The design of machine foundation involves careful study of the vibration characteristics of the foundation system. Relevant data required for the design and construction of the machine foundation of machine should be obtained from the manufacturer of the machine, prior to the start of design. All parts of machine foundation should be designed for maximum stresses due to the worst combination of vertical loads, torque, longitudinal and transverse forces, stresses due to temperature variation and the foundation dead load. In case, the machine foundation layout is partly built up of beam and column construction, straight bars should be provided both at top and bottom of the beams and the spacing of the stirrups should be close. The main foundation block should have the designed thickness and should be reinforced both at top and bottom, even if the reinforcements are not required from design considerations.
The general principles of machine foundation design are given below:
- The mass of the foundation block should be adequate to absorb vibrations and also to prevent resonance between the machine and the adjacent soil. This can be achieved by increasing the weight of foundation block in proportion to the power of the engines. Some authors suggest that for each break horse power of multicylinder engines, a minimum of 725 kg. weight of foundation should be provided for gas engines, 565 kg. for diesel engines and 225 kg. for steam engines. For single cylinder engines, the above value should be increased by 40 to 60%. As a thumb rule, the weight of the foundation should be at least 2½ times the weight of the whole machine.
- To avoid the possibility of differential settlement, the machinefoundation should be so dimensioned that the resultant force due to the weight of the machine and the weight of the foundation passes through the centre of gravity of the base contact area.
- The foundation should be stiff enough to have necessary rigidity, since the slightest deflection of foundation can cause considerable bearing troubles.
- To avoid transmission of vibration from the machine to the adjoining parts of the building, a gap should be left around the, machine foundation to isolate it from the adjoining parts of the building.
- As far as possible, overhanging cantilevers should be avoided. However, in situation where it is not possible to avoid cantilever projections, they should be designed for strength and rigidity against vibrations.(6) All units of machine foundation should be provided with reinforcement running both ways along the surface of the concrete block. The concrete cover to the reinforcement should not be less than 75 mm at the bottom, 50 mm on sides and 40 mm at the top. In case of foundation for steam turbo-generators, cover for the reinforcement at bottom, side and top of base slab should not be less than 100 mm.
- The amount of reinforcement in foundation units should not be less than 2 kg per cu. m of concrete for impact type or reciprocating type of machines, 50 kg per cu. m of concrete for rotary type of machines and 100 kg per cu. m of concrete for steam turbo generators.
- M 150 to M 200 grade of concrete can be used in the foundations and as far as possible, the-entire block should be concreted in one operation without construction joints.