Heavy vibrating machinery is any equipment having reciprocating or rotary masses as the major moving parts (such as reciprocating or rotary compressors, pumps, turbines) and having a gross plan area exceeding 2.5 m2, or a total weight of greater than 25kN.
Light vibrating machinery is any equipment having reciprocating or rotary masses as the major moving parts (such as reciprocating or rotary compressors, pumps, turbines) and having a gross plan area less than 2.5 m2 and total weight of less than 25 kN.
For light vibrating machinery dynamic design is not required. Static design shall be carried out according to the criteria below. Additionally the foundation weight shall not be less than 3 times the total rotary machine weight or 5 times the total reciprocating machine weight.
Consideration should be given to grouping light equipment on common flat slabs. In this case, adequate reinforcement is to be provided to prevent propagation of cracks due to vibration. The level of the foundation blocks shall allow cable and drain trenches to be located above it.
Machinery foundations shall have adequate surface reinforcement. The minimum surface reinforcement shall be 16 mm bars at 200mm spacing in both directions.
For foundations for heavy vibrating machinery, the minimum amount of internal reinforcement shall be 50 kg/m3. This reinforcement shall be triaxially arranged within the volume of the foundation. Reinforcement shall be adequate to limit crack widths to acceptable value in all conditions. Cracks shall in no circumstance exceed 0.3 mm width.
All machinery blocks shall be isolated by expansion joints from slabs and adjacent foundations to minimise the transmission of vibration to adjacent foundations, equipment, and buildings.
The following rules shall be considered in foundation design:
• shall be founded on suitable bearing strata, or a piled foundation adopted.
• Soil pressure or pile capacity, considering dead and live loads, shall not exceed 50% of the allowable figure.
• Foundation shall consist of a common foundation block supporting the equipment and its driver and the first pipe supports adjacent to the equipment ‘where practical to do so’ to allow exceptions.
• Pockets where vapour could accumulate are not permitted.
• The shape of beams and columns shall be uniform and rectangular.
• All parts of the machine supports shall be independent from the adjacent foundations and buildings.
• Concrete floor slabs, adjacent to machine foundations, shall be spaced a minimum of 20 mm from the foundation. The space between slab and foundation shall be filled with a flexible joint filler and sealer.
• The minimum thickness of the foundation base slab shall not be less than 1/10 of its maximum dimension.
• Foundations and supporting structures shall be designed as per CP2012 such that the natural frequency of the supporting structure including soil/structure interaction is either less than 0.7 or greater than 1.3 times the operating frequency range of the machine. Where this cannot be assured across the full operating frequency range(s), Suitable bearings shall be designed and provided to limit vibration to acceptable limits.
• The factor of safety against overturning shall be greater than or equal to 3.
• The design method shall be based on ACI 351.3R-04.
The common base block is to be sufficiently stiff to limit distortion within tolerance permitted by unit Machinery Vendor.
The dynamic modulus of elasticity (E’) of concrete to be used in the dynamic analysis is indicated in the following table:
TABLE 12.1 - THE DYNAMIC MODULUS OF ELASTICITY F’c (N/mm2) E’ (N/mm2)
25 30.000
35 34.000
45 37.000
55 39.000
12.1 DESIGN CRITERIA FOR RECIPROCATING MACHINERY
Design of foundations for reciprocating machinery shall be carried out in accordance with the following criteria:
a) The total foundation weight shall be at least 5 times the total weight of the machine.
b) The horizontal eccentricity in any direction between the centroid of the machine + foundation system and the centroid of the base contact area shall not exceed 5% of the respective base dimension.
Dynamic analysis shall be carried out as follows:
a) Natural frequencies in the modes being excited shall preferably be out of 0.7 to 1.3 times the disturbing frequencies of any machine on the foundation. If it is not possible to meet this requirement, frequencies within the above mentioned range may be accepted if the maximum calculated amplitudes are within the limits listed in the following point #e).
b) In case, the above mentioned frequency criteria cannot be achieved due to some unusual soil characteristics, damping is to be considered to restrict the calculated amplitude within allowable limit.
