Description of the Building and the Curtain-Wall System

California Pacific Medical Center (CPMC) Cathedral Hill Hospital, a major hos­pital with 550 patient beds, is to be constructed in downtown San Francisco (Fig. 2). The building roof is 251 ft (76.5 m) above grade. The gravity-load-resist­ing system consists of concrete fill on metal decks, and steel beams, girders, and columns. The lateral-load-resisting system above grade consists of moment – resisting steel frames with supplemental viscous wall dampers, whereas that below grade includes reinforced concrete floor diaphragms and shear walls. The foundation is designed as reinforced concrete spread footings. The building is

FIG. 2—Drawing of the building and its proposed location in San Francisco.

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comprised of three main components: the podium, the tower, and the rooftop equipment and central plant. A significant portion (75%) of the exterior clad­ding system of the tower, levels 7-15 on the north and levels 3-15 on the south, is comprised of a four-sided SSG unitized curtain wall.

A unitized curtain-wall system involves building individual units in the shop, then shipping them to the project site, and finally attaching them to the building floor slabs at each level. The unitized panel for this building is typically an 8-ft, 4-in. (2540-mm) – wide by 14-ft (4267-mm) or 17-ft (5182-mm) -tall unit in the tower as shown in Fig. 3, or 17 ft (5182 mm) tall at the podium (floors

1- 7) floors. There are three 8-ft, 4-in (2540-mm) panels and one 4-ft (1219-mm)- wide panel at each typical 29-ft (8839-mm) bay. All exterior glazing of the hospital are 1-in. (25.4-mm) -thick insulating glass units (IGU) consisting of two 0.25-in. (6.4-mm) panes of heat-strengthened glass with 0.5-in. (12.7-mm) air gap between. The IGUs are fully supported on all four sides by aluminum glazing frame members through structural silicone sealant. The factory coated

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finish on all extruded aluminum components of the curtain-wall system will be a two-part Kynar polyvinylidene fluoride (PVDF). PVDF [11] is a specialty resin plastic material in the fluoropolymer family and is used generally in applica­tions requiring the highest strength, and resistance to solvents, acids, bases, and heat, and it is the premier finish for curtain-wall and window aluminum extrusions. Because four-sided SSG systems rely on a chain of adhesion, the ad­hesion of a factory-applied coating (PVDF) to an aluminum mullion must have an adhesion failure pull-off value in excess of the structural sealant’s ultimate design strength. Laboratory testing has confirmed that the adhesion of a PVDF coating to an aluminum surface has an average pull-off strength of 215 psi (1482 kPa), which exceeds the maximum sealant tensile strength of 177 psi (1220 kPa).

There is a horizontal movement stack joint 7.75 in. (196.9 mm) above the top of the slab as shown in Fig. 1 at each floor accommodating lateral move­ment and vertical deflection. The stack joint that runs continuously around the full perimeter of the building is composed of two separate nested extrusions with air and water barriers. As shown in Fig. 1, there is a vertical stack move­ment joint at each side of the typical unit. There is some variation in the size of the glazing units, but this occurs at the east and west corner conditions referred to as day room areas. The mullions are 3 in. (76.2 mm) wide by 7 in. (177.8 mm) deep with 1 in. (25.4 mm) of insulating glass outboard. The vertical extruded aluminum fins project 1 in. (25.4 mm) from the face of the glass for both the in­termediate mullions and vertical joint mullions, whereas the horizontal fins are flush with the face of the glass for intermediate mullions, but project 0.625 in. (15.9 mm) from the face of the glass for horizontal movement joint mullions.

The unitized curtain-wall system is connected to the building slab at the top of the two primary vertical mullions by a pair of vertical hook plates (two per panel) attached to the nesting vertical stack mullions just below the horizontal stack joint. These vertical plates are hooked onto an extruded aluminum bear­ing plate with serrated edges, which in turn is bolted to the cast-in Halfen chan­nel anchors at the slab edge, as shown in Fig. 1. Additionally, the vertical hook plates will be mechanically fastened to this bearing plate once final alignment has occurred. The hook anchor plate incorporates height adjusting screws, which provide vertical alignment of the unit during installation. Slotted holes in the aluminum bearing plate allow for in-and-out alignment. The bottom (sill) horizontal mullion is mechanically engaged into the head mullion of the unit below (Fig. 4). The mechanical connection of the vertical stack mullions to the slab through the hook plates restrains the top of each unit from movement in the three translational component directions. The mechanical engagement of the sill to the head of the unit below restrains only the out-of-plane movement. Figure 4 shows typical details of the horizontal and vertical stack joints.