Building Design and Construction Handbook - Merritt - Ventech!

9.124 SECTION NINE
9.84 COLUMN TIES AND TIE PATTERNS
For full utilization, all ties in tied columns must be fully developed (for full tie
yield strength) at each corner enclosing a vertical bar or, for circular ties, around
the full periphery.
Splices. The ACI 318 Building Code provides arbitrary minimum sizes and maximum
spacings for column ties (Art. 9.83). No increases in size nor decrease in the
spacings is required for Grade 40 materials. Hence, the minimum design requirements
for splices of ties may logically be based on Grade 40 reinforcing steel.
The ordinary closed, square or rectangular, tie is usually spliced by overlapping
standard tie hooks around a longitudinal bar. Standard tie patterns require staggering
of hook positions at alternate tie spacings, by rotating the ties 90 or 180�. (‘‘Manual
of Standard Practice,’’ Concrete Reinforcing Steel Institute). Two-piece ties are
formed by lap splicing or anchoring the ends of U-shaped open ties. Lapped bars
should be securely wired together to prevent displacement during concreting.
Tie Arrangements. Commonly used tie patterns are shown in Figs. 9.52 to 9.54.
In Fig. 9.53, note the reduction in required ties per set and the improvement in
bending resistance about both axes achieved with the alternate bundled-bar arrangements.
Bundles may not contain more than four bars, and bar size may not exceed
No. 11.
Tie sizes and maximum spacings per set of ties are listed in Table 9.25.
Drawings. Design drawings should show all requirements for splicing longitudinal
bars, that is, type of splice, lap length if lapped, location in elevation, and layout
in cross section. On detail drawings (placing drawings), dowel erection details
should be shown if special large longitudinal bars, bundled bars, staggered splices,
or specially grouped bars are to be used.
9.85 BIAXIAL BENDING OF COLUMNS
If column loads cause bending simultaneously about both principal axes of a column
cross-section, as for most corner columns, a biaxial bending analysis is required.
For rapid preliminary design, Eq. (9.99) gives conservative results
M M x y
� � 1 (9.99)
Mox Moy
where M x, M y � factored moments about x and y axes, respectively
M ox, M oy � design capacities about x and y axes, respectively
For square columns with equal longitudinal reinforcement in all faces, M ox �
M oy, and the relation reduces to:
M � M
x y � 1 (9.100)
M ox
Because M x � e xP u and M y � e yP y, the safe biaxial capacity can be taken from