Dynamic Analysis of Multistory Buildings

This paper presents a comparison of the seismic forces generated from a Modal Response Spectrum Analysis (MRSA) by applying the provisions of two building codes, the 1997 Uniform Building Code (UBC) and the 2000–2009 International Building Code (IBC), to the most common ordinary residential buildings of standard occupancy. Considering IBC as the state of the art benchmark code, the primary concern is the safety of buildings designed using the UBC as compared to those designed using the IBC. A sample of four buildings with different layouts and heights was used for this comparison. Each of these buildings was assumed to be located at four different geographical sample locations arbitrarily selected to represent various earthquake zones on a seismic map of the USA, and was subjected to code-compliant response spectrum analyses for all sample locations and for five different soil types at each location. Response spectrum analysis was performed using the ETABS software package. For all the cases investigated, the UBC was found to be significantly more conservative than the IBC. The UBC design response spectra have higher spectral accelerations, and as a result, the response spectrum analysis provided a much higher base shear and moment in the structural members as compared to the IBC. The conclusion is that ordinary office and residential buildings designed using UBC 1997 are considered to be overdesigned, and therefore they are quite safe even according to the IBC provisions.

Strong ground motion from a nearby fault has frequency content in the same range as the natural frequencies of tall buildings. This may have serious repercussions and is the topic of this dissertation. Buildings are designed per building code standards. But, are the code provisions adequate? Strong motion from large earthquakes has been recorded only in recent times in the near-source region. Have the current codes used this information to update tall structure design guidelines? Considerable damage has been observed in tall buildings from the Northridge, Kobe, Turkey, and Taiwan earthquakes. How will tall buildings designed per the latest code regulations perform if they were to be shaken by any of these earthquakes? This thesis attempts to answer these questions. Tall buildings by their nature are computationally intensive to analyze. They consist of thousands of degrees of freedom and when subjected to strong ground motion from a nearby source, exhibit inelastic response. Modeling this inelastic response requires an iterative approach that is computationally expensive. Furthermore, a large class of buildings, classified as irregular, exhibits complex behavior that can be studied only when the structures are modeled in their entirety. To this end, a three-dimensional analysis program, FRAME3D, has been developed incorporating two special beam-column elements -- the plastic hinge element and the elastofiber element that can model beams and columns in buildings accurately and efficiently, a beam-column joint element that can model inelastic joint deformation, and 4-noded elastic plane-stress elements to model floor slabs acting as diaphragms forcing the lateral force resisting frames in a building to act as one unit. The program is capable of performing time-history analyses of buildings in their entirety. Six 19-story irregular steel moment frame buildings (with buildings 2A and 3A being variants of buildings 2 and 3, respectively) have been designed per the latest code (Uniform Building Code, 1997). Two of these buildings have reentrant corners and the other two have torsional irregularity. Their strength and ductility are assessed by performing pushover analyses on them. To assess their performance under strong shaking, FRAME3D models of these buildings are subjected to near-source strong motion records from the Iran earthquake (M[subscript w] = 7.3, Tabas Station) of 1978, the Northridge earthquake (M[subscript w] = 6.7, Sylmar Station) of 1994 and the Kobe earthquake (M[subscript w] = 6.9, Takatori Station) of 1995. None of the buildings collapsed under these strong events in the computer analyses. However, when compared against the acceptable limits for various performance levels in FEMA 356 document, the damage in terms of plastic deformation at the ends of beams and columns and at joints would render the buildings inadequate in terms of life safety in quite a few cases and would even indicate possible collapse in a couple of cases. Thus, in these terms, the code falls short of achieving its life safety objective, and the near-source factors introduced in the code in 1997 in recognition of the special features of near-source ground motion seem to be inadequate. The ductility demand, in terms of plastic rotation at the ends of beams and columns and in joints, on these buildings during this class of earthquakes is up to 6% of a radian, which is far greater than a typical limiting plastic rotation of 3% associated with fracture and consequent failure of large wide-flanged steel sections during experiments. Thus, if strength degradation due to fractures, local buckling, etc., were to be included in the analysis, then the results would likely to be worse, as far as the ability of these buildings to withstand these earthquakes without collapse is concerned. Due to damage localization, the peak drifts observed in the structure far exceeded the inelastic drift limit in the code of 0.02 (in some cases up to 3 times). This points to serious non-structural damage to facades, interior dry wall, etc. Furthermore, large roof permanent offsets after the events indicate significant post-earthquake repair requiring considerable disruption and building closure. Column yielding was minimal thus validating the strong-column weak-beam criterion in the code. Redundancy factors used to assess the redundancy in the system need to take into account the case of torsionally sensitive structures where frames in both principal directions are simultaneously activated. Stress concentration was not observed at the reentrant corners in L-shaped buildings. Finally, the data catalogued in this work could be useful for future code development and tall structure design guidelines.

