Effect of increasing floors number on the natural frequency compared with further masses added to steel building models (original) (raw)

A comparison study between base and forced excitation at the top floor for multi- story structure

AIP Conference Proceedings, 2020

Multistory buildings may subjected to many types of vibration excitation which affect their structures and properties. The period of vibration or natural frequency is an important parameter in building design as it governs the effect of wind or earthquake. A four story model has been constructed which simulates a multi-story shear structure (like a building). This model is connected to a shaker and a system of accelerometers to measure the dynamic response of that model and to make a comparison for two types of vibration excitations: base excitation and forced excitation at the top floor in order to investigate the dynamic response for the two cases. The results for the base excitation showed that the fundamental natural frequency was less than that for forced excitation at the top floor, and new natural frequencies have appeared in the spectrum. The level of vibration for the natural frequencies is higher compared to forced excitation. The overall level of vibration is higher too, and looks like the system became a self excitation. The mode shapes for the two cases were determined also, and compared to show that the more critical case is the base excitation.

The Influence of Lintel Beams and Floor Slabs on Natural Frequencies of the Tall Buildings Core - Numerical and Experimental Studies

Periodica Polytechnica Civil Engineering, 2015

This paper presents the analysis of influence of lintel beams and floor slabs on natural frequencies of tall buildings, braced by core walls. For that purpose a numerical procedure based on the Vlasov theory of thin-walled beams and transfer matrix method has been developed. In order to check the accuracy of the proposed method the calculated results are compared with those obtained by FEM and experiment. Based on these results the effects of the lintel beams and floor slabs on the natural frequencies of the tall buildings core are discussed Keywords thin-walled beam • core of tall buildings • transfer matrix method • coupled vibration analysis • experimental test

A continuous–discrete approach for evaluation of natural frequencies and mode shapes of high-rise buildings

International Journal of Advanced Structural Engineering

In this paper, a continuous-discrete approach based on the concept of lumped mass and equivalent continuous approach is proposed for free vibration analysis of combined system of framed tube, shear core and outrigger-belt truss in high-rise buildings. This system is treated as a continuous system (i.e., discrete beams and columns are replaced with equivalent continuous membranes) and a discrete system (or lumped mass system) at different stages of dynamic analysis. The structure is discretized at each floor of the building as a series of lumped masses placed at the center of shear core. Each mass has two transitional degrees of freedom (lateral and axial(and one rotational. The effect of shear core and outrigger-belt truss on framed tube system is modeled as a rotational spring placed at the location of outrigger-belt truss system along structure's height. By solving the resulting eigen problem, natural frequencies and mode-shapes are obtained. Numerical examples are presented to show acceptable accuracy of the procedure in estimating the fundamental frequencies and corresponding mode shapes of the combined system as compared to finite element analysis of the complete structure. The simplified proposed method is much faster and should be more suitable for rapid interactive design. Keywords High-rise building Á Combined system Á Continuous-discrete approach Á Equivalent continuous membranes Á Lumped mass system Á Eigenvalue problem Á Free vibration analysis & Reza Rahgozar

Vibration Analysis of a Residential Building

MATEC Web of Conferences, 2015

The aim of this paper is to present the results of a study regarding vibration problems in a 17 storey residential building during pile driving in its vicinity. The structural design of the building was checked according to the Brazilian standards NBR6118 and NBR6123, and using commercial finite element software. An experimental analysis was also carried out using low frequency piezo-accelerometers attached to the building structure. Structure vibrations were recorded under ambient conditions. Four monitoring tests were performed on different days. The objective of the first monitoring test was an experimental modal analysis. To obtain de modal parameters, data was processed in the commercial software ARTEMIS employing two methods: the Stochastic Subspace Identification and the Frequency Domain Decomposition. Human comfort was investigated considering the International Standard ISO 2631. The Portuguese standard, NP2074, was also used as a reference, since it aims to limit the adverse effects of vibrations in structures caused by pile driving in the vicinity of the structure. The carried out experimental tests have shown that, according to ISO2301, the measure vibration levels are above the acceptance limits. However, velocity peaks are below the limits established by NP2074. It was concluded that, although the structure has adequate capacity to resist internal forces according to normative criteria, it has low horizontal stiffness, which could be verified by observing the vibration frequencies and mode shapes obtained with the finite element models, and its similarity with the experimental results. Thus, the analyses indicate the occurrence of discomfort by the residents.

A simple analytic method for computing the natural frequencies and mode shapes of tall buildings

Applied Mathematical Modelling, 2012

A simple, yet accurate analytical approach based on energy principles is developed for quick computation of natural frequencies and mode shapes of multistory buildings constructed using framed tube, shear core and double belt trusses systems. The approach here is based on development of a continuum model that would be equivalent, in major motions, to the actual multistory building. Models studied here are cantilever beams with concentrated moments placed at belt truss locations. Governing equation and boundary conditions of the equivalent beam and moment system were derived using the energy method and Hamilton's principle. Separation of variables technique is then applied to model's partial differential equation to obtain the required eigensystem. Robustness and correctness of the proposed method are demonstrated through several numerical examples. Here, 40, 55 and 70-storey tall buildings with combined system of framed tube, shear core and double belt trusses, in which the results obtained from the proposed method, are compared with those obtained from three-dimensional analyses using SAP2000 software. Comparative analyses reveal that the proposed method is simple and efficient; and it provides reasonably accurate results quickly, a feature that is vital during the early stages of building design.

