Civil – Buildings – Designs.
Technical Circulars (TBD)
Design Circle’s Technical Note 7502
| | General Notes in respect of RCC Building designs for guidance of field Engineers | | 1 General 1.1 All Concrete for R.C.C. slabs, beams and footings, shall be of M150 grade unless other grade is
specifically mentioned in the schedule, for columns the grade of concrete shall be as indicated in the
schedule.
1.2 Mild steel bars to be used shall be tested mild steel bars conforming to grade 1 of I.S.: 432-1966. Such
steel is indicated as M.S in the Schedule (f )
1.3 High Yield strength deformed bars shall conform to I.S.: 1139-1966. or I.S.:1786-1966. Such steel is
indicated as H.Y.S.D. in the schedule. (f )
1.4 All bar diameters are in millimeter unless other wise mentioned.
1.5 For information regarding layouts, loads etc. refer to the structural note attached (Annexure I)
1.6 The design and other provisions are generally based on I.S.: – 456 which shall be referred for details, if
required. | | 2. COVER : 2.1 Clear cover at the end of a reinforcing bar shall be 25 mm or twice the bar diameter whichever is more.
2.2 Clear cover for longitudinal steel in columns shall be 40 mm or bar diameter whichever is more.
2.3 Clear cover for longitudinal bars in beams shall be 25 mm, or the bar diameter whichever is more, Clear
cover strips shall be minimum 13 mm.
2.4 Clear cover for reinforcement in slabs shall be 13 mm or the bar diameter whichever is more.
2.5 Clear cover at sides for reinforcement in footings and pile caps shall be minimum 50 mm, and at bottom
it shall be minimum 50 mm and 100 mm respectively for footings and pile caps.
2.6 In case of concrete members exposed to the saline or adverse atmosphere, the cover for various
members as indicated earlier shall be increased by 15 mm for faces of concrete members in contact with
earth, this extra cover shall be 15 mm over and above that indicated in paras 2.1 to 2.5. | | 3. SPLICES : 3.1 Splices at points of maximum tensil strees shall be avoided.
3.2 Lapped splices shall not be used for bars of 36 mm. diameter and above.
3.3 The lap length for reinforcement in beams, slabs and footing shall be as follows | Mild steel bars upto 40 mm. dia – | 58 times diameter | | Mild steel bars above 40 mm. dia – | 54 times diameter | | H.Y.S.D. bars upto 20 mm. dia – | 69 times diameter | | H.Y.S.D. bars above 20 mm. dia – | 63 times diameter |
3.4 (a) The lap length for reinforcement in columns in compression for various grades of concrete for mild
steel and H.Y.S.D. shall be as below :- Concrete Mix | No. of bar dia for M.S.
(1300 kg / Cm2) | No. of bar dia for H..Y.S.D.
(1750kg / Cm2) | 150 | 44 | 42 | 200 | 33 | 32 | 250 | 29 | 28 | 300 | 26 | 26 | 350 | 24 | 24* | 400 | 24* | 24* |
* Not less than 24
3.4 (b) The lap length of column reinforcement in tension shall be as specified by the designer.
3.5 Lap length shall be increased by 20 percent if,
a. Contact splices are spaced laterally closer than 12 times bar diameter.
b. Laps are located closer than 15 cm. to 6 time bar diameter from outside edge of the member.
c. More than one half the bars are lapped within a length of 40 bar diameters.
d. Lap is provided at the point of maximum stress. However in case the splice is enclosed for its full
length by stirrups as closely spaced spirals, the above extra length of lap should not be provided. 3.6 Contact splice should be formed by cranking one bar in such way that the axis of cranked bar coincides
with the axis of the other bar beyond the splice.
3.7 In columns where longitudinal bars are offset at a splice, the slope of inclined portion of bar shall not be
more than 1 in 6 and the portion of the bar above and below the offset shall be parallel to the axis of
column (refer sketch no. 3) | | 4 CLEARANCE 4.1 Horizontal distance between two parallel main reinforcing bars in reinforced concrete shall not be less
than the diameter of the bigger bar, and also it should be at least 6 mm more than the nominal maximum
size of course aggregate used in concrete.
