Building Vibrations
1.Introduction
Development of lighter and more sophisticated building and construction materials is a major breakthrough in the construction industry. However, this has one major setback; can these lighter and refined designs and material hold vibrations in the buildings? Definitely not, it therefore calls for development of critical views about building vibrations and ways to avoid such problems related to these vibrations.
Building vibrations can be categorised into: those arising from internal sources and those from external sources. Most internal vibrations are from machines for example cranes and from human activities for example walking. Vibrations from external sources are as a result of road traffic, construction activities for example blasting, earthquakes and sonic booms. Vibrations are a major cause of discomfort to the occupants of the buildings and even in worse scenarios can lead to lose of property and life when the buildings fail to host them.
To analyse vibrations in multistorey buildings, is generally assumed that floors exhibit no deformation in their planes. This assumption may not hold however in cases where the buildings has long and narrow plans for the floor. A good example of such plans is for example in schools, hospitals and houses. When buildings are observed during an earthquake, it is clear as the study will show that the deformation of the floors originate from the ground. (Arya, 2014) has in detail discussed the relation between floor shapes and vibration.
Conditions between buildings and the grounds are more complex in buildings with wide floor plans. The conditions are not usually uniform, the basements may be provided partially and the time lag for the earthquakes could occur at the foundations. The conditions lead then to lateral or torsional vibrations in the building (Karthika, & Gayathri, 2018).
This paper therefore seeks to understand the relation between the ground and the floors. Two dimensionally distributed multi-mass designs are considered, the vibrations between the building and the ground studied and the effects of the underground conditions on floor slab deformations elaborated. Numerical computation approach is adopted to present the tests and final results of evaluation for the different models of the building-soil system.
Building Soil System
The masses of the structure experienced at every floor are replaced in the direction of short sides “Y” by discrete masses as in figure 1 below. For the horizontal members, their lateral stiffness is calculated from the bending-shear stiffness of the slabs. The stiffness equivalent of the columns could be given by the wall’s and frame’s shear stiffness in scenarios where the building is relatively low. The soil’s rectangular prism is presumed for the level in which the building-soil interaction is active (Avilés, 1998, pp. 1523-1540).
CHAPTER TWO
2.0 Materials and Methods
2.1 Methods
Computational Model
1.Calculation of the Eigen values of the models:
2.Calculation of the participating factors for input of uniform force
3.Computation of the dynamic response with modal analysis by use of the Eigen values.
This is with regardless to natural periods of the system interaction of participation factors and the constant shear force of q=1 response.
Types of Building Vibrations
1.Adoption of q=1 in the procedure ensures that it is this coefficient that is adopted as a response spectrum of the white noise in place of the use of an individual measured earthquake wave.
CHAPTER THREE
3.0 RESULTS AND DISCUSSION
3.1 Computational Model
The prototype building to be examined here is a three-storey building about three and half meters with a reinforced concrete structure and without a basement.
Figure-3.1 the type of ground and buildings for analysis
The types of basements found under the prototype structure are set in three different models as illustrated in the figure 4.0 above. There are two cases in which the two bays of the basements are located at the end and center of the plan and the other one in which the basement is located on the entire plan (El-Khoury, & Adeli, 2013, pp. 353-360). The underground condition together with the building is assumed to consist of two layers of soil (Dutta, & Roy, 2002, pp. 1579-1594 ). The top layer being of V-100m/sec and pressure of 1.4 whereas the lower layer is of Vs=200 m/sec and pressure of 1.6. This condition is known as the prototype condition and will be referred to as that in this paper. The soils spring constant is k and is given in multi-mass systems in accordance to the equation where:
k=A.P.Vs2/d
A=Sectional Area as shown in figure 4.1 above
Vs=Velocity of S wave
D=distance between the adjacent masses
For a fruitful discussion on the effects of irregular underground parameters on the building soil system vibration, the features of the ground are investigated in relation to the prototype. Several assumptions are made in this analysis. It is assumed that the building’s damping coefficient is equal to the soil’s damping coefficient. The masses vertical displacement is also not raised.
3.2 Results
The natural periods together with the maximum values of the “Bu” obtained from the first order to the seventh order of the entire models shown in the table 1 below in which “Bu” is a product of the participation factor “B” and the Eigen vector “u”.
Table 1 Natural Period “t seconds” and “Bu max” (Taranath, 2016)
The Maximum Response Shear Force of the column, Q (ton), and The maximum response displacement of mass point, D (cm). This figure shows the maximum displacement of the different masses and also the maximum shear forces of the different columns of all models. The behaviours in the X and Y coordinates and or directions are independent for the structures analysed in the paper (De Stefano, & Pintucchi, 2008, pp. 285-308).
Relation between the Ground and the Floors
3.3 Discussion
a) Comparing the interaction system and independent system results
When we compare the natural periods of the Building Soil system “BO” with that of the prototype building “SO”, we conclude that both the first and second periods of the prototype building are longer by about thirty percent. Also comparing the natural periods of the “BO” and “SO” with the underground prototype “S”, we discover that the fourth and seventh period of the “SO” are close to the first period of “S”, this is in spite of the independence of the two (Trahair, 2017). It ultimately means that the fourth and seventh periods of “BO” are the drawn closer to the first period of “S”.
