Page 553 - Forensic Structural Engineering Handbook

BUILDING FACADES 15.5

due to the failure, and convincingly demonstrating the cause of failure and appropriateness
of the remediation is the responsibility of the investigating engineer or architect.

Stability-Related Failures

Building facades must resist a variety of externally and internally imposed forces. External
forces may include lateral loads from wind or earthquakes and vertical loads from the
facade’s own weight. The facade also may be subjected to localized internal forces imposed
by expanding elements embedded in the wall (e.g., corroding shelf angles or reinforcing
steel) or by differential planar movements between the facade element and the wall’s sub-
strate or the building’s structural frame. External forces are usually well understood, and
the response of the facade to these forces may be predictable. Wind, gravity, and even
earthquake loads can be quantified and analyzed. The facade’s mechanical properties and
anchorage can be designed to accommodate such forces with an appropriate factor of
safety. However, internal forces are more complex and may vary substantially from struc-
ture to structure and within the same structure, depending on the facade materials and con-
figurations employed. Such forces are sometimes overlooked or misunderstood, leading to
failures that are difficult to analyze.

Relative Stiffness. Many wall systems involve rigid facade materials that are weak in ten-
sion, with frames or anchor systems that are strong in tension but flexible. The distribution
of wind loads or other lateral forces in such a wall system is a function of the relative stiff-
ness of each wall component. For example, masonry claddings may resist virtually all
applied wind load while the light-gauge steel stud backup frame shares very little load until
the masonry cracks. Also, the flexibility of the wall tie or facade anchorage system will
affect the proportion of wind load shared with the backup wall. Disproportionate distribu-
tion of wind load can cause excessive cracking and deflection of the masonry in this type
of wall system.

Restrained Facade Expansion. Restrained expansion and contraction of the facade
material relative to the building’s structural frame is a frequent cause of facade stress fail-
ures. This type of distress is often related to cyclic thermal expansion and in some cases
moisture-related growth of the facade, coupled with resistance from the structural frame.
The frame does not expand as much due to these influences because it is sheltered from the
weather and relatively free from thermal fluctuations. Also, the weight of the building
induces elastic deformation, and shrinkage and creep occur in concrete building frames,
which further increase facade compressive forces by shortening the structural frame. Also,
with fired-clay masonry products, slow ceramic/moisture expansion of the facade con-
tributes to differential movement and compressive stress in the facade. These differential
movements accumulate in long or tall walls. Depending on how rigidly the cladding is
attached, this differential movement can create potential facade stress many times greater
than that due to normal gravity or wind loading. Figure 15.1 shows a building facade dam-
aged by restrained accumulated expansion. Expansion joints, which are designed to accom-
modate facade movement, create continuous gaps that are sealed with elastomeric
materials. Facade failures commonly occur when these expansion joints are inadequately
designed or are compromised by hard material or mortar blockage within the joint that
restricts free movement.
The symptoms of high stress in the facades are often readily recognizable. High verti-
cal stress may cause bulging or cracking, while compression buckling or vertical shear
cracks may occur at building corners where there is reduced lateral confinement of the facade.
Restrained horizontal facade movement causes step cracking and horizontal displacement