The Open Ground Plane In An Urbanizing World A Case Study of
CitiGroup Center
Jonathan Hopkins & Shawn Johnson
New York, New York
IDEA 3: Structural Precedent Analysis RWU SAAHP Arch 435 Fall 2011 Prof. Mete Turan TA Katie Kanakos
CitiGroup Center 601 Lexington Ave, MidTown, New York City Architect: Hugh Stubbins Jr. Engineer: William J. Messurier
Design Feature: Steel Frame Skyscraper without Columns at Corners
IDEA 3 Precedent Analysis -Theoretical Framework-Precedent Information-Structural Analysis-Design Solution-Conclusions-
Theoretical Framework - “The number of cities with population above one million has already risen from six to 19 in the past 40 years, and will continue to rise”1 - “By 2030 [the number of people living in towns and cities] will swell to almost 5 billion”2
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Yeang, Ken. “The Tall Building Typology and Cities” Reinventing the Skyscraper (Academy Press, 2002) pgs. 33-55 UNFPA. “Urbanization: A Majority in Cities” Linking Population, Poverty and Development (UNDESA, 2005)
Urbanized Population (2006)
Theoretical Framework
- “The presence of trees and grass has also been associated with reduced crime rates, possibly because residents of a well landscaped building spend more time outside, allowing them to get to know one another and develop a sense of community.”3 - “Humans living in landscapes that lack trees or other natural features undergo patterns of social, psychological and physical breakdown”4 ___________________________________________________________________________________________________________ 3
Davis, Dave. “Developing a Sense of Community” Designs and Codes that Reduce Crime around Mulitfamily Housing (Local Government Commission) p. 3 4 Staff. “Science Suggests Access To Nature Is Essential To Human Health” Science Daily (February 17, 2009)
CitiGroup Center, New York
Precedent Information
5
- CitiGroup Office Building Conceptualized in early 1970s - The site, across from CitiGroup HeadQuarters at 54th Street and Lexington Avenue, was occupied by St. Peter’s Church - A deal was struck to build the office tower while retaining the church’s location - This circumstance prevented conventional structuring of the office tower with corner columns - Stubbins and LeMessurier Proposed a 59-story building supported by enormous chevrons, each carrying and distributing the load of 6-8 stories to a central reinforced concrete column - The building is lifted off the ground 9-story by centrally located columns
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5
Morgenstern, Joe. “The Fifty-Nine Story Crisis” The New Yorker (May 29, 1995)
Assemblies & Construction
- Designed with Welded Joints - Structured for the 55-year Wind Storm - Calculations done with Perpendicular Wind Loads - Construction began in 1974 - The building was constructed outward from the core in 6-8 story sections - The structure is a steel frame - Floor Plates are structured with reinforced concrete - Corners cantilever 70 feet beyond columns - Aluminum and glass exterior cladding - 7th Tallest building in New York City at 915 feet with 59 floors
Crisis of 1978
- Construction Completed in 1977 - Contractors switched out welded joints with bolted joints - The Building deals with 45 degree angle wind loads - The Hurricane of ‘78 was a 100-year wind storm - Recalculations of loads revealed severe structural deficiencies - 2-inch steel plates were welded to each joint on the building to counteract the shearing forces on the bolted connections - There were serious uncertainties about the structural integrity of the building during the storm
Gravity Load Analysis
Structural Analysis
- 1000 Kip vertical loads acting downward on each 6-8 story section - “In the frames the [gravity] load is transferred from the minor columns to the mast column by diagonals at eight-story intervals”6 - The chevrons concentrate the loads to the center, thus freeing up the corners, which cantilever beyond the central columns 72 feet
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Smith, Bryan Stafford and Alex Coull. “Use of Large Scale Bracing” Tall Building Structures: Analysis and Design (John Wiley & Sons, Inc. 1991) p. 127
Compression
Tension
Moment
Structural Analysis
Wind Load Analysis
dx = 6.591 in
- “Wind Shear is collected by the core over eight-story-height regions and transferred to the braced outer frames at the base of each tier” - “At the base of the tower the entire shear is transferred back to the core and hence to the ground” - “Wind moment is carried mainly by the mast columns and legs in the faces normal to the wind, and partly by the core”7
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20 kip Horizontal Load
Deflection
7
Ibid.
Structural Analysis
Combined Load Analysis
- “Since the diagonals carry a significant part of the gravity loading, the structure may be classified as either a space truss or a braced frame”8 - CitiGroup Building is a relatively light-weight frame that is effected by lateral wind loads, which is compensated for with a mass damper (the first skyscraper to use one)
Shear
- The Brace frame is obstructive to facade elements like windows Tuned Mass Damper
Moment
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8
Ibid.
Axial
MultiFrame Model
Alternative proposal
Suspended, Reinforced Core Structure
Characteristics:
Suspended Frame10
- Large, Reinforced Central Core
- Horizontal Cantilevers at Roof Level
- Floor Slabs Suspended from Hangers
- Open Concourse Level
- “The Hangers, because they are in tension and consequently can be of high strength steel, have a minimum sized section...”9
Advantages: - More Open Ground Plane
Suspended Frame11
- Shorter Cantilever
- Freed Facade
- Reduced Deflection 5.635 vs 6.591
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9
Model
Moment
Shear Diagram
Smith, Bryan Stafford and Alex Coull. “Structural Form” Tall Building Structures: Analysis and Design (John Wiley & Sons, Inc. 1991) p. 50 10 Ibid. 11 Schodek, Daniel L. and Martin Bechthold. “Structural Systems: Design for Lateral Loadings” Structures (Pearson Education, Inc. 2008) p. 546
Conclusions CitiGroup Building, New York
- Inventive Solution to Existing Condition on a Site - Innovative Implementation of New Technology - Inadequate Lateral Load Calculations that did not account for 45 degree angle winds - Poor Construction Administration Oversight - Poorly Secured Joints - Braced Frame Restricts the Facade
Alternative Suspended Frame Solution
- Frees up the Facade - Opens up the Ground Plane - Reduced Deflection
Works Cited
Davis, Dave. Designs and Codes that Reduce Crime around Mulitfamily Housing (Local Government Commission) Morgenstern, Joe. The New Yorker (May 29, 1995) Schodek, Daniel L. and Martin Bechthold. Structures (Pearson Education, Inc. 2008) Science Daily (February 17, 2009) Smith, Bryan Stafford and Alex Coull. Tall Building Structures: Analysis and Design (John Wiley & Sons, Inc. 1991) UNFPA. Linking Population, Poverty and Development (UNDESA, 2005) Yeang, Ken. Reinventing the Skyscraper (Academy Press, 2002)
