Excellence in Engineering

"The engineering team overcame so many intricate and complex design challenges while working within the constraints of the existing structure. The multi-story sloped columns configured around an existing trainshed were also used to laterally brace the new structure. The strategically placed connections between the addition and the existing structure ensured a balanced lateral system without transferring significant load between either system." -- Fraser Reid, PE, CEng, MICE, Associate Principal, Buro Happold, 2025 IDEAS² Awards Judge

A midtown Manhattan office building needed a boost from its original 1960s form, and its steel frame lent itself to adaptation and longevity despite its age.

New steel elements incorporated at PENN 2 created a 75-ft by 450-ft addition called the Bustle that hovers 50 ft above the sidewalk on 14 dramatically sloped columns configured around an existing trainshed. Creative connections ensured the existing structure and addition do not transfer significant lateral load to each other.

Redeveloping PENN 2, formerly Two Penn Plaza, added inviting public spaces, improved office facilities, modern worker amenities, and updated aesthetics to the Penn District neighborhood and reconnects the building with street life. The 32-story building was constructed in 1968 following the original Penn Station’s demolition, replacing a monumental public space on a double-wide block on the west side of Seventh Avenue between West 31st and 33rd Streets.

The Bustle goes from the fourth to 10th floor and provides 100,000 sq. ft of double-height, column-free office space, 43,800 sq. ft of roof terraces, and an expansive, protected public plaza adjacent to Seventh Avenue. It takes advantage of the existing building’s setback from Seventh Avenue by using space not embraced in the 1960s design.

The addition stretches the width of the double block, extending almost 40 ft beyond the north and south ends of the tower, which are also set back from the sidewalk. The massing honors the former Penn Station but adds slender, widely spaced columns that replace the former station’s dense and regimented colonnade.

Smaller additions at the north and south ends are supported both by existing columns and new columns bearing over the existing trainshed framing. The north addition includes the columnfree, double-height Town Hall between the second and fourth floors that accommodates up to 280 people and supports a future pedestrian bridge spanning over PLAZA 33 to PENN 1. Other features include a triple-height entrance lobby, double-height corner loggias, a rooftop pavilion, and terraces.

Several years of conceptual studies and design development resulted in minimal disruption to the existing transit facilities and the hundreds of thousands of travelers who use them daily. Documentation of existing structures, including field surveys and material testing, combined with sophisticated analysis and advanced materials, allowed a modern addition to be built atop a century-old trainshed structure.

Adding Steel

Structural steel was the natural choice for the project. The original building is framed entirely in steel, with cinder concrete slabs and beam encasement to increase allowable stresses. That system is a precursor of today’s composite steel and concrete-filled metal deck and was easy to design and construct.

The triple-height entrance lobby, double-height corner loggias, and roof terrace and pavilion were created by selectively demolishing entire framing bays--except for members bracing existing columns--and reinforcing the columns for the increase in unbraced length. New infill framing was erected and the temporary bracing beams were removed.

Structural steel, even if more than a century old, can have remarkable reserve capacity. Engineers determined older steel locations where coupons could be cut and tested for tensile properties, chemical composition, and base metal notch toughness, per Appendix 5 of the AISC Specification for Structural Steel Buildings (AISC 360-22). Rivets were also tested.

The primary types of steel on the original structure were Penn Station’s 1906 ASTM A7/A9 60-ksi tensile strength (average) with a 30-ksi yield point and Two Penn Plaza’s ASTM A36 58-ksi tensile strength (minimum) with a 36-ksi yield point. Tests on both showed they met or exceeded current standards for ASTM A36. Some steel contained high concentrations of sulfur or silicon and was coated with lead-based paint, which was abated wherever necessary. Nevertheless, additions and reinforcement with steel were practical, efficient, and economical.

