RCC MEMBER DESIGN TIPS
A.BEAMS: OVERALL DEPTH OF BEAMS: SL. NO
MEMB ER
1.
PLINT H BEAM TIE BEAM FLOO R BEAM S GRID BEAM S
2. 3.
4.
SPAN/OVE RALL DEPTH RATIO 15 TO 18
18 TO 20 12 TO 15
20 TO 30
1. Beam sections should be designed for: a. Moment values at the column face & (not the value at centre line as per analysis) distance of d from the column column face. (not the b. Shear values at distance value at centre line as per analysis) c. Moment Moment redistri redistributio bution n is allowed allowed for static static loads loads only. only. d. For beams beams spanning spanning between between the the columns columns about about the weak 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. e. Moment Moment distribu distribution tion shall shall be done in in such a way that that 15% 15% of the support moments shall be added to the span moment without the support moments getting reduced. f. The sectio section n within within the span shall shall be designed designed for the the increase increased d span moment which will account for the concentrated & isolated loading that may act within one span. g. 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 1/3 of the +ve moment moment reinfo reinforcem rcement ent in SIMPLE 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 higher grade grade of concret concrete e if most of of the beams beams are doubly 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 reinforcement passes through the columns. This is for the reason that any reinforcement outside the column will be ineffective in resisting compression. 8”(200mm) or ¾ of effective 5. Restrict the spacing of stirrups to 8”(200mm) depth whichever is less.(for static loads) 6. Whenever Whenever possible possible try try to use T-beam T-beam or L-beam L-beam concept concept so as to avoid compression reinforcement. 7. Use a min. of 0.2% 0.2% for compressi compression on reinforce reinforcement ment to aid in controlling the deflection, creep and other long term deflections. 8. Bars of Secondar Secondary y beam shall shall rest rest on the bars bars of the Primar Primary y beam if the beams are of the same depth. The kinking of bars shall be shown clearly on the drawing. 9. Length Length of curtailmen curtailmentt shall be checked checked with with the require required d 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 The det detai aili ling ng for the the sec seconda ondary ry beam beam sh shal alll b be e done done so that hat it does not induce any TORSION on the main beam. 13. For For can canttilev ilever er beam beams s re reinfor nforce ceme ment nt at the su supp ppor ortt sh shall all be be given a little more and the development length shall be given 25% more. 14. As a shor hort cut, cut, bendi ending ng momen omentt for for a beam beam (par (parttial ially 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.n SLAB o 1. One- way simply supported slab 2. One-way continuous slabs 3. Two-way simply supported slabs 4.
Teo-way continuous slabs
SPAN/EFFE.DEPTH 30 35 38 for L/B=1.5 35 for L/B>1.5 40 for L/B=1.5 38 for L/B>1.5
1. Whenever Whenever the the slab thickness thickness is 150mm 150mm,, the bar diameter diameter shall shall be 10mm for normal spacing.(It can be 8mm at very closely spaced). 2. Slab thickness thickness can be 10mm,110m 10mm,110mm,12 m,120mm, 0mm,125m 125mm,15 m,150mm 0mm,, etc. 3. The maximum maximum spacing spacing of Main Main bar shall not not exceed exceed 200mm(8” 200mm(8”)) and the distribution bars @ 250mm(10”). 250mm(10”). 4. If the roof roof slab is supported supported by load bearing bearing wall(wi wall(without thout 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 the roof roof slab provide provide a min. of 0.24% 0.24% of slab slab cross cross sectiona sectionall 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 Section should be designe designed d for the column column moment moment values values at the beam face. 2. Use higher higher grade of of concrete concrete when the the axial load load is predominant predominant.. 3. Go for a higher higher section section proper properties ties when when the moment moment is is predominant. 4. Restrict Restrict the maximum maximum % of reinfor reinforceme cement nt 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 Provide laps at midhei midheight ght of column column to minimize minimize the damage damage due to moments(Seismic forces). 8. Avoid Avoid KICKER concre concrete te to fix column column form work work since it 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 Check the Factor Factor of Safety used used by the Geotechn Geotechnical ical engineer engineer for 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). IS-1893-2000(part-I). 4. Provide Provide always always PLINTH PLINTH BEAMS resting resting on natural natural ground 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 Always assume assume a hinged hinged end support support for column column footing footing for for analysis unless it it is supported supported by raft and on pile cap. The Common assumption of full fixity at the column base may only be valid valid for columns supported on RIGID RIGID RAFT 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 164Seismic design of Reinforced concrete and Masonry buildings by T.Paulay & M.J.N.Priestley.) 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 minimum reinfor reinforceme cement nt for the RCC RCC wall subject subject to BM shall shall be as follows: A. Vertic Vertical al reinfo reinforce rcemen ment: t: a) 0.0012 0.0012 of cross cross sectional sectional area for deformed deformed bars not larger than 16mm in diameter and with characteristic strength 415 N/mm^2 or greater. b) 0.0015 0.0015 of cross cross sectio sectional nal area area for for other other types types of bars. c) 0.0012 0.0012 of cross cross sectional sectional area area for welded welded fabric fabric not not larger than 16mm in diameter. Maximum horizontal spacing for the vertical reinforcement shall neither exceed three times the wall thickness nor 450mm. B. Horizontal Horizontal reinforc reinforcemen ement. t. a) 0.0020 0.0020 of cross section sectional al area for deform deformed ed bars not larger than 16mm in diameter and with characteristic strength 415 N/mm^2 or greater. sectional area for other types types of b) 0.0025 of cross sectional 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 He/t for a RCC wall shall not exceed 30 as per IS:456=2000,
where He is the effective height of the wall and t is the thickness of the RC wall. He 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 . 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. VIEREN VIERENDEE DEEL L GIRDER 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 a nd 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 1/61/4. The depth to span ratio of an arch is usually in the range of 1/40 -1/70.
5. FOLD FOLDED ED PL PLAT ATE: E: The typical depth /span ratio is in the range from 1/15 to 1/10. 6. FLAT FLATE E PLAT PLATE: E: A typical depth of a solid FLAT PLATE is 1/22 -1/18 of the effective span. 7. TWO-WA TWO-WAY Y RIBB RIBBED ED SLAB SLAB:: Supported on continuous stiff supports are in the range of 1/301/25 of the lesser effective span. 8. FLAT FLAT PLATE PLATE RIBBED RIBBED SLA SLAB: B: Typical depth of flat plate ribbed slabs are in the range of 1/201/17 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. ******************************************************************** **********************************************************
Before doing any model on computers a rule of thumb,in my opinion, is size of RCC column will be about 3 to 5 % of height of building. That is if building is 8 storied of near equal heights of floor 3M than building is about 24M tall. Probably column size will be between 750mm to 1200mm. However, spans in both direction will have to be considered[ Column spacing]. If spans in both direction is say 8M X8M than most probably 750mm X750mm X750mm may suffice. But if span is 14M in one direction and 4.5M in the other direction than you may be able to use 1/12 th of span and hence column size may be about 1.2M in 14M direction and 300mm to 400mm in that direction. Same is true for Steel columns columns but width of column[flange column[flange of column shall be 1/30th of height to keep latral buckling of flanges if you are using rolled sections.
