Design Seismic Action Research Articles

Evaluation of factors influencing the earthquake-resistant design of reinforced concrete frames according to Eurocode 8

The impact of factors which affect the performance of regular-reinforced concrete frame buildings designed according to the Eurocodes is investigated. The introduction of a sequence of variants of a structure, which gradually takes into account the design factors, makes it possible to measure the level of impact of an individual design factor on the global structural and performance parameters. The proposed procedure was applied to several regular reinforced concrete frame buildings, which were designed for ductility class medium and located in a region with moderate seismicity. A comparison between the reduction factor and the behaviour factor revealed that the behaviour factor defined in Eurocode 8, which significantly affects the design seismic action, is based more on the expert elicitation rather than on a firm scientific basis. Furthermore, based on the results of the investigated buildings it is shown that the design factors which are simply adopted in the design often have a significant impact on the resulting structures and their performance, whereas some other design factors, which require analysis of the structure and a lot of labour by engineers, often have only a minor effect on the performance of a structure.

The role of source and site effects on structural failures due to Azores earthquakes

The existing building stock in Azores islands (Portugal) was severely damaged during 1980 and 1998 earthquakes. Structural failure was probably caused by a combination of factors that are not yet well understood. Earthquake source characteristics, site effects and structural vulnerability may be some of those factors. However, it is very difficult to assess the influence of each factor on structural failure, mainly because recorded accelerograms used in nonlinear structural analysis are influenced by both source characteristics and site conditions. The only way to overcome this problem is to control each factor individually which can be done by using simulated accelerograms. In our previous work, stochastic ground motion simulations results were compared with earthquake records. Results seem to indicate that simulated accelerograms can match recorded accelerograms if proper source characteristics and geological site conditions are selected. In this work, simulated accelerograms were used for seismic nonlinear structural analysis. Simulations were carried out considering several 1980 Azores earthquake possible sources and for different geological site conditions. Simulated accelerograms were then used to evaluate the structural nonlinear behaviour of a reinforced concrete structure and of two masonry structures. The results of this work highlight the importance of site conditions and earthquake source characteristics to the determination of the design seismic actions of Azores islands. This work was performed in the scope of “Strong ground motion for Azores – SiGMA” project, financed by the Portuguese Science and Technology Foundation (PTDC/CTE-GIX/121957/2010).

Seismic Design of Precast I-Beam Bridges Based on Ductility

This study proposes a new configuration for the longitudinal direction of precast I-beam bridges aimed at the improvement of their earthquake resistance. The longitudinal response of the new bridge system is different from the response of the usual "floating deck" bridge configuration, the main characteristic of which is the different seismic movements between the deck and pier head. The proposed configuration includes the application of seismic stoppers that are used to transmit the longitudinal design seismic action to three contiguous piers, provided that serviceability requirements are preserved. The aforementioned seismic stoppers are combined with elastomeric bearings that allow the free rotation of the supported nodes. The deck is connected to the other piers through sliding bearings permitting its free movement in the longitudinal direction, while its transverse movement is constrained by stoppers. This methodology offers the opportunity to design precast I-beam bridges as ductile structures, provided that the longitudinal seismic action is transmitted to the three piers of the bridge. The result is that values of the behavior factor q ≤ 3,5 are obtained in both directions. The suggested modification is necessary before integrating a restraining system. This restraining system consists of four symmetrical bundles of steel bars, whose function is to improve the seismic behavior of the whole bridge system. These bundles are applied in the bridge sidewalks; they are anchored at their ends and bind the deck with the abutments’ wing walls, while their length between the anchorages is unbounded. The steel bundles act as a longitudinal struts and ties system. This proposed method improves the earthquake resistance of the structural system as the steel bundles are activated during earthquakes and restrain the seismic displacements. The above-mentioned procedures were applied in a precast concrete I-beam bridge and the results showed a significant reduction in the seismic displacements and loads of the bridge.

Assessment of concrete bridges subjected to ground motion with an arbitrary angle of incidence: static and dynamic approach

A nonlinear static analysis methodology for the derivation of a set of pushover curves for any angle of incidence of the seismic action (multidirectional pushover curves) for bridges is developed, wherein the interaction between axial force and biaxial moments at critical pier sections or biaxial shear forces at the bearings is taken into account. Dynamic pushover curves (base shear vs. peak deck displacement) for arbitrary angle of incidence of the excitation, are derived for both unidirectional (single-component) and bidirectional (dual-component) ground motion. It is found that neglecting the minor horizontal component leads to underestimation of bridge response, especially along the bridge principal directions and that the angle of incidence of bidirectional excitation affects bridge response, but to a lesser extent than in the case of unidirectional excitation. The proposed procedure is then applied to a straight symmetric bridge, its results are checked against those from response-history analysis, and is found to be sufficiently accurate for practical application. Using the derived results it is also found that the design of the selected bridge is safe since for the design bidirectional earthquake the bridge starts to behave inelastically (the first plastic hinge forms), while its failure occurs for about four times the design seismic action.

