RCC Design Tips

RCC MEMBER DESIGN TIPS

A.BEAMS:

OVERALL DEPTH OF BEAMS:

SL.NO	MEMBER	SPAN/OVERALL DEPTH RATIO
1.	PLINTH BEAM	15 TO 18
2.	TIE BEAM	18 TO 20
3.	FLOOR BEAMS	12 TO 15
4.	GRID BEAMS	20 TO 30

  

 1. Beam sections should be designed for:

1. Moment values at the column face & (not the value at center line as per analysis)
2. Shear values at distance of d  from the column face. (not the value at center 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.

B:SLAB

          EFFECTIVE DEPTH:          

Sl.no	SLAB	SPAN/EFFE.DEPTH
1.	One- way simply supported slab	30
2.	One-way continuous slabs	35
3.	Two-way simply supported slabs	38 for L/B=1.5 35 for L/B>1.5
4.	Teo-way continuous slabs	40 for L/B=1.5 38 for L/B>1.5

                       

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 100mm,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.

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 mid height 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.

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

      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.

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.