Effect of Vertical Ground Acceleration on Buildings

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Effect of Vertical Ground Acceleration on Buildings

1R. S. Patil, ²H.N. Rajakumara

1,2Department of Civil Engineering, RR Institute of Technology,Bengaluru-560090

Abstract— One of the main reasons behind those extensive damages to steel and RCC structures, designed by recent codes, has been claimed to be the effect of vertical ground acceleration on the behaviour of columns and other main load bearing elements, which has not been considered adequately in the corresponding design codes. In this project by considering a simple SDOF system, its seismic response has been identified in terms of response spectra plot.

Number of previously occurred earthquake data is collected and using MATLAB analysis, each earthquake is individually applied to the model for analysis using linear analysis method and response of building is plotted in terms of spectral displacement vs. angular frequency for all three directions. From each response plot peak value is collected and response spectra plot is drawn showing the response in all three directions. Finally conclusion is given for the effect of vertical ground acceleration on response of building comparing with the guidelines provided on available seismic design code of India and other countries.

Keywords—vertical ground acceleration; earthquake;

linear analysis method; response; peak value; MATLAB;

seismic response.

I. INTRODUCTION

India is one of the most disaster prone countries, vulnerable to almost all natural and manmade disasters.

About 85% area is vulnerable to one or multiple disasters and about 57% area is in high seismic zone including the capital of the country. Earthquake resisting design of building includes consideration of the effect of vertical and horizontal components of earthquake during design so as to make the structure seismic resistant. However very little consideration is given for the effect of vertical ground acceleration(VGA) in the codes like is 1893 (part 1): 2002 and is 13920: 1993. codes are more concentrated on the effect of horizontal component of earthquake on building and enough guidelines are provided for the design of buildings considering horizontal component.

Generally the peak horizontal acceleration will be in the range of 0.3g to 0.6g; where “g” is the acceleration due to gravity and in most of the cases the vertical component of a ground shaking will be in the range of 1/3^rd to 2/3^rd of the peak horizontal component, for the case of upward vertical excitation, the force will be acted against the gravity and there by a net reduction in the downward

action and for the downward excitation as the building is already designed for “g”, this will not demand further strengthening in either cases the building will be on safer side.

However in some special cases like, if considerable mass concentrated at the top of the structure, like huge water tanks, heavy decks in bridges etc. can produce unexpected rate dependent responses, which will cause damages to the lower part of the structure. In such cases vertical component should not be neglected during design of important buildings and other structures which are valuable to community in terms of cost and life safety. But in 1893 (part 1):2002 which provides the guidelines for seismic designing don’t have enough provisions for vertical component. So focus of the study is to develop a building model which is to be subjected to vertical ground acceleration as well as horizontal ground acceleration and plot the response for the same. From each response plot peak acceleration value will be collected and finally response spectra will be plotted. This spectra is very essential for designing the buildings.

II. OBJECTIVES

Several analytical studies on the effect of horizontal acceleration on buildings have been performed by various researchers using different models and assumptions. Also, many seismic design codes for designing of earthquake resistant buildings were developed throughout the world.

But these codes do not have sufficient consideration to the effect of vertical ground acceleration. Some codes have proposed relation between horizontal and vertical acceleration [for example, vertical acceleration is considered as 2/3^rd of the horizontal acceleration in IS 1893 (Part 1) : 2002]. It has been experienced in some cases that structure has failed even after designing the building considering all the criteria given in codes.

Negligence of vertical ground acceleration may be one of the reason.

The objectives of this study is to evaluate the effect of vertical ground acceleration on seismic performance of building using a realistic model in MATLAB, and propose protective methods to improve their seismic performance. The performance and its improvement will

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be evaluated through the response spectra plot in all three major directions.

The main objectives of the present study are:

 Development of the equation of motion for linear elastic SDOF system.

 Development of accurate and efficient analytical procedures to obtain the seismic response of the system.

 Perform MATLAB analysis to examine the effect of individual earthquake on building.

 Development of response spectra for all three direction components.

III. STRONG GROUND MOTION

A.CONCEPT OF GROUND MOTION

Earthquakes are the result of slippage along a fault plane often well below the surface of the earth. The presence of a fault indicates the possibility of an earthquake, though determination of its likelihood and size is still a very uncertain science.

Slippage along a fault line deep in the earth's surface may eventually result in "surface faulting," the crack or split on the earth's surface that provides the layman's vision of earthquakes. Surface faulting may result in large earth movements - perhaps several yards - and a building located across a surface fault is almost certain to suffer very severe damage however well it is designed.

