Beams Thumb Rules

ER.ATUL V.JOSHI. ‘SINDAGI COMPLEX’, FLAT NO.B-9,THIRD FLOOR, OLD EMPLOYMENT CHOWK, SOLAPUR-413001. (MAHARASHTRA) CONTACT:0217-2310508,9823110936

B.E.(CIVIL)MIE.CAT.CE

BEAMS (THUMB RULES) *Max.Bending Moment at Mid-span of Beam=(Load xSpanxSpan)/10 (for partially fixed) *Min.Depth of beam=Span of Beam in Ft +3 =------------inch Eg .For 40 ft span beam,depth =40+3=43 inch. ** For Every additional Point/Concentrated Loads, increase the depth of Beam by 3 to 9 inch. If Depth reduces, the width should be increased to maintain w.r.t. c/sl area of beam. * Width of the Beam =(Span of Beam in ft.)/3=------------in inch (Min.width of beam should be 9inch) Eg.Min.Width of 40 ft.span beam=40/3=13..33 inch say 15 inch OR Min.Width of Beam= (Depth of Beam in inch/3)=……….in inch Eg.Min.Width of Beam=(36inch/3) =12 inch ##If large span beam is resting on short span beam, then provide same size & steel of large span beam to short span beam. *Width of Point Loaded Beam= =(Span of Main beam in ft.+Span of secondary(pt.loaded)beam in ft)/3 =-------------inch * Depth of Point Loaded Beam= =(Span of Main beam in ft.+Span of secondary(pt.loaded)beam in ft +UDL in KN/M) /3 =------------- inch *Tension Steel Area Required=Width of Beam in inch x Depth in inch x4.5=------------sq.mm 1 avjoshi



(Here min.steel in Beam is 0.90% with Fe 415 grade steel considered) (Always provide min.2 bars of 12mm dia.tor steel for Tensile steel.) Intermediate Beams: *Area of Tensile Steel=(Width of Beam in inch XDepth in inch xspan in ft.)/4.20 =--------------sq.mm Eg.Area of Tensile steel=(9 inch x 24 inch x 22 ft.span)/4.20 = 1131 sq.mm OR Area Of Tensile Steel= Width in inch x Depth in inch x5.20(const.)=-------------sq.mm Eg.Min.area of tensile steel for 9”x12” beam=9x12x5.20cons.=561.0 sq.mm OR **Area of Tensile Steel= =(UDL in KN/M xWidth of Beam in inchXDepth in inch xspan in ft.)/175 =---------------sq.mm *Diameter of Tensile Steel: Min.Diam.= (Span in Ft.)/1.5 =----------------in mm Eg.for 36 ft span beam,better to provide min.steel dia.=36/1.5=24mm say 25mm NO.OF BARS REQUIRED FOR TENSION STEEL: *No.of bars reqd.=(Width of Beam in inchXDepth in inch xspan in ft.)/800=----------Nos. OR No.of bars reqd.=(Width of Beam in inchXDepth in inch)/40=----------Nos. OR **No.of bars reqd.=(Area Reqd.in sqmm x8 cons.)/(100x Diam.of Bars in mm)=----------Nos. OR **No.of bars reqd= (Width of Beam in inch XDepth in inch x Span in ft.x10) (Tensile Stg.of Steel in N/sqmm xDiam.of Steel bar in mm.) =-------------Nos. 2 avjoshi



STONE LINTELS: *Depth of Stone Lintel: Depth= Span in ft.+1inch =-----------------inch Eg. Depth of 4ft.span lintel=4+1=5 inch OR Depth of Stone Lintel=(Span in mt)/12+25mm =----------------inch Eg.Depth/Thk.of 12 ft.(3.65m)span stone lintel=(3.65/12)+25=0.30m+25mm=12’+1”=13inch. *Width of Stone Lintel: Width= Span in ft./3 =-----------------inch Eg.Min.width of 4ft.stone lintel=4/3=1.33 inch.say min.3inch (Thus for 4ft.Door or Window opening ,we can provide stone lintel of size 3”x5’’ or Wall thk.x5”) TIMBER BEAMS: *Max.Bending Moment=(LoadxSpanXSpan)/8=f x z =(f x Breadth xDepth xDepth)/6 (Here For Sal Wood f=Safe Comp.Stress=105 kg/sq.cm & for Indian Teak Wood f=100kg/sq.cm) *Depth of Wooden Beam: Depth=(Span in cm/25)+5cm=…………….cm Eg.Depth of 12ft(3.65m=365cm)span timber beam=(365cm/25)+5cm=19.6cm=196mm=7 .66 inch. OR Depth of Wooden Beam=Span in ft./1.5=--------------inch Eg.Depth of 15ft.span wooden beam=15/1.5=10inch OR Depth of Wooden Beam=Span in Ft.-5inch=----------inch Eg. Depth of 10ft.span wooden beam=10-5=5inch *Width of Wooden Beams: Width=0.50 to 0.75 x Depth Eg.For 6”depth wooden beam,min.width should be 3inch 3 avjoshi