Note : In congested portion of the member the size of the coarse aggregate can be reduced and large size of
aggregate can be used beyond the congested reinforcement in the same member. The clearance
mentioned above is minimum. If possible more clearance should be provided. 4.2 The minimum vertical distance between two tiers of main reinforcing bars in case of beams or two rows of
main reinforcing bars of column shall normally be, 15 mm., maximum size of the coarse aggregate or the
maximum size of the bar, whichever is the greatest. | 5 BEAMS
(The anchorage length given for beams are for concrete of grade M150) 5.1 Anchorage of bottom reinforcement - Straight bars at botom shall be continued for a length of [8d+hook]
beyond face of the support unless otherwise mentioned. However, if the diameter of the bottom bar of the
adjacent beam with bottom at the same level, is same, then these bars may be kept continuous. Surplus
bars may however be stopped. 5.2 Arrangement for supporting share stirrups.
5.2.1 Anchor bars as specified in the schedule of beam should be provided in the central half span of the
beam only. In the remaining end portion (1/4th span on either side) of beams, use of extra steel
and / or bent up bars should be made for supporting shear stirrups, in case the extra steel
specified at support is also same as the anchor bars (usually two bars of 12 mm f ). As provided in
the middle half of the span, the later may be continued to the support to form the extra steel of
same size (two bars of 12 mm f ). If no extra steel is specified at supporter nor there are bend up
bars, anchor bars should be continued for supporting shear rings.
5.2.2 In case of beams supported on other beams, anchor bars or extra steel shall be continued over the
supporting beams into the adjoining span or well anchored in supporting beam if it is an end span. 5.3 Anchorage of steel at top of support
a) In case of all beams, whether resting on columns or supported on other beams, all steel provided at top
(whether bent up bars or extra steel) shall be well anchored either by continuing these in the adjoining
span or by anchoring it, in the supporting columns or beams if it is on end support. The length of
anchorage in case of mild steel and high yield strength deformed steel shall be as follows. Mild Steel bars upto 40 mm dia - | 58 times diameter | Mild steel bars above 40 mm dia - | 54 times diameter | HYSD bars upto 20 mm dia - | 69 times diameter | HYSD bars above 20 mm dia - | 63 times diameter |
These lengths are inclusive of hooks. Hooks, where provided shall be as per the Standard hooks shown
in I.S.:2502 – 1963.
b) For beams framing into columns and designed for wind / seismic forces. The bottom bars also must be
anchored for lengths as specified in 5.3 (a) above.
5.4 Arrangement of reinforcement near support in case of two adjacent beams in same line and having top at
the same level shall be as follows:-
a) Bend up bars should be continued at top for a length of (0.25L + hook) in the adjacent span, where ‘L’
is the span of the adjacent beam (measured from centre to centre of support).
b) Extra steel at top should be provided for a length of (0.25 L + hook) on either side of the support, where
‘L’ is the span of the beams on either side of support (measured from centre to centre of support)
5.5 Arrangement of reinforcement near support where the beam ends at that support should be as follows :-
a) Bent bars and extra steel at top shall be anchored in the support for a length as specified in 5.3 (a)
above.
b) The extra steel, if any, at the end support should be provided for (0.25 L + hook) or for a length as
specified in 5.3 (a) above, whichever is more.
5.6 Incase where the adjacent beams meeting at a column are not at the same level at top, each beam shall
be considered as ending at that support for the purpose of providing reinforcement and anchorage near
supports.
5.7 The length of (0.25 L + hook ) mentioned in paras 5.4, 5.5 and 5.6 above for continuing the bent up bars
or the length of extra steel shall be in case be less than that specified in 5.3 (a) above.
5.8 Extra steel-in case of column supporting two beams in one line, extra steel to be provided at the top shall
be more of the two values specified in the schedule for two beams.
5.9 In case of a secondary beam supported by another main beam having its bottom at a higher level then the
top of the supporting beam, the gap between the two shall be filled in by concrete of M-150 (or the mix of
beam if richer than M - 150 is used for the beam) for the full width. Bottom bars of such secondary beams
should then be continued in this concrete block (on the face avail from the secondary beam) for a length
as mentioned in para 5.1 above. Top steel at the support of such secondary beams shall pass through
this concrete block and continue in the main supporting beam for anchorage length as per para 5.3 (a)
above. If the height of this concrete block is more than 60 cm., 4 bars of 12 mm. may be provided as
vertical reinforcement in the concrete block and top bars can be anchored adequately in this concrete
block (refer sketch no. 5).
