December 28, 2017
Madison Square Park Tower is a 61-story residential tower that reaches 777 feet at its pinnacle. The ultra-luxury condominium building is located at 45 east 22nd Street in Manhattan’s historic Flatiron District near Madison Square Park and only one block away from the Flatiron building.
The building consists of 83 simplex, duplex, full-floor and penthouse apartments and amenity spaces that include automated parking, a library and a basketball court, all totaling 260,000 square feet. The building is clad with a sculptural glass façade that sits atop a five-story stone-clad base intended to harmoniously blend with the surrounding neighborhood. The tower is sculpted so that the floor plate is as small as 62-feet-wide by 52-feet-deep near the base which gives a maximum slenderness ratio of about 13:1. Compounding the overall slenderness, the tower cantilevers outward as it rises to maximum floor plate of 94-feet-wide by 52-feet-deep.
The gravity system is comprised of flat slabs spanning from the interior shear wall core to perimeter columns with varying cantilevers because of the complex tower geometry. Floor plates vary from ten to 12-inches-thick at residential levels as the spans lengthen with increasing height. Thicker slabs are used at mechanical areas including a 20 inch slab at the west side of the roof to support a 1.2 million pound tuned mass damper system. The lateral system is comprised of one full-height shear wall core centered on the south side of the building with wall thicknesses ranging from 42 inches at the base to 24 inches at the roof. The core is connected to perimeter columns at the 33rd floor mechanical level with a one-story outrigger/belt wall.
Given the combination of complex geometry (every plate above the sixth floor is a unique shape) and required column-free interiors, a two-way flat plate system was chosen. The system provides flexibility in locating columns (including sloped and walked columns) while providing maximum ceiling height for floor to ceiling glass walls.
The greatest challenge of the project was the design of the lateral system to resist the required forces and to meet strict drift and acceleration requirements, while resisting the inherent gravity overturning resulting from the top-heavy design. A system of high-strength concrete shear walls, 14000 psi at the base down to 8000 psi at the roof, coupled to perimeter columns mid-height of the building with the belt wall was selected. The design of this system was accompanied by wind tunnel shaping studies to find the optimum configurations. While the concrete system alone meets the drift requirements, a tuned mass damper at the roof level was added to reduce building accelerations to acceptable levels.
The building is supported on a 50-foot-wide by 80-foot-deep mat that is eight-feet-thick. The mat bears on 20 tsf bedrock and includes 32 270 ton rock anchor tie-downs to resist overturning. The mat is reinforced with four layers of rebar top and bottom and local shear reinforcement as required.
Construction below the 11th floor included numerous structural transfer elements as the tower reduces to its minimum footprint at the seventh floor. From the seventh to 11th floor, the west side of the tower cantilevers 16.5-feet-out over an adjacent building. The system is design as a multi-story bracket braced to the core, with the base of the bracket (compression) reacting through eight-feet-wide by two-feet-deep bracing beams and the top of the bracket reacting (tension) through groups of #14 GR97 SAS threaded bars.
Above the 11th floor, the building was typically construction with a fast-track, two-day pour cycle – alternating the pour of the vertical concrete elements and the horizontal concrete elements. This allows for a shortened construction schedule that wouldn’t be possible with other structural systems. Throughout the project, GR75 vertical reinforcement is used in the shear core and columns to reduce reinforcement congestion. The belt wall at the 33rd floor mechanical level is constructed with 2-foot-thick perimeter walls that couple to the core to the perimeter columns. Large openings for mechanical louvers were accommodated with careful analysis and added reinforcement. The tuned mass damper is located at the west side of the roof level and its weight bears half on the core and half on the slab. A system of a 20 inch slab with 48-inch-wide by 40-inch-deep beams is utilized at the slab support to provide adequate strength for the 1.2-million-pound steel damper assembly.
The design and construction of this tower was a dynamic and challenging process that could not have been achieved successfully without the cooperation and responsiveness of all design consultants and construction team members.