PROJECT REPORT

On

EFFECTS OF EARTHQUAKE ON FOUNDATIONS AND DESIGN FOR EARTHQUAKES

FOUNDATION ENGINEERING

By

ROHIT SAHAI 1012135591

Under the guidance of

PROFESSOR ÖMER BILGIN(Assistant Professor)

NOVEMBER 2014

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TABLE OF CONTENT

CHAPTER TITLE PAGEABSTRACT iiiLIST OF TABLES ivLIST OF FIGURES v

1 INTRODUCTION 11.1 CHARACTERISTICS AND TYPES OF WAVES 1

1.1.1 Compression or P-Waves 11.1.2 S-Waves 11.1.3 Love Waves 21.1.4 Rayleigh Waves 2

1.2 CAUSES OF EARTHQUAKE 31.3 EARTHQUAKE LOADS ON STRUCTURES 31.4 SEISMIC RISK ZONE 31.5 SEISMIC RESPONSE OF BUILDING SYSTEM 4

2 METHODOLOGY 52.1 SOIL PROFILE TYPE FOR A BUILDING SITE 5

2.1.1 Inertia Forces on Structures 52.1.2 Flow of Inertia Forces to Foundations 5

2.2 SEISMIC DESIGN REQUIREMENTS 52.2.1 Calculations of Base Shear Due To Earthquake 62.2.2 Overturning Moment Due to Earthquake 8

2.3 RESPONSE CONTROL 92.3.1 Base Isolation and Isolating Devices 92.3.2 Rubber Pads 102.3.3 Lead Rubber Bearings 102.3.4 Spherical Sliding Base Isolation 10

2.4 RETROFITTING 112.4.1 Adding Shear Wall/Infill Wall 112.4.2 Adding Bracing 112.4.3 Adding Wing Wall/Buttress 122.4.4 Wall Thickening 122.4.5 Supplement Damping 132.4.6 Masonry Foundation Retrofit 13REFERENCES 14

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EFFECTS OF EARTHQUAKE ON FOUNDATIONS AND DESIGNING FOR EARTHQUAKES

R. Sahai1, A.M. A.S.C.E, 1University of Dayton, Department of Civil and Environmental Engineering, 929-H Wilmington Avenue, Dayton, OH 45420; PH (937) 607-8864; email:

sahair1@udayton.edu

ABSTRACT

Uncertainties involved in the characterization and seismic response of soil foundation structure systems along with the inherent randomness of the earthquake ground motion result in very complex (and often controversial) effects of soil foundation structure interaction on the seismic response of structures.

The earthquake responses of structures are usually analyzed under the assumption that the foundation is firmly bonded to the soil. Such analyses often predict a base overturning moment that exceeds the available overturning resistance due to gravity loads, which implies that a portion of the foundation mat or some of the individual column footings would intermittently uplift during the earthquake. Therefore, it is a vital subject to investigate the influence of uplift on earthquake response of structures including the effect of different kinds of seismic waves evolved during an earthquake.

The paper provides an introduction to principles of seismic design, including strategies for designing earthquake resistant buildings to ensure safety and security of building occupants and assets.

Retrofitting is the process of modifying something after it has been manufactured. The primary purpose of earthquake retrofitting is to keep your home from being displaced from its concrete foundation making the building safer and less prone to major structural damage during an earthquake. Hence, the expectation of improving amenities of the buildings which is achieved due to the development of new technology that allows significant reduction in energy and loss of property will be studied precisely in the paper.

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LIST OF TABLES

TABLE TITLE PAGE

2.1 Seismic Coefficient Ca 6

2.2 Seismic Coefficient Cv 6

2.3 Value of R factor for most common structural systems 7

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LIST OF FIGURES

FIGURE TITLE PAGE

1 Propagation of P-waves 1

2 Propagation of S-waves 2

3 Propagation of Love waves 2

4 Propagation of Rayleigh waves 2

5 Intensity of major earthquake activity in the United States 4

6 Effect of inertia in a building 5

7 Base Isolation Technique 9

8 Structure of elastomer rubber pad 10

9 Lead rubber bearing 10

10 Spherical Sliding Base Isolation 11

11 Shear wall and Infill wall 11

12 Cross bracings 12

13 Wing wall/Buttress 12

14 Wall thickening 12

15 Supplement Damping 13

16 Strengthening of foundations 13

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CHAPTER 1

INTRODUCTION

1.1 CHARACTERISTICS AND TYPES OF EARTHQUAKE WAVES

An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.

