Applicability of the Pushover Method for the Seismic Assessment of URM Structures in Groningen

Abstract

In Groningen, seismic activity has increased due to the extraction of gas in the area. A large-scale research campaign has been launched with the aim to assess and safeguard structures in the region. However, an accurate assessment of these buildings turned out to be a challenge, due to the nonlinear behaviour of the masonry and the dynamic nature of a seismic load. A Nonlinear Time History (NLTH) analysis takes into account both these factors, but the computational demand of such a method is considerable. Another method that is widely used to analyse the seismic response of a structure is the Modal Response Spectrum (MRS) method. The computational demand of this method is considerably less compared to NLTH, but nonlinear material behaviour is only taken into account in an indirect manner via a behaviour factor, and the results are considered to be too conservative. A third method is the Nonlinear Pushover (NLPO) method. It takes nonlinear material behaviour into account and compared to NLTH, NLPO is computationally more efficient. Furthermore, an advantage is that it separates capacity from demand. Even though the NLPO method is commonly applied worldwide, its validity still needs to be proven for the Groningen case. Both objectives were studied by looking into a single case study, consisting of a low-rise URM apartment building. The behaviour of the structure is characterised by a weak and strong direction, in which the weak direction is characterised by a relatively low stiffness and lateral capacity compared to the strong direction. The seismic response of the structure is determined according to the MRS, NLPO and NLTH methods. Furthermore, the NLPO analyses are executed using two different computational discretisation methods, namely continuum FEM and macro EFM. DIANA is used as a FEM solver for the MRS, NLPO and NLTH analyses and 3MURI is used for the EFM model. Moreover, a modal and uniform lateral load pattern are taken into account for the NLPO analyses. The conclusions which are drawn from the case study can generally be applied to low-rise URM apartment buildings in Groningen. However, it must be noted that significant alterations in geometry and building materials might influence the results. Furthermore, modelling assumptions have been applied, and it is important to note that the possible influence of these assumptions, may partially limit the extent of the conclusions. Moreover, several limitations are inherent to the studied methods, and cannot be accounted for somehow. All analyses are performed by incrementally increasing the seismic load until one of the near collapse limit state criteria according to NPR 9998 is met. Furthermore, three target displacement methods are evaluated: the capacity spectrum method according to NPR 9998, the regular N2-method, included in the Eurocode 8, and an adaptation of the N2-method which is developed specifically for URM structures by Guerinni. The performance of the structure according to each of the methods is studied subsequently, by looking into the force-displacement behaviour, displacement profile and damage at failure, failure mechanisms and the maximum admissible seismic load. Two significant disadvantages of macro EFM were identified when comparing the results of the NLPO analyses using 3MURI and DIANA. First, the fact that out-of-plane behaviour is not taken into account in 3MURI could significantly influence the behaviour of a structure in terms of base shear capacity, which is especially true when structures are characterised by an extremely low total length of piers in the in-plane direction. Furthermore,DIANA allows for a more gradual softening behaviour, which helps the post-peak force redistribution. As a consequence, the maximum admissible seismic load according to DIANA could be higher. However, despite the two aforementioned disadvantages of the macro EFM method as implemented in 3MURI, all other relevant results of both methods are similar. The fact that the two identified disadvantages of 3MURI can only result in more conservative results, suggests that macro EFM, as implemented in 3MURI, is a suitable computational discretisation method for the seismic assessment according to the NLPO method for low-rise URM apartment buildings in Groningen. However, it should be taken into account that the conservativeness of 3MURI could lead to a significantly larger amount of required retrofitting, in comparison with DIANA. The applicability of the NLPO method is reviewed by comparing the results of the MRS, NLPO and NLTH methods. Similar behaviour of the structure according to the NLPO and NLTH method was captured, which suggests that the NLPO method is a suitable analysis method for the studied typology. The maximum admissible seismic load using the target displacement method according to NPR 9998 is in-line with the NLTH analysis. However, the governing load case made use of a uniform load pattern, which returns a structural behaviour different than that obtained by NLTH analyses, as can be seen from the force-displacement behaviour. If only the capacity curve according to the modal load pattern would be considered, then the allowable seismic load according to NLTH is similar to that of NLPO using the target displacement method of Eurocode 8. Furthermore, from the results of the case study can be concluded that the choice of target displacement method has a significant influence on the maximum admissible seismic load. For the case study, NPR 9998 is more conservative in the strong direction, and Eurocode 8 is more conservative in the weak direction. Regarding the MRS method, very conservative results were found. A reason that was found for these conservative results is that the prescribed behaviour factor by NPR 9998 is too low for the case study when compared to that derived from the NLPO analysis. However, even with a larger behaviour factor, the results according to the MRS method would still be conservative in the weak direction.