5.10 Flange steel : Incase of slabs cast monolithically with beam, main reinforcement of which is parallel to
length of beam, additional reinforcement called flange steel shall be provided at top across the beam as shown in sketch No2. This reinforcement should be 60 percent of main reinforcement of the slab or 0.3
percent and 0.2 percent of the cross section of the slab for M.S. and H.Y.S.D. respectively (inclusive of
available reinforcement of bent bars from adjoining slabs, in any), whichever is more. The spacing of such
bars shall not be more than three times the effective depth of slabs.
5.11 Cantilever beams.
5.11.1 In case of cantilver beams at least two bars shall be bent down at the free end continued at
bottom into the supporting beams or columns as the case may be for length as per para 5.1 above
5.11.2 Top reinforcement in cantilever beams shall be anchored in the supporting columns or continued in
the adjoining beam at the same line (if any) for sufficient anchor length as per para 5.3 (a) above or
(0.25 L + Hook) whichever is more ‘L’ is the span of the adjoining beam between centre to centre of
the support.
5.12 Stirrups - The cut ends of the stirrups should project 8 times diameter beyond the end of the bend, unless
otherwise, mentioned or for torsion. For beams subjected to torsion, the projection of stirrups shall be as
shown in sketch No. 8. | 6 SLABS
(The anchorage lengths given for slabs are for concrete of grade M150)
6.1 Alternates bars of main steel shall be bent up at 0.2 L from supprots and continued in the adjoining span
for a distance of (0.25 L+ hook) or for a length specified in 5.3 (a) above whichever is more. The bottom
bars shall be continued for a lenght as given in 5.1 above.
6.2 Extra steel, is any shown in schedule, shall be provided across a beam on either side for a length as
specified in 6.1 above, measured from centre to support.
6.3 In case steel at top of support cannot be continued in the adjoining span, it shall be anchored in the
supporting beam for anchorage length as specified in 5.3 (a) above.
6.4 In case of staircase without a beam at junction of landing and staircase flight, arrangement of steel in
waist slab and landing should be as shown in sketch No. 6
6.5 In case of cantilever slabs every third bar shall be bent down at the free edge and continued into the
supporting beam or continuos slab for a distance as per para 5.1 above. For typical details of cantilever
slab see sketch No. 4A.
6.6 In case of cantilever slabs, the main reinforcement at the top shall be continued for a length as specified
in 6.1 above into the supporting beam or adjacent continuos span.
6.7 Over discontinuous edges of two way slabs extra steel bars with 6 times the spacing of bottom main
steel at centre shall be provided at top for a length of (0.25 L+ Hook) as anchorage length as per 5.3 (a)
above whichever is more.
6.8 For details of arrangement of main reinforcement and corner reinforcement required for torsion in two way
slabs, refer to Annexure III | 7 COLUMNS 7.1 Column bars shall be bent and anchored into the beam if the column stops at that beam. The length of
anchorage shall not be less than 44 times diameter of the bar for mild steel and 42 times diameter for
high yield strength deformed steel (for concrete mix M150)
7.2 In case the section of the column above is reduced in comparison to column below by providing offsets,
the longitudinal bars from the lower column shall be bent and taken into the column above. The slope of
the inclined portion of the bar with the axis of the column shall not exceed 1 in 6 and the portion of bar
above and below the offset in the column shall be parallel to the axis of the column. Where the offset is
excessive so that it is not possible to bent the bars at a slope less than 1 in 6, splices of vertical bars
adjacent to the offset face shall be made by separate dowels overlapped as above. Spacing of lateral
ties shall be halved along the length of the splice. For mode of placement of dowels, refer sketch No. 3
7.3 Column section as specified in the schedule should be continued up to top of beams resting on it.
Proposed reduction in section, if any, should be carried out from the top level of the supported beams.
