500 Walnut

Philadelphia, PA, United States

Extensive building enclosure support for a luxury residential tower 

500 Walnut is a 27-story luxury residential tower in Philadelphia. Designed by architect Cecil Baker + Partners, the building is situated across the street from Independence Hall, a UNESCO World Heritage site and the place where the U.S. Constitution was signed.


  • The Challenge

    The building’s profile is slightly irregular, as though slices had been removed from its corners. The resulting shape lets sunlight reach Independence Hall to the north, and creates impressive views from condominium balconies. The design also ensures that the view of Independence Hall from the Liberty Bell remains framed by open sky.

    The smooth, prismatic shape of 500 Walnut minimizes its visual competition with nearby historic buildings, while giving the building its own architectural character. The facades facing south, away from Independence Hall, needed to respond to the traditional street grid and achieve high levels of energy efficiency. Acoustic separation between floors was another priority.

    Two key technical components of the overall building enclosure solution were a customized, flush window wall and a solid wall of metal panels and punched windows. Both components needed to be prefabricated off-site – and both assemblies could be seen up close from the condominium balconies, so it was vital that they communicate quality and craftsmanship to potential purchasers.

  • Our Approach

    Starting in Schematic Design, our building enclosure consultants worked with the design team to devise enclosure systems that demonstrated strong craftsmanship and reliability. We used a “design assist” approach, involving contractors during final documentation of the enclosure; this technique boosts efficiency and controls costs by engaging contractors before the design is fully complete in order to coordinate the architect’s final details and the contractor’s early shop drawings.

    A number of technically demanding assemblies required especially detailed attention. These included the flush window wall slab edge covers (a solution that was unconventional at the time); complicated geometry at the tower’s projecting, angular “bay windows”; a parapet that needed to match the window wall; and a “pilot house” at the tri-level penthouse. Our team also needed to drive the design of the interface that combined the two largest components of the enclosure: the window wall system and the metal panel system.

    Another significant challenge: we supported the waterproofing of the building’s deep foundation, immediately adjacent to the foundations of existing buildings. We also waterproofed several large roof terraces constructed over condominiums at the fourth floor and the penthouse. An indoor swimming pool on the third floor required special attention; there we contained odor and humidity with a special vapor barrier and with a triple-glazed window wall and skylights.

    Not only were our enclosure consultants involved in the design assist, laboratory testing, and submittal reviews, but we made frequent visits to the window wall fabrication plant to ensure that quality control programs were effective. The quality of the insulating glass unit (IGU) fabrication was an area of particular focus, as the structural silicone glazing exposed some parts that would normally be concealed. Our team reviewed and approved dozens of IGU at the factory, indicating acceptable units and rejecting others – annotating directly on the glass the reasons for rejection. This provided clear guidance for the expected level of quality and informed the glass fabricators’ own quality control program for the benefit of the project.

  • The Outcome

    Pre-sales of the condominiums were brisk, and owners started moving into 500 Walnut in late fall of 2017. The sleek window wall system provides great performance at a lower cost than a comparable curtain wall, and with a higher level of acoustic separation. The combination of the metal wall panels and flush window wall provides a high-end appearance that differentiates the project from other condominiums. Careful design and quality control ensured conformance with tight tolerances, creating an impression of craftsmanship and quality throughout the building.