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(7) Lateral ties must enclose all longitudinal bars in compression
to insure their full development. These ties must conform to the following:
(a) The ties shall be at least No. 3 bars for longitudinal bars
No. 8 or smaller and at least No. 4 bars for No. 9 longitudinal bars or
greater.
(b) To insure the full development of the ties, they shall be
closed using 135degree hooks. The use of 90degree bends is not
recommended.
(c) The vertical spacing of the ties shall not exceed 16
longitudinal bar diameters, 48 tie diameters, or 1/2 of the least dimension
of the column section.
(d) The ties shall be located vertically not more than 1/2 the
tie spacing above the top of footing or slab and not more than 1/2 the tie
spacing below the lowest horizontal reinforcement in a slab or drop panel.
Where beams frame into a column, the ties may be terminated not more than 3
inches below the lowest reinforcement in the shallowest of the beams.
(e) The ties shall be arranged such that every corner and
alternate longitudinal bar shall have lateral support provided by the corner
of a tie with an included angle of not more than 135 degrees and no bar
shall be farther than 6 inches clear on each side along the tie from such a
laterally supported bar.
(8) The above requirements for the lateral ties are to insure
against buckling of the longitudinal reinforcement in compression. However,
if the section is subjected to large shear or torsional stresses, the closed
ties must be increased in accordance with the provisions established for
beams.
f.
Design of Spiral Columns
(1) Spiral columns may be subjected to significant bending moments
about both axes and should therefore be designed for biaxial bending.
However, due to the uniform distribution of the longitudinal reinforcement
in the form of a circle, the bending moment (or eccentricities) in each
direction can be resolved into a resultant bending moment (or eccentricity).
The column can then be designed for uniaxial bending using Equations (63)
and (68) for the regions where compression and tension controls the design,
respectively.
(2) Since spiral columns show greater toughness than tied columns,
particularly when eccentricities are small, the minimum eccentricity for
spiral columns is given as 0.05D in each direction rather than 0.1h in each
direction for tied columns. The resultant minimum eccentricity for a spiral
column is then equal to 0.0707D. Therefore, if computations show that there
are no moments at the ends of a column or that the computed resultant
eccentricity of the axial load is less than 0.0707D, the column must be
designed for a resultant minimum eccentricity of 0.0707D.
(3) To insure proper behavior of a spiral reinforced column, the
longitudinal reinforcement must meet the same restrictions given for tied
columns concerning minimum and maximum area of reinforcement, smallest and
largest reinforcing bars permissible and the minimum clear spacing between
bars. The only difference is that for spiral columns the minimum number of
longitudinal bars shall not be less than 6 bars.
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