UFC 4-023-03
25 January 2005
CHAPTER 3
DESIGN STRATEGIES
The progressive collapse design requirements employ two design/analysis
approaches: Tie Forces (TF) and Alternate Path (AP). This chapter discusses the
general procedures for these approaches.
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TIE FORCES.
In the Tie Force approach, the building is mechanically tied together,
enhancing continuity, ductility, and development of alternate load paths. Tie forces are
typically provided by the existing structural elements and connections that are designed
using conventional design procedures to carry the standard loads imposed upon the
structure.
Depending upon the construction type, there are several horizontal ties that
must be provided: internal, peripheral, and ties to edge columns, corner columns, and
walls. Vertical ties are required in columns and load-bearing walls. Figure 3-1
illustrates these ties for frame construction. Note that these "tie forces" are not
synonymous with "reinforcement ties" as defined in the 2002 version of the Building
Code Requirements for Structural Concrete from the American Concrete Institute (ACI
318-02) for reinforced concrete design.
The load path for peripheral ties must be continuous around the plan
geometry and, for internal ties, the path must be continuous from one edge to the other.
Along a particular load path, different structural elements may be used to provide the
required tie strength, providing that they are adequately connected; for instance, an
internal tie strength may be provided by a series of beams on a beam line, provided that
the connections to the intermediate elements (girders, beams or columns) can provide
the required tie strength. Likewise, vertical ties must be continuous from the lowest
level to the highest level. Horizontal ties to edge columns and walls do not have to be
continuous, but they must be satisfactorily anchored back into the structure. For
buildings that are composed of separate sub-structures or that incorporate expansion
joints that create structurally independent sections, the tie force requirements are
applied to each sub-structure or independent section, which are treated as separate
units. Note that all tie force paths must be geometrically straight; changes in direction
to accommodate openings or similar discontinuities are not allowed.
3-1.1
Load and Resistance Factor Design for Tie Forces.
Following the Load and Resistance Factor Design (LRFD) approach, the
design tie strength provided by a member or its connections to other members is taken
as the product of the strength reduction factor, Φ, and the nominal tie strength Rn
calculated in accordance with the requirements and assumptions of applicable material
specific codes, including an over-strength factor, Ω, as applicable. (Note that for wood
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