However, the probability of a building location straddling a line of surface rupture is relatively low compared to the probability of a building location that will be affected by ground motion caused by fault slippage. The epicentre is the point on the earth's surface directly above where the faulting and energy release first begins. Since the faulting plane is not necessarily exactly vertical, and since the fault may rupture along a considerable distance, shaking at the epicentre may not be the most intense, although it will almost certainly be among the more heavily shaken areas in a given earthquake.

Ground motion is the movement of the earth's surface from earthquakes or explosions. Ground motion is produced by waves that are generated by sudden slip on a fault or sudden pressure at the explosive source and travel through the earth and along its surface.

Fig.1 A plot of recorded ground motion data The ground motion that is transmitted through the base of a building, then, has a random form, but sometimes an emphatic direction. The motion originates in four clearly defined types of waves created by a fault rupture. These are the primary, or P wave, which is the fastest, traveling at about 8km/s, or 18,000 mph, and arrives first. It has the form of a sound wave that, as it spreads out, alternately pushes and pulls at the ground material. The second type of wave is the secondary or S wave; this shears the rock sideways at right angles to the direction of travel. The third type is a surface wave called the Love wave, which is similar to a secondary (S) wave with no vertical displacement; it moves the ground from side to side horizontally parallel to the earth's surface, at right angles to the direction of propagation, and produces horizontal shaking. The fourth type of wave, also a surface wave, is known as the Rayleigh wave; in this the disturbed material moves both vertically and horizontally in a vertical plane pointing in the direction in which the waves are traveling.

Of the two surface waves, Love waves generally travel faster than Rayleigh.

The nature of the ground motion that affects the building can be summarized in a conceptual way as follows. The waves that create motion emanate from the line of fault rupture, and so approach the building from a given direction. The nature of the waves and their interactions is such that actual movement at the ground will be random:

predominantly horizontal, often with some directional emphasis, and sometimes with a considerable vertical component. The actual horizontal ground displacement is small, generally measured in fractions of an inch, except in the immediate area of the fault rupture where displacements of several feet may occur.

Determination of peak value of the ground motion is very important for the seismic design of the building so as to give the maximum resistance to the building against possible ground acceleration during an earthquake.

Although, many earthquake design codes are available throughout the world, all these codes have given the primary consideration to the horizontal acceleration of ground. The reason being, generally horizontal acceleration has more effect on structure compared to the vertical acceleration. But this was just an assumption made by the researchers based on the analysis of previously occurred earthquakes and their effect on structure. But there is no clear logic behind that, that the vertical acceleration will always be lower than the

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horizontal acceleration. So in this chapter the effect of vertical ground acceleration on building and comparison of the effect of horizontal and vertical acceleration in terms of response spectra plot is discussed. From the response spectra plot it is very easy and understandable to conclude weather the statement given in all the codes for vertical acceleration applicable for all the cases or any further improvement need to be done in clause given for vertical acceleration.

B. PEAK GROUND ACCELERATION

Peak ground acceleration (PGA) is a measure of earthquake acceleration on the ground and an important input parameter for earthquake engineering, also known as the Design Basis Earthquake Ground Motion (DBEGM).It is the maximum amplitude of the ground acceleration time-history. In terms of structural response, it corresponds to the peak value of the absolute acceleration of a single degree of freedom (SDOF) system with infinite stiffness, that is, with a natural period of vibration equal to zero. This parameter does not necessarily provide a complete representation of the severity of the earthquake, in terms of its potential to induce structural damage. Other parameters such as the Effective Peak Ground Acceleration (EPGA) and the Effective Peak Ground Velocity (EPGV) have been proposed as alternatives to quantify the severity of the ground motion. Several definitions and different physical interpretations have been proposed for these parameters, which have been employed to define design ground motions for use in model building codes. They were introduced in the Applied Technology Council (1978) seismic provisions as convenient normalizing factors for construction of design response spectra for ground motions of normal duration. The EPGA was defined as proportional to the spectral ordinates corresponding to periods within the range of 0.1 to 0.5 sec, while the EPGV was defined as proportional to the spectral ordinate corresponding to a period of about 1 sec. The constant of proportionality (for a 5 percent damping spectrum) was set at a standard value of 2.5 in both cases. The EPGA and EPGV are related to peak ground acceleration and peak ground velocity but are not necessarily the same as or directly proportional to peak acceleration and velocity.

