A Practical Guide to Soil-Structure Interaction FEMA P-2091 / December 2020 FEMA P-2091 / December 2020 A Practical Guide to Soil-Structure Interaction Prepared by APPLIED TECHNOLOGY COUNCIL 201 Redwood Shores Parkway, Suite 240 Redwood City, California 94065 www.ATCouncil.org Prepared for FEDERAL EMERGENCY MANAGEMENT AGENCY Michael Mahoney, Project Officer Robert D. Hanson, Subject Matter Expert Washington, D.C. APPLIED TECHNOLOGY COUNCIL Jon A. Heintz, Program Executive, Program Manager Ayse Hortacsu, Project Manager PROJECT TECHNICAL COMMITTEE WORKING GROUP MEMBERS Bret Lizundia (Project Technical Director) Timothy D. Ancheta C.B. Crouse Laurel Jiang Steve Harris Ricardo Henoch Boris Jeremic Gyimah Kasali Jonathan P. Stewart Lisa M. Star Michael Valley Christos Tokas Jiejing Zhou PROJECT REVIEW PANEL Peter Lee Sissy Nikolaou* Robert Pekelnicky Payman Khalili Tehrani *ATC Board Representative Notice Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of the Applied Technology Council (ATC), the Department of Homeland Security (DHS), or the Federal Emergency Management Agency (FEMA). Additionally, neither ATC, DHS, FEMA, nor any of their employees, makes any warranty, expressed or implied, nor assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, product, or process included in this publication. Users of information from this publication assume all liability arising from such use. The contents of this document do not have the force and effect of law and are not meant to bind the public in any way. This document is intended only to provide clarity to the public regarding existing requirements under the law or agency policies. Cover image – Photo of project site showing soil conditions and foundation and superstructure under construction. Preface Modern building codes include criteria for how ground motions enter a structure’s foundation and affect the building response. This soil-structure interaction (SSI) can make a substantial difference in how buildings behave during earthquake shaking and how they should be designed, yet there is relatively little implementation of SSI effects by practicing structural engineers. Provisions are available in ASCE/SEI 7-16 and in ASCE/SEI 41-17 that can be used to address SSI. In addition, a research report prepared by the National Institute of Standards and Technology (NIST) published in 2012 is also a valuable resource. In 2019, the Applied Technology Council (ATC), with funding from FEMA under Task Order Contract HSFE60-17-D-1005, commenced the ATC-144 project. The objective of this project was to develop this document, A Practical Guide to Soil-Structure Interaction, to present information regarding SSI as implemented in code provisions but in an easy-to- understand, concise format targeted toward practicing engineers to help them determine when SSI effects are of importance and show them examples of how to implement them in design. ATC is indebted to the leadership of Bret Lizundia, Project Technical Director, and to the members of the ATC-144 Project Team for their efforts in developing this Guide. The Project Technical Committee, consisting of C.B. Crouse, Steve Harris, Boris Jeremic, Bret Lizundia (Chair), Jonathan Stewart, and Michael Valley, performed the technical development efforts and served as primary authors. Tim Ancheta, Laurel Jiang, Gyimah Kasali, and Lisa Star assisted in the development of the design examples, and Ricardo Henoch and Jiejing Zhou provided detailed review of the design examples as members of the Project Working Group. Chris Tokas developed many of the figures in the document. The Project Review Panel, consisting of Peter Lee, Sissy Nikolaou, Robert Pekelnicky, and Payman Khalili Tehrani provided technical review, advice, and consultation at key stages of the work. The names and affiliations of all who contributed to this report are provided in the list of Project Participants. FEMA P-2091 Preface iii ATC also gratefully acknowledges Michael Mahoney (FEMA Project Officer), and Robert D. Hanson (FEMA Subject Matter Expert) for their input and guidance in the preparation of this document. Carrie J. Perna (ATC) provided report production services. Ayse Hortacsu Jon A. Heintz ATC Director of Projects ATC Executive Director iv Preface FEMA P-2091 Table of Contents Preface ........................................................................................................... iii List of Figures ............................................................................................... ix List of Tables ................................................................................................xv 1. Introduction ...................................................................................... 1-1 1.1 Overview .................................................................................. 1-1 1.2 Types of Soil-Structure Interaction Modeling .......................... 1-1 1.3 Purpose of the Guide ................................................................ 1-2 1.4 Scope of the Guide ................................................................... 1-2 1.5 Target Audience for the Guide ................................................. 1-2 1.6 Key SSI Terminology ............................................................... 1-3 1.7 Tips for Understanding and Implementing SSI ........................ 1-4 1.8 Organization of the Guide ........................................................ 1-5 2. Situations Where SSI is Important ................................................. 2-1 2.1 Overview .................................................................................. 2-1 2.2 Large Building Footprint Reduces Design Forces ................... 2-1 2.3 Substantial Foundation Embedment Reduces Design Forces ....................................................................................... 2-2 2.4 High Structure-to-Soil Stiffness Ratios Will Lengthen Period and Change Design Forces ....................................................... 2-2 2.5 Foundation Rocking Impacts Superstructure Behavior ............ 2-4 3. Rule of Thumb Test for Inertial SSI Significance ......................... 3-1 3.1 Overview .................................................................................. 3-1 3.2 Key Equation ............................................................................ 3-1 3.3 Determining Average Effective Shear Wave Velocity ............. 3-2 3.4 Determining Fundamental Period of the Structure ................... 3-3 3.5 Example .................................................................................... 3-4 3.5.1 Building Description ................................................... 3-4 3.5.2 Average Effective Shear Wave Velocity ..................... 3-6 3.5.3 Fundamental Period of the Structure ........................... 3-7 3.5.4 Structure-to-Soil Stiffness Ratio ................................. 3-7 3.6 Comparisons Using Site Class ................................................. 3-7 4. Base Slab Averaging ........................................................................ 4-1 4.1 Overview .................................................................................. 4-1 4.2 Key Equations .......................................................................... 4-2 4.3 Determining Effective Foundation Size ................................... 4-3 4.4 Determining whether Base Slab Averaging Applies ................ 4-3 4.5 Limitations and Issues .............................................................. 4-4 4.6 Example .................................................................................... 4-5 FEMA P-2091 Table of Contents v 5. Embedment Effects .......................................................................... 5-1 5.1 Overview................................................................................... 5-1 5.2 Key Equation ............................................................................ 5-1 5.3 Determining Depth of Embedment ........................................... 5-2 5.4 Limitations and Issues .............................................................. 5-6 5.5 Example .................................................................................... 5-7 6. Foundation and Soil Flexibility ....................................................... 6-1 6.1 Overview................................................................................... 6-1 6.2 Soil Properties for Flexibility Calculations .............................. 6-2 6.3 Vertical and Rotational Springs ................................................ 6-3 6.3.1 Method 1 – Rigid Foundation and Flexible Soil .......... 6-3 6.3.2 Method 2 – Flexible Foundation and Nonlinear Flexible Soil ................................................................. 6-8 6.3.3 Method 3 – Flexible Foundation and Linear Flexible Soil ............................................................................... 6-8 6.4 Horizontal Springs .................................................................... 6-9 6.4.1 Method 1 .................................................................... 6-10 6.4.2 Resistance due to Passive Pressure ............................ 6-12 6.4.3 Resistance due to Friction or Cohesion ..................... 6-13 6.5 Bounding Analyses ................................................................. 6-14 6.6 Vertical and Rotational Spring