CIRCULAR BEAMS: *Depth of Circular Beams:Depth=Radius of Beam in Ft.+6 inch=-----------------inch Eg.Depth of 12 ft.radius beam= 12+6=18 inch OR Depth of Circular Beam= Perimeter of Beam in ft./4=---------inch Eg.Depth of 8ft radius beam=2X

=12.56 inch say 14 inch

*Width of Circular Beam=Depth in inch/2=---------inch Eg.Width of 8ft radius beam=12.56/2=6.28 inch say 8 inch CANTILEVER BEAMS:(subjects to porch loads, light wts.etc) *Depth of the cantilever beam=3 x Cantilever span in ft.=--------------inch Eg.Depth of 6ft.cantilever span beam=3x6ft =18inch *Width of the cantilever beam=2+ Cantilever span in ft.=--------------inch Eg.Width of 6ft.cantilever span beam=2+6ft =8inch OR Width of the cantilever beam=Depth of Beam in inch/2=--------------inch Eg.Width of cantilever beam=30inch/2=15inch TOP STEEL FOR CANTILEVERS: *Diam.of Top (Main)steel=Width of Cantilever Beam in inchx1.5=----------mm(Min.Dia.=12mm) OR Diameter of Top Steel=Cantilever Span in ft.x2=------------mm Eg.for 12ft.cantilever beam,min.Diam.of steel reqd.=12 x2=24mm say 25mm NO.OF BARS AT TOP REQUIRED: *No.of bars at top=(Cantilever span in ft.x UDL in KN/sqm.)/(Diam.of top steel in mm)=------nos. 4 avjoshi



Eg.No.of Bars at 3ft.cantilever=(3ftx25kn/m)/12mm dia.=6.25 nos. OR No.of bars at top=

(Cantilever span in ft.x UDL in KN/mx400)

(Diam.of top steel bar in mmxWidth of Beam in inchx Depth of Beam in inch) =----------------nos. Eg.No.of Bars =(10ft.x 11.62kn/mx400)/(20mm dia.x12inchx24inch)=8 nos. BOTTOM STEEL FOR CANTILEVER BEAMS: *Bottom steel diameter= Top Steel dia. OR Bottom steel diameter=Top steel diam.in mm-2=------------mm Eg.Bottom steel dia.=20mm-2=18mm NO.OF BARS AT BOTTOM REQUIRED: *No.of bars at Bottom=

(Cantilever span in ft.x UDL in KN/mxWidth in inch)

(Diam.of bottom steel bar in mmxWidth of Beam in inchx Depth of Beam in inch) =-----------nos. OR No.of bars at Bottom= (Top Bar dia.in mm x No.of top bars ) (Diam.of bottom steel bar in mmx3) =-----------nos. Eg.No.of bottom bars =20mm x8 nos./3x16mm=3.33nos. say 4nos. Stirrups(Rings): *Diameter of stirrups=(Width of cantilever beam in inch-4) =---------------mm 5 avjoshi

Min.dia.should be 6mm



OR Diameter of stirrups=(Cantilever span in ft.x UDL in KN/m)/12 =---------------mm Eg.Diam.of stirrups=12ft.x11.62 kn/m/12=11.62mm=12mm here 4 legged 8mm stirrups may be provided. Spacing of Stirrups: (Max.spacing should not be more than 9 inch) *Spacing =

(Diam.of Stirrups x UDL in KN/mxWidth in inch) 4X (Cantilever Span in ft.x Depth of Beam in inch)

=--------------------inch Eg.spacing of 6mm dia.stirrups= (6mm x 11.62kn/m x 9inch)/(4cons.)x(4ft.x 12inch)=3.26 inch

