similarly found at a design firm.

Chicago Spire: Background and Location

The Chicago Spire is 2,000 ft tall on a strip of land between the Ogden Slip and the mouth of the
Chicago River (Figure 1.2). The tower contains approximately 3 million square feet of upscale
condominiums and amenities. The basement has 7 below-grade parking levels on top of rock
socketed caissons. The structural engineer of record was Thornton Tomasetti. The project was put
on hold in 2008 with only its foundation completed. Upon completion, the Chicago Spire would be
the tallest building in the Western Hemisphere.
Spanish architect Santiago Calatrava was the architect and engineer for the project. The design
highlights a spiraling exterior supported by an exterior column grid and a concrete core. Calatrava
compared the design to an imaginary smoke stack from a campfire lit by the indigenous Native
American tribes of Chicago.

Project Scope

The Master of Engineering team was charged with providing a complete structural design of the
gravity and lateral system as well as foundation design, connections and details, and an analysis of
the effects from creep and shrinkage. The project required the eighteen team members to
collaborate as sub-teams to complete assigned deliverables, utilizing full engineering knowledge
and experience. The design of the cylindrical superstructure entailed expanding design limits into
unfamiliar areas through self-learning and provided resources.
The design project was split into ten deliverables through the academic school year. Deliverables
include submittal of white-paper reports, annotated engineering calculations, structural drawings,
and finite element models. Local and professional advisors provided design support through biweekly
teleconferences and daily correspondence.
Structural design was supplemented by academic field trips pertinent to tall building design.
ASPIRE traveled to New York City in November of 2011. This trip included a presentation by
Silverstein Properties and a site tour of Four World Trade Center, a 72-story skyscraper designed
by Leslie E. Robertson and Associates. In December 2011, team members visited the Rowan,
Williams, Davies, Inc. (RWDI) wind tunnel testing facility in Guelph, Ontario, Canada. RWDI is a
leader in the field of wind tunnel testing, and has performed dynamic analysis for some of the
world’s tallest structures, including the Chicago Spire.

Design Criteria

Preliminary analyses required ASPIRE to study relevant tall building design. In adjunction with
architectural constraints, this research was utilized to select specific structural systems for gravity
and lateral systems.

Tall Building Design

For years engineers have furthered the practice of structural engineering, designing increasingly
taller skyscrapers to meet the demanding vision of architects and owners. As buildings grow, more
efficient, specialized structural systems are needed to handle the loads. One of the first buildings to
clearly demonstrate the potential of the skyscraper was the Empire State Building, reaching 1250
feet in 1931 through the use of a standard riveted steel frame with simple portal bracing (Binder
2006, 42). Engineering has progressed onward from this simple system, reducing the amount of
material used while simultaneously increasing the height. One way of achieving this is through the
use of a high-density concrete core with outriggers and belt trusses at mechanical floors. This
strategy has allowed buildings like the Shanghai World Finance Center and the Burj Khalifa to soar
to heights over one and two thousand feet, respectively.
The Chicago Spire’s specifications are demanding, defining a building that is truly unique.
Outriggers and belt trusses for lateral restraint are limited to the mechanical floors, isolated
throughout the structures elevation. The residential floors contain spacious floor plans with evenly
spaced columns in a ring around the core. Cantilever beams extend from the radial frame to the
façade, allowing for unobstructed views in all directions.
For a building as slender as the Chicago Spire, the lateral system is often the limiting factor in
selecting a design. As buildings increase in height, the structural frames continue to decrease in
average weight per square foot. This is possible due to interaction between interior/exterior
components; high strength low-alloy steel; composite construction; wind tunnel tests; and concrete
improvements in reinforcement and strength.
A vital piece of the Chicago Spire’s design is a system to resist lateral loads. Lateral loads are more
variable than gravity loads and increase significantly with building height. Lateral systems are
designed with the tower’s strength, stability, and rigidity in mind. For tall buildings, serviceability
usually controls the design. Inter-story deflection, also called floor-to-floor drift, and dynamic
effects, such as vortex shedding and vibrations, are concerns for slender buildings.