Near‐source seismic hazard and design scenarios

SUMMARYEarthquakes damage engineering structures near, relatively to the rupture's size, to the source. In this region, the fault's dynamics affect ground motion propagation differently from site to site, resulting in systematic spatial variability known as directivity. Although a number of researches recommend that records with directivity‐related velocity pulses should be explicitly taken into account when defining design seismic action on structures, probabilistic seismic hazard analysis (PSHA), in its standard version, seems inadequate for the scope. In the study, it is critically reviewed why, from the structural engineering point of view, hazard assessment should account for near‐source effects (i.e., pulse‐like ground motions), and how this can be carried out adjusting PSHA analytically via introduction of specific terms and empirically calibrated models. Disaggregation analysis and design scenarios for near‐source PSHA are also formulated. The analytical procedures are then applied to develop examples of hazard estimates for sites close to strike–slip or dip–slip faults and to address differences with respect to the ordinary case, that is, when pulse‐like effects are not explicitly accounted for. Significant increase of hazard for selected spectral ordinates is found in all investigated cases; increments depend on the fault‐site configuration. Moreover, to address design scenarios for seismic actions on structures, disaggregation results are also discussed, along with limitations of current design spectra to highlight the pulse‐like effects of structural interest. Finally, an attempt to overcome these, by means of disaggregation‐based scenarios specific for the pulse occurrence case, is presented. Copyright © 2012 John Wiley & Sons, Ltd.

Flexural test of precast high-strength reinforced concrete pile prestressed with unbonded bars arranged at the center of the cross-section

In this study, a prestressed reinforced concrete pile that uses high-strength material to increase the pile’s flexural capacity was developed. The main structural characteristics of the developed pile include (1) the neutral axis is constantly near the centroidal axis of the pile, even if the longitudinal reinforcement yields due to a flexural moment, because the pile has a high axial compressive force that is induced by prestressed steel bars, and hence, the concrete in the compression region can contribute to increasing the flexural strength of the pile; and (2) the flexural strength of the pile increases because the high-strength concrete is confined by high-strength spirals and carbon-fiber sheets in combination with concrete infilling, and, together, these modifications provide a sufficiently high lateral-confinement pressure. The results of bending tests demonstrate that the proposed prestressed reinforced concrete pile with carbon-fiber sheets and concrete infilling had a much higher flexural capacity than a conventional precast concrete pile. In addition, an analytical approach is presented that can be used to obtain the relationship between the bending moment and the curvature of the proposed pile. Even if concrete bridge systems are constructed on strata that can experience soil liquefaction, such as very soft soil, bridge foundations that use the proposed piles could remain undamaged under the design seismic action.

Hysteretic model for steel energy dissipation devices and evaluation of a minimal-damage seismic design approach for steel buildings

This paper evaluates an alternative seismic design approach for steel structures that concentrates damage in easy-to-replace steel energy dissipation devices and protects the main structural members from yielding with capacity design rules. This approach can reduce damage repair costs and downtime, and, can be further enhanced by using rate-dependent dampers in parallel to steel devices to achieve drift reduction and protection of drift-sensitive non-structural elements. A model for steel energy dissipation devices is proposed and calibrated against experimental results. In particular, the Bouc-Wen model is modified to capture the combined kinematic and isotropic hardening in the hysteresis of steel devices. The model is found able to accurately predict the experimentally obtained hysteresis and is implemented in the OpenSees software for use in seismic response analysis. A simplified seismic design procedure is proposed and used to design a prototype steel building equipped with steel devices and viscous dampers according to explicitly defined minimal-damage performance objectives. Seismic analyses results indicate the accuracy of the design procedure and confirm that the building is able to achieve immediate occupancy under the design seismic action and rapid return to occupancy under the maximum considered seismic action. The same building is designed as a conventional steel MRF according to EC8. Results of seismic analyses show that repair of damage in the main structural members of the conventional MRF may not be financially viable in the aftermath of the design and maximum considered earthquakes.

