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D. ACI 318-05 Slab Design Principles

The ACI 318-05 Slab Design Brief is for any slab modeled with finite elements regardless of boundary restraint conditions.

Slabs are designed for flexure in two orthogonal directions for both hogging and sagging moments. Designs for shear or punching shear checks are not included.

Analysis results for plate elements that form a ‘Slab’ are used to generate design moments with respect to the axes of the slab. These moments are then used to generate required reinforcement contours. Note that no account is taken of the requirements for embedment or development of reinforcement.

Tip: The level of accuracy of results depends on the meshing density used to model the slabs. It is recommended that users examine the accuracy of the model at the analysis stage and ensure satisfactory results are obtained prior to the design stage.

Design for Flexure

Longitudinal and transverse reinforcement are calculated for both sagging and hogging moments on the basis of the slab thickness and parameters defined in the Design Brief. Compression reinforcement is provided where required.

The design of a slab is based on an envelope of design forces. Hence the program determines the required area of steel at the center of each element, for both the maximum hogging and sagging moments in the two orthogonal directions. (i.e., along the Slab/Region local axes)

Reinforcement calculation is based on the ACI 318-05 recommendations for the design of one-way slab systems assuming a unit width together with the elements thickness.

Slab end restraint conditions are not catered for at the design stage, however to ensure accuracy of design results, they must be incorporated in the analytical model of the structure.

As has been stated earlier, in this release of the RC designer, shear checks have not been catered for.

Code Clauses Implemented

The list of code clauses used from ACI 318-05 is as follows:

Chapter 3 - Materials

  • 3.3.2 (c)

Chapter 7 - Details of Reinforcement

  • 7.6.1
  • 7.6.5

Chapter 9 - Strength and Serviceability Requirements

  • 9.1.1
  • 9.3.1
  • – Sections are assumed to be tension controlled

Chapter 10 - Flexure and Axial Loads

  • 10.2.2 but not deep flexural members
  • 10.2.3
  • 10.2.4
  • 10.2.7
  • 10.5.4 – but with spacing not greater than twice the slab thickness (assumes two-way slab as 13.2.2)
  • 10.6.4

Chapter 13 – Two Way Slab Systems

  • 13.2.2