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[husseins20]

Dear All,

We are aware that concrete usualyy needs to be provided with minimum reinforcement. However, in the event where the load is minimum is it still necessary to provide reinforcment.?

The scenario is as follows:-

Machine is 18Tons on 4 legs i.e each leg 4.5t = 45kn ( Static load only) No dynamic loading

Leg size is 210x210 sqaure

The machine is resting on ground on a Mass concrete pedestal of 500x500 x500 of Grade 20

Can you please advise if it is acceptable.

[vikram.jeet]

Mass concrete 5x5x5m resting  on ground ,,  

Presume that foundation is at at least at a depth which can able to take care of  ( 5*2.4 = 12.5 t/m2 self wt + 18 /25 t/m2 machine wt + any  pr due to eccentricity of machine wt wrt centerline mass conc pedestal)


Dead wt and ht of foundation is substantial,, it must be checked for moments arising from EQ forces on self wt + machine load.

Minimum surface reinforcement must be provided,  but if EQ moments warrant higher reinf sd per design may be provided,  that is to say that if stresses on tension side are less than that for M20,  then minimum otherwise as per calculations.

[husseins20]

Dear Vikram,

Thank you for the feedback.

TO clear out the pedestal is 500mmx500mmx500mm

the Room Size is 4m x 4m

Thus we can deduce the following:-

1. Pedestal load  4 x 3.125KN ( Per pedestal) = 12.5KN
2. Machine Load = 180KN

Stress on soil = 192.5KN/16 = 12KN/m2 + Self weight of base 10KN/m2

Total load on soil = 22KN/m2 <<< 150KN/m2

Stress on mass concrete base = 45 x 1.6 x 10^3/210x210 = 1.6N/mm2 << 13N/mm2

Punching  = 45 x 1.6 x 10^3/(4*210) x500  = 0.17N/mm2 << 3.5N/mm2


Trust i have got the above numbers correct.

Would appreciate if there is any missig check.

rgds

[vikram.jeet]

Dear Er,,   Revised version

First and foremost is to seek pardon fm you for misreading the dim as 5000mm instead of 500mm ,, Age factor and small sise of letters, sometimes cause slip,  inconvenience regretted ,

Presume there are four maas conc block foundations,  0.75x0.75x0.75 depth  ,,
Self wt of each = 0.75*0.75*0.75*2400 = 1012 kg
Machine load = 4500 kg

Normal Condition -
Total load  = 1012+4500 = 5512 kg  each block foundation
Base pr = 5.512/(0.75*0.75) = 9.8 T/m2

,, see soil report and compare with gross sbc

Seismic condition :
Taking zone Iv in illusttation
Ah = (0.24/2) * 2.5 * (1.5/2.5) = 0.18

Moment due to horz Seismic force on machine = O. 18*18.0* (centerline ht above top of mass conc block)
Say =0.18* 18.0* (say 1.5 ) = 4.86 tonne m
Assuming Leg s are spaced 1.0 m apart,, you workout on actual
Additional Reaction on found blocks  per block
= +/-  4.86/1.0) *1/2 = 2.43 tonne

Max base pr

Each block found
Moment at base of block = 0.18*4.5*0.75 = 0.61 tm
Passive less active relief Mo = [(3 - 0.33) * 1800 * 0.75^3  /6 ]*0.75 / fos 1.5 = 170kgm

Net moment = 0.61 - 0.17 = 0.44 tm

Vertical seismic force = 2/3* 0.18 * 5.512=  0.66 tonne

Base Pressure max
= (5.512+2.43+ 0.66) /(0.7*0.75) +  
(0.44* 6) /(O.75*0.75^2)/6
= 15.3+ 6.3 =21.6 t/m2  compare fm soil report (1.25*gross sbc )
Base  pr minimum
=(5.512 - 2.43 -0. 66) /(0.75*0.75)  - 6.3
2.422/(0.75*0.75) - 6.3
= 4.3 - 6.3= negative cause upliftment of footing
e = 0.44/2.422= 0.18 m
Contact length x = 3*(0.75/2 - 0.1   = 0.585 , I. E 0.585/0.75 *100= 78%
Revise block foundation size to say 800*800*750  and check

Contact length shall be at least 80% .

[vikram.jeet]

Dear Er
Pl see revised version,, as corrected/updated  today in preeceeding post

[sakumar79]

Dear sirs,

IMHO,

1. Since the pocket size is only 500 mm deep, the pocket will probably rest on filled up soil and not even on natural earth.

2. For 45 kN over .5x.5 pedestal, the pressure on earth from the pocket is about 20t/sqm. As a thumb rule, we can consider the bearing capacity on moderately good soil and well consolidated filling as 5 t/sqm.

Hence, the pedestal size should not be less than 1 m x 1 m instead.

If possible, I would suggest that four legs have a common pedestal so that settlement will be more or less uniform on all the legs.

Earthquake aspects as noted by Er Vikram Jeet sir should also be considered.

Yours sincerely
S Arunkumar

[sakumar79]

Dear Sirs,
   In continuation of my earlier post, I also feel that with minimum thickness of 500 mm based on earlier calculation, the load will get properly distributed to the soil for a bearing pressure of 5 t/sqm and may not require reinforcement if there are no major earthquake forces to be considered.

Yours sincerely
S Arunkumar

[vikram.jeet]

As regards provision of reinf in block foundation :

Going by illustration given earlier :
Forces on block foundation under EQ case :
  W = 2.422 (min)
M = 0.61 tm (ignoring soil relief for stress check)


Tensile stress =
2.422*1000/(75*75)  - 0.61*100000*6/(75*75^2)
= 0.43  -  0.87 = -  0.44  kg/cm2

Though it is within limits,, still some 8mm dia about 12nos vertical at periphery + 8mm dia ring at periphery and open links 2x2 to address local stresses around Anchor bolts.