Influence of higher modes in nonlinear seismic analysis of building structures

In the Ph. D. thesis, the extension of the N2 method for the structures with important higher mode effects along the elevation has been proposed. The estimation of higher mode effects is based on assumption that the structure remains in the elastic range when vibrating in higher modes. In other words, it is assumed that the higher mode effects in the inelastic range are the same as in the elastic range. Based on this assumption, the computational procedure was prepared. The influence of higher modes is determined by the standard elastic modal analysis and applied in terms of correction factors, which are used for the adjustment of the results obtained by the usual pushover analysis. The proposed procedure is consistent and compatible with the extended N2 method for plan-asymmetric buildings. A single standard elastic modal response spectrum analysis can provide correction factors for taking into account the higher mode effects both in plan and in elevation. We consider this point as the main advantage of the proposed approach. In the first part of the thesis, the proposed procedure was applied to nine planar steel frame building with different number of stories using different intensities of ground motion. The results, in particular storey drifts, are compared with the results of nonlinear response history analysis, with the results of other simplified nonlinear methods (MPA and MMPA), and with the results obtained by pushover analysis without consideration of higher modes. A considerable influence of higher modes on storey drifts can be observed at the upper part of the structures. The extended N2 method, as well as the other two methods (MPA and MMPA), which approximately take into account the higher mode effects, are able to substantially improve the accuracy of the results compared to results without considering the influence of higher modes (basic N2 method). The accuracy of predictions of the three approximate methods (proposed N2, MPA and MMPA), varies with the intensity of ground motion (i.e. with the magnitude of plastic deformation) and is different for different sets of ground motions and for different structural systems. In the second part of the thesis the practice-oriented procedure for seismic evaluation of building structures, based on the extended N2 method (taking into account higher modes effect in plan and in elevation) is presented, together with the application of this procedure to an existing multi-storey reinforced concrete buildings. Although several problems may occur during the analysis procedure, the N2 method provides, in combination with data on seismic capacity, a feasible tool for rational yet practical seismic evaluation of building structures for multiple performance objective

Comprehensive permanent remote monitoring system of a multi-span highway bridge

As part of the reconstruction of a multi-span viaduct on a Slovenian highway, a permanent remote monitoring system with over 200 sensors was established. Several parameters are monitored on different parts of the viaduct by means of temperature sensors, accelerometers, strain gauges, long-gauge deformation and Fibre Bragg Grating (FBG) sensors. In this way strains, frequencies and temperatures on external prestressed beam cables, carbon fibre rebarsused for the flexural strengthening of a deck overhang, pier caps and prestressed beams are measured and stored into the on-site central data acquisition system. This paper presents architecture of the permanent bridge monitoring system and preliminary results of the measurements

Eurocode 8: Seismic Design of Buildings - Worked examples

This document is a Technical Report with worked examples for seismic design of buildings following the Eurocodes. It summarizes important points of the Eurocode 8 for the seismic design of concrete and steel buildings including foundations utilizing a common generic building as a basis. An overview of EN 1998 with focus on the performance requirements and compliance criteria for structures, ground conditions and seismic actions is presented at the first section. An introduction to the example reinforced concrete building with its geometrical and material properties as well as the main assumptions for analysis and the detailed structural analysis calculations are presented in the second chapter of the report. Specific rules for design of the building for ductility and the design of concrete foundation elements are presented in the following chapters. For the sake of completeness, the details of design and detailing of the same example as a steel building with three different configurations, namely; with (i) steel moment resisting frames, (ii) composite steel concrete moment resisting frames, and (iii) composite steel concrete frames with eccentric and concentric bracings is also presented afterwards. Key concepts of base isolation is summarized by utilizing the example building. Seismic performance assessment and retrofitting according to EN 1998-Part 3 is explained as the last past of the report. The reinforced concrete/steel building as worked example analyzed in this report was prepared and presented at the workshop “Eurocode 8: Seismic Design of Buildings” that was held on 10-11 February 2011 in Lisbon, Portugal. The workshop was organized by JRC with the support of DG ENTR and CEN and in collaboration with CEN/TC250/Sub-Committee 8 and the National Laboratory for Structural Design (Laboratorio Nacional de Engenharia Civil - LNEC, Lisbon). The document is part of the Report Series ‘Support to the implementation, harmonization and further development of the Eurocodes’ prepared by JRC in collaboration with DG ENTR and CEN/TC250 “Structural Eurocodes”.JRC.G.5-European laboratory for structural assessmen

Measurements of bridge dynamic amplification factor using bridge weigh-in-motion data

The dynamic component of bridge traffic loading is commonly taken into account with a Dynamic Amplification Factor (DAF)—the ratio between the dynamic and static load effects on a bridge. In the design codes, this factor is generally more conservative than in reality. Recently a new method of cal- culation of this factor had been developed. Data from 15 different bridges have been analysed since then and this paper presents the results of the analyses. The background for Bridge Weigh-in-Motion is given, and the most recent method for DAF calculation is described. The sites from which the data originated are presented, and the selection of data discussed. The results of the analyses are presented and discussed and some examples of DAF calculations are shown. Data from the considered sites have invariably demonstrated a DAF decrease with increasing axle load. This is a significant result, especially for assessment of existing structures, since it is beneficial to use measured structural param- eters to optimise structural analysis

Determination of characteristic internal forces and moments in road bridges from weigh- in-motion data

