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Design of Slab on Grade for Connex structure above it.
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karansohal875
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PostPosted: Wed Sep 07, 2022 6:28 am    Post subject: Design of Slab on Grade for Connex structure above it. Reply with quote

Greetings all!
I am working on a project in which I have to design a foundation for a Connex/Porta-Cabin. Plan dimensions of the foundations are 41ft x 9ft.
I am thinking to design a 5inch slab with its edges thickened upto 18inch to 24inch to hold anchor bolts.
Any suggestions or recommendations for this? Will it be designed as a slab on grade over beams a its ends? or some other approach?

Thanks
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vikram.jeet
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PostPosted: Wed Sep 07, 2022 8:22 am    Post subject: Reply with quote

Potta  cabin is light wt structure and s B c is not a problem even if structure rest just at existing  G L.

Under wind case , it will be subjected wind pressures on pota walls and light wt roof of m s sheeting . But space is small span 9 '  and length is 41'.

Proposed slab with peripheral beams is a sound option.

OTHER OPTIONS

Annular rcc  footing of small width as per design dictate on either side  and flooring  pcc in-between space

Small pits  with pcc Block foundations of size as per design dictate at  locations of anchor boltings.
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karansohal875
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PostPosted: Wed Sep 07, 2022 9:33 am    Post subject: Reply with quote

Thanks vikram

Can you help me how this slab and peripheral beams will be designed? I mean slab will be designed with kn/m2 load over it(might need min reinf) what about those beams?

PFA

vikram.jeet wrote:
Potta  cabin is light wt structure and s B c is not a problem even if structure rest just at existing  G L.

Under wind case , it will be subjected wind pressures on pota walls and light wt roof of m s sheeting . But space is small span 9 '  and length is 41'.

Proposed slab with peripheral beams is a sound option.

OTHER OPTIONS

Annular rcc  footing of small width as per design dictate on either side  and flooring  pcc in-between space

Small pits  with pcc Block foundations of size as per design dictate at  locations of anchor boltings.



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vikram.jeet
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PostPosted: Wed Sep 07, 2022 11:55 am    Post subject: Reply with quote

Will try to give just a rough illustration  which u can refine.

Roof width say = 9 + 3 = 12 feet  ( taking 1.5' proj on either side )
Ht = 4.0 m
Wall thickness = 0.15m


Case 1  DL + LL case


DL = 15 kg / m2
LL =  2/3 rd * 75 = 50 kg/m2

DL +  LL = 15 + 50 = 65 kg/m2

Roof weight per meter  on each support = 65 * 3.66 /2 = 119 kg
Pota wall wt per m run  say @ 900 kg  / m3 = 900 *  4.0*0.15 = 540 kg**
** wt can vary based on material but just taken for illustration
Slab wt  per m on each support = 3.05/2  *0.12 * 2500 =  477 kg
Beam wt  per m = 0.45*0.45 *2500 = 506 kg
Floor finish over slab wt each side = 0.05*2400*3.0/2 = 180kg
Floor zLL @ 300 kg/m2  on each side = 300*3.0/2 = 450 kg

Total wt  per m = (119 + 540)+(477+506  +180+ 450)= 2272 kg  each side

Base area = 9' + 2* 6" = 10 '

Base pressure  = 2272  *2  / ( 3.05 *1.0) = 0.54 t/ m2
Net pressure fm bottom = ( 119 + 540 ) *2/( 3.05 *1.0) = 0.44t/m2

BM in base slab at mid span considering simply supported
Tension top =  0.44* 2.90^2/8 = 0.48tm /m
Taking single   mesh centrally placed
Eff depth ~ 125 /2 = 62.5mm
Mu/bd^2 = 0.48*1.5 *100000/( 100*6.25^2)/10.2 =  1.81
p st ~ 0.50  ( for exact refer sp16)
Ast = 0.50*6.25 = 3.12 cm2/m

Will check wind load case and see ast required and higher of two to be provided

R cc beams are not stressed in  above DL + LL case



Case 2 DL + WIND LOAD      - - -  NEXT POST


Last edited by vikram.jeet on Thu Sep 08, 2022 2:30 am; edited 1 time in total
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vikram.jeet
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PostPosted: Wed Sep 07, 2022 2:57 pm    Post subject: Reply with quote

Case 2 DL + WL

Taking  wind pressure  = 100 kg/m2  for illustration  purposes
(Pl work out as per IS 875 )

Roof slope 1 in 10  

Pi ~ 0.8 p
Pe ~ 0.5p

Pi + pe =1.3 p  , it will be 1.3p on windward half of roof and little say 1.0 p on leeward half  , but for illustration take 1.3p all over

Wind Uplift force on each wall  of roof = 1.3*100 *3.66/2 = 238 kg/m

Wind force on windward wall = 1.2 * 100*4.0 = 480 kg   /m
Wind moment at base = 480* 4.0/2 = 960 kgm /m

DL On roof on each side  =  3.66/2 * 1.0 * 15  *80%=  22 kg/m
Wall wt = 900 * 4.0 *1.0 *0.15  *80%= 432 kg

Base slab + beam wt + finishes each side = 477 +506 + 180  = 1163 kg

Total wt = 22 + 432 + 1163 = 1617 kg  < 1.2* 238

Structure is OK  stability against Wind uplift

Load /m  without base slab & finishes  on sides = 506 kg > 238 kg
Hence base slab is not stressed due to wind uplift

