Simplified Design for Columns - Simplified Design of Reinforced Concrete Buildings - page 147

Chapter 5
Simplified Design for Columns

5.1 INTRODUCTION

Use of high-strength materials has had a significant effect on the design of concrete columns. Increased use of
high-strength concretes has resulted in columns that are smaller in size and, therefore, are more slender.
Consequently, in certain situations, slenderness effects must be considered, resulting in designs that are more
complicated than when these effects may be neglected.

For buildings with adequate shearwalls, columns may be designed for gravity loads only. However, in some
structures—especially low-rise buildings—it may not be desirable or economical to include shearwalls.
In these situations, the columns must be designed to resist both gravity and lateral loads. In either case, it is
important to be able to distinguish between a column that is slender and one that is not. A simplified design
procedure is outlined in this chapter, which should be applicable to most columns. Design aids are given to
assist the engineer in designing columns within the limitations stated.

5.2 DESIGN CONSIDERATIONS

5.2.1 Column Size

The total loads on columns are directly proportional to the bay sizes (i.e. tributary areas). Larger bay sizes mean
more load to each column and, thus, larger column sizes. Bay size is often dictated by the architectural and
functional requirements of the building. Large bay sizes may be required to achieve maximum unobstructed
floor space. The floor system used may also dictate the column spacing. For example, the economical use of a
flat plate floor system usually requires columns that are spaced closer than those supporting a pan joist floor
system. Architecturally, larger column sizes can give the impression of solidity and strength, whereas smaller
columns can express slender grace. Aside from architectural considerations, it is important that the columns
satisfy all applicable strength requirements of the ACI Code, and at the same time, be economical. Minimum
column size and concrete cover to reinforcement may be governed by fire-resistance criteria (see Chapter 10,
Tables 10-2 and 10-6).

5.2.2 Column Constructability

Columns must be sized not only for adequate strength, but also for constructability. For proper concrete
placement and consolidation, the engineer must select column size and reinforcement to ensure that the
reinforcement is not congested. Bar lap splices and locations of bars in beams and slabs framing into the column
must be considered. Columns designed with a smaller number of larger bars usually improve constructability.