Under such circumstances, the material internal (hysteretic) damping shall not be considered higher than 3%. The total damping (Material + Geometric) shall be limited to 20% in the case of torsional mode of vibration. For other modes of vibration, damping provisions of ACI 351.3R-04 are to be followed.
c) Primary forces, couples and moments shall be applied at machine speed for calculation of primary amplitudes.
d) Secondary forces, couples and moments shall be applied at twice the machine speed for calculation of secondary amplitudes.
e) Total amplitude shall be calculated by combining, in the worst conditions, primary and secondary amplitudes.
f) Time step dynamic analysis (Time-History) shall be carried out to cater for all the six modes of vibration of the block foundation using a 6 x 6 matrix in case of damped vibration of foundation. This shall be in line with sections 4.3.4.2 and 4.3.4.3 of ACI 351.3R-04. “Dynamics of Bases and Foundations” by D. D. Barkan referred in CP 2012-1 may be referred to.
Total peak to peak amplitudes on the foundation shall not exceed 0.05 mm, unless specified otherwise by the machinery manufacturer.
12.2 DESIGN CRITERIA FOR ROTATING MACHINERY
Rotary machines may be supported either on a direct foundation or on an elevated structure.
Weight of basement (foundation and elevated structure) shall be at least three times the weight of the machinery.
In case, frequency criteria cannot be achieved due to some unusual soil characteristics, damping is to be considered to restrict the calculated amplitude within allowable limit. Under such circumstances, the material internal (hysteretic) damping shall not be considered higher than 2%. The total damping (Material + Geometric) shall be limited to 20% in the case of torsional mode of vibration. For other modes of vibration, damping provisions of ACI 351.3R-04 are to be followed.
Higher values of damping factor may be considered in loading conditions in which the loads are significantly higher than that during normal operation.
Dynamic analysis may be dispensed with if the mass of rotating elements is less than 1/100 of the mass of the whole system (machine + foundation).
Model shall be defined in such a way to correctly describe the foundation behaviour up to 1.5 fmax, where fmax is the maximum operating speed.
Natural frequencies of the system, machine + foundation, shall be calculated in accordance with the criteria stated below.
Number of natural frequencies to be calculated shall be defined so that the highest natural frequency calculated is at least 10% higher than the maximum operating frequency. (This criterion may be relaxed for operating frequencies higher than 75 Hz.)
However, depending on the analysis model, the number of natural frequencies, n, to be calculated shall meet the following:
n = 10 for two dimensional models in which only displacements out of the plane are considered and in which vibration in one direction has influence in other directions.
n = 6 for two dimensional symmetrical models in which only vertical displacements are considered and in which vibration in one direction has only secondary influence in other directions (the system may be represented by independent models).
The assessment of vibration behaviour shall be checked as follows:
First order natural frequency (lowest frequency):
f1<= 0.8 fm or f1 >= 1.25 fm where fm is the lowest service frequency.
Higher order natural frequencies:
fn <= 0.9 fm and fn+1 >= 1.1 fm
If the higher order criteria are not satisfied it shall suffice that fn is less than fm where n is either 6 or 10.
Time step dynamic analysis (Time-History) shall be carried out to cater for all the six modes of vibration of the block foundation using a 6 x 6 matrix in case of damped vibration of foundation. This shall be in line with sections 4.3.4.2 and 4.3.4.3 of ACI 351.3R-04. “Dynamics of Bases and Foundations” by D. D.
Barkan referred in CP 2012-1 may be referred to.
12.3 ANALYSIS OF VIBRATIONS DUE TO UNBALANCED FORCES 12.3.1 Exciting Forces
Unbalanced forces are to be provided by the machine manufacturer and shall be used for the dynamic response of the foundation.
In case of absence of such information unbalanced forces may be calculated on the basis of a nominally balanced machine as follows:
a) Operating state
The balance quality of machine shall be assumed one grade lower than that for the relevant machine group.
F = Mώ2e = F(ώe)ώ where: ώ= speed in rad/sec, (ώe) = quality of balance of machine All forces shall be considered to be applied at the bearings.
b) Malfunctioning state
Forces due to malfunctioning shall be assumed 6 times the values for operating state and shall be used for the static design and stability checks of the structure.
12.3.2 Allowable Displacements
If available, allowable displacements are given by the machine manufacturer shall be used to check the structure.
In the absence of such information the maximum amplitudes, effective at the bearings, may be assumed for the particular machine group as follows:
a) Operating state:- The value associated with the operating frequency which is one grade higher than that guaranteed by the manufacturer shall be taken as the amplitude under service conditions.
b) Malfunctioning state:- The amplitude in case of malfunctioning shall be assumed to be 6 times the values used for the operating state.