It is because span of pota cabin small

However slab is subject to Moment transfered from wall = 960 kgm/m
Mu/bd^2 = 1.2*960*100/ ( 100*6.25^2)/10.2 = 2.89 n/mm 2
Slab may need thickening at edges and 150 th slab would be OK at edges

Revised Mu/bd^2 ~ 2.1
Pst  ~ 0.59  ;  Ast = 0.59 *7.5 = 4.43 cm2/m

Hence a mesh of 8mm dia at 150mm c/c plus extra bars on each side 8mm dia @ 300 mm c/c providing alternately in regular mesh bars
Reinf provided 3.33 + 3.33/2 = 5.0 cm2/m  
Extra bar length = 3.0x0.3 = 0..9m

Or it is better to provide 8mm dia @ 120 cm c/c mesh centrally  placed  

Beam design : see next post
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vikram.jeet
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PostPosted: Thu Sep 08, 2022 4:18 am    Post subject: Reply with quote

Beam design :

Assuming steel columns at 4.0m c/c
Wind case

Uplift force on columns = 238* 4.0 = 952 kg
Net uplift force on column = 952 -22*4 = 864 kg
Wind moment on column  from cladding = 960*4 = 3860 kgm

Hence steel column to be designed for P tension  and moment 3860kgm

Beam design ( Beam 450x450mm )
Vertical Downward DL  On beam per m = 506+432= 938 kg /m
Base pr = 0.938/0.45 = 2.08 t/m2
Under normal cond itionns ( non wind) , base pr is balanced by udl on beam

Due to wind Net  Downward wt = 938 - 238 = 700 kg/m
Base pr = 0.700/0.45 = 1.56 t/m2

BM at support/ span  - - - taking same as str is mini str
= (938 - 1.56*0.45) * 4.0^2/10 = 381kgm  - - small value but see for twisting moment below
Torsion moment = 960kgm/m
Torsion moment in beam at at column = 960 *4.0/2 = 1980 kgm
( Moment of 3860kgm  from column will be taken by grade slab through twisting of beam , Grade slab being thin member will have local punching shear failure if beam is not provided)

Pl see this rough illustration  and do exact calcs


Last edited by vikram.jeet on Thu Sep 08, 2022 11:11 pm; edited 1 time in total
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karansohal875
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PostPosted: Thu Sep 08, 2022 5:31 am    Post subject: Reply with quote

Thanks a lot Vikram Sir for your guidance.
I have few doubts, mentioned below.

  • Which length (2.90m) you considered while calculating BM in base slab at mid span?
  • Don't we have to consider EQ loads?
  • The moment and shear due to lateral loads will be taken care by anchor bolts which will be around 10 inch deep. These lateral forces should be considered in design of slab or beams under slab?

vikram.jeet wrote:

BM in base slab at mid span considering simply supported
Tension top =  0.44* 2.90^2/8 = 0.48tm /m
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vikram.jeet
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PostPosted: Thu Sep 08, 2022 8:45 am    Post subject: Reply with quote

karansohal875 wrote:
Thanks a lot Vikram Sir for your guidance.
I have few doubts, mentioned below.

  • Which length (2.90m) you considered while calculating BM in base slab at mid span?
  • Don't we have to consider EQ loads?
  • The moment and shear due to lateral loads will be taken care by anchor bolts which will be around 10 inch deep. These lateral forces should be considered in design of slab or beams under slab?

vikram.jeet wrote:

BM in base slab at mid span considering simply supported
Tension top =  0.44* 2.90^2/8 = 0.48tm /m



Eff Span is taken as 9' clear span + 6" =  2.75+0.15 = 2.90 m

EQ  - Since this is very light wt structure   it will not govern over Wind

Anchor bolts are there to transfer forces and moments from column base to Rcc beam . These are transferring devices , but ultimately everything is to be borne by MOTHER EARTH and Rcc beams and slab are also there to receive forces and moments  fm anchor bolts and transfer finally to underlying strata.
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vikram.jeet
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PostPosted: Thu Sep 08, 2022 9:20 am    Post subject: Reply with quote

Grade slab


It is designed here  for vertical loading from wall at ends . BM worked out is conservative since base pr is  small wrt to sbc of  3- 5 t/m2 at ground surface and even these wall loads may not stress base slab .

  ( Rest other loads viz self wt and finishes wt are in form of udl  over entire area , the upward base pressure fm bottom  due to these will be counter balanced by uniform loading pressure from top while working BM at mid span of grade slab , hence not figured in BM calcs )



However wind moments from walls transfered thru bolts are to be completely taken by base slab .
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PostPosted: Sun Sep 11, 2022 4:31 am    Post subject: Re: Design of Slab on Grade for Connex structure above it. Reply with quote

karansohal875 wrote:
Greetings all!
I am working on a project in which I have to design a foundation for a Connex/Porta-Cabin. Plan dimensions of the foundations are 41ft x 9ft.
I am thinking to design a 5inch slab with its edges thickened upto 18inch to 24inch to hold anchor bolts.
Any suggestions or recommendations for this? Will it be designed as a slab on grade over beams a its ends? or some other approach?

Thanks



Please check whether subsoil is swelling shrinking soil. It can move the building up and down. Even if the structure is not affected, the service connections, especially sanitary lines, can give some problem

S.P.Srinivasan
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