This document summarizes research on modeling soil-structure interaction in OpenSees. It describes developing a modeling tool using a "monkey-tail" model of a vibrating footing on an elastic soil half-space. The model is implemented and validated in OpenSees. Case studies of two building frames with and without soil-structure interaction show increased drift when interaction is considered. Finally, the soil amplification factor depends on period, intensity, and soil stiffness.

1. Modelling of Soil-Structure Interaction in OpenSees:
A Practical Approach for Performance-Based Engineering
Smail Kechidi, Researcher, University of Porto
Aires Colaço, Ph.D. Student, University of Porto
Mário Marques, Post-Doctoral Researcher, University of Porto
José Miguel Castro, Assistant Professor, University of Porto
Pedro Alves Costa, Assistant Professor, University of Porto
OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017

2. 1. Background and Motivation
2. Soil-Structure Interaction Modelling Tool
• Formulation of the Problem
• Model Implementation in OpenSees
• Validation of the Proposed Model
3. Case Studies
• Description of the Buildings
• EC8-based Ground Motion Record Selection
• Seismic Performance Assessment
Approaches I and II (with and without Soil-Structure
Interaction)
Approach III (Soil-Structure Interaction and site effect)
• Soil Factor
4. Conclusions
2/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Overview

3. Background and Motivation
Layered Soil
Input motion
Bedrock motion
Free
surface
motion
???
3/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017

4. • Motivation for the research presented herein:
• Building codes discourage designs that allow the mobilisation
of the foundation capacity.
• Development of a well-calibrated modelling tool, coupled with
parameter selection protocols cast in a simplistic fashion.
• Encourage the modelling of SSI within the community of
practicing engineers
• Is the prevailing belief, saying that ignoring soil-structure
interaction leads to conservative results, always valid?
4/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Background and Motivation

5. • Formulation of the Problem
Kbub + Cb ̇ub + Mb ̈ub = Fb
us
b
= Δub + u
̇us
b
= Δ ̇ub + ̇u
̈us
b
= Δ ̈ub
+ ̈u
�Fb = −Fs = −(KsΔu𝑏𝑏 + CsΔ ̇𝑢𝑢𝑏𝑏 + MsΔ ̈𝑢𝑢𝑏𝑏
𝐾𝐾 𝑎𝑎𝑎𝑎
𝐾𝐾 𝑎𝑎𝑎𝑎
𝐾𝐾 𝑏𝑏𝑏𝑏 𝐾𝐾 𝑏𝑏𝑏𝑏 + Ks
𝑢𝑢a
𝑢𝑢b
+
𝐶𝐶 𝑎𝑎𝑎𝑎
𝐶𝐶 𝑎𝑎𝑎𝑎
𝐶𝐶 𝑏𝑏𝑏𝑏 𝐶𝐶 𝑏𝑏𝑏𝑏 + Cs
̇𝑢𝑢a
̇𝑢𝑢b
+
𝑀𝑀𝑎𝑎𝑎𝑎
𝑀𝑀𝑎𝑎𝑎𝑎
𝑀𝑀𝑏𝑏𝑏𝑏 𝑀𝑀𝑏𝑏𝑏𝑏 + Ms
̈𝑢𝑢a
̈𝑢𝑢b
=
0
Ksu + Cs ̇𝑢𝑢 + Ms ̈𝑢𝑢
5/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Soil-Structure Interaction Modelling Tool

6. Monkey-tail modelVertical vibrating surface footing
resting on a homogeneous elastic
half space
6/21
• Model Implementation in OpenSees
OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Soil-Structure Interaction Modelling Tool

7. 7/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Soil-Structure Interaction Modelling Tool
• Model Implementation in OpenSees

8. 8/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
• Validation of the Proposed Model
Soil-Structure Interaction Modelling Tool

9. Vs30 = 800 m/s Vs30 = 360 m/s
Vs30 = 180 m/s Vs30 = 100 m/s
• Validation of the Proposed Model (SDOF)
9/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Soil-Structure Interaction Modelling Tool

10. Vs30 = 180 m/s Vs30 = 100 m/s
• Validation of the Proposed Model (Oscillator)
10/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Soil-Structure Interaction Modelling Tool

11.  Gravity designed frame “GB”
 Seismically design frame “SB3” (Lisbon, soil class C and q = 4)
• Description of the Buildings
11/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Case Study

12. GB frame T1= 1.61 sec SB3 frame T1= 0.87 sec
• EC8-Based Ground Motion Record Selection (SelEQ)
12/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Case Study

13. ug(t)ug(t)
Fixed bases
(Approaches I)
SSI consideration using
the Monkey-tail model
(Approaches II)
• Seismic Performance Assessment
13/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Case Studies

14. GB fame
2%/50 yrs.
10%/50 yrs.
• Approaches I and II
SB3 fame
14/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Case Studies

15. Layered Soil
Rock
Input motion
1-
Deconvolution
process
2- Convolution
process
Bedrock motion
Free
surface
motion
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• Approach III
OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Case Studies

16. Vs30=360 m/s Vs30=180 m/s
• Approach III
16/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Case Studies

17. 2%/50 yrs.
10%/50 yrs.
• Approach III (SB3 frame)
17/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Vs30=360 m/s Vs30=180 m/s
Case Studies

18. Vs30=360 m/s Vs30=180 m/s
18/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
• Approach III
Case Studies

19. 2%/50 yrs.10%/50 yrs.
19/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Soil Factor

20. • Using lumped parameter models can be seen as a good
option considering the compromise between accuracy of
the solution and reduction of the computation effort.
• Drifts considering the Soil-Structure Interaction are even
larger in the lower stories than those of the fixed-base
case.
• Dependence of the amplification soil factor on the
fundamental period of vibration, seismic intensity level
and soil stiffness.
20/21OpenSees Days Europe 2017, First European Conference on OpenSees Porto, Portugal, 19–20 June 2017
Conclusions

21. Thank you for your attention!
kechidi@fe.up.pt