Stuart Palmer - Soil-Interaction Starts Here

Delegates attending the NZSEE Annual Technical Conference this week will be hearing a lot about how important dynamic soil-structure interaction is when designing buildings to withstand earthquakes. But at T+T we believe this starts long before the earthquake strikes, with the dynamic interaction between geotechnical engineers and structural engineers a critical part of the process to design safe, cost-effective and sustainable buildings.  

When undertaking a soil-structure interaction (SSI) analysis, there are four key steps that our project teams work through: 

1. The geotechnical and structural engineer meet and agree the level of detail and method for the SSI analysis.  
2. The geotechnical engineer provides information to model the foundations’ vertical load-displacement behaviour.   
3. The structural engineer applies this information to the analysis of the structure.  
4. The structural and geotechnical engineer then jointly review the conclusions to check that the modelled behaviour aligns with the intention and expectations. The models and analysis are refined as necessary.   

More detail on this process is provided in a series of guidance documents which T+T helped to develop in collaboration with our industry colleagues: 

• SESOC interim design guidance document “Design of Conventional Structural Systems” 
• MBIE / NZGS guidance, Module 4 “Earthquake resistant foundation design” 
• NZSEE guidance document “Earthquake design for Uncertainty” 

Some of the key aspects from these guidance documents regarding SSI are summarised below. 

Why is SSI important? 

Soil-structure interaction (SSI) can have a significant influence on the seismic performance of a building, and if this is not understood then it can lead to unrealistic prediction of the forces imposed on structural elements and the displacements of the structure. As illustrated in the examples below from the SESOC Guidance, if the foundation has a stiff/strong response then this tends to result in large forces in the structure, but small displacements. Conversely, if the foundation has a flexible/weak response then this tends to reduce forces in the structure, but increase displacements. This can be particularly important for design of buildings with slender shear walls or braced frames. 

How do we know when SSI analysis is necessary or worthwhile? 

The structural and geotechnical engineers should be collaborating right from the very start of a project, as the ground conditions can be important when choosing the optimum structural form for the building. As part of this process, the geotechnical engineer should establish the expected displacement of the foundations under earthquake loading, and the structural engineer should consider the implications of this displacement on the performance of the structure. The design team can then decide whether or not more detailed SSI analysis would be useful to help refine the building design. 

What options are available for modelling the response of foundations? 

Even after detailed ground investigations are undertaken, there remains substantial inherent uncertainty in predicting the response of the foundations and the underlying ground. And the response can also vary over the course of earthquake shaking depending on how the ground and building react to the imposed dynamic loading. This means that it is important to understand the response of the structure across a range of potential foundation stiffnesses.  

Depending on the level of complexity warranted by the particular structure being designed, there are several options available for modelling foundation response: 

• Evaluate the load-displacement behaviour using a first-principles approach which combines the contributions of various loadpaths in proportion to their stiffness. 
• Model the foundation as a purely elastic element (i.e. constant stiffness, non-yielding). This model is available in most routine structural engineering software packages, and can be useful for modelling the stiffer end of the range of potential foundation responses.  

• Model the foundation as an elastic-plastic element (i.e. constant stiffness up to a yield point). This can be modelled in some routine structural engineering software packages by manually adjusting the stiffness during the analysis to represent an elastic-plastic response. This can be useful for modelling the softer end of the range of potential foundation responses. 
• Undertake a non-linear soil spring analysis (i.e. stiffness varies depending on displacement). This option is not usually available in the software packages used for routine structural engineering design, and tends to only be considered for projects where a detailed understanding of the foundation response is critical. 

In all cases the geotechnical engineer should review the loads determined by the structural analysis to check that they are consistent with what was assumed when the foundation stiffness parameters were derived. In some cases an iterative process between the geotechnical and structural analysis may be required to establish a solution with compatible loads and displacements, especially for raft foundations where the structural design of the raft influences the distribution of imposed stresses within the soil profile. 

The analysis should also consider the potential for spatial variability across the footprint of the structure, both in terms of the ground stiffness and the response of the structure above to deformation. It is typically not possible to reliably predict how these vary across the footprint, so a pragmatic approach is for the structural and geotechnical engineers to work collaboratively to identify a series of plausible unfavourable distributions of stiffness and test the structural design against these scenarios. 

How can we make the most of this collaboration between geotechnical and structural engineers? 

Our project teams often find that simple (but to-scale) sketches are valuable communication aides. Examples of useful sketches include:  

• The soil profile and foundations (ground model)  
• Graphs of load-displacement behaviour  
• A cross section of the structure and foundations  

It is also important to make sure the feedback loop happens. After the structural analysis the geotechnical engineer and structural engineer should discuss the conclusions of the analysis and what was critical to design. This provides the opportunity to check that the geotechnical parameters have been applied as intended and to review/revise parameters if they appear unreasonably onerous to the structure. This ongoing process of collaboration also provides a great opportunity to learn and for the project team to refine the approach for next time.