When high frequencies are present in the ground motion, the EPGA may be significantly less than the peak ground acceleration. In general, if one examines the ratio between the spectral ordinate at period 0.2 sec and the corresponding PGA value at individual locations in the national probabilistic hazard maps, the value of the ratio is variable and generally less than 2.5. Newmark and Hall (1982) characterized the effective peak acceleration as the acceleration value that is most closely related to structural response and to damage potential of an earthquake. That is, this concept of effective peak acceleration is intended to reflect the actual damage potential of the seismic excitation, which cannot be accurately described only by the peak value of the ground acceleration. The definition

of the effective peak acceleration therefore must take into account not only the amplitude of the excitation, but also its frequency content and the type and characteristics of the general structural system under consideration.

Unlike the Richter and moment magnitude scales, it is not a measure of the total energy (magnitude, or size) of an earthquake, but rather of how hard the earth shakes in a given geographic area (the intensity). The Mercalli intensity scale uses personal reports and observations to measure earthquake intensity but PGA is measured by instruments, such as accelerographs, and it generally correlates well with the Mercalli scale.

Peak ground acceleration of earthquake includes two parameters namely Peak Horizontal Acceleration (PHA) and Peak Vertical Acceleration (PVA).

(i) Peak Horizontal Acceleration

The peak horizontal acceleration (PHA) is the most commonly used type of ground acceleration in engineering applications, and is used to set building codes and design hazard risks. In an earthquake, damage to buildings and infrastructure is related more closely to ground motion, rather than the magnitude of the earthquake. For moderate earthquakes, PGA is the best determinate of damage; in severe earthquakes, damage is more often correlated with peak ground velocity.

IS code and all other similar seismic design code has given enough guidelines for designing of structure considering horizontal acceleration of earthquake. So in this study we will not discuss horizontal acceleration further as our interest is on the vertical acceleration and its effect on building.

(ii) Peak Vertical Acceleration

It is a well Known fact that the civil engineering structures are subjected to the three dimensional earthquake ground motions. But it is only the horizontal motion which has been extensively studied and considered in the design Process whereas the vertical component of the ground motion has generally been neglected in design and hardly studied from hazard point of view. Also most of the Prevailing building codes including NBC 105, IS 1893, UBC 97 and many other codes worldwide assume the vertical component of the ground motion to be 1/2 to 2/3 of the horizontal component. However, in recent destructive earthquakes such as the 1989 Loma Prieta, 1994 Northridge, 1995 Kobe and 1999 Chi-Chi, it was found that vertical ground motion may equal or even significantly exceed the local horizontal ground motion.

In such situations, most existing code specifications must be considered un-conservative. In recent years many authors has highlighted this fact and done significant researches to identify and quantify the damaging potential of the vertical component of ground motion. Many studies

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reported data showing that the vertical peak acceleration may be even higher than the horizontal value. Others have attributed the observed failure on the Reinforced concrete structures to the reduction of shear strength caused by vertical ground motion effects. As recently shown by Kunnath et al. (2008), vertical motion may magnify and potentially create reversal of bending moment in longitudinal bridge girders. Widespread phenomenon of bearing failure and deck unseating, as observed during the recent earthquakes, was partially attributed to the destructive impact of vertical motions. However effects on vertical acceleration on response of the long span cable stayed bridge and its steel tower was found to be slight (Shrestha, 2009; Abdel raheem, Hayashikawa and Aly, 2002). Based on a large body of available studies, it is possible to conclude that vertical shaking may escalate the axial column force, cause an increase in the moment and shear demand, and amplify plastic deformation, extend plastic hinge formation and finally diminish the ductility capacity of structural component.

This part of the thesis concerns the importance of vertical ground motion in the near field of large earthquakes.

Recently there has been an increase in interest about vertical ground motions because buildings have become more architecturally unique and more structurally complicated, base isolation systems are being increasingly employed which may become unstable if there is uplift at any of the isolation elements and also sensitive equipment mounted on floors may be adversely affected by amplified vertical ground motions. In this thesis SDOF models, which include the effect of vertical excitation, are studied.

The vertical component of earthquake ground motion is associated with the arrival of vertically propagating P-waves, while the horizontal component is more of a manifestation of S-waves. As the wavelength of P-waves is shorter than that of S-waves, the vertical component of ground motion has much higher frequency content than the horizontal component. Although the energy content over the frequency range of the vertical ground motion is lower than that of the horizontal motion, the energy tends to be concentrated in a narrow, high frequency band. Such high frequency content leads to large amplifications in the short period range, which often coincide with the vertical period of RC structures. Thus, significant response amplifications are caused, especially with regard to forces as opposed to displacements (Elnashai and Papazoglou, 1997). The significance of the vertical component of ground motion is often characterized by the V/H peak ground acceleration ratio. Many design codes suggest scaling of a single spectral shape, originally derived for the horizontal component, using an average V/H ratio of 2/3. This procedure was originally proposed by Newmark in 1973. As a result, all components of motion have the same frequency content. However, the frequency content is demonstrably different, as discussed above.