Earthquakes are measured using observations from seismometers. Seismic waves are propagating vibrations that carry energy from the source of the shaking outward in all directions.

There are many different seismic waves, but all basically of four types:

Compression or P waves (for primary) Transverse or S waves (for secondary) Love waves Rayleigh waves

1.1.1 Compression or P-Waves

P-waves are the first waves to arrive on a complete record of ground shaking because they travel the fastest (their name derives from this fact - P is an abbreviation for primary, first wave to arrive). They typically travel at speeds between ~1 and ~14 km/sec (shown in figure 1).

Fig. 1: Propagation of P-waves

1.1.2 S-Waves

Secondary, or S waves, travel slower than P waves and are also called "shear" waves because they don't change the volume of the material through which they propagate, they shear it. S-waves are transverse waves because they vibrate the ground in the direction "transverse", or perpendicular, to the direction that the wave is traveling (shown in figure 2).

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Fig. 2: Propagation of S-waves

1.1.3 Love Waves

Love waves are transverse waves that vibrate the ground in the horizontal direction perpendicular to the direction that the waves are traveling (shown in figure 3). Love waves are dispersive waves and they are formed by the interaction of S waves with Earth's surface and shallow structure. The speed at which a dispersive wave travels depends on the wave's period. In general, earthquakes generate Love waves over a range of periods from 1000 to a fraction of a second, and each period travels at a different velocity but the typical range of velocities is between 2 and 6 km/second.

Fig. 3: Propagation of Love waves

1.1.4 Rayleigh Waves

Rayleigh waves are the slowest of all the seismic wave types and in some ways the most complicated. Like Love waves they are dispersive so the particular speed at which they travel depends on the wave period and the near-surface geologic structure, and they also decrease in amplitude with depth. Typical speeds for Rayleigh waves are on the order of 1 to 5 km/s (shown in figure 4).

Fig. 4: Propagation of Rayleigh waves

1.2 CAUSES OF EARTHQUAKES

The earthquakes are caused by various natural and artificial phenomenon’s:

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Inertial forces generated by severe ground shaking. By direct fault displacement at the site of a structure. By landslides, or other surface movements. Large-scale tectonic changes in ground elevation. By seismically induced water waves such as seismic sea waves (tsunamis) or

fluid motions in reservoirs and lakes.

1.3 EARTHQUAKE LOADS ON STRUCTURES

Earthquake forces are inertia forces, created at every molecule of mass in every member of the structure as the structure is being shaken by earthquake motions.

The earthquake creates both lateral motions and vertical motions in a structure. Under Newton’s second law relating force F, mass m and acceleration a (F=ma), it is the rate of acceleration of these motions that governs the magnitude of the earthquake forces. In general, vertical earthquake motions can produce vertical inertia forces as high as 20% of the dead load, acting either upward or downward. Similarly, lateral earthquake motions can produce lateral inertia forces as high as 30% or even 40% of the dead weight of the building, acting laterally in any direction.

Structures are typically designed for vertical gravity loads of 100% dead load plus 100% live load, with a nominal margin of safety of roughly 70% to failure load. Consequently, the additional vertical load created by an earthquake (25% of dead load) is not regarded as serious overload. In general, the vertical load produced by an earthquake is considered to be within acceptable limits for a one time load and special measures are not needed to account for the vertical load.

The base shear created by earthquake forces on a structure is an inertia force. In earthquake design, the inertia force is computed as a factor times the dead weight of the structure. The only loads that can contribute to base shear are the loads that will be accelerated by the earthquake motion. The live loads are usually loose, or at least so loosely fastened that they will not be accelerated at the same rate as the structural frame. Even a small amount of slippage of an object will reduce the inertia force so sharply that it will make very little contribution to base shear. Though only fixed loads produce inertia forces, codes do require a small percentage of live load be included with the dead load.

1.4 SEISMIC RISK ZONES

The location and intensity of major earthquake activity in the United States is charted in figure 5.

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Fig.5: Intensity of major earthquake activity in the United States

1.5 SEISMIC RESPONSE OF BUILDING SYSTEMS

The magnitude of the earthquake inertia forces on a structure will vary with the natural period of oscillation of the structure. The type of structural system thus has a bearing on the magnitude of the inertia forces and the consequent base shear.

Codes separate the various structural systems in common use into groups that have similar responses. Each system is assigned a response factor R (shown in Table 2.3). The response factor R take into account the relative rigidity of the structural system. A very flexible structure will sway when subjected to motions at its base, thereby reducing the base shear considerably. In contrast, a low rigid building having a stiff structural system can actually undergo accelerations as much as 2×1/2 times as large as ground accelerations.