Engineering design earthquakes from multimodal hazard disaggregation

To define reference structural actions, engineers practicing earthquake resistant design are required by codes to account for ground motion likely to threaten the site of interest and also for pertinent seismic source features. In most of the cases, while the former issue is addressed assigning a mandatory design response spectrum, the latter is left unsolved. However, in the case that the design spectrum is derived from probabilistic seismic hazard analysis, disaggregation may be helpful, allowing to identify the earthquakes having the largest contribution to the hazard for the spectral ordinates of interest. Such information may also be useful to engineers in better defining the design scenario for the structure, e.g., in record selection for nonlinear seismic structural analysis. On the other hand, disaggregation results change with the spectral ordinate and return period, and more than a single event may dominate the hazard, especially if multiple sources affect the hazard at the site. This work discusses identification of engineering design earthquakes referring, as an example, to the Italian case. The considered hazard refers to the exceedance of peak ground acceleration and 1s spectral acceleration with four return periods between 50 and 2475 year. It is discussed how, for most of the Italian sites, more than a design earthquake exists, because of the modeling of seismic sources. Furthermore, it is explained how and why these change with the limit state and the dynamic properties of the structure. Finally, it is illustrated how these concepts may be easily included in engineering practice complementing design hazard maps and effectively enhancing definition of design seismic actions with relatively small effort.

Seismic evaluation of an existing complex RC building

The seismic evaluation of existing buildings is a more difficult task than the seismic design of new buildings. Non-linear methods are needed if realistic results are to be obtained. However, the application to real complex structures of various evaluation procedures, which have usually been tested on highly idealized structural models, is by no means straightforward. In the paper, a practice-oriented procedure for the seismic evaluation of building structures, based on the N2 method, is presented, together with the application of this method to an existing multi-storey reinforced concrete building. This building, which is asymmetric in plan and irregular in elevation, consists of structural walls and frames. It was designed in 1962 for gravity loads and a minimum horizontal loading (2% of the total weight). The main results presented in terms of the global and local seismic demands are compared with the results of non-linear dynamic response-history analyses. As expected, the structure would fail if subjected to the design seismic action according to Eurocode 8. The shear capacity of the structural walls is the most critical. If the shear capacity of these elements was adequate, the structure would be able to survive the design ground motion according to Eurocode 8, in spite of the very low level of design horizontal forces. The applied approach proved to be a feasible tool for the seismic evaluation of complex structures. However, due to the large randomness and uncertainty which are involved in the determination of both the seismic demand and the seismic capacity, only rough estimates of the seismic behaviour of such structures can be obtained.

Multi-hazard performance assessment of a transfer-plate high-rise building

Many urban areas are located in regions of moderate seismicity and are subjected to strong wind. Buildings in these regions are often designed without seismic provisions. As a result, in the event of an earthquake, the potential for damage and loss of lives may not be known. In this paper, the performance of a typical high-rise building with a thick transfer plate (TP), which is one type of building structure commonly found in Hong Kong, is assessed against both earthquake and wind hazards. Seismic-and wind-resistant performance objectives are first reviewed based on relevant codes and design guidelines for high-rise buildings. After a brief introduction of wind-resistant design of the building, various methodologies, including equivalent static load analysis (ESLA), response spectrum analysis (RSA), pushover analysis (POA), linear and nonlinear time-history analysis (LTHA and NTHA), are employed to assess the seismic performance of the building when subjected to frequent earthquakes, design based earthquakes and maximum credible earthquakes. The effects of design wind and seismic action with a common 50-year return period are also compared. The results indicate that most performance objectives can be satisfied by the building, but there are some objectives, such as inter-story drift ratio, that cannot be achieved when subjected to the frequent earthquakes. It is concluded that in addition to wind, seismic action may need to be explicitly considered in the design of buildings in regions of moderate seismicity.

Adapting the structural design actions standard for the seismic design of new industrial plant

In the late 1970’s it was recognised that the seismic provisions of the current NZS 4203:1976 did not readily apply to the types of structures normally used within the land based processing facilities of the “heavy industries” such as petrochemical and oil and gas processing plants impending under the “Think Big” regime.
 Since the 1984 revision to NZS 4203, there have not been any publicly available New Zealand guidelines on how to interpret the earthquake provisions of the various versions of NZS 4203 (and now AS/NZS 1170) that would update the 1981 publication created by the Ministry of Works for the Ministry of Energy, “Seismic Design of Petrochemical Plants”.
 There are overseas publications that have considered the differences in the typical structural systems necessary to support the equipment and distributive systems needed to process industrial feedstock. How they behave seismically has been reviewed and recommendations made on the methods to be used to determine the design seismic actions. Such standards as ASCE 7 and FEMA 450 incorporate these in a specific manner relating to the design of industrial plant.
 With the advent of new oil and gas processing plants in Taranaki, this paper takes the opportunity to review AS/NZS 1170 and adapt these overseas guidelines for the seismic design of new industrial plant in New Zealand. The background for these guidelines will be presented with examples of typical industrial structural systems and their seismic actions. This is with the aim of re-establishing a basis of seismic design for industrial plant within the framework of the new standards AS/NZS 1170.0 and NZS 1170.5.