Furthermore, the 2/3 rule for V/H is unconservative in the

near field and over conservative at large fault distances.

Studies such as Abrahamson and Litehiser (1989), Ambraseys and Simpson (1996), Elgamal and He (2004), and Bozorgnia and Campbell (2004), among others, provided evidence to confirm the lack of conservatism of the 2/3 scaling factor. Alongside the V/H ratio, it is prudent to study the relationship between the timing of peak response in the horizontal and vertical components of ground motion. The early arrival of the vertical motion may cause shakedown of the structure prior to the arrival of horizontal motion, thus, significantly affecting the structural response. On the other hand, the coincidence of vertical and horizontal peaks would cause high levels of distress in structural members. Many records show that significant vertical ground motion occurs earlier than horizontal motion, while others exhibit near coincidence in time. This characteristic of vertical motion is dependent on magnitude, source distance, site conditions, travel path, and type and depth of source. Collier and Elnashai (2001) investigated the time interval by using records from the Imperial Valley (1979) and Morgan Hill (1984) earthquakes. Thirty-two records at various distances but with similar site conditions were considered. The study concluded that the time interval increases with distance from source and should be taken as zero for a distance of 5 km or less from the source. The records are selected with source distances less than 50 km, relatively large interplate earthquakes (Mw ≥ 6) and with peak acceleration of 0.1 g or more. The distribution of V/H ratios indicates that the assumption of a V/H ratio of 2/3 seriously underestimates actions on structures near the source and overestimates the actions at large distances. It is also observed that the V/H ratios for 97% of the ground motions is no higher than 2.0.

IV. MODELING AND ANALYSIS

A. EARTHQUAKE DATA COLLECTION

In the seismic design of any structure it is necessary to consider the earthquake load along with the dead load and live load of the structure. For a structure to be constructed as earthquake resistant structure earthquake load need to be determined from the previously occurred earthquake by averaging large number of earthquake with different magnitude and frequency. So during seismic design primary step is to collect the information of as large number as possible the previously occurred earthquake from different regions. In this study we have collected earthquake data for 10 different regions occurred in between 1986 to 2001. After collecting the earthquake data it is necessary to convert all the data’s to the vector form using MATLAB command.

B. Plotting of Response of Building

Next step is to plot the response spectra for each earthquake for all the three directions. Response plot gives the spectral displacement vs. frequency curve. From

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the plot it is very easy to determine the displacement of structure for varying angular frequency.

V. RESULTS AND DISCUSSIONS

Response spectra plot is prepared in MATLAB which gives displacement vs angular frequency graph. From the plot, ratio of horizontal and vertical acceleration is determined and compare with the guidelines of 2/3^rd rules given in clause 6.4.5 of IS 1893:2002.

i. Response spectra for Bhuj Earthquake

Fig. 2 Response Spectra

In fig.2 it can be seen that response in Y and Z direction co-insides with each other. So the ratio of vertical to horizontal acceleration for Z and Y direction is 1 which is more than the provision given in IS 1893 (Part 1) : 2002.

So it does not follow the IS code guidelines. On the other hand ratio of Z and X direction is 4/6.4= 0.625 when angular frequency is close to zero and as angular frequency increases ratio becomes higher. So we can conclude that from over all consideration, IS code guidelines is not applicable in this case.

ii. Response spectra for India-Burma Border Earthquake (NE INDIA)-1

Fig 3 Response Spectra

In fig.3 both the ratio is more than 2/3 initially and in no case the ratio is less than 1 which means IS code guidelines is not applicable here.

iii. Response spectra for India-Burma Border Earthquake (NE INDIA)-2

In fig.4 both the ratio is more than 2/3 initially and in no case the ratio is less than 1 which means IS code guidelines is not applicable here.

iv. Response spectra for India-Burma Border Earthquake (NE INDIA)-3

In fig.5 ratio of spectral displacement for Z to Y direction is almost equal to 2/3 when angular frequency is nearer to zero and as frequency increases ratio also increases and after a certain angular frequency ratio becomes more than 1. Similarly ratio of Z to X direction is very much higher than 2/3^rd when angular frequency is nearer to zero and with the increasing angular frequency ratio goes on reducing but in no case the ratio becomes less than 1 which means IS code guidelines is not applicable here.

v. Response spectra for India-Burma border earthquake (NE INDIA)-4

In the fig.6 it can be seen that both the ratio is more than 2/3 which means IS code guidelines is not applicable in this case.

