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ACI ACI R Chair of subcommittee that prepared this? Past chair. Yelena S. Louis Jack Moll Ira W. Smalley Philip A. Smith W. Tod Sutton? Alan Wiley Robert L. Rowan, Jr.? William E. Rushing, Jr. This report presents to industry practitioners the various design criteria and methods and procedures of analysis, design, and construction applied to dynamic equipment foundations.
Keywords: amplitude; concrete; foundation; reinforcement; vibration. All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by electronic or mechanical device, printed, written, or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors.
This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom.
Reference to this document shall not be made in contract documents. It is the responsibility of the user of this document to establish health and safety practices appropriate to the specific circumstances involved with its use. ACI does not make any representations with regard to health and safety issues and the use of this document. The user must determine the applicability of all regulatory limitations before applying the document and must comply with all applicable laws and regulations, including but not limited to, United States Occupational Safety and Health Administration OSHA health and safety standards.
When excessive, such vibrations may be detrimental to the machinery, its support system, and any operating personnel subjected to them.
Many engineers with varying backgrounds are engaged in the analysis, design, construction, maintenance, and repair of machine foundations. Each of these participants has inputs and concerns that are important and should be effectively communicated with each other, especially considering that machine foundation design procedures and criteria are not covered in building codes and national standards. Some firms and individuals have developed their own standards and specifications as a result of research and development activities, field studies, or many years of successful engineering or construction practices.
Unfortunately, most of these standards are not available to many practitioners. As an engineering aid to those persons engaged in the design of foundations for machinery, the committee developed this document, which presents many current practices for dynamic equipment foundation engineering and construction. This document provides general guidance with reference materials, rather than specifying requirements for adequate design.
Where the document mentions multiple design methods and criteria in use, factors, which may influence the choice, are presented. For the purposes of this document, dynamic equipment includes the following: 1. Rotating machinery; 2. Reciprocating machinery; and 3. Impact or impulsive machinery. Site conditions such as soil characteristics, topography, seismicity, climate, and other effects; 2.
Machine base configuration such as frame size, cylinder supports, pulsation bottles, drive mechanisms, and exhaust ducts; 3. Process requirements such as elevation requirements with respect to connected process equipment and hold-down requirements for piping; 4. Anticipated loads such as the equipment static weight, and loads developed during erection, startup, operation, shutdown, and maintenance; 5. Erection requirements such as limitations or constraints imposed by construction equipment, procedures, techniques, or the sequence of erection; 6.
Operational requirements such as accessibility, settlement limitations, temperature effects, and drainage; 7. Maintenance requirements such as temporary access, laydown space, in-plant crane capabilities, and machine removal considerations; 8. Regulatory factors or building code provisions such as tied pile caps in seismic zones; 9. Economic factors such as capital cost, useful or anticipated life, and replacement or repair cost; Environmental requirements such as secondary containment or special concrete coating requirements; and Recognition that certain machines, particularly large reciprocating compressors, rely on the foundation to add strength and stiffness that is not inherent in the structure of the machine.
These machines are characterized by the rotating motion of impellers or rotors. Unbalanced forces in rotating machines are created when the mass centroid of the rotating part does not coincide with the center of rotation Fig. This dynamic force is a function of the shaft mass, speed of rotation, and the magnitude of the offset. The offset should be minor under manufactured conditions when the machine is well balanced, clean, and without wear or erosion.
Changes in alignment, operation near resonance, blade loss, and other malfunctions or undesirable conditions can greatly increase the force applied to its bearings by the rotor. Because rotating machines normally trip and shut down at some vibration limit, a realistic continuous dynamic load on the foundation is that resulting from vibration just below the trip level.
Individual inertia forces from each cylinder and each throw are inherently unbalanced with dominant frequencies at one and two times the rotational frequency Fig. Reciprocating machines with more than one piston require a particular crank arrangement to minimize unbalanced forces and moments. A mechanical design that satisfies operating requirements should govern. Individual cylinder fluid forces act outward on the cylinder head and inward on the crankshaft Fig.
For a rigid cylinder and frame these forces internally balance, but deformations of large machines can cause a significant portion of the fluid load to be transmitted to the mounts and into the foundation. Particularly on large reciprocating compressors with horizontal cylinders, it is inappropriate and unconservative to assume the compressor frame and cylinder are sufficiently stiff to internally balance all forces.
Such an assumption has led to many inadequate mounts for reciprocating machines. This shock loading is often transmitted to the foundation system of the equipment and is a factor in the design of the foundation. Closed die forging hammers typically operate by dropping a weight ram onto hot metal, forcing it into a predefined shape. While the intent is to use this impact energy to form and shape the material, there is significant energy transmission, particularly late in the forming process.
During these final blows, the material being forged is cooling and less shaping takes place. Thus, pre-impact kinetic energy of the ram converts to post-impact kinetic energy of the entire forging hammer. As the entire hammer moves downward, it becomes a simple dynamic mass oscillating on its supporting medium. This system should be well damped so that the oscillations decay sufficiently before the next blow. Timing of the blows commonly range from 40 to blows per min. The ram weights vary from a few hundred pounds to 35, lb kN.
Open die hammers operate in a similar fashion but are often of two-piece construction with a separate hammer frame and anvil. Forging presses perform a similar manufacturing function as forging hammers but are commonly mechanically or hydraulically driven. These presses form the material at low velocities but with greater forces. The mechanical drive system generates horizontal dynamic forces that the engineer should consider in the design of the support system.
Rocking stability of this construction is important. Figure 2. Mechanical metal forming presses operate by squeezing and shearing metal between two dies. Because this equip- Fig. Speeds can vary from 30 to strokes per min.
Dynamic forces from the press develop from two sources: the mechanical balance of the moving parts in the equipment and the response of the press frame as the material is sheared snap-through forces.
Imbalances in the mechanics of the equipment can occur both horizontally and vertically. Generally high-speed equipment is well balanced. Low-speed equipment is often not balanced because the inertia forces at low speeds are small. The dynamic forces generated by all of these presses can be significant as they are transmitted into the foundation and propagated from there.
The ability to use such a foundation primarily depends on the quality of near surface soils. Block foundations are nearly always designed as rigid structures.
Combined blocks are more difficult to design because of the combination of forces from two or more machines and because of a possible lack of stiffness of a larger foundation mat. Elevation allows for ducts, piping, and ancillary items to be located below the equipment. Tabletop structures are considered to be flexible, hence their response to dynamic loads can be quite complex and depend both on the motion of its discreet elements columns, beams, and footing and the soil upon which it is supported.
The effectiveness of isolators depends on the machine speed and the natural frequency of the foundation. Details of this type of support are provided in Section 4. The springs are then supported on a block-type foundation. This arrangement has a dynamic effect similar to that for tabletops with vibration isolators. Other types of equipment are spring mounted to limit the transmission of dynamic forces.
In this situation, dynamic machines are usually designed with a supporting inertia block to alter natural frequencies away from machine operating speeds and resist amplitudes by increasing the resisting inertia force. Piles are generally used where soft ground condi- Fig. Piles use end bearing, frictional side adhesion, or a combination of both to transfer axial loads into the underlying soil.
Transverse loads are resisted by soil pressure bearing against the side of the pile cap or against the side of the piles. Various types of piles are used including drilled piers, auger cast piles, and driven piles. Yet, behind this straightforward definition lies the need for careful attention to the interfaces between machine, mounting system, and concrete foundation. The loads on machine foundations may be both static and dynamic.
351.3R-04: Foundations for Dynamic Equipment (Reapproved 2011)
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