UFC 4-151-10
10 September 2001
6-2.1
Simplifying Design Assumptions. Structural design commonly employs
simplifying assumptions intended to make the design effort more manageable. These
assumptions are invariably conservative and often result in substantial excess strength,
for example, in presenting the distribution of concentrated loads to a slab, and in the
structural design of variable sections.
6-2.1.1
Distribution of Concentrated Loads to a Slab. Conventional procedures,
such as those described in the references of MIL-HDBK-1002/3, DM-2.04, and the
American Association of State Highways and Transportation Officials (AASHTO),
underestimate distribution of concentrated loads applied to a slab. Previous discussion in
paragraph 6-1 relates here.
6-2.2
Locations of Weakened Sections. Members are proportioned for
maximum stress conditions. The section required at points of maximum stress frequently
is carried for the full length of the member to minimize the costs of fabrication or of form
work, or for aesthetic reasons. If the deterioration of a member is localized and does not
occur at a point of maximum stress, the "strength" of the overall member may not be
impaired by exhibited deterioration.
6-2.3
Changed Design Methodology. If a structure was originally designed
based on an elastic analysis, re-analysis based on ultimate strength, plastic redistribution
of moments, or moment distribution based on the concept of yield line theory will
frequently yield a greater analytical capacity.
6-2.4
Design Live Loads. Design live loads are seldom realized in practice.
Therefore, original design loadings should be assessed and compared to actual load
requirements for continuing validity.
6-2.5
Excess Section. Designs often contain excess strength by way of
provision of sacrificial metal, rounding member sizes to the next heavier section, or to a
lighter but stronger section, or for satisfaction of requirements for minimum thickness of
metal or limiting deflection. Piling, in particular, is often sized beyond its structural
requirement to resist driving stresses or for load transfer to the soil.
6-2.6
Change of Structural Action. The structural response to an applied load
may differ from that assumed during design. Ordinary beams and slabs are a common
case in point. These are proportioned on the basis of pure flexural behavior. However,
except for large ratios of span to depth, pure flexural action is not realized, and the
member resists the load, at least partly, by catenary action or arching. Composite actions
between separate different structural elements may develop over time, which had not yet
developed during original construction. Yield points may develop, changing the moment
diagram and reactions, thus increasing some and decreasing others, with the changes
often being non-critical.
6-2
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