Steel’s aesthetic advantages were also important. MdeAS Architects desired an expansive protected public plaza beneath the Bustle and slender, widely spaced columns to support it. The sloped columns’ arrangement and their cast steel end connections’ simplicity perfectly matched the architect’s and owner’s visions. The sloped columns, other round columns, truss diagonals, pin connections, and supporting gussets were all considered showcase elements and were fabricated, shipped, and erected to architecturally exposed structural steel (AESS) Category 4 (showcase elements).

Aesthetics were crucial in the Bustle framing as well. Tapered plate girders at the Bustle’s lowest level create an LED-lit faceted soffit, adding captivating visuals. The widely spaced perimeter columns and compact floor framing maximize the open feeling of the office space from inside and outside. The perimeter truss diagonals provide the necessary load transfer and stiffness while minimizing obstructed views through the two-story curtainwall.

Trainshed Tactics

The north, south, and east building expansions are supported on the original Penn Station trainshed. The station’s above-ground portions were demolished in the 1960s to make way for Madison Square Garden and Two Penn Plaza and the below-grade structure remained mostly intact, with excess structural capacity due to the removal of the above-ground structures.

The original columns and foundations were used to their fullest extent to limit existing structure reinforcement within the trainshed and corresponding train service disruptions. Consequently, the sloped column locations were meticulously calibrated to land in specific locations on the sidewalk-level transfer structures to distribute the loads to the existing columns without exceeding their capacity. A new column to carry the load to bedrock was needed at only one location.

The transfer structures consist of plate girders and heavy wideflange sections divided into seven individual platforms that bear directly on the trainshed roof, which simplified installation and precluded disturbing the tracks below. The framing is integrated into the elevated plaza--constraining its depth and arrangement--and encased in concrete for stability and weather protection.

Working above and within Penn Station and the trainshed required coordinating with multiple agencies, primarily Amtrak. With the trainshed’s tight clearances and an order to minimize service disruptions and track outages, the structural design had to be as compact as possible, with work performed from above wherever practical.

The only existing structural drawings available for Penn Station and the trainshed were not original; they had been redrawn in the 1940s. Field investigations and condition surveys were performed as needed to verify critical existing structures affected by the work.

Load Management

The sloped columns support the east and north sides of the Bustle; one sloped and six vertical columns support its west edge, adjacent to the tower. Brackets field-welded to existing steel columns support the Bustle at the building center, and field-welded steel plates increased their load capacity.

All sloped columns are 24-in.-diameter HSS with a cast steel pin connector at both ends. The complete end connections, including clevis, pin, and gusset, were fabricated in the shop and the gussets field-welded to accommodate their tight tolerances better. A high-performance coating system protects the columns from weather.

The Bustle’s lowest level is framed as a cantilevered tabletop. Steel plate girders span east to west from the face of the tower and over the top of the sloped columns to the exterior. In the north-south direction, a line of plate girders snakes across the tops of the sloped columns, cantilevering at their ends. The Bustle exterior is diagonalized between the fourth and eighth floors to form a multispan truss to collect vertical loads tributary to the plate girders and their supporting columns. The diagonals are 14-in.-diameter hollow structural sections (HSS) with cast steel pin connectors.

Laterally, the Bustle and other additions are self-supporting to avoid triggering an upgrade to the lateral force resisting system of the existing tower--which was designed well before modern wind and seismic design codes were developed. All loads required are per the current building code, and loads on the existing building were not increased beyond the level that would trigger an upgrade to the building’s lateral force-resisting system.

All column-to-beam joints incorporate moment connections, and the sloped columns act as diagonal braces. Connections at the sixth, eighth, and 10th floors ensure stability without transferring significant lateral load. At the fourth floor, an isolation joint uncouples the existing building from the addition and prevents lateral load from flowing to the sloped columns. The brackets that vertically support the Bustle allow horizontal slip for the expected deflections.

The additional lateral loads are resisted by existing north-south concrete retaining walls in the trainshed and existing east-west moment-resisting frames in PENN 2. The plaza concrete slab and girder encasement are reinforced for the additional shear. At sidewalk level, steel plates were anchored to the trainshed roof and spliced together to form a load path from the plaza framing to the resisting elements. A reinforced concrete slab to transfer the loads at sidewalk level would have been too thick.