7.4 All bars of the column in lower section should be continued in the upper section of the column for length
corresponding to the mix of the upper section as given in para 3.4 above. | 8 FOOTING : 8.1 In case of column footings for which dowel bars have been specified, column bars should be started from
top of footing. Dowel bars shall be taken above the top of footing to the extent of lap length
corresponding to the mix of the column section as given in para 3.4 above. (Sketch No.1)
8.2 In case of pile caps or column footing resting on piles, reinforcement of piles shall be anchored for 44
times diameter of the pile bar into the pile cap. For details of a typical pile cap see sketch No. 1A. | 9 CONSTRUCTION JOINTS :
(Refer sketch No. 7)
9.1 The construction joints shall be few as possible and interval between two construction joints should be
decided on one full days output of concrete mixer / mixers on the job.
9.2 Time interval between the new concrete and old one shall normally not exceed 24 hours. Proper
precautions as laid down in the clause 20.1.4.2 of I.S.: 456-1964 shall be taken when laying new
concrete.
9.3 Construction joints should primarily be placed at a section of minimum or low shear and the face at the
joint should be normal to the compressive forces in the member.
9.4 Inclined feather joints in slabs and beams should be avoided, except where concreting will be continued
in a few minutes.
9.5 Construction joints in various structural members like slabs, beams and column should be located as
below
a) SLABS :
i) In slabs spanning in one direction, construction joints should be located near about mid span if the
joints run at right angles to the main reinforcement. If the joints are to be located parallel to the main
reinforcement, they should preferably be within the middle third but shall be clear from the flange of
the beam designed as Tee or Ell beam.
ii) In case of two-way slabs, the joints should be located in the middle third of either span.
iii) Construction joints in cantilever slabs should be parallel to the main reinforcement in the slab and
should never be located at the face of the support. b) BEAMS :
i) Construction joints in beams should be located near about mid span within the middle third at right
angles to the span, joints in slab and beam should be staggered.
ii) Where a secondary beam joins a main beam, the construction joint in the main beam should not be
located within a distance equal to twice the width of the secondary beam, from the junction point on
either side. c) COLUMNS : The operation of placing concrete in columns in each lift should be as continuos as
possible but the rate of vertical progress shall not exceed about 1 metre in half an hour, the depth of
concrete placed in a single batch should not preferably exceed 1 metre. Concreting of columns
should be stopped 10 cm. to 15 cm. below the level of the soffit of beams running into the column.
The portion of the column between the stopping off level and top of slab should be concreted along
with the beam. The top of each lift of concrete or any other construction joint in vertical column should
be truly horizontal. If the column is inclined to the vertical, joints should be at right angles to the
column axis. | 10 MISCELLANEOUS : 10.1 Forwork :- A proper and definite sequence of operations has to be followed in removing the centering
and shuttering. The simplest principle of striking the centering is to adopt such a sequence as will
make the structure behave as designed, for details, reference is invited to the "Code of practice for
design and construction for form work of concrete" published by Government of Maharashtra.
10.2 Expansion joints :- For details of expansion joint and method of their laying and fixing, reference is
invited to "standard specifications" (Red Book) Published by Government of Maharashtra.
10.3 Building located in seismic areas (Zones III & IV)
i) When the design is carried out taking in account the seismic forces for zone III & zone IV as per
IS:1893-1970, the provisions shall be as per schedule supplied for the various R.C. members.
ii) When the design is done without taking into account seismic forces for zone III & zone IV as per
I.S.:1893-1970 the reinforcement in R C members shall be modified as stipulated in Government letter
No. BDC-3870/87584-K, dated 14 January 1972 with recommended modifications as given in
Annexure II.
iii) All the reinforcement shall be anchored for full anchorage length as stipulated in para 5.3(a) above and
in manner as specified on paras 5.3 (b) to 5.7 above depending upon the position. 10.4. In case of any difficulty in applying the provisions of this technical note or interpretation of drawing
supplied by Design Circle or any excessive congestion of steel reinforcement at any junction, reference
shall be made to designs circle for clarification /solution.
10.5 This note supersedes earlier note No. 101 and 168 issued by design circle in 1969 and 1963. Encl.:- Annexure –I (Structural note)
Annexure –II (Seismic Provision)
Annexure – III (Two way slabs)
Annexure –IV (Sketches – 1 to 8) | | | 7502 ANNEXURE –I | (Accompaniment to Design Circle,s Technical Note No. 7502) | STRUCTURAL NOTE. 1. LAYOUTS :- The layouts indicate the positions of columns, beams and slabs. The orientation of length and
breadth of rectangular columns should be as shown in the lay out, columns are serially numbered as C-1,
C-2, ...... etc. The beams framing in these columns are numbered as 1.1, 1.01, 1.2, 1.02, etc. The
secondary beams are numbered as 1.3, 1.4 etc.
