1 September 1999
failure of one bay of bracing will not cause a serious torsional response which could jeopardize
the bracing on the other sides of the building. A single row of bracing could be considered to
have redundant characteristics if each bracing element is designed to carry both tension and
compressive loads. This is not, however, as reliable as a system containing two or more
braced bays. The practice of limiting the bracing on a side of a building to a single bay should
be avoided if possible. A single braced bay consisting of tension-only bracing is unacceptable.
In a building without redundant components, every component must function as intended to
preserve the overall structural integrity of the building.
d. Ductility. It is desirable, and required by code, that structures be ductile to avoid brittle
failure mechanisms which could lead to an unexpected failure. Ductility is imperative for the
design of earthquake resistant structures.
(1) Structural steel is an inherently ductile material. The ductility of steel structures is
achieved by designing connections to avoid tearing or fracture and by ensuring an adequate
path for loads to travel across the connection. Detailing for adequate stiffness and restraint of
compressive braces, outstanding legs of members, compression flanges, etc. must be
provided to avoid local and global instability by buckling of relatively slender steel members
acting in compression. Deflections must be limited to prevent overall frame instability due to P-
∆ effects. Steel bracing systems must be configured such that bracing forces can not distort
columns in a manner that would amplify P-∆ effects Refer to TI 809-04 for additional
information on acceptable and unacceptable bracing systems).
(2) Less ductile materials, such as concrete and unit-masonry, require steel
reinforcement to provide ductility. Concrete structures should be designed to prevent brittle
failure mechanisms such as compressive failure, shear failure, anchorage failure, and bond
failures from occurring. Compressive failures in flexural members can be controlled by limiting
the amount of tensile reinforcement or by providing compressive reinforcement and confining
the reinforcement with closely spaced transverse reinforcement such as spirals, or stirrup ties.
Confinement increases the strain capacity and the compressive, shear and bond strengths of
concrete and masonry. Shear failures in concrete and masonry can be prevented through the
use of adequate shear reinforcing. Anchorage and splice failures can be controlled by
providing adequate splice development length, or by providing suitable mechanical or welded
connections. Masonry walls must be adequately reinforced and anchored to floors and roofs.
Additional information on the detailing of structures to provide a ductile response to earthquake
ground motions can be found in TI 809-04.
e. Quality of Materials and Construction. It has been observed that the quality of
materials used in construction and the quality of construction are important factors, which can
determine whether or not a building will survive extreme loadings due to earthquakes or wind.
Testing and inspection programs are necessary to ensure the finished structure meets code
requirements. It is the designer's responsibility to assure all testing and special inspections
required by code are part of the contact documents.