Rational Limits for Longitudinal Steel in Concrete Columns

ASCE-ACI Committee 441, Concrete Columns, Subcommittee on Longitudinal Reinforcement Limits

Document Type: Proceeding Paper

Part of: Structures Congress XII


Current limits for the amount of longitudinal reinforcement in concrete columns were established more than 60 years ago. Commercial grades of reinforcement in 1930 were about half as strong as those in general use today. Common values fc' for concrete design strength similarly were about half the values in general use today. The ACI318-89 lower limit of 1% gross area was advised in 1930 in order that creep deflections under sustained service load should not cause any smaller amount of steel to yield under service load. A study of creep and shrinkage deformations under sustained force is shown to indicate that rational lower bound values less than 1% are dependent on the steel yield strength fy and the concrete strength fc'. Also, minimum reinforcement should be adequate to limit concrete cracking, again a function of fc' and yield strength of longitudinal reinforcement. Cross section geometry, splicing requirements, and constructability will control longitudinal reinforcement upper limits without being cited as a reason for any specific limit. An expression for an upper limit to longitudinal reinforcement, derived to insure against concrete cracks from steel restraint to concrete shrinkage, shows that no cracking should be anticipated for most combinations of fc' and fy. Recommendations are made for changes to ACI318 Clause 10.9.1.

Subject Headings: Reinforced concrete | Steel columns | Concrete columns | Strength of materials | Steel | Cracking | Load factors | Shrinkage (material)

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