The slabs are numbered as 200S1.1, 200S1.2, 200S2.1, 200S2.2, etc. 200S-1.1, 200S1.2 will mean one
way slabs Nos. 1 and 2 with live load of 200 Kg. / M2. 200S2.1, 200S2.2 will mean to way slabs Nos. 1 and
2 with live load of 200 Kg. / M2.
The lay out should always be read in conjunction with the Architects plan. Any discrepancy noticed should
be brought to notice of designs circle for clarification before execution 2. LOADS ASSUMED
a) Dead Loads
i) Concrete ... ... ... ... ... ... ... 2400 Kg / cu.m.
ii) Brick Masonary ... ... ... ... ... 1920 kg. / cu.m
iii) Floor finish ... ... ... ... ... ... ... 100 kg. /m2
iv) 10 cm thick B.B coba ... ... ... 192 kg./ m2
v) B.B. coba on slopping W.C. ... 500 kg./ m2
10 cm thick B.B. coba is assumed over sanitary blocks and terraces and roof slabs with parapet walls.
No B.B. coba load is assumed over canopy, weather shades and roof slabs and terraces without parapet
walls unless specifically mentioned b) LIVE LOADS, WIND LOADS AND SEISMIC LOADS
Live loads and wind loads are provided for as per provisions in I.S.875 unless specific requirement are
communicated by the field officers. Seismic forces where-ever required are provided for as per the
requirements of IS:1893. 3) PERMISSIBLE STRESSES ASSUMED
(a) concrete Mix | M150 | M200 | M250 | M300 | M350 | Ulitmate strees in Kg / Cm2 | 150 | 200 | 250 | 300 | 350 |
For M-150
Shear 5 Kg / cm2
Local Bond 10 kg / cm2
Average bond 6 kg / cm2
All concrete to be of M150 mix unless otherwise specified.
b) Mild steel conforming to Grade I of I.S.: 432 - 1966
Working stress Tension 1400 kg / cm2
Working stress Compression 1300 Kg/ cm2
Ulitmate stress 2600 Kg / cm2
c) H.Y.S.D. bars conforming to I.S.: 1786 - 1966 or I.S.: 1139 – 1966.
Working stress Tension 2300 Kg / cm2 for dia upto 20 mm.
2100 kg/ cm2 for dia above 20 mm.
Working stress compression 1750 kg / cm2
Ultimate stress 4250 Kg / cm2 | | | | | | | Annexure II | (Accompaniment to Designs Circles Technical note no. 7502) | No. BDG3870/8/87584 –K Building and Communications Department,
Sachivalaya, bombay – 32 BR.
Date 14th January 1972 To,
Superintending Engineer of Circles under the Building and Communications Department,
(except the Superintending Engineer, Designs, Circle B&C, Bombay) Sub.:- Construction of Buildings seismic zones
Ref. :- The Chief Engineers letter of even No., dated the 6th April 1971. 2. Recommendation for designing C.D. works minor bridges and major duties for seismic effects have been issued under Government Letter RRN 5070 / 44565 – G, dated 21st September 1970.
Similarly recommendation for building having load bearing , walls have also been issued under Chief Engineer- Buildings letter referred to.
A question of farming recommendations for designing building with R.C.C. frame was under consideration of this Department for some time past on careful and critical examination of this issue by the Superintending Engineer, Design Circle B & C Bombay, the following recommendations are made for designing R.C.C Structures (upto 3 storied) in different seismic zones. i) Building in Zone 1st & 2nd Structures in these zones need not be designed for seismic effect as already indicated in Chief Engineers Letter under reference ii) Building in zone 3rd & 4th
General a) The foundation should rest on such type of soil having a minimum safe bearing capacity of 30 t /
sq.m A temp should be made to rest foundation on this strata even by providing deeper
foundation if there is a possibility of obtaining such strata within a reasonable depth the cases
will to have be studied and solved on individual merits.
