Sponsors: Sponsored by ACI Committee 351 Editor: Carl A. Nelson This special publication grew out of the Technical Session entitled “Application of ACI 351-C Report on Dynamic Foundations,” held at the ACI Spring 2019 Convention in Québec City, Québec. Following this event, Committee 351 decided to undertake a special publication with contributions from those session participants willing to develop their presentations into full-length papers. Three papers included in the current publication were contributed by these presenters and their coauthors, with six additional papers provided by others. All but one of the papers deal with the subject matter of ACI 351.3—Foundations for Dynamic Equipment—updated in 2018. The one exception (the paper of Wang and Fang on wind turbine foundations) provides valuable information to engineers dealing with a lack of consistent design criteria among various codes for reinforced concrete foundations subjected to high-cycle fatigue loads. I would like to thank the members of ACI Committee 351 for their support, in particular the current main Committee and Subcommittee C Chairpersons Susan Isble and Dr. Mukti L. Das, respectively. I also wish to express my gratitude to the authors for their perseverance through the difficult circumstances of 2020, and to the reviewers who generously contributed their time and expertise to this publication. Last, but not least, I want to thank my wife Cindy for tolerating me (and the growing piles of paper) over the past several months as the deadline approached. Carl A. Nelson On behalf of ACI Committee 351 Minneapolis, December 2020

The availability of high-speed computers at a reasonable price resulted in various sophisticated analysis and design methodologies for the elevated flexible pedestal (Tabletop) foundations replacing the rule-of-thumb processes based on static equivalent principle which used to produce either unsafe or overly conservative structures. A thorough study is undertaken with four typical models usually selected for static and dynamic analyses in the structural/mechanical engineering practice. It is found that all the models provide comparable results. However, each one has its own advantages and disadvantages. These will be highlighted in the Summary and Concluding Remarks of this paper.

This paper discusses steps for both computing vibration from equipment foundations using the elastic halfspace theory and then computing the decrease in vibration amplitude from the foundation to receivers. The steps are demonstrated on an existing foundation at a project site in Ohio that was subjected to dynamic loading from a hydraulic vehicle test rig. Several approaches are discussed to estimate the dynamic shear modulus of different soils, along with a methodology to establish an equivalent dynamic shear modulus for soils with varying shear wave velocities. Vibration transmission through the soil can affect people and sensitive equipment both near and far from the source. This paper shows a hybrid method and an SRSS method to compute the vibration attenuation through the near field and far field. The calculated results for this site were found to be very close to the measured values. Finally, vibration levels are compared for variations in stiffness, damping and attenuation to evaluate the sensitivity to calculations and/or field measurements. Variations in stiffness result in a nearly proportional change in vibration level while variations in damping and attenuation produce relatively small changes in the results.

This paper presents a case study involving the structural analysis and design of an elevated foundation plinth to support multiple pieces of rotating machines with different operating weights and speeds. The equipment is used to operate a high-speed balancing testing facility for turbines and rotors that are located within an adjacent testing chamber. This project comprised of several layout and design challenges, including vibration and resonance concerns, effects of multiple operating frequencies, plinth shape, and pile foundation effects. Major concern was to maintain the high precision and strict tolerance limitations required by the high-speed balancing operations. Elevated machine foundations integral with other structures possess many natural frequencies, both locally and globally. The traditional design rules-of-thumb are not adequate for analyzing and designing elevated machine foundations. A computer-based finite element analysis method is required to identify the multiple natural frequencies of a complicated foundation structure. The strength design of a machine foundation can become very challenging when trying to implement code requirements that are mostly applicable to building elements and not to massive concrete foundations. This study recognizes the need for the development of a design standard to include special design requirements for mass concrete machine foundations.

This paper discusses the factors that affect the dynamic response of machine foundation systems, which include (1) the soil dynamic properties, (2) the geometric properties of the foundation, (3) mass of the machine and foundation, and (4) the amplitude and frequency of the applied dynamic loads. The primary objective in any machine foundation design is to limit the foundation response below a specific amplitude threshold. A foundation response exceeding this limit may adversely affect the performance of the machine and damage the machine internals, resulting in costly repairs and lost revenue. Also, the excessive vibrations may result in structural degradation of the foundation, additional excitation stresses on the machine, and increase the compressor unbalance loading. This paper presents dynamic analysis results of a four-cylinder compressor foundation originally designed without consideration for soil-foundation interaction and suffering from excessive vibration. The foundation block supports a 4-cylinder Dresser-Rand compressor, suction and discharge bottles, a crank, and a driving motor with a total weight of approximately 300 kip (1334 kN). A three-dimensional, finite element model representing the soil–foundation system was developed to determine the dynamic characteristics and assess the foundation response under applied dynamic loading from the compressor crank. Results showed that the response of the soil-foundation system is governed by the response of the individual support piers (blocks) and not the global foundation response. This paper also provides a recommended modification to the foundation geometry to reduce the effect of the individual piers' local modes and enhance the foundation dynamic performance.