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RCC MEMBER DESIGN TIPS

BEAMS

                                                                                 OVERALL DEPTH OF BEAMS:

 1. Beam sections should be designed for:

1. Moment values at the column face & (not the value at centre line as per analysis)
2. Shear values at distance of d  from the column face. (not the value at centre line as per analysis)
3. Moment redistribution is allowed for static loads only.
4. For beams spanning between the columns about the weak axis, the moments at the end support shall be reduced more and distributed and the span moments shall be increased accordingly to account for the above reduction.
5. Moment distribution shall be done in such a way that 15% of the support moments shall be added to the span moment without the support moments getting reduced.
6. The section within the span shall be designed for the increased span moment which will account for the concentrated & isolated loading that may act within one span.
7. Moment redistribution is not allowed if
   1. moment co-efficient taken from code table
   2. designed for earthquake forces and for lateral loads.

2. At least 1/3 of the +ve moment reinforcement in SIMPLE SUPPORTS & ¼ the +ve moment reinforcement in CONTINUOUS MEMBERS shall extend along the same face of the member into the support, to a length equal to Ld/3. (Ld-development length)
3. Use higher grade of concrete if most of the beams are doubly reinforced. Also when Mu/bd^2 goes above 6.0.
4. Try to design a minimum width for beams so that the all beam reinforcement passes through the columns. This is for the reason that any reinforcement outside the column will be ineffective in resisting compression.
5. Restrict the spacing of stirrups to 8”(200mm) or ¾ of effective depth whichever is less.(for static loads)
6. Whenever possible try to use T-beam or L-beam concept so as to avoid compression reinforcement.
7. Use a min. of 0.2% for compression reinforcement to aid in controlling the deflection, creep and other long term deflections.
8. Bars of Secondary beam shall rest on the bars of the Primary beam if the beams are of the same depth. The kinking of bars shall be shown clearly on the drawing.
9. Length of curtailment shall be checked with the required development length.
10. Keep the higher diameter bars away from the N.A(i.e. layer nearest to the tension face) so that max. lever arm will be available.
11. Hanger bars shall be provided on the main beam whenever heavy secondary beam rests on the main beam.(Try to avoid the hanger bar if secondary beam has less depth than the main beam, as there are enough cushions available).
12. The detailing for the secondary beam shall be done so that it does not induce any TORSION on the main beam.
13.  For cantilever beams reinforcement at the support shall be given a little more and the development length shall be given 25% more.
14. As a short cut, bending moment for a beam (partially continuous or fully continuous) can be assumed as wl^2/10 and the same reinforcement can be detailed at span and support. This thumb rule should not be applied for simply supported beams.

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SLAB

          EFFECTIVE DEPTH:          

1. Whenever the slab thickness is 150mm, the bar diameter shall be 10mm for normal spacing.(It can be 8mm at very closely spaced).
2. Slab thickness can be 10mm,110mm,120mm,125mm,150mm, etc.
3. The maximum spacing of Main bar shall not exceed 200mm(8”) and the distribution bars @ 250mm(10”).
4. If the roof slab is supported by load bearing wall(without any frames) a bed block of 150/200mm shall be provided along the length of supports which will aid in resisting the lateral forces.

5.     If the roof is of sheet(AC/GI) supported by load bearing wall (without any frames) a bed block of 150/200mm shall be provided along the length of supports except at the eaves. The bed block is provided to keep the sheets in position from WIND.

6.     For the roof slab provide a min. of 0.24%  of slab cross sectional area reinforcement to take care of the temperature and other weathering agent and for the ponding of rain water etc since it is exposed to outside the building enclosure.

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 COLUMN

1. Section should be designed for the column moment values at the beam face.
2. Use higher grade of concrete when the axial load is predominant.
3. Go for a higher section properties when the moment is predominant.
4. Restrict the maximum % of reinforcement to 3.
5. Detail the reinforcement in column in such a way that it gets maximum lever arm for the axis about which the column moment acts.
6. Position of lap shall be clearly mentioned in the drawing according to the change in reinforcement. Whenever there is a change in reinforcement at a junction, lap shall be provided to that side of the junction where the reinforcement is less.
7. Provide laps at midheight of column to minimize the damage due to moments(Seismic forces).
8. Avoid KICKER concrete to fix column form work since it is the weakest link due to weak and non compacted part.

