Page 299 - Forensic Structural Engineering Handbook

9.10 CAUSES OF FAILURES

detailing or installation of soft joints below shelf angles often results in unintended com-
pressive forces and related distress within the wall. The design and placement of rein-
forcement and lateral anchorage is also critical in controlling the load paths within the wall
system because defects can result in instability or excessive stress (see Fig. 9.5).

FIGURE 9.5 Defective masonry wall tie installation (pintles not engaging eyes).

Masonry systems are particularly susceptible to distress due to defects in detailing of the
structural and architectural elements. Defects in the weather-resisting systems often result
in premature failure, such as porous bricks or deficient terracotta glazing, which leads to
increased water absorption. Improper detailing and installation of flashing are notorious
defects that result in leaks and trapped water within masonry walls. Additional defects
commonly found in masonry walls include insufficient weep holes and excessive mortar
droppings that block weep holes.

Deterioration. Deterioration in exterior masonry walls usually propagates at an increas-
ing rate due to weathering factors. Specific weather-related mechanisms include freeze-
thaw damage, salt recrystallization, and embedded steel corrosion. Stress factors and
incompatibility factors also contribute to accelerating the deterioration of masonry, such as
cracking or crushing of masonry at lintels.
Various types of masonry units share similar deterioration mechanisms that include
spalling, cracking, and delamination. This deterioration allows increased water infiltration,
commonly associated with a reduction in structural integrity. The deterioration can accel-
erate due to freeze-thaw damage that follows the water absorption.
Embedded steel elements may corrode in a moist environment, and the corrosion prod-
ucts result in material expansion in the form of pack-rust. Pack-rust may expand several
times the thickness of the original steel, and the swelling induces stress in the surrounding
masonry units, an effect known as “rust-jacking.” In addition, the corrosion of embedded
steel components results in a reduction in the structural capacity of the exterior wall. In