AN IMPROVED METHOD OF MATCHING RESPONSE SPECTRA OF RECORDED EARTHQUAKE GROUND MOTION USING WAVELETS

Dynamic nonlinear analysis of structures requires the seismic input to be defined in the form of acceleration time-series, and these will generally be required to be compatible with the elastic response spectra representing the design seismic actions at the site. The advantages of using real accelerograms matched to the target response spectrum using wavelets for this purpose are discussed. The program RspMatch, which performs spectral matching using wavelets, is modified using new wavelets that obviate the need to subsequently apply a baseline correction. The new version of the program, RspMatch2005, enables the accelerograms to be matched to the pseudo-acceleration or displacement spectral ordinates as well as the spectrum of absolute acceleration, and additionally allows the matching to be performed simultaneously to a given spectrum at several damping ratios.

Pseudodynamic Tests On Full Scale Masonry Walls

To investigate the performance of unreinforced masonry shear walls under earthquake loadings some full scale pseudodynamic tests have been carried out at the Department of Concrete Structures of the Technical University Munich. Thus simulating the main shear wall in the first floor of a four-story building, which was supposed to be constructed on medium soil conditions with an assumed peak ground acceleration up to 1,6 [mls2]. According to the corresponding elastic response spectrum an earthquake ground acceleration timehistory was generated artificially and given as input data to the computer calculating the pseudodynamic algorithm. The used accelerogram with a duration of 9 seconds characterises regions of low seismicity where the design seismic action results from short focal distance earthquakes with low magnitudes. With time and load progress significant stiffness reductions appeared and, as a consequence, the eigenfrequency decreased. Compared with uncracked specimen, the shift in the elastic response spectrum led to reduced loads. The link between the degree of stiffness-reduction and reduction of resulting seismic loads was properly specified using the pseudodynamic test method. The tests indicated, that the current design standards for masonry, based on conventional design methods, were too conservative.

Formulation of elastic earthquake input energy spectra

SUMMARY The object of this paper is to introduce a procedure for the determination of elastic design earthquake input energy spectra taking into account the influence of magnitude, soil type and distance from the surface projection of the fault. Firstly, an accurate selection of a large set of representative records has been realized. Secondly, the construction of the design input energy spectra has required determining the spectral shapes and a normalization factor which measures seismic hazard in terms of energy. This factor, denoted as the seismic hazard energy factor, has been defined as the area under the earthquake input energy spectrum in the period interval between 0)05 and 4)0 s. Finally, due to the importance of the source-to-site distance in the evaluation of the input energy, an investigation into the attenuation of the seismic hazard energy factor has been carried out. ( 1998 John Wiley & Sons, Ltd. The fundamental need to improve the reliability of the current procedures of earthquake-resistant design of structures has led to the recognition of methodologies based on energy criteria as e⁄ective tools for a comprehensive interpretation of the behaviour observed during recent destructive events. Energy-based design involves considering two essential aspects: the first is related to the establishment of Design Earthquakes, while the second concerns the evaluation of the actual energy absorption and energy dissipation capacities of structures. The aim of this work is to introduce a proposal which could contribute to the resolution of the first of the above-mentioned aspects, namely the definition of a design seismic action as a function of appropriate parameters providing a measure of the energy actually transferred from soil to structures during seismic shaking. The commonly adopted design approach in terms of forces, based on both elastic and inelastic response spectra, as results from the analysis and interpretation of the observed structural behaviour is open to criticism. The most controversial and uncertain aspect of the conventional design procedures specified by the di⁄erent codes is represented by the interpretation of the elastic design spectrum as a measure of destructiveness and, by the definition of the elastic response reduction factor as a function of a presumed inelastic behaviour. This factor, also known as the behaviour factor, although it is based on the comparison between elastic and inelastic response spectra and non-linear structural analysis, is still substantially assigned by the codes according to empirical criteria. Furthermore, uncertainty often arises in various steps of these

Repair and retrofit of a six storey reinforced concrete building damaged by the earthquake in south-east Sicily on the 13th December 1990

This paper presents the studies that led to the seismic retrofitting of a six storey reinforced concrete building damaged by the 1990 earthquake in south-east Sicily. A seismological study, available geological information and extensive geotechnical tests led to the definition of the ultimate design earthquake for the site. A detailed damage survey and a numerical damage simulation together with available information on the design and construction history denounced the highly deificient condition of the building against the design seismic action. After having established, through a campaign of destructive and non-destructive tests, the main structural characteristics of the materials and of the structural elements, joints included, seven different retrofitting systems are presented and discussed. Finally the recommended solution is presented in some detail and the methods of analysis and the conducted safety cheks are briefly described.