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High Rise Design: Accounting for Column Length Changes

by Mark Fintel, (M.ASCE), Dir.; Advanced Engrg. Services Dept., Portland Cement Assoc., Skokie, Ill. 60077,
S. K. Ghosh, (M.ASCE), Principal Structure Engineer; Advanced Engrg. Services Dept., Portland Cement Associate, Skokie, Ill. 60077,


Serial Information: Civil Engineering—ASCE, 1984, Vol. 54, Issue 4, Pg. 55-59


Document Type: Feature article

Abstract:

In the late 1950's and early 1960's, the height of concrete buildings jumped from 20 to 60 stories. During the same period, architects introduced the use of exposed columns that are subject to thermal movements. Such changes made it necessary for the designer of high-rise buildings to consider column length changes due to elastic stresses and creep caused by gravity loads, drying shrinkage, and temperature variations of exposed columns. While column length changes within a single story may be minor, they are cumulative. When multiplied by a large number of stories, they become substantial. The total shortening affects such nonstructural elements as pipes and elevator rails attached to the concrete. Cladding details are also affected by overall changes in column lengths. Structural effects are caused by differential movements only and not by total movements; they are moments induced into the forcibly distorted slabs or beams and the accompanying moments in the columns. In the 1960's, the structural solutions for effects of shrinkage and creep required extensive longhand computations. A procedure for determining elastic or inelastic column length changes in tall structures has now been updated, computerized and make applicable to concrete, steel and composite structures. The updated procedure separately considers elastic and creep shortenings due to gravity loads and shrinkage shortening. While elastic and creep deformations depend upon loading history, member size and reinforcement, the shrinkage component is independent of loading and depends on member size and on reinforcement. Time and material properties are important variables in all computations of shortening.

Subject Headings: Gravity loads | Shrinkage (material) | Elastic analysis | Creep | Building design | Material properties | High-rise buildings

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