Removing an existing column between the ground and fourth floors further opened the lobby. New columns added one bay to either side and bear on new girders below the ground floor. Existing fourth-floor framing was replaced with beams spanning between the new columns on both sides. Diagonal members between the fourth and fifth floors create a truss to transfer load to the new columns. The 6-ft-deep ground floor plate girders transfer load back to the original location. This double-cantilever girder acts like a well-balanced seesaw; the tips are braced laterally but free to deflect vertically.

After the truss connections were locked in, jacks relieved load from the existing column. The new columns were shimmed tight and the existing column removed. When the jacks were released, the load transferred to the new columns and back to the existing column, leaving the distribution of vertical load essentially unchanged.

Columns and Conversions

Columns spring from the tabletop’s perimeter to create a column-free interior. Along the tower’s face, wide-flange columns align with the existing columns at about 20 ft on center. On the exterior, the 14-in.-diameter HSS columns are spaced at about 36 ft. They support beams spanning east-west at the sixth and eighth floors, creating the double-height volumes.

The triple-height lobby was created by removing second- and third-floor bays. Eliminating beam-to-column joints redistributed lateral load and required reinforcement of existing moment connections around the lobby. Removing the existing framing of the north- and southwest corner bays between the 14th and 29th floors created the double-height loggias. On even-numbered floors, new framing supports the increased dead and live loads of the terraces. On odd-numbered floors, the corner columns were reinforced for the resulting increase in unbraced length, prior to removal of the spandrel beams.

Converting the tower’s roof to terrace space and adding a multi-purpose pavilion required reinforcing the existing framing. New beams were added between existing beams, and existing girders were augmented with tee sections field-welded to their bottom flanges. Extending the high-rise and freight elevators was also required. The existing machine room was demolished, the shafts extended, and a new machine room was constructed. Existing columns were reinforced as needed.

Construction Considerations

Studying existing structures helped plan the Bustle’s steel erection sequence. The long spans, cantilevers, double cantilevers, and multiple levels of supporting steel could deflect, shift, or vibrate during erection. That meant tolerance issues for fitting up the steel framing, curtain wall, and finishes were considered.

Temporary supports were installed for the tabletop using the permanent transfer platforms, other existing trainshed columns, and the existing concrete retaining wall. Temporary girders were installed where needed for two tower cranes.

The tabletop framing was erected using tower cranes, with supports at the cambered tips of the cantilevers. Next, the concrete fill on metal deck was placed. Concurrently, the sidewalk steel plates were installed and the plaza concrete completed. Once the tabletop steel had been checked and adjusted and the concrete had cured for 28 days, the temporary supports were removed. The tower cranes then erected the remainder of the Bustle using the tabletop as a stable platform.

Throughout the design process, the construction manager brought together the structural engineers and contractors to evaluate the work and suggest modifications to streamline fabrication, delivery, site logistics, erection, and connections to best use the shop’s methodologies and personnel. Engaging the steel casting supplier resulted in simpler connections to fabricate and erect and satisfied the aesthetic vision.

PENN 2 steel work was completed in December 2023. The $650 million building opened in 2024, satisfying its stakeholders’ stringent requirements and meeting budget. The redevelopment is a sustainability win by reusing an existing facility. A complete demolition and rebuild would have been easier but would have increased waste, new material creation, and train station impact. Instead, the project team adapted a 20th-century building to attain 21st-century environmental performance.

Owner: Vornado Realty Trust, New York
General contractor: Turner Construction Company, New York
Architect: MdeAS Architects, New York
Structural engineer: Severud Associates Consulting Engineers, PC, New York
Steel team:
Fabricator/detailer: Crystal Steel Fabricators, Delmar, Del. *AISC full member, AISC-certified fabricator*
Erector: Skanska USA Civil Northeast, Inc., East Elmhurst, N.Y.
Casting supplier: CAST CONNEX, Toronto *AISC associate member*