Analysis of Concrete Structures Using a Reorganized ACI 318 Building Code

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Analysis of Concrete Structures Using a
Reorganized ACI 318 Building Code

• D Rogowsky
• AECOM
• SEI Congress, Austin, May 1, 2009

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Disclaimer

• Opinions expressed are my own, rather than those
  of ACI, ACI 318, or ACI 318C
• Will share information from open public meetings
• Final decisions on what ACI 318-14 look like are
  made by ACI 318 main committee
• Hard to predict what main committee will do

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Experiment

• Step 1 Cross your arms

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Experiment

• Step 2- Cross your arms the other way

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Experimental Observation

• Change is not always comfortable!

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Follow-up Experiment

• Keep your arms crossed wrong for a period of
  time
• Uncross them
• Re-cross them wrong again
• Observation If you practice the change enough,
  it will feel right!

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Overview

• 60 Second experiment
• Background
• Current situation in ACI 318
• Options for change
• Reoganization
• What may go out
• What may come in
• Closure

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Background

• Base case is ACI 319-08
• Major reoganization for ACI 318-14
• Chance to make code easier to use
• Chance to remove material
• Obsolete
• Textbook material
• Chance to add material
• Modern methods of analysis

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Current Basis for ACI 318 Analysis Provisions

• Elastic analysis
• Limited redistribution
• It shall be permitted simplifications to avoid
  detailed deflection calculations (h/L)
• Approximate Analysis Coefficients, Direct Design,
  Equivalent Frame
• It shall be permitted simplifications for time
  dependent and second order effects

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Implied Plastic Analysis

• Shear Friction (upper-bound plasticity theorem)
• Strut-and-Tie Models (lower-bound plasticity
  theorem)
• Direct Design Method (upper-bound plasticity
  theorem with some tweeks)

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Reorganization

• ACI 318-08 has analysis provisions distributed in
  various Chapters
• ACI 318-14 should have all explicit analysis
  provisions in one Chapter
• Eg. Currently, designer sets-up frame analysis
  relative stiffnesses in Chapter 8 only to revise
  to absolute cracked stiffnesses in Chapter 10.

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What Might Come Out?

• Equivalent Frame
• Direct Design
• Approximate Analysis Coefficients

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What Might Come In?

• Rules for dealing with Finite Element Analysis
• mXY
• Coarseness of mesh
• Widths of design strips
• Stress concentrations
• Shear stresses around columns
• Cut-off points
• Going from the 8 stress resultants for a shell
  element to design of reinforcement

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Mxy, the Twisting Moment in Slabs

• Not usually an issue in regular orthogonal
  structures
• An issue when the rebar is not oriented in the
  principle moment directions
• Irregular structures which are frequently
  analyzed with FEA
• Some regular looking structures also have

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Coarseness of Mesh

• How coarse can a mesh be and still produce
  acceptable results?

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Widths of Design Strips

• Are column strips and middle strips still
  meaningful?
• What about other situations?

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Stress Concentrations at Discontinuities

• How to deal with stress concentrations?
• Singularities
• Re-entrant corners
• Concentrated supports
• Concentrated loads

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Shear Stresses Around Columns

• Punching shear stresses around columns need to be
  treated with caution
• Out-of-plane shear stresses from FEA are not
  necessarily compatible with conventional punching
  shear calculations

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Cut-Off Points

• After converting bending moments to steel
  requirements point by point throughout the slab,
  how do you determine where bars should be
  cut-off?
• In which direction does one apply the d-shift
  rule to account for through thickness shear lag?
• Uncertainty about locations of points of
  inflection due to loading and modelling
  assumptions

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Dealing With the 8 General Stress Resultants from
a Shell Element

• 3 in-plane stresses
• 3 plate bending moments
• 2 out-of-plane shears

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Closure

• Things will change
• Progress requires change
• Not all change is progress
• Guaranteed that few will like the reorganized
  code initially

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• You know that you are in trouble when you have
  to look to a code to tell you what to do.
• Hrabok, 1979