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DDEESSIIGGNN OOFF AAXXIIAALLLLYY LLOOAADDEEDD CCOOLLUUMMNNSS

Categories of ColumnsConcrete columns can be roughly divided into the following three categories:

1. Short compression blocks or pedestals. If the height of an upright compression

member is less than three times its least lateral dimensions it may be consideredto be a pedestal.2. Short reinforced concrete columns. Should a reinforced concrete column fail due

to initial material failure, it is classified as a short column. The load that it cansupport is controlled by the dimensions of the cross section and the strength ofthe materials of which it is constructed.

3. Long reinforced concrete columns. Should the length of a column be increased,the chances that it will fail by buckling will be increased. A column that fails bybuckling is said to be a long column.

Types of Columns

A plain concrete column can support very little load, but its load-carrying capacity will begreatly increased if longitudinal bars are added. Under compressive loads columns tendnot only to shorten lengthwise but also to expand laterally due to the Poisson effect. Thecapacity of such members can be greatly increased by providing lateral restraint in theform of closely spaced closed ties or helical spirals wrapped around the longitudinalreinforcing.

Reinforced concrete columns are referred to as tied or spiral columnsdepending on the method used for laterally bracing or holding the bars in place. If thecolumn has a series of closed ties, as shown in Figure 8.1(a), it is referred to as a tiedcolumn. These ties are effective in increasing the column strength. They prevent thelongitudinal bars from being displaced during construction and they resist the tendency

of the same bars to buckle outwards under load, which would cause the outer concretecover to break or spall off. Tied columns are ordinarily square or rectangular, but theycan be octagonal, round, L-shaped, and so forth.

If a continuous helical spiral made from bars or heavy wire is wrapped around thelongitudinal bars, as shown in Figure 8.1(b), the column is referred to as a spiralcolumn. Spirals are even more effective than ties in increasing a columns strength.The closely spaced spirals do a better job of holding the longitudinal in place, and theyalso confine the concrete inside and greatly increase its resistance to axialcompression. Spiral columns are usually round, but they also can be made intorectangular, octagonal, or other shapes.

Composite columns, illustrated in Figures 8.1(c) and (d), are concrete columnsthat are reinforced longitudinally by structural steel shapes, which may or may not besurrounded by structural steel bars, or they may consist of structural steel tubing filledwith concrete (commonly called lally columns).

Failure of Tied and Spiral ColumnsShould a short, tied column be loaded until it fails, parts of the shell or covering concretewill spall off and, unless the ties are quite closely spaced, the longitudinal bars willbuckle almost immediately as their lateral support (the covering concrete) is gone. Such

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Ties

LongitudinalBars

(a) Tied Column

Spirals

LongitudinalBars

(b) Spiral Column

(c) Composite Column

(d) Composite Column

Figure 8.1 Types of Columns

failures may often be quite sudden, and apparently they have occurred rather frequentlyin structures subjected to earthquake loadings.When spiral columns are loaded to failure, the situation is quite different. The

covering concrete or shell will spall off but the core will continue to stand, and if thespiral is closely spaced, the core will be able to resist an appreciable amount ofadditional load beyond the load that causes spalling. As a result, the spalling off of theshell of a spiral column provides a warning that failure is going to occur if the load isfurther increased.

NSCP Requirements for Cast-in-Place ColumnsThe NSCP specifies quite a few limitations on the dimensions, reinforcing, lateralrestraint, and other items pertaining to concrete columns. Some of the most importantlimitations are listed in the paragraphs to follow.

1. The area of longitudinal reinforcement may not be less than 1% or more than 8%of the gross area Ag section. (NSCP Section 410.10.1 )

2. The minimum numbers of longitudinal bars permissible in compression members(NSCP 410.10.2) are as follows: 4 for bars within rectangular or circular ties, 3 forbars within triangular ties, and 6 for bars enclosed by spirals.

3. The NSCP does not directly provide a minimum column cross-sectional area butto provide the necessary cover outside of ties or spirals and to provide thenecessary clearance between longitudinal bars from one face of the column tothe other it is obvious that minimum widths or diameters of about 200 to 250millimeters are necessary.

4. When tied columns are used, the ties shall not be less than 10mm diameter insize for 32mm diameter or smaller, and at least 12mm diameter in size for 36mmdiameter bars and bundled longitudinal bars. (NSCP Section 407.11.5.1)

5. The vertical spacing of ties should not exceed 16 times the longitudinal bardiameter, 48 times the diameter of the ties, or the least lateral dimension of thecolumn. (NSCP Section 407.11.5.2)

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Figure 8.2 Typical Tie Arrangements

150mm max 150mm max

150mm max

150mm max

150mm max 150mm max

>150mm> 150mm

>150mm

>150mm

> 150mm > 150mm

>150mm

>150mm

> 150mm > 150mm

150mm max150mm max 150mm max

>150mm

>150mm

150mm max 150mm max

>150mm

>150mm

150mm max

6. The ties must be arranged so that every corner and alternate longitudinal barshave lateral support provided by the corner of a tie having an included angle not

greater than 0135 and a bar shall be not farther than 150 mm clear on each sidealong the tie form such a laterally supported bar. (NSCP Section 407.11.5.3)(See Figure 8.2 for the tie arrangements for various column sections)

