Analysis of Vibro-Isolated Building Excited by the Technical Seismicity of Traffic Effects
Abstract
By location of a building in the vicinity of underground tube structure the effect of train operation excites the groundborne vibration. The solution of vibration transfer from the subsoil environment to the building structure is demonstrated using the example of a multistorey reinforced concrete building. The application of an elastic layer at foundation base level is used in order to eliminate excessive vibrations of these technical seismicity effects. The building is loaded by the non-stationary history of vibrations in accelerations. The measured time histories in acceleration were selected and then the typical history was used as an input for a dynamic analysis of the structure. Two 3-D numerical models of the building take into account the individual storeys, firstly modelled with vibro-isolation of building and secondly without this elastic part. The elastic layer was considered as the elastic subsoil of the Winkler-Pasternak model below the whole area of the upper part of the dual foundation plate and as the elastic support for columns and walls above the piles on the upper foundation plate level. The response prediction for vibro-isolated and non-isolated structure is compared and discussed. In the conclusion, the methodology of vibro-base isolation is evaluated.
Keywords
Download Options
Introduction
A calculation model (Fig. 1) of an entire structure was designed for an analysis of the structure, including the underground storeys and the vibro-isolation rubber layer. The computational model is placed on a multiple-layer subsoil structure, on the level of the floor of the 3rd underground storey.
The character of the vibrations generated by transport depends particularly on vehicle weight, driving speed, how the vehicles move and in what way and direction [1, 3, 4]. Another parameter is the “evenness“ of the vehicle trajectory, in terms of whether it concerns the quality of the pavement surface or the horizontal and vertical railway alignment, the way in which the rails are fastened, the composition of the pavement courses, etc. The magnitude of the vibrations is influenced not only by the vibration parameters at the source but also by the composition of the environment on the way from the source to the threatened building structure, in particular the composition of the geological environment and its mechanical properties, i.e. stiffness, wave propagation velocity, distance-dependent damping, etc. Last but not least, the magnitude of the vibration may be amplified or damped by the building structure itself and by its foundations, in particular the frequency tuning of the threatened structure [2].
As far as safety is concerned, this level of vibrations generated by standard traffic is of no significance for current buildings, with the exception of historical or decrepit structures. Major cracks may originate due to the passage of very heavy vehicles or the operation of construction plant (such as vibration rollers) on new construction sites in the proximity of existing buildings.
Before transport-generated vibrations begin to cause damage to threatened structures a more serious problem may arise, i.e., the impact of vibrations on the people dwelling within these structures. Vibrations of this type usually exceed the safety limits specified by hygienic standards well before cracks and fissures originate in the structure.
Conclusion
In this paper we set out to assess the effect of building vibro-isolation on the transfer of vibrations due to traffic from the subsoil environment. The maximum measured intensities of the vibrations at the construction site were used as a nonperiodic load of the building by technical seismicity caused by traffic effects. Optimum distribution of the vibro-isolation in the foundation structure was designed on the basis of a calculation of the static and dynamic response of the building. The calculation was also used to predict the floor vibration on individual storeys, and the time courses of the vibration at selected points were determined.
The paper has compared the calculated responses for an isolated building and a non-isolated building. The computed vibration histories of the examined building reveal that the vibrations of the isolated structure are decreased in all the aboveground storeys. The effectiveness of the vibro-isolation is determined by the frequency tuning of the isolated structure.
The calculated vibration forecast was compared with the final measurement results after the building was constructed (Fig. 4). This measurement indicated good agreement between prognosis and final observations. The frequency spectra show clearly that the main reason for the reduction in vibration is the shift of the excitation frequencies in the soil medium to the lower interval of frequencies of the vibration response due to the vibro-base isolation of the whole building.