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vi. Response spectra for Chamoli Earthquake (NE INDIA)-1

From fig.7 it can be seen that ratio for Z to X direction is almost equal to 0.5 when angular frequency is close to zero and as frequency increases ratio goes on increasing and after a certain value of angular frequency ratio becomes 1. Similarly for Z to Y direction the ratio is 0.005/0.006=0.83 when angular frequency is zero and as frequency increases ratio goes on increasing and after a certain value of angular frequency ratio becomes 1 which is more than 2/3 which means IS code guidelines is not applicable in this case.

vii. Response spectra for Chamoli earthquake (NE INDIA)-2

In fig.8 it can be seen that the ratio is more than 2/3 in both the cases which means IS code guidelines is not applicable in this case.

viii. Response spectra for Uttarkashi Earthquake

In fig.9 it can be seen that the ratio is more than 2/3 in both the cases which means IS code guidelines is not applicable in this case.

ix. Response spectra for India-Bangladesh border earthquake

In fig.10 it can be seen that in no case the ratio is less than 1 which means IS code guidelines is not applicable in this case.

x. Mean response spectra for all the Earthquakes

Fig. 11 Mean Response Spectra

Mean response spectra plot is drawn by taking peak value of all the individual response spectra plot. In this plot it can be seen that maximum value of spectral displacement in Z- direction is about 0.092 when the angular frequency is close to zero and as angular frequency increases spectral displacement goes on reducing. Similarly spectral displacement in Y- direction is about 0.042 and with the increasing angular frequency the value goes on reducing sharply up to a certain value of angular frequency and beyond that spectral displacement reduces slightly with the increasing angular frequency and follows almost a horizontal path. Similarly spectral displacement in X- direction is about 0.02 when angular frequency is closed to zero and with the increasing angular frequency up to a certain angular frequency and beyond that spectral displacement reduces with the increase in angular frequency. So from the plot it can be seen that maximum ratio of spectral displacement in Z to X direction is 0.092/0.02=4.6 and as angular frequency increases ratio decreases but nowhere the ratio is less than 1 which means this ratio is more than 2/3^rd provisions given in IS code.

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Similarly maximum ratio of spectral displacement in Z to Y direction is 0.092/0.042=2.19 and as angular frequency increases ratio decreases but nowhere the ratio is less than 1 which means this ratio is more than 2/3^rd provisions given in IS code. So from the study it can be concluded that guidelines provided in IS 1893 (Part 1) : 2002 is not applicable in this case.

VI. CONCLUSIONS

Results show that the effect of vertical component of ground motion is very extensive in many cases and can lead to serious uplift problem. Since we have used small number of earthquake data in this study so it is not possible to reach on the result that the guidelines provided on IS 1893 (Part 1) : 2002 and similar other seismic design codes are not correct for giving satisfactory result all the times. But we have seen that for the cases considered in the study, the ratio of vertical to horizontal acceleration do not match with the guidelines of IS 1893 (Part 1) : 2002 clause 6.4.5. So it can be concluded that as safety is more important rather than cost, it is important to concentrate towards vertical ground acceleration and present code should be revised thoroughly to get final conclusion on this.

The analysis carried out in the study is linear dynamic analysis without consideration to ductility in the members and nonlinear behaviour of the elements and building as a whole. To arrive at a proper conclusion an extensive study considering nonlinear analysis need to be carried out over a large amount of data. Moreover the effects of such vertical ground acceleration and its effects on a building need to be considered separately by considering floor response spectra obtained using the similar procedure adopted in the present study.

Nevertheless this study gives a fair idea about the need for such an extensive study and also establishes a step by step procedure which can be followed coupled with extensive numeric and experimental analysis to arrive at a firm conclusion.

C. Authors and Affiliations

R.S. Patil is presently working as an Assistant Professor in Department of Civil Engineering, RR Institute of Technology, Bangalore. Guided Several M. Tech Projects. Thought Structural Dynamics, Earthquake resistant structure, Advanced RCC, Construction Project Management, Pre-stressed Concrete in post graduate level.

Dr. H. N. Rajakumara is presently Professor & Head in Department of Civil Engineering, RR Institute of Technology, Bangalore. 17 years of Teaching and Research Experience, guided BE, M.Tech and PhDs.

With effective class room management skills and Mentoring Skills. Authored 2 Books and recognized PhD

Guide/Supervisor for Anna University, Tamil Nadu and Visvesvaraya Technological University, Karnataka.

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