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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

One of major challenges for the US wind industry is the lack of consistent fatigue design criteria. ASCE/AWEA RP2011 recommends several design codes for fatigue analysis of land-based wind turbine support structures. However, it does not provide discussions on the differences and limitations of these codes. The purpose of this paper is to present our findings on the application of fatigue design codes including Model Code 2010 (MC10), Eurocode 2 (EC2), Det Norske Veritas (DNV), and ACI 215. Comparison of the design results from using these codes/standards are summarized. Due to lack of consistency in the design standards, evaluation results may vary greatly, which can be confusing and inconclusive at times. In addition, this study shows that there will be significant differences on fatigue design adequacy depending on which analysis method is used: the average sectional method or finite element method, the two principal methods used to analyze fatigue. A number of suggestions and critical comments are also provided in this paper for helping development of more consistent fatigue analysis and design criteria for wind turbine foundations.

The “Notional Pile” formulation is developed for modeling a group of piles in a foundation. It is a new procedure for foundation modeling for dynamic analysis in conformance with ACI 351.3R. It is an augmentation of the well-known Novak procedure. Foundation stiffness is represented as a set of notional pile elements. This differs from conventional procedures in which the pile group stiffness is represented by a set of springs lumped at one point. With notional piles and finite element modeling of the cap, flexible-cap modes of vibration can be extracted. With conventional procedures, only lower-frequency rigid body modes can be extracted. Notional piles distribute stiffness more realistically and enable cap-pile interaction. A specific case is used to illustrate the new procedure. For that case, the cap did not have a regular distribution of mass or stiffness. Dynamic loads were applied with considerable eccentricity, at multiple locations and with multiple frequencies. Notional piles accommodated these irregularities. The notional pile formulation was validated by comparing measured to computed foundation responses. The comparison was good but not great. The foundation was to be reconfigured for new machinery. The retrofit design was modeled using notional piles. Responses were computed and compared to applicable limits.

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.

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.