Page 36 - Design of Reinforced Masonry Structures

INTRODUCTION 1.5

of steel reinforcement for concrete masonry began between 1930 and 1940. In the ensuing
years, reinforced concrete masonry became a viable construction practice for single and
multistory buildings such as schools, hospitals, hotels, apartment complexes, commercial,
shopping centers, and industrial and office buildings. One of the tallest modern reinforced
concrete masonry structures is the 28-story Excalibur Hotel in Las Vegas, Nevada. The
load-bearing walls of this four-building complex are built from concrete masonry of 4000 psi
compressive strength [1.19]
Research on masonry continues in the United States in an organized way. In 1984, the
Technical Coordinating Committee for Masonry Research (TCCMR) was formed for the
purpose of defining and performing both experimental and analytical research and develop-
ment necessary to improve structural masonry technology [1.20]. Masonry construction,
which evolved as masons’ creations, turned into engineered construction based first on
empirical design and later on engineering principles. From 1984 to 1994, 19 researchers
conducted the most extensive research program ever into the development of a limit states
design standard for the design of masonry buildings in seismic areas [1.21].

1.5 UNREINFORCED AND REINFORCED
MASONRY

Unreinforced masonry has been in use in the United States as in the rest of the world
for many centuries. The early masonry structures were unreinforced and built to support
only the gravity loads; lateral forces from wind and earthquakes were ignored (for lack of
basic knowledge of dynamic forces). The massiveness of these structures provided stabil-
ity against lateral loads. Stone masonry dams and reservoirs are examples of unrfeinforced
masonry structures that resisted water pressure through their massiveness. However, the lat-
eral load resisting capability of ordinary masonry structures had always been questionable.
In the western United States, the inherent weakness of unreinforced masonry structures to
resist lateral loads was clearly exposed during the 1933 Long Beach earthquake (M 6.3). *
Although strong enough to resist gravity loads, these structures proved incapable of provid-
ing the required lateral resistance to seismic forces. Thus, in the ensuing period, reinforcing
of masonry construction was codified, resulting in the modern form of engineered rein-
forced masonry construction. A significant advantage of reinforced masonry was dramatic
reduction in the thickness of walls that were designed to resist dynamic lateral loads due
to wind and earthquakes.
Poor performance of unreinforced masonry was evident during the October 1, 1987
Whittier Narrows earthquake (M 6.3) and the October 17, 1989 Loma Prieta earthquake
(M 7.1) [1.21] in the United States, and during many earthquakes in other parts of the
world. In the January 17, 1994 Northridge earthquake (M = 6.7), hundreds of unreinforced
w

masonry structures were severely damaged and some simply collapsed. Many engineered
reinforced masonry structures and retrofitted unreinforced masonry structures also were
severely damaged during this earthquake, due presumably to poor engineering design, lack
of proper detailing, or as a result of poor workmanship and quality control. Extensive
destruction of unreinforced masonry structures during these earthquakes again called atten-
tion to, among other factors, poor tension and shear resistance of unreinforced masonry.

*
The devastating effects of this earthquake served as wake-up call to the rapidly growing Southern California
region from which evolved a new profession: earthquake engineering. The 1933 earthquake led to the passage of two
laws in California—one outlawing the construction of unreinforced brick buildings in the State of California and the
other requiring that all school buildings be designed to meet specific earthquake resistance standards.