Question

The principal design criteria for foundations for reciprocating machinery are as follows:

1. The natural frequency should be at least 40% away from the operating speed of the machine.

2. The amplitude of motion of the foundation should not exceed 0.2 mm.

3. The pressure on soil should be within the respective permissible values.

4. For preliminary design, the maximum pressure on soil due to static load, alone may be taken as 0.4 times the corresponding safe bearing capacity.

1. 1, 2, 3, and 4 are correct
2. 1, 3, and 4 are correct
3. 3 and 4 are correct
4. 2, 3, and 4 are correct

Answer 1

Correct Answer - Option 1 : 1, 2, 3, and 4 are correct

Concept:

Design criteria for foundations for reciprocating type machines:

As per IS 2974 (Part 1):1982, cl.5.

The following are the design criteria for foundations for reciprocating machinery:

• When the number of similar machines to be installed side by side in close spacing and soil conditions, a mat foundation should be provided such that deformation is minor as compared to the resultant of vibration.
•  The mass of the foundation should be greater than the mass of the whole machine.
• The foundation shall be so dimensioned such that resultant force due to mass of the machine and foundation passes through the CG of the base of the contact area.
• The eccentricity of the foundation system along both directions should not exceed 5% of the length of the corresponding side of the contact area.
• The highest natural frequency of foundation shall be higher than 40% of the highest disturbing frequency while the lowest natural frequency of the foundation shall be lower than 150% of the lowest disturbing frequency.
• The amplitudes of the vibrations at the upper edge shall not exceed 0.20mm in both directions.
• The soil stress below the foundation under dead loads only shall not exceed 80% of the allowable bearing pressure of the static loading.
• For preliminary design, the maximum pressure on soil due to static load alone may be taken as 0.4 times the corresponding safe bearing capacity

Frequency Ratio (f/f_n):

• It is the ratio of operating frequency to the natural frequency.
• For reciprocating machinery, it should be greater than 0.40.