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Slender Columns

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Columns

Cast-in-place concrete columnstypically carry both axial loads

and moments.

Interior columns have momentdue to the difference in themoments on the adjacent spans

Exterior columns have larger

moments because they only havea slab or beam framing into oneside.

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Effect of Shape

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Reinforcing Configurations

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Slender 

A slender column is defined as a column that has asignificant reduction in its axial-load capacity due tomoments resulting from lateral deflections of the

column. In the derivation of the ACI Code, “a significant

reduction” was arbitrarily taken as anything greaterthan about 5%

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Basis of Slender 

Lateral deflections of a slender column cause an increase in the columnmoments,

These increased moments cause an increase in the deflections, which n turn

lead to an increase in the moments.

If the axial load is below the critical load, the process will converge to astable position. This is referred to as a second-order process

Two different types of second-order moments act on the columns in aframe:

P-δ moments. These result from deflections, of the axis of the bent column

away from the chord joining the ends of the column

P-Δ moments. These result from lateral deflections, of the beam – column joints

from their original undeflected locations

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Most building columns fall in the short-column category. Exceptions

occur: in industrial buildings and in buildings that have a high first-floor story for

architectural or functional reasons.

Some bridge piers and the decks of cable stayed bridges

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Differences in moments

Pin-ended columns are rare in cast-in-place concrete construction, butdo occur in precast construction.

the columns supporting the back of a precast grandstand.

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Braced and Unbraced

Most concrete building structures are braced (nonsway) frames

the bracing provided by considerably stiffer elements than the

columns themselves: shear walls,

stairwells, or

elevator shafts

Unbraced frames are encountered:

near the tops of tall buildings, where elevator core may be discontinued

in industrial buildings where an open bay exists.

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