BEAMS THR’WHICH CANTILEVER PORCH PROJECTED: *Volume of Beam should be equal or greater than volume of cantilever porch. *Depth of the beam=5 x Cantilever span of porch/canopy slab in ft.=----------------inch. Eg.Beam depth reqd. thr’ which cantilever porch of span 6ft.is projected=5 x6ft.=30 inch OR Depth of beam=4(cantilever span of porch slab in ft.)+Span of Beam in ft. =---------inch Eg.Depth of 20ft beam thr’which 12ft.canopy is projected=4x12ft.+(20ft)=68inch OR Depth of beam=(UDL in KN/m on beam +Span of Beam in ft.)+1.5 x (cantilever span of porch slab in ft.)=---------inch eg.Depth=(20kn/m +20ft.beam span)+1.5x10ft.cant.porch=40+15=55 inch Width of Beam: Width=Cantilever porch span in ft.+3=---------inch Eg.Beam width=12ft.+3 =15inch

6 avjoshi



BOTTOM & TOP STEEL : *Diameter of bar= Cantilever porch span in ft.x2=--------------mm Eg.Diam.of steel for beam=10ft.x2=20mm. NO.OF BARS AT BOTTOM REQUIRED: *No.of bottom bars= (2 x Span of Beam in ft.)+(2 xCant.porch span in ft.)/(Diam.of bar in mm) =--------------nos. Eg.Beam with 20ft.span & cant.porch of 10ft. with 20mm bottom bars Nos.=2x20ft.+2x10ft.cant./20mm=3 nos. *Diameter of stirrups=[Cantilever porch span in ft.+( Beam span in ft x UDL in KN/m)]/40 =-----=--------mm Eg.Dia.of stirrups=(10ft.+(20ft.x25kn/m))/40=12.75mm .Here we can provide 10mm -4legged stirrups. ##(Note-Generally porch main steel is continued in the beam in a Ring/stirrup shape for reqd. development length). *Spacing Of Stirrups =Min.4 inch(100mm)at support & Max.8 inch(200mm)towards mid-span. PLINTH BEAMS: *Width of Plinth beam=Span in ft./3=----------------inch *Depth of Plinth beam=Span in ft./1.5=-------------inch.(Min.Depth should be 12inch=300mm) OR Depth of Plinth Beam=Span in ft.-9=---------inch (Note: For M20 grade Beams: Each & every increment in Depth of beam by 3inch(75mm)with same width & min.main steel percentage of 0.90%,increases Moment of Resistance capacity of Beam by 25 KN-m) 7 avjoshi



Eg. For M20 Beam of size 230mm x300 mm with 0.90% steel:Moment of Resistance=57kn-m And For M20 Beam of size 230mm x375 mm with 0.90% steel: Moment of Resistance=89 kn-m (Here increment in depth with same width is 375-300=75mm) BOTTOM & TOP STEEL OF PLINTH BEAMS: *Area of Bottom Steel =3 x Area of Plinth Beam in sq.inch =------------------in sq.mm.(Min.dia.should be 12mm tor) Eg.For 9inch x12 inch plinth beam,Area of Bottom steel= 3x 9x12=324 sq.mm *Area of Top Steel =1.5 x Area of Plinth Beam in sq.inch =------------------in sq.mm.(Min.dia.should be 10 mm tor) Eg. For 12inch x21 inch plinth beam, Area of top steel= 1.5x 129x21=378 sq.mm (Generally ,same steel area of bottom steel is provided at top to resist upward earth pressure) Note:Plinth beams should freely rest on ground(or min.6inch (150mm)below g.l. If they are provided at plinth level, they has to be designed for sagging moments due to downward loads of brick wall, lofts resting on walls etc. Notes: *All Thumb Rules are created on the basis of Experience , Calculations ,Data available from various books and guidance given by Senior Engineers/Technocrats. *Thumb Rules are practicable and easy to work out basic/ initial sizes of structural members for Quantity calibration, estimation and to cross check the safety of structural designs. *For Big & Complicated structures, actual calculations should be done along with use of these Thumb Rules.

8 avjoshi



*Thumb Rules are invented by considering Factored Load,Wind Pressure,Seismic Forces, Corrosion of material, Worksmanship &Practical Convenience. *More suggestions and invention on these Thumb Rules are always welcome. *Thumb Rules are created by help of SI,MKS,FPS,CGS unit systems therefore sufficient knowledge of all unit systems are must. *Adopt sizes & steel for the higher/safer side with thoroughly judgment.

9 avjoshi

Beams Thumb Rules

Recommend Documents