| Building Facade Assessment Wacker Washington
Plaza is a high-rise office building located in downtown Chicago.
The building is clad with polished granite panels and an aluminum
curtainwall storefront. Chicago experiences extreme freeze-thaw
cycles that cause trapped moisture to expand when it freezes,
exerting forces on building materials (e.g., masonry, concrete,
stone) causing it to spall and eventually delaminate. This frequently
occurs on building façades where the cladding is the
only line of defense against water infiltration behind the building
façade and into the building. Along the plaza level,
many of the panels displayed varying degrees of distress at
locations along the perimeter of the panel, first observed as
staining. The staining coincided with the anchor locations for
the granite panels, which then manifested into spalling, and
in some cases delamination of the granite panel. Water and moisture
infiltrating behind the granite panels caused corrosion and
expansion of the panel anchors known as "rust jacking."
The corroding anchors placed stresses on the panel that caused
the panel to spall and, eventually, delaminate. Exponent recommended
that the granite panels on the plaza deck be removed and new
stainless-steel anchors and granite panels be installed.
 Roof Failure On a Sunday evening in 1991, a San Francisco Bay Area bowling alley filled with bowlers heard a loud noise from the ceiling area accompanied by noticeable deformations of the ceilings panels. The alley operator quickly evacuated the building. A few hours later, partial collapse of the 1958 roof structure occurred. Exponent investigated the cause of the collapse and determined it had resulted from a bending failure of a 100-foot long, 4-foot deep, simply supported, glue-laminated beam. The loads on the beam at the time of the collapse were very light, consisting of only the dead loads (roofing, structure self weight, ceiling tile, and sprinkler). Exponent retrieved the failed glulam beam, which was placed and reconstructed for examination of the fracture patterns and failure surfaces. Samples were taken from the beam for testing of the glue-line between laminations. We found no evidence of decay or poor glue-line strength. Exponent's investigation determined that the glue-laminated beams were properly designed for the allowable strength of the lumber as reflected in 1958 standards. However research over the subsequent thirty years revealed that the strength of lumber assumed by 1950s standards was overly optimistic. The allowable stresses under current codes would be only 60 percent of the allowable stresses at the time of manufacture. We found that the stresses under long-term dead loads alone were potentially greater than the ultimate strength of the beams, which explained the collapse.
 Concrete
Assessment A Los Angeles, California, condominium complex
suffered damage from the January 17, 1994 Northridge earthquake.
The complex, an elevated slab structure, consists of a parking
area at ground level, a concrete slab at the second story
supported by circular concrete columns with drop panels, and
three levels of wood-framed dwelling units supported by the
slab. During the earthquake, the stucco exterior finish of
the wood-framed portion sustained damage, and the elevated
slab suffered cracking to the concrete masonry block walls
that provided lateral force resistance. Exponent conducted
a load test to determine if the structure was in danger of
collapse and would require extensive repair. We performed
full-scale load tests meeting American Concrete Institute
(ACI) requirements and safety guidelines. The ACI load was
applied to a 940-square-foot area. Loading occurred by placing
55 gallon drums over the test area and filling them with water.
Maximum load was applied for 24 hours and then removed. Slab
deflections were measured continuously throughout the load
test, and maximum deflections were on the order of 0.25 inches.
Exponent determined the slab was in good condition.
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