Suspension-type structures are of excellent seismic performance. It is expected to realize a better anti-seismic effect by integrating the elastic constraints that can absorb the seismic energy with the suspension-type structures. A structure consisting of vertical suspension and lateral elastic support was designed according to seismic damage characteristics for engineering structures, and some hypotheses were made to its stress conditions, which were properly simplified. Next, a basic mechanical model was established, followed by the structural dynamics analysis, so as to obtain the design value of elastic rigidity needed to ensure the minimum vibration amplitude. In the end, the seismic response characteristics and optimal aseismic design of the entire structure were acquired through an equivalent analogy. This study can be taken for reference in the new-type aseismic structural design based on the abovementioned model.

Earthquake codes have been revised and updated depending on the improvements in the representation of ground motions, soils and structures. These revisions have been more frequently seen in recent years. One of the key changes in earthquake codes has been performed on the design spectra. In this paper, the design spectra recommended by Turkish Earthquake Code and three other well known codes (Uniform Building Code, Eurocode 8, and International Building Code) are considered for comparison. The main purpose of this study is to investigate the differences caused by the use of different codes in the dynamic analysis and seismic verification of given types of buildings located at code defined different sites. The differences in expressions and some important points for elastic and inelastic spectra defined by the codes are briefly illustrated in tables and figures. Periods, base shears, lateral displacements and interstory drifts for the analyzed buildings located at code defined ground type are comparatively presented.

The seismic provisions of the 1997 Uniform Building Code (UBC) are examined from the perspective of achieving performance-based earthquake engineering of structural wall buildings. It is shown that although strain limits are present in the 1997 UBC, the drift ratio limits generally govern design. The conflict between assumed force reduction factors and actual ductility demand at the design limit state controlled by drift is also explored. Through the use of design examples and dynamic inelastic time history analysis it is shown that attempts to achieve performance-based engineering with a force-based approach such as that described in the 1997 UBC will inevitably not be possible. As an alternative, it is shown that a simple and more rational direct displacement-based approach would better achieve the objectives of performance-based earthquake engineering.

In this study, the framework of performance-based earthquake engineering is used to predict the seismic collapse risk of California's older (non-ductile) and modern (ductile) reinforced concrete frame structures. Collapse performance assessments are conducted for two sets of structures, those designed according to an out-dated building code, the 1967 Uniform Building Code (UBC), and those designed according to modern building code provisions, the 2003 International Building Code (IBC). Each set includes 2, 4, 8 and 12 story reinforced concrete frame buildings, designed as space and perimeter frame systems. These preliminary results provide measures of collapse safety of reinforced concrete frame structures and are used to evaluate the level of safety provided by modern building codes, and to quantify differences in safety between older and moden structures, answering questions such as "how safe are code-conforming reinforced concrete frame structures?" and "how much more likely are existing reinforced concrete frames to collapse in an earthquake?"

