One Post Office Square is an existing 42-story office building constructed in the 1980s in the heart of Boston’s financial district. The existing building structure, originally designed by LeMessurier, includes a unique lateral system featuring of a series of stub columns at the building perimeter the building that form an efficient steel moment frame above Level 7. Architecturally, the building’s original precast façade and small windows were appropriate at the time of construction but are out of step with current offerings at the top end of the office market. The Repositioning project, beginning construction in 2019, undertakes a complete transformation of the existing building to restore it as a premier office tower. In addition to providing a completely new façade comprised of elegant, high-performance glass curtain wall, central to the Repositioning project is the demolition of the existing 7-story attached parking garage and construction of a new 19-story addition in its place. The new addition adds over 200,000sf of column-free space to the building and includes accommodations for a fully automated parking system. SidePlate moment connections are used throughout the steel frame for the addition to provide a robust lateral system that prevents the new addition from increasing loads on the existing tower structure. As well as serving as structural engineer for the building, LeMessurier also worked on Construction Logistics to help develop the careful construction phasing required to enable tenant occupancy during construction.