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(2) The use of exterior columns would normally be restricted to use
in framed structures to transfer roof and floor beam reactions to the
foundation. Normally, only tied columns would be used since they are
compatible with the placement of wall and beam reinforcement. Exterior
columns are not normally required for flat slab structures since roof and
floor loads are uniformly transmitted to the exterior walls.
b.
Design of Exterior Columns.
(1) Exterior columns are generally designed as beam elements.
The axial load on these columns may be significant, but usually the effect
of the transverse load is greater. The column will usually be in the
tension controls region (e > eb) of the PM curve (Figure 35) where the
addition of axial load increases the moment capacity of the member.
Consequently, the design of an exterior column as a beam, where the axial
load is neglected, is conservative.
(2) Since an exterior column is a primary member which is
subjected to an axial load, it is not permitted to attain large plastic
deformations. Therefore, the lateral deflection of exterior columns must be
limited to a maximum ductility, Xm/XE, of 3.
6.
EXAMPLE PROBLEMS.
a.
Design of a Beam.
Problem:
Design an interior beam of a roof subject to an overhead blast
loading.
Given:
(1) Structural configuration.
(2) Pressuretime loading.
(3) Deflection criteria.
(4) Material properties.
Solution:
(1) Calculate dynamic strength of materials from Table 53 of
NAVFAC P397 and Equations (32) and (33).
(2) Assume reinforcing steel and concrete cover.
(3) Check maximum and minimum steel ratios using
Equations (36), (37), and (42)
(4) Determine moment capacity of the sections using
Equations (34) and (35)
(5) Find ultimate resistance of the beam using Table 5.
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