Seismic isolation is an effective design strategy to mitigate the seismic hazard wherein the structure and its contents are protected from the damaging effects of an earthquake. This paper presents the Hangar Project in Sabiha Gokcen Airport which is located in Istanbul, Turkey. Seismic isolation system where the isolation layer arranged at the top of the columns is selected. The seismic hazard analysis, superstructure design, isolator design and testing were based on the Uniform Building Code (1997) and met all requirements of the Turkish Earthquake Code (2007). The substructure which has the steel vertical trusses on facades and RC H shaped columns in the middle axis of the building was designed with an R factor limited to 2.0 in accordance with Turkish Earthquake Code. In order to verify the effectiveness of the isolation system, nonlinear static and dynamic analyses are performed. The analysis revealed that isolated building has lower base shear (approximately 1/4) against the non‐isolated structure.

Most seismic codes specify empirical formulas to obtain the fundamental period of buildings. The equations specified in present IS codes, are according to the available data on the time period of buildings measured from their recorded accelerograms. Shear-wall dominant reinforced concrete buildings, constructed, using codes specification are commonly built in different countries, facing a substantial seismic risk, in spite of their high resistance against ground motions. Current seismic code provisions including the Uniform Building Code (International Conference of Building Officials, Whittier, CA, 1997) and the Indian Seismic Code (Criteria for earthquake resistant design of buildings, fifth revision, 2002) are considered to evaluate the effect of time period on seismic behavior of building.In this study, time period obtained by code formulas are compared with those obtained by modal analysis in SAP2000. Also the top story displacement (as an adequate parameter of determination the seismic performance of building) correspond to the values of mentioned time period are estimated using uniform building code and software respectively. It is observed that current empirical equation for calculating the time period of RCC buildings is rather inaccurate. Also it is shown that the time period has very effective influence on seismic performance of building.

A computer program that automatically designs and details an optimum-cost, one-way solid slab, reinforced concrete building is presented. Skip-loading, lateral loads (including consideration of torsional stiffness of the building frame), and live-load reductions are all considered. The program is completely automatic, and no preliminary member sizing or dead-load estimates are required. Special attention is paid to simplicity and minimization of input data. Output is selective and in a readily understood format. Problem formulation is presented in relation to the basic considerations of synthesis i.e., requirements, technology, and criterion. Attention is paid to important aspects of decentralization of, and iteration through, various parts of the program such as gravity frame, lateral-load frame, detailing, costing, etc. All analysis and design is in accordance with the Uniform Building Code and the ACI Building Code. Detailing is in accordance with the Manual of Standard Practice for Detailing Reinforced Concrete Structures. The inherent advantages include moderate cost, practical flexibility, improved design, complete details, reduced errors, and readily checked output.

Piping system response induced by thermal-hydraulic transient: A review of recent analysis methods and experiments

<div class="htmlview paragraph">This paper describes a software system called FREPS (Forced REsponse Prediction System) that integrates structural dynamic, steady and unsteady aerodynamic analyses to efficiently predict the forced response dynamic stresses in axial flow turbomachinery blades due to aerodynamic and mechanical excitations. A flutter analysis capability is also incorporated into the system. The FREPS system performs aeroelastic analysis by modeling the motion of the blade in terms of its normal modes. The structural dynamic analysis is performed by a finite element code such as MSC/NASTRAN. The steady aerodynamic analysis is based on nonlinear potential theory and the unsteady aerodynamic analysis is based on a linearization of the non-uniform potential mean flow. The program description and presentation of the capabilities are reported herein. The effectiveness of the FREPS package is demonstrated on the High Pressure Oxygen Turbopump turbine of the Space Shuttle Main Engine. Both flutter and forced response analyses are performed and typical results are illustrated.</div>

Results are summarized of analytical and experimental studies conducted on physical and analytical models of a 15‐story reinforced concrete framecoupled wall structure which was identified as typical of medium‐rise office construction in the western United States and designed in accordance with provisions of the 1973 version of the Uniform Building Code. The primary objectives are to evaluate whether or not: (1) The state‐of‐the‐practice leads to a design that accords with the criteria envisioned by seismic design documents; and (2) the state‐of‐the‐art of analysis of reinforced concrete frame‐wall structures provides reliable analytical assessments of the seismic behavior of structures. The results indicate that designs based on the Uniform Building Code provisions may in several important ways be inadequate and that analytical techniques currently used to evaluate such designs similarly suffer from shortcomings.

