CEMP-E
TI 809-02
1 September 1999
loadings, experience greater damage than regularly configured systems. Irregular structures
are structures having one or more plan irregularities or one or more vertical irregularities. Plan
irregularities are defined in FEMA 302, Table 5.2.3.2. Vertical irregularities are described in
FEMA 302, Table 5.2.3.3. Plan irregularities, such as large reentrant corners or differences in
stiffness between portions of diaphragms, produce stress concentrations and high localized
forces. Other plan irregularities can cause an undesirable torsional response to lateral loads.
Buildings that are T, L, U-shaped, or cruciform configurations are examples of irregular plans.
Often seismic joints, which separate the various wings of buildings, are provided to allow each
wing to perform as an individual structure. This is often more practical than using a rigorous
three-dimensional analysis to determine how the wings of the building will interact if connected
together. Vertical irregularities, such as an abrupt change in stiffness from one story to the
next, or offsets in the lateral force resisting systems from one story to the next, can also
produce stress concentrations and high-localized forces. Regular structural system
configuration should be encouraged for new designs, especially for buildings located in high
seismic areas. It should be realized however that an regular configuration is not always
possible. In such cases the designer must recognize the effect each particular irregularity will
have on structural response and make a conscious effort to ensure his design will meet all
strength and serviceability requirements.
b. Load Path Continuity. Direct and continuous load paths should be provided to assure
that all loads to which the structure is subjected can be delivered from the load point of
application to the foundation. All elements and connections along the load path must have
sufficient strength, stiffness, and deformation capability to deliver the loads to the foundation
without impairing the buildings ability to perform as a unit. When connection is required to
develop the strength of the connected members (such as in the design of earthquake-resistant
systems), the effect member over-strength will have on system performance must be
considered. Different parts of the building should be adequately interconnected to resist
extreme loads, to prevent progressive type failures, and resist foundation settlement. Beams
and girders should be adequately tied to columns, and columns should be adequately tied to
footings. Concrete and masonry walls should be adequately anchored to floors and roofs for
lateral support. Diaphragms that distribute lateral loads to vertical resisting elements must be
adequately tied to those elements. Collector or drag struts should be provided to collect shear
forces and deliver them to shear-resisting elements, such as shear walls or other bracing
elements, that may be spaced at various intervals around the diaphragm. Shear walls must be
adequately tied to floor and roof slabs and to footings. Non-structural elements such as
exterior cladding and interior stairs should be isolated from structural load paths to assure
loads are delivered as intended from the point of application to the foundation. Rigid non-
structural features if not properly isolated can attract loads that will most likely damage non-
structural elements and in the process create unintended load paths that can damage
structural elements.
c. Redundancy. Redundancy of load paths is a desirable structural system
characteristic, especially with respect to earthquake-resistant design. Redundancy means that
when one structural element or system fails a new load path develops allowing the loads once
carried by the failed structural element or system to be safely transferred to another primary or
secondary system thereby preventing progressive collapse of the structure. Redundant
systems if properly designed will also prevent the formation of unwanted load paths. For
instance if two or more bays of lateral bracing are provided on each side of a building the
2- 5
Previous Page