b) Plinth beam shall in variable to provided
c) The column should be connected to each other by R.C.C. beams in both the directions as fare
as possible.
d) The height of columns should not exceed 4 meter
e) Cantilever balconies, Chajjas, etc should be avoided far as possible.
f) Structural members supporting the cantilever elements should have anchorage at the supports
of these members.
g) The anchor bars of two No. 12 mm. Provided through out at top of beams should be properly
anchored in supporting columns or continued at top in the adjacent beams if any. 2) Building with ground floor or ground + first upper storey structure a) The precautions listed at (i) above should
be followed and the RCC design may be prepared without considerations of seismic in the design. 3) Building with ground + second upper storey
a) The precautions listed at (i) above should be taller and the RCC designs may be prepared without
consideration of seismic forces in design by observing the additional precautions listed below
b) The Reinforcement in columns should be equal to the maximum reinforcement required at terrace level
second floor, first floor, or plinth level. The section of the columns may be changed from floor to floor but
reinforcement should be same through out.
c) All RCC beams shall be provided with minimum top and bottom steel as shown below which should be
well anchored at the supports or adequately continued in adjacent beams if any. The section of the
beams may change depending upon the span. But the minimum reinforcement shall be as under
i) Beams with span more than 4.572 meter (15’0") – 2 Nos. 20 mm
ii) Beams with span 4.572 meter (15’0") or less – 2 Nos. 16 mm 4) Building with ground less three or more upper storeys a) The design shall be got preapred from Designs Circle.
Note :- The above instructions do not apply to structures located within a radius of 50 Km. From Koyna. Signed M.K. Noble Chief Engineer (B) and Joint Secretary to
Government of Maharashtra Buildings and
Communications Department Copy forwarded to the Superintending Engineer, Designs Circle Bombay for information with reference to his letter no. BLDD (I) / PRC/ UOR/ 3, dated 16th August 1971. It is presumed the copies of seismic map have been supplied to all Superintending Engineers as instructed vide para (II) of the endorsement below Chief Engineers letter under reference please confirm. | | Modification to para 2(II) 3 of Government Letter no. BDG-3870/87584-K dated 14th January 1972 recommended by Superintending Engineer, Designs Circle | Original Provision | Modified Provision | Vide Ces letter dated 14th January 1972 3) (a) the precautions listed are (i) above should be followed and the RCC designs may be prepared without consideration of seismic forces in the design by observing the additional precautions listed below. | | 3) (b) The reinforcement in column should be equal to the maximum reinforcement required at the terrace level second floor, first floor, or plinth level. The section of columns may be changed from floor to floor but reinforcement should be same through out. | 3)(b) (i) Reinforcement in column should be equal to the maximum reinforcement required at the terrace level second floor, first floor, or plinth level. The sections of column may be changed from floor to floor but reinforcement should be same through out except in column section between plan and footing. (ii) The reinforcement worked out in column section between plinth and footing considering dead load, live load and moments, should be further increased by 30% | 3) (c) All RCC beams shall be provided with minimum top and bottom steel as shown below which should be well anchored at the support or adequately continued in the adjacent beams if any. The sections of the beams may change depending upon the span the minimum reinforcement shall be as under i) Beams with span more than 4.572 m (15’0") to Nos. –20 mm ii) Beams with span 4.572 m (15’0") or less – 2 Nos. 16 mm | 3) (c) (i) All RCC beams shall be provided with minimum top and bottom steel as shown below which should be well anchored at the support or adequately continued in the adjacent beams if any. The sections of the beams may change depending upon the span the minimum reinforcement shall be as under 2 Nos – 20 mm for beams with span more than 4.572 m (15’0")’ 2 Nos – 16 mm for beams with span more than 4.572 m (15’0")’ or less (ii) An additional top reinforcement of 2 nos. 16 mm for beams having span upto 15’ and 2 Nos. 20 mm for beams with span more than 15’ should be provided at top through out in addition to the actual top reinforcement and any extra reinforcement required at support. | | 3) (d) The spacing of stripups for beams and binders for columns, or distance of 1 m from each joint should be half the spacing provided in normal design |
Vide C.E’s circular no. BDG – 3870 / 87584-Desk X dated 20th November 1975
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