7. Ties should be located vertically not more than one half a tie spacing above thetop of footing or slab in any story and should also be spaced not more than onehalf a tie spacing below the lowest horizontal reinforcement in slab above.(NSCP Section 407.11.5.4)

8. Column ties should have hooks as specified in Section 407.2.3 of the NSCP.(NSCP Section 407.11.5.6)

9. The NSCP (Section 407.11.4.3) states the clear spacing of spirals may notexceed 75 mm or be less than 25 mm.

10. For cast-in-place construction, the size of spirals should not be less than 10mmdiameter. (NSCP Section 407.11.4.2)

11. Should splices be necessary in spirals, they are to be provided by welding or by

lapping the spiral bars or wires with a length of the larger of 48 times the bardiameter and 300mm. (NSCP Section 407.11.4.5)12. Section 407.11.4.6 of the NSCP states that spirals should extend from top of

footing or slab in any story to level of lowest horizontal reinforcement in memberssupported above.

13. The ratio of spiral reinforcements

should not be less than'

g cs

c y

A f0.45 1

A f

.

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Comments on Economical Column DesignReinforcing bars are quite expensive, and thus the percentage of longitudinal reinforcingused in reinforced concrete column is a major factor in their economy. This means thatunder normal circumstances a small percentage of steel should be used (perhaps in therange of 1.5 to 3%). This can be accomplished by using larger column sizes and/or

higher-strength concretes.In general, tied columns are more economical than spiral columns particularly ifsquare or rectangular columns are to be used. Of course, spiral columns, high-strengthconcretes, and high percentages of steel save floor spaces.

As few different column sizes as possible should be used throughout a building.This consistency of sizes will provide appreciable savings in formworks and labor costs.

Design FormulasSection 424.7.1 of the NSCP states that the service axial load capacity of columns maynot be greater than the following values:

For spiral columns,

'c g s y sP 0.255 0.85f A A f A

For tied columns,

'c g s y sP 0.224 0.85f A A f A

The equations presented here for tied and spiral columns are applicable only forsituations where there is no moment or the moment is sufficiently small so that eis lessthan 0.10h for tied columns or 0.05h for spiral columns and provided it is a shortcolumn. Should the e values be greater than the limiting values and/or should thecolumns be classified as long ones, it will be necessary to use the procedures describedin the later sections.

Example 1. (Tied Column)Design a square tied column to support an axial dead load DL of 580 kN and an axial

live load LL of 800 kN. Assume 2% longitudinal steel is desired, cf ' 27.6 MPa and

yf 414 MPa. For lateral ties, use yf 276 MPa.

Solution:1. Calculate Design Load P ,

P DL LL 580 800 1380 kN2. Select Column Dimensions,

'

c g s y sP 0.224 0.85f A A f A

3

g g g1380x10 0.224 0.85 27.6 A 0.02A 414 0.02A

g A 197012mm2

Assume square section, B = L

gB A 197012 443.86 mm

Say, use 450mm x 450mm column

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Figure 8.3 Column Section Detail

450 mm

450 mm

Vertical Bars: 8 25mm bars

Lateral Ties: 2 sets - 10mm at 400 mm O.C.

40 mm

40 mm

3. Select Longitudinal Bars,

2

g A 450 202500mm2

3

s s1380x10 0.224 0.85 27.6 202500 A 414 A

s A 3610.55mm2

Using 25mm bars 2b A 490.9 mm ,

sb

A 3610.55n 7.35

A 490.9Say, use 8 25mm diameter bars

4. Design of Ties (Assuming 10mm bars),Spacing: (a) 48 x 10 = 480 mm

(b) 16 x 25 = 400 mm(c) Least dimension of column = 450 mm

Say, use 10mm ties at 400 mm O.C.5. Draw Column cross section,

Example 2. (Spiral Column)Design a round spiral column to support an axial dead load D of 800 kN and an axial

live load of 1340 kN. Assume 2% longitudinal steel is desired, cf ' 27.6 MPa and

yf 414 MPa. For lateral ties, use yf 276 MPa.

Solution:1. Calculate Design Load P ,

P DL LL 800 1340 2140 kN2. Select Column Dimensions,

'

c g s y sP 0.255 0.85f A A f A

3

g g g2140x10 0.255 0.85 27.6 A 0.02A 414 0.02A

g A 268371mm2

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600 mm

Dc = 520 mm40mm 40mm

Figure 8.4 Column Section Detail

Vertical Bars: 10 25mm bars

Spirals: 10mm at 40 mm O.C.

2g

g

4 2683714AD A ;D 584.55

4mm

Say, use 600mm diameter column3. Select Longitudinal Bars,

2

g A 600 2827434 mm2

3

s s2140x10 0.255 0.85 27.6 282713 A 414 A

s A 4506mm2

Using 25mm bars 2b A 490.9 mm ,

sb

A 4506n 9.18

A 490.9Say, use 10 25mm diameter bars

4. Design of Spirals (Assuming 10mm bars), cD D 80 600 80 520 mm

Minimum

2'

g cs

2c y

600A f 27.640.45 1 0.45 1 0.014911 A f 276

5204

Spacing: (a)

s

c s

4 78.54As 40.50

D 520 0.014911Ok!

(b) s < 75 mm(c) s > 25 mm

Say, use 10mmspirals at 40 mm O.C.

5. Draw Column cross section,