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

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.

Dynamic pile group effect can either increase or decrease the response of pile-supported structures. This paper presents the results of a three-dimensional finite element model of the pile-to-pile interaction that considers the effect of the surrounding soil to determine the dynamic stiffness and damping for vertical end bearing pile groups subjected to vertical harmonic loading. The results were generated for a wide range of the dimensionless frequency parameter (ao) for a 9x9-pile group with three different spacings: 2-, 4-, and 6-pile diameter. Both the stiffness and the damping showed an oscillatory behavior with the dimensionless frequency parameter ao, as well as with the soil shear modulus. Also, the group efficiency was determined as a function of the pile spacing and the soil shear modulus. The efficiency factor for the stiffness can be as high as 1.15 and as low as 0.7 and for the damping as high as 3.75 and as low as 0.4 as a function of the dimensionless frequency parameter ao.

A more than 50-year old Steam Turbine/Generator (STG) table-top concrete foundation was retrofitted to support a new STG/Condenser unit. This new machine unit is set on a sub skid with spring/damper assemblies underneath and located on existing concrete table top columns. This paper presents a case study of the seismic design and evaluations of the existing foundation structure that were performed to assess and qualify the structure’s service and strength capabilities. Based on these evaluations, modifications to the existing STG foundation were minimized allowing the cost effective reuse of the existing foundation resulting in significant savings for the overall installed cost of the project.

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.