lecture20 - Lecture20Columns July23,2003 CVEN444...

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    Lecture 20 – Columns Lecture 20 – Columns July 23, 2003 CVEN 444
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    Lecture Goals Lecture Goals Definitions for short columns Columns
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    Analysis and Design of “Short”  Analysis and Design of “Short”  Columns Columns General Information Vertical Structural members Transmits axial compressive loads with or without moment transmit loads from the floor & roof to the foundation Column:
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    Analysis and Design of  Analysis and Design of  “Short” Columns “Short” Columns General Information Column Types: 1. Tied 2. Spiral 3. Composite 4. Combination 5. Steel pipe
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    Analysis and Design of  Analysis and Design of  “Short” Columns “Short” Columns Tie spacing h (except for seismic) tie support long bars (reduce buckling) ties provide negligible restraint to lateral expose of core Tied Columns - 95% of all columns in buildings are tied 2245
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    Analysis and Design of “Short”  Analysis and Design of “Short”  Columns Columns Pitch = 1.375 in. to 3.375 in. spiral restrains lateral (Poisson’s effect) axial load delays failure (ductile) Spiral Columns
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    Analysis and Design of “Short”  Analysis and Design of “Short”  Columns Columns Elastic Behavior An elastic analysis using the transformed section method would be: st c c nA A P f + = For concentrated load, P uniform stress over section n = E s / E c A c = concrete area A s = steel area c s nf f =
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    Analysis and Design of “Short”  Analysis and Design of “Short”  Columns Columns Elastic Behavior The change in concrete strain with respect to time will effect the concrete and steel stresses as follows: Concrete stress Steel stress
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    Analysis and Design of  Analysis and Design of  “Short” Columns “Short” Columns Elastic Behavior An elastic analysis does not work, because creep and shrinkage affect the acting concrete compression strain as follows:
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    Analysis and Design of  Analysis and Design of  “Short” Columns “Short” Columns Elastic Behavior Concrete creeps and shrinks, therefore we can not calculate the stresses in the steel and concrete due to “acting” loads using an elastic analysis.
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    Analysis and Design of  Analysis and Design of  “Short” Columns “Short” Columns Elastic Behavior Therefore, we are not able to calculate the real stresses in the reinforced concrete column under acting loads over time. As a result, an “allowable stress” design procedure using an elastic analysis was found to be unacceptable. Reinforced concrete columns have been designed by a “strength” method since the 1940’s. Creep and shrinkage do not affect the strength of the member. Note:
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    Behavior, Nominal Capacity and  Behavior, Nominal Capacity and  Design under Concentric Axial loads Design under Concentric Axial loads Initial Behavior up to Nominal Load - Tied and spiral columns.
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lecture20 - Lecture20Columns July23,2003 CVEN444...

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