Rule of Thumb

 

Rules of thumb for Structural design of members CHAPTER TWO RULES OF THUMB FOR REINFORCED CONCRETE BEAMS 1. General 

-  All beams of the same types having approximately (equal span ( ±) 5% variation), magnitude of loading, support conditions and geometric property are grouped together. The heaviest beam of the group is considered for the design. In the preliminary beam design, values of reaction at both ends are worked out for all the loadings acting on the beam. -  Selecting different joist depths and beam sizes for each floor may result in minor savings in materials, but specifying the same depth for all floors will achieve major savings in forming costs. -  Beams should be of sufficient depth to avoid the necessity for excessive compression reinforcement and to ensure that economical amounts of tension and shear reinforcement are provided. This will also facilitate the placing of concrete. -  Beams shall be treated as:   A rectangular beam if it does not support any slab on o n either side.   As L-beam if it supports a slab on one side.   As T-beam if it supports slab on both sides. 

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-  Depth of Girder or Beam = Span/12 (or anything between span/15 to span/10 is good to start with). Depth of beam includes thickness of slab. -  For Cantilever beam, depth= Span/5 -  Beam width= [C/C distance between adjacent slabs (perpendicular to the span direction)]/12. Generally architect will ask to restrict width to 250 mm. -  Suppose you got the width of the beam as 350mm by using the above stated thumb rule and the effective depth of the beam was 700 mm (as per the effective depth thumb rule). Then, use the following equation to determine the increased effective depth. 350 x 700^2=250 x d^2. -  Normally Width of beam = (0.35- 0.67) of (depth) , Minimum 250 mm -  Limiting steel reinforcement ratios for tension-controlled members: members:   Minimum steel reinforcement reinforcement ratios = 0.005, AS= 0.005x bd   Maximum steel reinforcement reinforcement ratios = 0.02, AS= 0.02x bd   Minimum Main bar diameter= 16 mm, max Spacing= 200 mm 

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Shear reinforcement, minimum stirrups diameter= 10 mm, max Spacing= the smaller of width, depth/2 and 250 mm. -  The approximate reinforcement quantity quantity in beams (150 - 250 kg/m3) 

 

2. Beam and Girder-Supported One-Way Solid Slab

-  Girder depths should be more than beam depth or at least the same. Therefore, the girder should span along the shorter direction -  Girder depth= Span/12. Beam depth= Span/15 -  Girder and beam width = (0.4-0.67) depth, Minimum (250 mm for beam, 300 mm for Girder 3. Two-way Solid Slab

-  Beam depth= Span/15. Beam width = (0.4-0.67) depth, Minimum 250 mm  4. One-Way Band Beam Slab

-  Beam depth = (2.0 to 2.5) (slab thickness). Beam width = Width = (0.25 to 0.3) (center-to center beam spacing). Minimum 500 mm 5. One-Way Joist Slab

-  Joist depth = Span/18. Joist width = (150 -200 mm mm)) -  Beam depth = Span/18. Beam width = (1.75-2) (1.75-2) (Joist width), width), Minimum 300 mm 6. Two-Way Joist (Waffle) Slab

-  Joist depth = Span/20. Joist width = (150 -200 mm mm))