workday. The eastern elevation was clad in the same material for aesthetic reasons. The modern facade, installed in 2010, is made of low emissivity glass that resembles the original facade. The modern curtain wall contains two layers of glass panes, which are more resistant to shattering in case of a bombing. The original curtain walls were cantilevered from the building's superstructure and were attached to the concrete floor slabs. Each of the original windows were aluminum sash windows, separated by aluminum mullions that projected slightly from the facade. The sash windows were compatible with conventional window cleaning equipment. The modern curtain walls are hung from the superstructure via outrigger plates, and they contain projecting aluminum mullions similar to those on the original sash windows. The western and eastern elevations are each divided vertically into ten bays, each measuring wide. Within each bay are seven panels, each measuring wide and tall. Three of the original curtain-wall panels are preserved in the Museum of Modern Art. The windows on different stories are separated by spandrel panels. The inner faces of the spandrels are painted black, insulating the building while also giving the impression of depth. The curtain walls originally contained weep holes, which were intended to prevent the windows from cracking. Most of the weep holes were plugged in 1952 and 1953 because the building's shape and its susceptibility to high winds frequently caused rainwater to leak into the building. Floors 6, 16, 28, and 39 contain pipe galleries rather than glass panels, since these stories include mechanical equipment. The facades of these mechanical stories consist of latticed panels, except on floor 39, where there is a mechanical penthouse behind an open-air grille. The architect Henry Stern Churchill wrote that the mechanical penthouse was "a very simple shape and could very well have been left visible". Marble slabs The narrower northern and southern elevations are made of masonry clad with Vermont marble. These elevations rise as unbroken slabs and do not contain any openings. The building's steel superstructure, including steel bracing, was concealed within these marble slabs. According to Harrison, the marble walls not only allowed the Secretariat Building to be seen as a monument, but also reduced competition between staff members who wanted corner offices. Structural features The foundation includes concrete piers that extend down to the underlying bedrock. Steel pilings are used at points where the bedrock is more than