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FOOTING

1. Never assume the soil bearing capacity and at least have one trial pit to get the real site Bearing capacity value.
2. Check the Factor of Safety used by the Geotechnical engineer for finding the SBC.
3. SBC can be increased depending on the N-value and type of footing that is going to be designed. Vide IS-1893-2000(part-I).
4. Provide always PLINTH BEAMS resting on natural  ground  in orthogonal directions connecting all columns which will help in many respect like reducing the differential settlement of foundations, reducing the moments on footings etc.
5. Always assume a hinged end support for column footing for analysis unless it is supported by raft  and on pile cap.

     The Common assumption of full fixity at the column base may only be valid for columns supported on RIGID RAFT   foundations or on individual foundation pads supported by

      short stiff piles or by foundation walls in Basement. Foundation pads supported on deformable soil may have considerable rotational flexibility, resulting in column forces in the  

      bottom storey quite different from those resulting from the assumption of a rigid base. The consequences can be unexpected column HINGES at the top of lower storey

      columns under seismic lateral forces. In such cases the column base should be modeled by a rotational springs. (Ref:page 164-Seismic design of Reinforced concrete and

      Masonry buildings by T.Paulay & M.J.N.Priestley.)

      Also refer the Reinforced concrete Designer’s Handbook by Reynold where it is clearly mention about the column base support.

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R.C.C.WALLS

1. The minimum reinforcement for the RCC wall subject to BM shall be as follows:
   1. Vertical reinforcement:

a)     0.0012  of cross sectional area for deformed bars not larger than 16mm in diameter and with characteristic strength 415 N/mm^2 or greater.

b)     0.0015  of cross sectional area for other types of bars.

c)      0.0012 of cross sectional area for welded fabric not larger than 16mm in diameter.

Maximum horizontal spacing for the vertical reinforcement shall neither exceed three times the wall thickness nor 450mm.

2. Horizontal reinforcement.

a)     0.0020 of cross sectional area for deformed bars not larger than 16mm in diameter and with characteristic strength 415 N/mm^2 or greater.

b)     0.0025  of cross sectional area for other types of bars.

c)      0.0020 of cross sectional area for welded fabric not larger than 16mm in diameter.

Maximum vertical l spacing for the vertical reinforcement shall neither exceed three times the wall thickness nor 450mm.

NOTE: The minimum reinforcement may not always be sufficient to provide adequate resistance to effects of shrinkage and temperature.

_2.     The H_e/t for a RCC wall shall not exceed 30 as per IS:456=2000, where H_{e is the effective height of the wall and t is the thickness of the RC wall.}  H_{e for a braced wall will be :}

a)     0.75 H, if the rotations are restrained at the ends by floors where h is the height of the wall.

b)     1.0h .

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MISCELLANEOUS

     Ref: (Principle of structures by Ariel Hanaor).

     1. TRUSS:

         The Depth to span ratio for a truss is h/L=10. Beyond a certain optimal value, increase in structural depth increases weight. The same principle applies to trusses. An optimal depth/span ratio for a planar truss is approximately 1/10. Although forces in the CHORDS decrease with increasing depth, forces in the WEB are practically UNCHANGED and increasing the depth increases the lengths of these members. Approximately half the web members are in COMPRESSION and increasing their lengths reduces their efficiency due to the increased susceptibility to BUCKLING.

3. VIERENDEEL GIRDER:

The span to depth ratio=1/8 to 1/10 are typical.

The compression on top chord or tension in the bottom chord for a UDL loading is C=T= qL^2/8h where q is the udl and h is the depth.

4. CABLE:

 A structure in pure TENSION having the funicular shape of its load is termed as Cable.

4.ARCH:

Let us now invert the shape of a cable under a given load, that is the sag at any point is turned into a rise. The point is now above the chord joining the end points by the  

 same amount it was previously below it. A structure built according to the funicular shape in COMPRESSION is termed as an ARCH.

 The optional rise to span ratio for an arch is in the range of 1/6-1/4. The depth to span ratio of an arch is usually in the range of 1/40 -1/70.

5. FOLDED PLATE:

 The typical depth /span ratio is in the range from 1/15 to 1/10.

6. FLATE PLATE:

A typical depth of a solid FLAT PLATE is 1/22 -1/18 of the effective span.

7. TWO-WAY RIBBED SLAB:

Supported on continuous stiff supports are in the range of 1/30-1/25 of the lesser effective span.

8. FLAT PLATE RIBBED SLAB:

  Typical depth of flat plate ribbed slabs are in the range of 1/20-1/17 of the lesser effective span.

