Understanding Structures and Earthquakes in Groningen

Abstract

Rapid extraction of gas in the north-eastern Groningen province of the Netherlands has led to an increase in the occurrence of induced earthquakes in the region due to subsidence of gas bearing sandstone layers. This process manifests itself in the form of ground motions at the surface. Netherlands, historically being an inactive tectonic zone, has not paid much attention to detail structures withstand lateral seismic forces in the past. This has led to an alarming situation amongst the residents and government authorities since damage has been reported in the form of claims for compensation. The predominant presence of old masonry houses has further aggravated the situation because of quasi-brittle material characteristics weak in tension. A large-scale research campaign was launched after the historical seismic event at Huizinge in 2012 with an aim to assess and safeguard building structures in the region although much of the research has been focussed on behaviour of masonry houses. NPR 9998 which serves as a national guideline in the Netherlands for seismic assessment and retrofitting was published and is continuously being updated with the latest developments. However, it is equally important to address other typology of structures in terms of material and geometry. With this objective, it was decided to start with a fundamental study on the seismic analysis methods with specific regards to steel structures.

The present thesis provides a comprehensive review of the lateral behaviour of affected structure initially and the fundamental differences in the induced earthquakes when compared with deep tectonic earthquakes. This is followed by state-of-the-art of linear and nonlinear seismic analysis methods which forms the basis of guidelines & codes presented in the NPR 9998 and EN 1998 context. Further, an understanding on the generation of seismic action in response spectrum format from recorded ground motions which is the most widely adopted one across seismic design codes worldwide. The case study adopted for this study is a steel office building preliminary designed for non-seismic actions. Global seismic demands are determined using linear-static and linear-dynamic analysis methods with verification of specific criteria to be satisfied for safety of steel structures. Modelling parameters and methodologies are discussed in detail with regards to using simplified numerical models for analysis based on recommendations from Eurocodes and Internaltional codes. A variation model to assess the likely performance level using nonlinear static pushover analysis for a specific intensity of ground motion in terms of peak ground acceleration was made. Conclusions in the form of applicability of analysis methods are made towards the end with affected structures primarily vibrating in the fundamental mode, the present study can serve as a reference guide for a practicing engineer carrying out seismic analysis. Discussions about the background of design principles is made alongside the analysis for a clear understanding.

This thesis is expected to fill the knowledge gap for a design engineer carrying out seismic assessment of structures in the Groningen region of the Netherlands by providing a fundamental understanding of seismic demands imposed on a structure and assessment of capacity deficiency by carrying out non-linear pushover analysis. Recommendations based on NPR, Euro codes and International codes have been made to simplify numerical modelling of the structure. Similar analysis can be undertaken for other types of structures prone to be affected by induced earthquakes by adopting corresponding material nonlinear models and considering level of interaction with the ground in terms of soil-structure interaction where the same may lead to modification of structural response.