PurposeThe purpose of this paper is to briefly summarize and review the theories and methods of complex structures’ dynamic reliability. Complex structures are usually assembled from multiple components and subjected to time-varying loads of aerodynamic, structural, thermal and other physical fields; its reliability analysis is of great significance to ensure the safe operation of large-scale equipment such as aviation and machinery.Design/methodology/approachIn this paper for the single-objective dynamic reliability analysis of complex structures, the calculation can be categorized into Monte Carlo (MC), outcrossing rate, envelope functions and extreme value methods. The series-parallel and expansion methods, multi-extremum surrogate models and decomposed-coordinated surrogate models are summarized for the multiobjective dynamic reliability analysis of complex structures.FindingsThe numerical complex compound function and turbine blisk are used as examples to illustrate the performance of single-objective and multiobjective dynamic reliability analysis methods. Then the future development direction of dynamic reliability analysis of complex structures is prospected.Originality/valueThe paper provides a useful reference for further theoretical research and engineering application.

The dynamic analysis of structures with uncertain parameters presents an attractive field of structural health monitoring in many cases of technological interest. In the dynamic analysis of hydraulic structures, such as existing dams, modeling assumptions, resulting inaccuracies, and changes in seismic loading are typically the main sources of uncertainties. Many hydraulic structures of concrete can be subjected to seismic loads. However, it is necessary to take haphazard or random phenomena as crucial considerations when assessing the security of these structures or planning new ones. This paper shows computational analysis for the characterization of the behavior of a concrete gravity dam under seismic loads, which are considered sources of uncertainties. The multi-linear regression methodology was performed and applied to evaluate the dynamic response of the considered structure. Numerous nonlinear time history analyses based on Latin Hypercube Sampling were realized to investigate the effect of uncertain parameters on the dynamic response. These analyses were applied to two types of seismic actions, the near and far earthquakes, which act on a concrete gravity dam. Then, a sensitivity analysis was used for each random variable to quantify its risk and clarify its influence on the dynamic behavior of the dam. Results divulge that for near-fault cases, major variables affecting the global sensitivity across all limit states are the Young’s modulus of soil and concrete. On the other hand, for far-fault cases, the important variables influencing the global sensitivity index include the compressive strength of concrete, Young’s modulus of soil, and cohesion.

PurposeThe safety assessment of engineering structures under repeated variable dynamic loads such as seismic and wind loads can be considered as a dynamic shakedown problem. This paper aims to extend the stress compensation method (SCM) to perform lower bound dynamic shakedown analysis of engineering structures and a double-closed-loop iterative algorithm is proposed to solve the shakedown load.Design/methodology/approachThe construction of the dynamic load vertexes is carried out to represent the loading domain of a structure under both dynamic and quasi-static load. The SCM is extended to perform lower bound dynamic shakedown analysis of engineering structures, which constructs the self-equilibrium stress field by a series of direct iteration computations. The self-equilibrium stress field is not only related to the amplitude of the repeated variable load but also related to its frequency. A novel double-closed-loop iterative algorithm is presented to calculate the dynamic shakedown load multiplier. The inner-loop iteration is to construct the self-equilibrated residual stress field based on the certain shakedown load multiplier. The outer-loop iteration is to update the dynamic shakedown load multiplier. With different combinations of dynamic load vertexes, a dynamic shakedown load domain could be obtained.FindingsThree-dimensional examples are presented to verify the applicability and accuracy of the SCM in dynamic shakedown analysis. The example of cantilever beam under harmonic dynamic load with different frequency shows the validity of the dynamic load vertex construction method. The shakedown domain of the elbow structure varies with the frequency under the dynamic approach. When the frequency is around the resonance frequency of the structure, the area of shakedown domain would be significantly reduced.Research limitations/implicationsIn this study, the dynamical response of structure is treated as perfect elastoplastic. The current analysis does not account for effects such as large deformation, stochastic external load and nonlinear vibration conditions which will inevitably be encountered and affect the load capacity.Originality/valueThis study provides a direct method for the dynamical shakedown analysis of engineering structures under repeated variable dynamic load.