Prispevek obravnava določitev notranjih statičnih količin cestnih mostov iz podatkov tehtanj vozil med vožnjo (meritev WIM, ang. weigh-in-motion), ki edina zagotovijo celovito in nepristransko sliko o tovornem prometu na merjenem cestnem odseku. Tovrstni podatki so ključni za določitev realnih učinkov prometnih obtežb, s katerimi dokazujemo zadostno varnost mostov, tudi starih in poškodovanih. Takšne mostove bi bilo treba ob upoštevanju obtežnih shem iz sodobnih pravilnikov za mostove pogo- sto zapreti ali jim omejiti prometno obtežbo. Predstavljene so metode, s katerimi iz podatkov WIM izračunamo karakteristične vrednosti notranjih statičnih količin. Posebej smo analizirali metodo konvolucije in rezultate primerjali z rezultati ekstrapolacije ekstremnih vrednosti, najbolj pogostega postopka za napovedovanje maksimalnih pričakovanih vplivov prometa v izbranem obdobju, ter rezultati obsežnih numeričnih simulacij. Veliko pozornost smo namenili izbiri vhodnih parametrov ter načinu od- čitavanja karakterističnih vrednosti, ki bistveno vplivajo na rezultate ekstrapolacij in simulacij. Rezultati kažejo, da daje metoda konvolucije, ki je računsko neprimerno manj zahtevna od ostalih uporabljanih metod, primerljive rezultate. Sočasno so le-ti manj občutljivi za subjektivno izbiro uporabljenih parametrov. Bistveni zaključek analize je tudi, da za zanesljiv račun karakte- rističnih notranjih statičnih količin mostov zaradi prometa potrebujemo s sistemom WIM izmerjene osne pritiske in medosne razdalje vsaj 100.000 tovornih vozil.The paper deals with the determination of the internal forces and moments in road bridges from the weigh-in-motion (WIM) data. WIM measurements are the only means that provide a comprehensive and unbiased picture of freight traffic on a me- asured road section. The WIM results are crucial for calculating the actual effects of traffic loads, a key parameter in assessing the structural safety of old and deteriorated bridges. Such bridges would often be closed or moved when analysed with load models from the current design codes. The paper presents methods for calculating the characteristic values of load actions calculated using WIM data. We focused on the convolution method and compared the results with the extreme value extra- polations, the most common procedure for predicting the maximum expected impact of traffic load, and extensive numerical simulations. We paid close attention to the selection of input parameters and the determination of characteristic values that significantly affect the extrapolation and simulation results. Finally, we have shown that the convolution method, which is com- putationally far less demanding than other more commonly used methods, yields comparable results, which are, at the same time, less sensitive to the subjective choice of parameters. The main conclusion of the research is that for a reliable calculation of the characteristic static internal forces and moments in road bridges, we need WIM-measured axle loads and spacings of at least 100,000 heavy goods vehicles

In situ consideration of resistance of bridge girder according to EC2 with AEM

The paper presents a case study of a considerably cracked and degraded bridge in Slovenia: with the implementation of in-situ measurements under bending and shear and the use of a non-destructive acoustic emission technique. Despite the existing crack system, the latter was able to detect microstructural changes. These were characterised by low values of average frequency (AF), as well as lower values of the rise time-amplitude ratio (RA), and energy. A correlation between shear capacity and acoustic activity was observed. This promises to expand the use of AE in the process of assessing of the load-bearing capacity of existing concrete structures

Model updating concept using bridge Weigh-in-Motion data

Finite element (FE) model updating of bridges is based on the measured modal parameters and less frequently on the measured structural response under a known load. Until recently, the FE model updating did not consider strain measurements from sensors installed for weighing vehicles with bridge weigh-in-motion (B-WIM) systems. A 50-year-old multi-span concrete highway viaduct, renovated between 2017 and 2019, was equipped with continuous monitoring system with over 200 sensors, and a B-WIM system. In the most heavily instrumented span, the maximum measured longitudinal strains induced by the full-speed calibration vehicle passages were compared with the modelled strains. Based on the sensitivity study results, three variables that affected its overall stiffness were updated: Young’s modulus adjustment factor of all structural elements, and two anchorage reduction factors that considered the interaction between the superstructure and non-structural elements. The analysis confirmed the importance of the initial manual FE model updating to correctly reflect the non-structural elements during the automatic nonlinear optimisation. It also demonstrated a successful use of pseudo-static B-WIM loading data during the model updating process and the potential to extend the proposed approach to using random B-WIM-weighed vehicles for FE model updating and long-term monitoring of structural parameters and load-dependent phenomena

Using weigh-in-motion data to determine bridge dynamic amplification factor

The dynamic component of bridge traffic loading is commonly taken into account with a Dynamic Amplification Factor (DAF) – the ratio between the maximum dynamic and static load effects on a bridge. In the design codes, this factor is generally higher than in reality. While this is fine for new bridges that must account for various risks during their life-time, it imposes unnecessary conservativism into assessment of the existing well defined bridges. Therefore, analysis of existing bridges should apply more realistic DAF values. One way of obtaining them experimentally is by bridge weigh-in-motion (B-WIM) measurements, which use an existing instrumented bridge or culvert to weigh all crossing vehicles at highway speeds. The B-WIM system had been equipped with two methods of obtaining an approximation to the static response of the. The first method uses the sum of influence lines. This method relies on accurate axle identification, the failure of which can have a large influence on the DAF value. The other method uses a pre-determined low-pass filter to remove the dynamic component of the measured signal; however an expert is needed to set the filter parameters. A new approach that tries to eliminate these two drawbacks has been developed. In this approach the parameters for the filter are determined automatically by fitting the filtered response to the sum of the influence lines. The measurement of DAF on a typical bridge site agrees with experiments performed in the ARCHES [1] project: dynamic amplification decreases as static loading increases