Civil Engineering Reference
In-Depth Information
Higher strength concretes can be used more economically in columns than in beams.
Under ordinary loads, only 30 to 40% of a beam cross section is in compression, while the
remaining 60 to 70% is in tension and thus assumed to be cracked. This means that if a
high-strength concrete is used for a beam, 60 to 70% of it is wasted. For the usual column,
however, the situation is quite different because a much larger percentage of its cross sec-
tion is in compression. As a result, it is quite economical to use high-strength concretes
for columns. Although some designers have used concretes with ultimate strengths as
high as 19,000 psi (as at Two Union Square in Seattle) for column design with apparent
economy, the use of 5000- to 6000-psi columns is the normal rule when higher strengths
are specified for columns.
Grade 60 reinforcing bars are generally used for best economy in the columns of
most structures. However, Grade 75 bars may provide better economy in high-rise struc-
tures particularly when they are used in combination with higher strength concretes.
In general, tied columns are more economical than spiral columns, particularly if
square or rectangular cross sections are to be used. Of course, spiral columns, high-
strength concretes, and high percentages of steel save floor space.
As few different column sizes as possible should be used throughout a building. In
this regard, it is completely uneconomical to vary a column size from floor to floor to sat-
isfy the different loads it must support. This means that the designer may select a column
size for the top floor of a multistory building (using as small a percentage of steel as pos-
sible) and then continue to use that same size vertically for as many stories as possible by
increasing the steel percentage floor by floor as required. Furthermore, it is desirable to
use the same column size as much as possible on each floor level. This consistency of
sizes will provide appreciable savings in labor costs.
The usual practice for the columns of multistory reinforced concrete buildings is to
use one-story-length vertical bars tied together in preassembled cages. This is the pre-
ferred procedure when the bars are #11* or smaller, where all the bars can be spliced at
one location just above the floor line. For columns where staggered splice locations are
required (as for larger size bars), the number of splices can be reduced by using preassem-
bled two-story cages of reinforcing.
Unless the least column dimensions or longitudinal bar diameters control tie spac-
ings, the selection of the largest practical tie sizes will increase their spacings and reduce
their number. This can result in some savings. Money can also be saved by avoiding inte-
rior ties such as the ones shown in the bottom two rows of columns in Figure 9.4. With no
interior ties, the concrete can be placed more easily and lower slumps used (thus lower
cost concrete).
In fairly short buildings, the floor slabs are often rather thin, and thus deflections may
be a problem. As a result, rather short spans and thus close column spacings may be used.
As buildings become taller, the floor slabs will probably be thicker to help provide lateral
stability. For such buildings, slab deflections will not be as much of a problem, and the
columns may be spaced farther apart.
Even though the columns in tall buildings may be spaced at fairly large intervals,
they still will occupy expensive floor space. For this reason, many designers try to place
many of their columns on the building perimeters so they will not use up the valuable in-
*#36 in SI.