Where the ground consists of soft soil, the 4th and the 7th periods of the soil building systems are also very close to the 1st and 3rd periods in the soft ground. The 4th to 4th periods of the “SO” are shorter compared to the 1st and 3rd periods in the soft ground.
In the figure below, the maximum value of “Bu” in the order of the model “BO”“SO” and “SI”. The third mode shows the transverse in the “X” direction. The torsional motions are represented by the 2nd and 6th modes and the “Bu max” for the modes equalling to zero for the uniform input waves. Other modes aside from the once discussed above are the transverse modes that take place in direction “Y’.
b) Vibration Features of the Buildings with Numerous Basements
The figure below will illustrate the shear force of the columns in top stories for all the models. It is clear from this illustration that the shear force in the model with the same basement plan decreases to nearly two-thirds at the column ends and also to one-third at the center column; this is in comparison with that of buildings without the basement “BO” (L. N. Hao & Zhang, 2012, pp. 351-353).
Figure 4.3.2 Effects of basement location on maximum shear force response of the columns.
The model with two bays of basement at the end of the building “B2e”, the torsional vibrations and the shear force of the columns located above the basement’s inner end is maximum for every storey. The shear force distribution is not uniform in the “B2e’ buildings. This makes the shear force at the center of columns to be about double that at the building’s end (1st storey), and nearly three times in the 3rd storey. It is therefore clear that the shear force for the columns of the structure containing a partial basement is more than that of buildings without basement “BO” and “B8” (Ellis, & Jeary, 1980).
c) The Vibration Features of the Structures on Inclined Layers or Dislocation
On the prototype ground conditions, the response values for the buildings without basements “BO” are not any different from the values of the inclined buildings “S2”. Torsional vibrations in these buildings appear slightly despite their inclination. The response value for all modes “S3” on hard soils inclined buildings are torsional giving a non-symmetrical value. Also the maximum shear force is greater on soft soil than for buildings without basements (Lu, 2018, pp. 26-38). The distribution of stiffness is uniform hence giving the response values similar on every floor (Di Sarno, 2011, pp. 1673-1702).
Figure 3.3.3 Effects of the ground conditions, with the maximum shear force response showing for the 1st story.
4.0 Conclusion
In conclusion, the paper has clearly and illustrated the different scenarios of analyses of vibrations in multistorey buildings. The analysis evaluated storey buildings with basements, with partial basements and without basements in different soil conditions, hard soil and soft soil. The findings of the report can hence be summarized as follows:
- The high order mode of in the building-soil system should not be neglected when the period of the system is near the ground’s prominent period.
- In spite of its shape, buildings with basements will general effect decrease in shear force. Shear force in the columns is not distributed uniformly in buildings with partial basements and hence a maximum value is taken for the columns above the basement on each of the floors.
- For the inclined buildings, the vibration modes are torsional and the response values are unevenly distributed for each floor.
References:
Arya, A.S., Boen, T. and Ishiyama, Y., 2014. Guidelines for earthquake resistant non-engineered construction. UNESCO.
Avilés, J. and Pérez?Rocha, L.E., 1998. Effects of foundation embedment during building–soil interaction. Earthquake engineering & structural dynamics, 27(12), pp.1523-1540.
De Stefano, M. and Pintucchi, B., 2008. A review of research on seismic behaviour of irregular building structures since 2002. Bulletin of Earthquake Engineering, 6(2), pp.285-308.
Di Sarno, L., Chioccarelli, E. and Cosenza, E., 2011. Seismic response analysis of an irregular base isolated building. Bulletin of Earthquake Engineering, 9(5), pp.1673-1702.
Dutta, S.C. and Roy, R., 2002. A critical review on idealization and modeling for interaction among soil–foundation–structure system. Computers & structures, 80(20-21), pp.1579-1594.
Ellis, B.R., Jeary, A.P. 1980. “A review of recent work on the dynamic behaviour of tall buildings at various amplitudes”, Proceedings of the 7th World Conference on Earthquake Engineering, Istanbul.
El-Khoury, O. and Adeli, H., 2013. Recent advances on vibration control of structures under dynamic loading. Archives of Computational Methods in Engineering, 20(4), pp.353-360.
Karthika, A.P. and Gayathri, V., 2018. LITERATURE REVIEW ON EFFECT OF SOIL STRUCTURE INTERACTION ON DYNAMIC BEHAVIOUR OF BUILDINGS.
N. Hao and Y. S. Zhang, 2012 “Natural Vibration Period Analysis of Multi-Storey Reinforced Concrete Frame Structure”, Applied Mechanics and Materials, Vols. 226-228, pp. 351-354,
Lu, Y., Hajirasouliha, I. and Marshall, A.M., 2018. An improved replacement oscillator approach for soil-structure interaction analysis considering soft soils. Engineering Structures, 167, pp.26-38.
Selvadurai, A.P., 2013. Elastic analysis of soil-foundation interaction. Elsevier.
Trahair, N.S., 2017. Flexural-torsional buckling of structures. Routledge.
Taranath, B.S., 2016. Structural analysis and design of tall buildings: Steel and composite construction. CRC press.
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