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machine foundation frequency ratio as per CP 2012

 
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bagmass
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PostPosted: Wed May 19, 2010 4:07 pm    Post subject: machine foundation frequency ratio as per CP 2012 Reply with quote

dear all

       we are designing an Block foundation for Diesel and Gas generator. we got information from our vendor which i given below

Engine Reciprogating type with Isolaters below the machine.
engine dimesnion  9*2.38*3.588m

Machine C.G lies at 3.89 in length , 1.19 in width wise * 1.83 m height.

Total weight 100600kg

the harmonic forces transfered to the

6.5 Hz   -- torque 9.1kNm
12.5 Hz   -- torque 8.2kNm
75 Hz   -- torque 29.4kNm

machine speed - 750 rpm
4290 Kw.

soil data dynamic shear modulus 175.5mpa , density of soil 1.95 gm/cm3 & poissons ratio of soil 0.439

we designed the foundation as CP 2012: part 1: 1974.

As per this code they are asking for a frequency limitation to keep away the machine/foundation ratio by either 0.5 or above 2 for impartant installation and 0.6 or 1.5 for lesser impartant installation and with isolaters 3.

Normally we used to keep the frequency ratio 0.7 or 1.3. as this logic we arrived a block size of 11.5 * 4.3 * 2.4. depth of foundation 2.1m.

My question is

1. is it mandotary to consider codal requirement  or we can keep +/- 30% ratio.

2. if needs to follow codes value is there any method or ways to increase or reduce frequency to get optimul size.


please guide us to solve this problem.

Thanks & Regards

Bagmass
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suraj
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PostPosted: Wed May 19, 2010 5:55 pm    Post subject: Operating Frequency? Reply with quote

Operating Frequency?
Before raising any question, please have the value of Operating Frequency from relevant vendor.
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IntPE(India)Suraj Singh FIE Civil
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bagmass
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PostPosted: Thu May 20, 2010 5:07 am    Post subject: Reply with quote

Dear suraj sir

Operating frequency of the machine is 12.5 Hz.

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bagmass
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suraj
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PostPosted: Sun May 23, 2010 2:40 pm    Post subject: Machine Foundation Frequency Reply with quote

Machine Foundation Frequency
To avoid resonance to occur, frequency ratio limitations are defined by codes.
0.5  & 2 are the limits.
Ask vendor to provide frequency in terms of ratio.
Calculate the foundation soil frequecy or natural frequency.
In case, it is not achievable, increase the foundation size & obtain the relevant altitude.
In case, resonance occurs, amplification of altitude shall lead to failure of system as it happens during earth quake.
Limits are required to be maintained.
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bagmass
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PostPosted: Mon May 24, 2010 12:22 pm    Post subject: Reply with quote

Dear suraj sir

       Thanks for your reply. we have problem in increasing the width. i have to play with either changing length or depth only. we tried a lot and not able to achieve  codal values. so we are looking for some options or suggestions from you and from our sefians to satisfy codal requirements.


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Bagmass
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suraj
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PostPosted: Mon May 24, 2010 2:31 pm    Post subject: Machine Block Foundation Reply with quote

Machine Block Foundation
Did you try combining generator foundation with alternator foundation as one unit?
Also, try increasing length & depth of block.
What basis did you consider quantity of soil as an interactive with foundation?
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bagmass
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PostPosted: Mon May 24, 2010 2:48 pm    Post subject: Reply with quote

dear sir

         Actually the Alternator, rotor and engine combined together resting on Rigid Steel skid. the skid is supported on isolaters and below isolaters we have block foundation which supports the complete system. we have Dynamic shear modulus, poissons ratio and density for the site based on cross hole test results by our geotech consultant. we calculated the coefficient of elastic uniform compression based  on Barkan's equation for site specific dynamic Elastic modulus derived from shear modulus,poissons ratio and density of soil. All other coefficients based on Barkan. Because CP2012 recommends to use Barkan's equation.

we analysed the block foundation with this combined system only. we have problem to reduce the frequency to 0.5 and increasing to 2. we are getting +/-40% only.


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Bagmass
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suraj
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PostPosted: Mon May 24, 2010 3:02 pm    Post subject: Machine Foundation Reply with quote

Machine Foundation
What I understand from reply is that system adopted is different than it should be.
Block that is indicated is like a raft for combined equipment.
Steel skid placed meant for all but, it should be only for equipment generator.
Alternator massive column should be monolithic with raft.
Isolation mechanism should have been made only for the generator.
Not indicated what type of base over isolation system is provided.
According to technical requirement, there should be a massive block over the isolation system embedded into with the isolation steel plate.
Please, check whether or not, massive block has been considered.
Massive block is too significant for such structure to allow dampening effect.
Barkan's formula is perfectly right in this case.

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