Page 552 - Forensic Structural Engineering Handbook

15.4 MATERIAL-SPECIFIC FORENSIC ANALYSES

Curtain Walls
Beginning in the late nineteenth century, the need for taller buildings led to the develop-
ment of the structural steel frame. Exterior walls in higher rise buildings typically no longer
served to support the floors and roof but instead were needed primarily to protect the build-
ing’s interior from weather. These lightweight “curtain walls” consist of an exposed
weather barrier and a backup wall or structural support system, which accommodates grav-
ity and lateral loads transferred from the facade. Curtain walls can be barrier or drainage
systems; the former resist water penetration at the outer surface, while the latter accommo-
date some water penetrate by incorporating mechanisms to divert moisture to the exterior.
In addition, curtain walls can be rain screen systems, in which the exterior cladding serves
as a screen and diverts most water away from the building. However, some water is
expected to pass through open joints in the screen and is then resisted by an air/moisture
barrier at the backup wall. Rain screen walls can be pressure equalized or back ventilated;
each approach reduces the pressure drive at the exterior plane of the cladding system,
resulting in a lower volume of water reaching the air/moisture barrier. Successful design of
contemporary walls requires consideration of loads and forces acting upon the cladding
system, water penetration and air infiltration resistance, thermal efficiency, and movement
of the facade relative to the building’s structural frame.
The contemporary facade must be designed and connected to the structure in a manner
to support its own weight and to resist any anticipated lateral and planar forces with an ade-
quate factor of safety. Facades may be made from a wide variety of materials and systems,
including thin stone, precast concrete panels, metal panels, composite panels, metal and
glass curtain wall components, etc. Each type of facade has distinct benefits and potential
problems. There is a great deal of flexibility in the choice of materials, appearance, and con-
figuration of building facades. Industrial facilities may simply require a utilitarian envelope
of light-gauge metal panels over an economical building frame. High end commercial, res-
idential, and institutional buildings, however, may require a highly stylized envelope com-
bining glass and architectural metal, masonry, or thin stone panels.

MASONRY FACADES

Two broad categories of masonry facade failures include stability failures (e.g., complete
wall collapse, displacement of portions of the facade, or loose pieces) and serviceability
failures (e.g., water leakage, air infiltration, condensation, materials failures, etc.). A build-
ing collapse is an obvious failure; however, a facade that appears to be intact but cannot
sustain normal wind loads because of missing or corroded anchors behind the wall surface
is also a failure. Such failures can be more problematic because they are difficult to predict
and may result in collapse without warning. Loss of function of building facades, such as
air or moisture infiltration, is far more common than failures resulting in a catastrophic col-
lapse that claims life and property. Collectively, the economic consequences of such ser-
viceability failures may be far greater than those of collapses.
Patterns or characteristics of facade cracking, bulging, delamination, and leakage usu-
ally offer clues to their causes. Recognizing the symptoms of failure and patterns of distress
is essential to identification of the cause of failure. Facades are subjected to a wide variety
of forces and influences that can lead to failure. Deterioration of cladding materials or
anchorages undermines the ability of the facade to resist normal loads. External forces of a
magnitude or orientation not anticipated in design can damage facade and connection ele-
ments. Defects introduced during construction may affect the facade’s ability to perform as
designed. Identifying the source of failures, establishing the means of correcting damage