9. DOMES:

  The structural depth of DOMES is the full height of the dome from base to crown. Depth to span ratio range from as low as 1/8 for shallow domes to ½ for deep domes.

  A depth /span ratio of 1/5-1/4 is a common value which is near optimal for many applications.

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The detailing of reinforcement is as important as the analysis and design of any RCC members. Specially it is true in the design of  structures against the SEISMIC forces. The most and very important aspect of detailing is well documented in the text book on “SEISMIC DESIGN OF REINFORCED CONCRETE AND MASONRY BUILDINGS by T.Paulay and M.J.N.Priestley. The text extraction is given below for the structural engineers who need to know more about the importance of the above issue.

 Page: 157:

The spacing of the transverse reinforcement is as important as the quantity to be provided. For this reason, recommended maximum spacings of sets of transverse ties along a member, required for four specific purposes, are summarized here.

1. To provide shear resistance: Except as set out in section 3.3.2(a)(vii):

                        In beams          s≤0.5d  or 600mm(24”)

                        In columns        s≤0.75h or 600mm(24”)

                        In walls             s≤2.5b_w  or 450mm(18”)

2. To stabilize compression bars in plastic Regions: As described in section 4.5.4 for beams, but also applicable to bars with diameters db in columns and walls[ Section 5.4(e)]:

                        s≤6.0d_b,     or    s≤d/4,    s≤ 150mm(6”)

3. To provide confinement of compressed concrete in potential plastic regions: As described in sections 3.6.1(a),4.6.1(e)M AND 5.4.3(E).

                              s_h≤ b_c/3  ,   s_h ≤ h_c/3 ,        s_h ≤6 d_b,   s_h≤180mm(7”).

4. At Lapped splice : As described in Section 3.6.29B),4.6.10 and 4.6.11(f) for the end regions of columns where plastic hinges are not expected to occur:

                       s≤8.0d_b,  s≤200mm(8”).

Page:208:

The diameter of stirrup ties should not be less than 6mm(0.25”) and the area of one leg of stirrup tie in the direction of potential buckling of longitudinal bars should not be less than

                  A_te=∑ A_b f_y  s

                            _{_____________(Mpa)}

                        16 f_yt  100

For design purpose it is convenient to rearrange the above equation in the form:    A_te/s  =  ∑ A_b f_y  /1600 f_yt  (mm^2/mm)  

Where    A_b is the sum of the areas of the longitudinal bars reliant on the tie, including the tributary area of any bars exempted from being tied in accordance                 

                   with the proceding section.   

              A_te is the area of the stirrup tie in mm^2.                                                                                                                                                   

                f_y    is the yield strength of longitudinal bars.

                f_yt   is the yield strength of tie bars                                                                                                            

Page 128:

Because of the reversal of shear forces in members affected by earthquakes, the placing of stirrups at an angle other than 90^Ө to the axis of such members is generally impractical.

The choice of the angle 45 ^Ө for the plane of the diagonal tension failure in the region of potential plastic is a compromise.

Please note that in IS 13920 it is not recommended to use single bent up bars.

Minimum shear reinforcement:

Current codes (NewZeland)  require the provision of minimum amount of shear reinforcement in the range of 0.0015 ≤A_v/b_wsC0.0020 in members affected by earthquake forces.

ii) Spacing of stirrups: To ensure that potential diagonal tension failure planes are crossed by sufficient sets of stirrups, spacing limitations such as set out below, have been widely used. The spacing s should not exceed:

1.     In beams:

In general :0.5d or 600mm(24”)

When (v_i-v_c) > 0.07 f^’_c: 0.25d or 300mm(12”).

2.     In columns:

When P_u/A_g≤0.12 f^’_c; as in beams

When P_u/A_g > 0.12 f^’_{c: 0.75’ or 800mm(24”).}

_{3.      In walls,}

_{2.5 times the wall thickness or 450mm(18”).}

_{Spacing limitations to satisfy requirements for the confinement of compressed concrete and the stabilizing of compression bars in potential plastic hinge regions are likely to be more restrictive.}

_Page:233:

            _{Design of transverse Reinforcement:}

_{(a)    General considerations: There are four design requirements that control the amount of transverse reinforcement to be provided in COLUMNS:}

_{1.      Shear strength;}

_{2.      Prevention of buckling of compression bars;}

_{3.      Confinement of compressed concrete in potential plastic hinge  regions or over the full length of column subjected to very large compression stresses and;}

_{4.      The strength of lapped bar splices.}

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