The document, “The Seismic Assessment of Existing Buildings” July 2017 (The Guidelines) provides a technical basis for engineers to carry out seismic assessments of existing buildings in New Zealand. The Guidelines support seismic assessments for a range of purposes, including whether or not a building is earthquake prone in terms of the Building Act 2004. Section C4 “Geotechnical Considerations” of The Guidelines stresses the need for foundation (geotechnical) aspects to be considered as part of the seismic assessment. This paper summarises the main points presented in Section C4 and particularly emphasises the need for structural and geotechnical engineers to work collaboratively in undertaking a seismic assessment. The intent is to provide an overview and to emphasise key points, but not to replace the need to read The Guidelines and attend relevant courses.
Does this sound familiar?
The structural engineer; “All I want is a report confirming its subsoil class C”.
And the geotechnical engineer; “Liquefaction is triggered at 30%ULS shaking. The building therefore must be 30%NBS.”
The objective of Section C4 is to get structural engineers to work in conjunction with geotechnical engineers to identify and assess geotechnical aspects which could influence the behaviour of the building; i.e. it’s more than just subsoil class.
And for geotechnical engineers to work in conjunction with the structural engineers to jointly develop an understanding of the interaction between the soil and the structure; i.e. it’s not all about the ground (eg triggering of liquefaction), it’s about how the structure behaves as a consequence.
The objective is for us to work together so that geotechnical aspects are considered and an understanding is developed of how, or if, the geotechnical aspects effect the overall behaviour of the building.
Key principles of a seismic assessment include; that the focus of the assessment is on stability of the structure and life safety. As part of that we need to consider the behaviour of the foundations and assess whether or not that foundation behaviour could influence structural stability and life safety.
In ultimate limit state design it is normal to apply a load and resistance factored design approach. The foundation capacity is reduced by a strength reduction factor to provide some reliability and to limit deformations. The assessment approach is quite different. We are interested in the probable behaviour of the foundations and the building. Parameters are selected accordingly, and strength reduction factors are not applied. A displacement based approach is likely to be applied. The Guideline suggests simplified elastic plastic models be applied.
Geohazards to be considered in the seismic assessment of a building include;
- lateral and vertical load/displacement behaviour of the foundations,
- the possibility of liquefaction and its associated effects,
- slips or retaining wall movement removing support to foundations.
These geohazards occur beneath the building footprint and are to be considered in assessing the %NBS rating of the building. Retaining wall displacements, slips and rockfall originating from beyond the building with the potential to impact the building are also to be considered, but do not influence the %NBS rating of the building. This is in the same way that the stability or %NBS rating of a neighbouring building does not influence the %NBS rating of the building being assessed.
The principle of step change is another key aspect of an assessment, and particularly in relation to geotechnical considerations.
Soils can be subject to an abrupt and large reduction in strength with an increase in loading, deformation or earthquake shaking. This is a geotechnical step change. Examples of this include liquefaction, brittle failure of a rock anchor or a brittle underslip. An important aspect of an assessment is to identify potential geotechnical step changes and whether or not they could result in instability of the building and significant life safety hazard ie, whether or not they could result in a structural step change. The geotechnical engineer needs to assess;
- the capacity of the foundation before the step change,
- the %ULS shaking or deformation which triggersthat step change, and
- the residual capacity of the foundation after the step change.
In the assessment the structural engineer is to allow for the geotechnical step change occurring at a %ULS shaking or deformation of ½ of that of the trigger. This is to provide some resilience in the %NBS rating. Assessment of the buildings behaviour post the geotechnical step change could become critical to %NBS rating of the building. The structural and geotechnical engineers must jointly assess the stability of the building allowing for the residual foundation capacity. For example if liquefaction is assessed to be triggered at 60%ULS shaking AND it is assessed that with the residual behaviour of the foundations the building would be unstable, potentially endangering life, then the associated score would be 30%NBS.
Collaboration and Iteration.
Figure 1 below is taken from Section C4 of The Guidelines. It outlines the process recommended for building assessment including geotechnical considerations. The circled portion of the flow diagram is highlighting an iterative process of assessment and review to develop an understanding of the behaviour of the building including its foundations. Each iteration includes conversations between the structural and geotechnical engineers. The objective is to focus any geotechnical work on what matters. That is what matters for the stability of the structure and life safety. The following subsections illustrate this further.
Figure 1: Collaboration and iterations during a seismic assessment.
The first step is for the structural engineer to develop a general understanding of the structures form, load paths and potential structural weaknesses. And for the geotechnical engineer to develop an understanding of the ground model and identify potential geotechnical issues, on the basis of a desk top study. Certainly not to produce a geotechnical report at this stage. A simple sketch of the likely ground model and a list of potential geotechnical issues is all that is required. Figure 2 above presents an example.
Figure 2: Ground Model
The structural and geotechnical engineers then meet and compare notes. They work through each of the geotechnical issues and identify which of them could be material to the assessment of the structure. And for those which could be material what additional information is necessary to assess them further. The output of the meeting would be agreement as to what further geotechnical input is required for the next iteration.
A useful question at the end of the meeting is how would we categorise the assessment; dominated by the structure, by interaction between the soil and the structure, or dominated by geotechnical considerations. This will help focus the level of further geotechnical input required. A majority of assessments are likely to be structurally dominated such that little or no further geotechnical input is required.
A possible outcome from that first meeting is that the load displacement behaviour of the foundations could be critical to the behaviour of the structure and deserves further consideration. Figure 3 describes how that further consideration could be undertaken by the geotechnical engineer and communicated to the structural engineer.
The first thing the geotechnical engineer needs to consider is whether the load displacement behaviour is likely to be ductile i.e. resistance increases with displacement; like indicated by the dashed line in Figure 3. If non-ductile or brittle behaviour is possible then a step change needs to be considered.
The next thing the geotechnical engineer needs to consider is how to model the foundation behaviour for application to a structural assessment. Keep it simple. The Guideline suggests a simple elastic plastic model as indicated by the solid line in Figure 3. The geotechnical engineer needs to nominate the limiting resistance R. This would be assessed as it would be for design but a strength reduction factor is not applied. Section C4 of The Guideline and NZGS Module 4 provides further guidance.
Figure 3: Modelling Foundation Behaviour
The uncertainties we need to deal with in geotechnical engineering are large. In assessment the uncertainties are greater than in design. The presence of the existing building constrains access for investigations or makes them very expensive. Often the depth and size of foundations are unknown. Consequently as part of the iterative and collaborative assessment process the structural and geotechnical engineers need to undertake sensitivity checks to identify any critical issues in relation to stability of the structure and life safety. The geotechnical engineer needs to provide some “what if” scenarios as indicated on Figure 4 and the structural engineer needs to test these in terms of impact on the structure. If a possible foundation behaviour scenario is found to be critical then further assessment or investigation may be required. The message to the geotechnical engineer is that an expensive investigation programme may not be necessary.
Figure 4: Managing Uncertainty
Don’t focus on producing a geotechnical report. Encourage structural and geotechnical engineers to work together to identify geohazards which could influence structural stability and life safety. This is going to require us to talk to each other right from the very beginning of a project. Use sketches of the structure, the ground model and of load displacement behaviours to aid our communication. Once the critical issues have been identified and assessed the conclusions can be presented in a report.