1.1.2Short Columns
1.Behavior of Axially Loaded Short Column
When an axial load is applied to a reinforced concrete short column ,the concrete can be considered to behave elastically up to a low stress of about (1/3) ft'.Two different types of failure occur in columns ,depending on whether ties or spirals are used .A tied column fails at the load Na (figure 4.25) .At this load ,the concrete fails by crushing or shearing outward along inclined planes ,and the longitudinal steel bars fail by bucking outward between ties ,as shown in Figure 4.26a .The column failure occurs suddenly ,much like the failure of a concrete cylinder .
In a spirally reinforced column ,when the same load Na is reached ,the longitudinal steel and the concrete within the core are prevented from moving outward by the spiral .The concrete in the outer shell ,however ,not being so confined ,does fail ; i.e. ,the outer shell spalls off when Na is reached ,as shown in Figure 4.26b. It is at this stage that confining action of the spiral has a significant effect ,and if sizable spiral steel is provided ,the load will ultimately fail the column by causing the spiral steel to yield .The axial strain when the column fails can be much larger than that at which the shell spalls off .
If the load on the column is increased to reach its ultimate strength ,the concrete will reach the maximum strength and steel will reach its yield strength , fy .See Figure 4.27. The ultimate bearing capacity of a short column reinforced with longitudinal bars and lateral ties can be written as follow
Na = fy' As' + fc Ac
where ,
As' =reinforcement area
fy' =reinforcement yield strength
Ac =concrete section area
fc =concrete prismatic compressive strength
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