Design & Detailing of Beam

Detailing of beams and slabs reinforcements has a huge impact on strength, durability and cost optimization. Reinforcement details of RCC beam and slab should tell abut the length of main reinforcement, effective cover to reinforcement, type and number of shear reinforcement and diameter of reinforcement.

Cantilever beam

Beam is the generally horizontal reinforced members which transmit the loads (mostly of slab) to columns or to another beam. Design of beam involves sizing and finding the quantity of steel based on requirement of strength and serviceability. 

The design of beam is mostly based on bending moment of beam (Calculated based on loading and support conditions). Beam is firstly design for bending moment, followed by design for shear. Then check for Deflection and Crack width are applied for ensuring safe design.
Before proceeding to design of beams, We will discuss about basic like effective span, cover, reinforcement requirements for beam in this module.

A single span beam supported on masonry wall is an example of simple supported beam. This beam deflect toward downward side throughout (thus has Sagging Moment (+ve)) and has zero moment at ends. If beam is casted monolithically with slab, then slab will act as flange and resist the bending moment. If slab is present at both sides, then beam is called as T- beam and if slab is present at only one side, then beam is called as L- beam.
If beam is continuous over a number of supports, then beam will deflect  downward at centre and also try to deflect upward at supports. Thus beam has Sagging Moment (+ve)
at centre and Hogging Moment(-ve) at supports.

Effective Span 

The effective span of beam is that length of beam which is susceptible to torsional buckling due to applied bending moment. It is based on different supporting conditions and calculated as shown below:

a.Simple Supported Beam

                     
Effective span= clear span + effective depth  
                                          or
                         centre to centre distance between supports
   whichever is lesser.

b.Continuous Beam

Case.1 If width of support < 1/12th of clear span

             Effective span = Same as Simple supported beam

Case.2 If width of support >1/12th of clear span
            
            (a). When one end is fixed and other end continuous or both continuous.
                     
                   Effective span = clear span

            (b). When one end is simply supported and other is continuous.
                 
                  Effective span =  clear span+ d/2
                                                or
                                              clear span+1/2th of width of support
                  whichever is lesser.


c.Cantilever Beam

       
       Effective span = clear span + d/2

d.Overhanging Portion

     
      Effective span = centre of support to free end ( clear span + 1/2th of width of support)

Size of Beam

a.Depth of beam

According to Limit state design, We always design for under reinforced section as it give early warning before collapse of any RC section under worst conditions, if happen. So it is desirable to keep the depth such that percentage of steel is around 75 percent of that required in a balance section. It also satisfy the limit state of deflection.

Thus the depth of beam is selected between 1/12th to 1/15th of span for simple supported beam and 1/15th to 1/20th of span for continuous beam. For ease, we kept the depth in multiple of 50.

b.Width of beam

The beam accommodate bars at bottom as tensile reinforcement. The width of beam should be such that bars are placed at bottom in layer with sufficient spacing and also ensure min. side cover.

Width is also kept in multiple of 50. The normal width size of beam is 200, 250, 300, 350 (all in mm). We generally chose the 230 mm as width of beam as it can easily flush with 9" thick wall.

(Note:- IS 456 also specifies that size of beam should be such that it satisfy slenderness ratio condition for lateral stability. For these purpose, min. 200 mm width of beam is recommended by IS code.)

c.Cover to Reinforcement

IS code recommends min. nominal cover to different exposure conditions as shown below:

           Exposure Conditions                     Min. Cover(mm)

       

            Mild                                                 20

            Moderate                                        30

            Sever                                              45

            Very Sever                                      50

            Extreme                                          75

Reinforcement Requirement    

a. Main Reinforcement  

1.In Tension side of beam, the minimum numbers of bars is 2 and maximum 6 bars is used      in a single layer.
2.The minimum diameter of bar to be used as reinforcement is 10 mm.
3.The minimum area of tension reinforcement shall not be less than

 where Ast = area of steel reinforcement
            b    = width of beam
            d    = effective depth
            fy   = yield strength of steel.

4.Maximum area of tension reinforcement shall not be greater than 4 percent of gross              sectional area (0.04bD).

5. Reinforcement should be of same type and of same grade.

b.Shear Reinforcement

1.These type of reinforcement should passes by outer side of tension and compression            reinforcement.

2.If there is no steel bar in compression side, then atleast 2 bars of diameter 10 mm are            used to support shear reinforcement.

3.The spacing of shear reinforcement should not exceed from (0.75d or 300mm, whichever      is lesser).

4.Minimum shear reinforcement in form of stirrups shall greater than 

   

5.Steel bar of diameter 6, 8, 10 or 12 mm should be used as shear reinforcement.

6. Generally two legged shear stirrups are used. But if spacing is very less, then we can use four legged stirrups as reinforcement.

 

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