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D. CP65 Beam Design Principles

The CP 65 Beam Design Brief is for single or multi span, prismatic, rectangular solid or tee shaped members.  The member sections must be defined as PRISMATIC sections in the STAAD.Pro data file.  

Refer to D. Suitable Member Properties for more details.

Beams are designed for flexure, and shear only.  Each member is divided into equally spaced sections and the locations of maximum positive and negative moments along each element that makes up the member.  The user can specify the number of segments to be considered between 4 and 25 for each member.

Design for Flexure

The main (longitudinal) reinforcement is calculated for both sagging and hogging moments on the basis of the section profile and parameters defined in the Design Brief.  Compression reinforcement is provided where required.

The design of a beam is based on an envelope of design forces and thus at each of the defined sections, the program determines the required area of steel for both the maximum hogging moment and maximum sagging moment at that section.

The beam is then divided into sub-beams, those that can use the same cage

  1. Same size
  2. Same covers

For each sub-beam, the sections that have the largest sagging and hogging moments are identified and the most efficient reinforcement is calculated for the range of bars specified in the Design Brief.  The programs limits 8 bars in any one layer and uses a maximum of 2 layers.

The program then goes along the beam and checks each section to see how many bars from the critical sections can be removed.  The bars are only removed at the section if they are not required for compression reinforcement or would result in failure in a crack check.

Design for Shear

The shear reinforcement is designed to resist the major axis shear force envelope, Fz, acting through the beam.  The minor axis shear and torsional forces are not considered.

The number of shear legs and the shear link size is specified in the Design Brief. Therefore the required spacing for minimum links can be defined. The program then checks each section to determine the shear stress, v, and concrete shear capacity, vc . From this, the section is classified as either minimum link or a high shear section. Adjacent sections of the same type are grouped into zones. For non minimum link zones, the shear links are designed for the maximum shear force within that zone.

If necessary, additional legs may be added to the shear links in order to restrain tension or compression reinforcement.

Minimum shear links required,V, for shear forces between these values

Anchorage and Bond Lengths

Anchorage and bond lengths are calculated in accordance with the requirements of clause 3.12.8.  They can be displayed graphically on the Main Reinforcement diagram and are used for the schedule table.

Code Clauses Implemented

The following lists the code clauses used from CP65-1999.

3.0 Design and detailing: Reinforced concrete

  • 3.1.7 Strength of materials controlled by brief
  • 3.3 Concrete cover to reinforcement controlled by brief
  • Analysis of section Program Conforms

Design for Flexure

  • Design formulae for rectangular beams.  Program Conforms
  • Design formulae for flanged beams where the neutral axis falls below the flange.  Program Conforms

3.4.5 Design shear resistance of beams

  • Shear stress in beams.  Program Conforms
  • Shear reinforcement: form, area and stress.  Program Conforms
  • Concrete shear stresses.  Program Conforms
  • Spacing of links.  Program Conforms
  • Enhanced shear strength near supports.  Program Conforms
  • Shear and axial load.  Program Conforms

3.4.6 Deflection of beams

  • Span/effective depth ratio for a rectangular or flanged beam.  Program Conforms
  • Long spans.  Program Conforms
  • Modification of span/depth ratios for tension reinforcement.  Program Conforms
  • Modification of span/depth ratios for compression reinforcement.  Program Conforms
  • Minimum percentage of reinforcement.  Program Conforms
  •  Maximum percentage of reinforcement.  Program Conforms
  • Arrangement of links for containment of beam or column compression.  Program Conforms reinforcement

3.12.11 Spacing of reinforcement

  • Minimum distance between bars.  Program Conforms
  • Maximum distance between bars in tension.  Program Conforms
  • Clear distance between bars in tension.  Program Conforms
  • Clear distance between face of beam and nearest longitudinal bar in tension.  Program Conforms
  • Bars near side faces of beams exceeding 750 mm overall depth.  Program Conforms