A methodology of determining the natural frequencies of low-rise buildings

2020

We live in a world that is constantly faced with natural disasters such as earthquakes, floods, tsunamis and other geological processes. These catastrophes besides causing material damage, are sometimes fatal to people's lives. Therefore, different types of measurements are performed, various studies are made, with the one same goal - to predict disasters, to mitigate and their consequences, and to find a way to protect ourselves from them. One of the most important parameters for design of earthquake resistant structures is the natural frequency of the structure. The main purpose of our research is to find the natural frequencies, i.e. the fundamental periods of objects. Determining natural frequencies, we can find dynamic forces acting on the structure so we can design the constructive elements to resist to these forces. We will do this in order to determine at certain excitation (such as explosions or earthquakes) which frequencies of excitations are dangerous to the construc...

Experimental vibration analysis for a 3D scaled model of a three-floor steel structure

Latin American Journal of Solids and Structures, 2012

In this paper we present an experimental study of a three dimensional physical model of a three-floor structure subjected to forced vibrations by imposing displacements in its support. The aim of this work is to analyze the behavior of the building when a dynamic vibration absorber (DVA) is acting. An analytic simplified analysis and a numerical study are developed to obtain the natural frequencies of the structure. Experiments are carried out in a vibrating table. The frequency range to be experimentally analyzed is determined by the first natural frequency of the structure for which the DVA damping effects are verified. The equipment capabilities, i.e. the frequencies, amplitudes and admissible load, limit the analyses. Nevertheless, satisfactory results are obtained for the study of the first mode of vibration. The effect of different amplitudes of the imposed support motion is also analyzed. In addition, the damping effect of the DVA device is evaluated upon varying its mass and its location in the structure. The characteristic curves in the frequency domain are obtained computing the Fast Fourier Transformation (FFT) of the acceleration history registered with piezoelectric accelerometers at different checkpoints for the cases analyzed.

Identification of Natural Frequency of Low Rise Building on Soft Ground Profile using Ambient Vibration Method

Journal of Physics: Conference Series

Natural frequency is the rate at which a body to vibrate or oscillate. Application of ambient vibration (AV) excitation is widely used nowadays as the input motion for building predominant frequency, fo, and ground fundamental frequency, Fo, prediction due to simple, fast, non-destructive, simple handling operation and reliable result. However, it must be emphasized and caution to isolate these frequencies (fo and Fo) from spurious frequencies of site-structure effects especially to low rise building on soft ground deposit. In this study, identification of fo and Fo by using AV measurements were performed on ground and 4-storey primary school reinforced concrete (RC) building at Sekolah Kebangsaan (SK) Sg. Tongkang, Rengit, Johor using 1 Hz of tri-axial seismometer sensor. Overlapping spectra between Fourier Amplitude Spectra (FAS) from and Horizontal to Vertical Spectra Ratio (HVSR) were used to distinguish respective frequencies of building and ground natural frequencies. Three dominant frequencies were identified from the FAS curves at 1.91 Hz, 1.98 Hz and 2.79 Hz in longitudinal (East West-EW), transverse (North South-NS) and vertical (UD) directions. It is expected the building has deformed in translational mode based on the first peak frequency by respective NS and EW components of FAS spectrum. Vertical frequency identified from the horizontal spectrums, might induces to the potential of rocking effect experienced by the school building. Meanwhile, single peak HVSR spectrum at low ground fundamental frequency concentrated at 0.93 Hz indicates to the existence deep contrast of soft deposit. Strong interaction between ground and building at similar frequency (0.93 Hz) observed from the FAS curves on the highest floor has shown the building to behave as a dependent unit against ground response as one rigid mass.

Estimating fundamental frequencies of tall buildings

2007

Empirical estimates of the fundamental frequency of tall buildings vary inversely with their height, a dependency not exhibited by the various familiar models of beam behavior. This paper examines and explains this apparent discrepancy by analyzing the consequences of using two models to estimate such natural frequencies: A two-beam model that couples the bending of a classical cantilever to that of a shear beam by imposing a displacement constraint; and a Timoshenko beam in which the Euler-Bernoulli beam model is extended by adding a shear-displacement term to the classical bending deflection. A comparison of the two beam models suggests that the Timoshenko model is appropriate for describing the behavior of shear-wall buildings, while the coupled two-beam model is appropriate for shear-wall-frame ͑e.g., tube-and-core͒ buildings, and that the coupled-beam model comes much closer to replicating the parametric dependence of building frequency on height.