The structural systems that are most likely to utilize shallow spread footings are the low diaphragm shear wall structures; in these structures, walls carry all lateral loads. Typically, these structures are low and rigid, having heights less than 65 feet and periods less than 0.7 seconds.

The moment resistant frames are more likely to be used for taller structures; they are not often competitive in cost for lower buildings.

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CHAPTER 2

METHODOLOGY

2.1 SOIL PROFILE TYPE FOR A BUILDING SITE

Earthquake forces are called inertia forces which are related to the mass, stiffness and energy absorbing characteristics of the structure. The magnitude of the earthquake inertia forces is also dependent on the type of soil at the site as well as its strength and its depth.

2.1.1 Inertia Forces on Structures

Earthquake causes shaking of the ground. So a building resting on it will experience motion at its base. From Newton’s First Law of Motion, even though the base of the building moves with the ground, the roof has a tendency to stay in its original position. But since the walls and columns are connected to it, they drag the roof along with them. This tendency to continue to remain in the previous position is known as inertia. In the building, since the walls or columns are flexible, the motion of the roof is different from that of the ground as shown in figure 6.

Fig. 6: Effect of inertia in a building

2.1.2 Flow of Inertia Forces to Foundations

Under horizontal shaking of the ground, horizontal inertia forces are generated at level of the mass of the structure (usually situated at the floor levels). These lateral inertia forces are transferred by the floor slab to the walls or columns, to the foundations, and finally to the soil system underneath.

2.2 SEISMIC DESIGN REQUIREMENTS

The two most important elements of concern to a structural engineer are calculation of seismic design forces and the means for providing sufficient ductility.

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Based on the risk zone Z of a building site, the soil profile type for the site and the type of structural system to be used, an average seismic coefficient for the structure is defined by the Tables of values for two such coefficients Ca and Cv are given in Table 2.1 and Table 2.2, respectively.

Table 2.1: Seismic Coefficient Ca

Soil Profile    

Seismic Zone Factor, Z    

Type Z = 0.075 Z = 0.15 Z = 0.2 Z = 0.3 Z = 0.4

SA 0.06 0.12 0.16 0.24 0.32 Nv

SB 0.08 0.15 0.2 0.3 0.40 Nv

SC 0.09 0.18 0.24 0.33 0.40 Nv

SD 0.12 0.22 0.28 0.36 0.44 Nv

SE 0.19 0.3 0.34 0.36 0.36 Nv

SF     See Footnote1    1Site-specific geotechnical investigation and dynamic site response analysis shall be performed to

determine seismic coefficient for Soil Profile Type SF

Table 2.2: Seismic Coefficient Cv

Soil Profile    

Seismic Zone Factor, Z    

Type Z = 0.075 Z = 0.15 Z = 0.2 Z = 0.3 Z = 0.4

SA 0.06 0.12 0.16 0.24 0.32 Nv

SB 0.08 0.15 0.20 0.30 0.40 Nv

SC 0.13 0.25 0.32 0.45 0.54Nv

SD 0.18 0.32 0.40 0.54 0.64Nv

SE 0.26 0.50 0.64 0.84 0.96 Nv

SF     See Footnote1    1Site-specific geotechnical investigation and dynamic site response analysis shall be performed to

determine seismic coefficient for Soil Profile Type SF

2.2.1 Calculations of Base Shear Due to Earthquake

In recognition of all the foregoing influences, Code specifies the value of the design base shear V based on the average acceleration of the superstructure:

V=C v IRT

W (1)

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Where,

Cv is an average seismic coefficient specified by Code, given in Table 2.2

I is the importance factor

R is the interactive response factor specified by Code, given in Table 2.3

T is the natural period of the structure

W is the dead weight of the structure

For simplicity in all following discussions, the importance factor I is again taken at its base value of 1.0.

Table 2.3: Value of R factor for most common structural systems

S.No. Lateral force resisting system description R1 Special Moment Resisting Frame Systems. 8

2Dual System With Special Moment Resisting Frames which are capable to resist at least 50% of Prescribed Seismic Force 7.5

3Dual System With Special Moment Resisting Frames which are capable to resist at least 25% of Prescribed Seismic Force. 6.5

4Dual System With Special Moment Resisting Frames which are capable to resist at least 10% of Prescribed Seismic Force. 5.5

5Bearing Shear Wall System without Special Moment Resisting Frames 4.5

For the low structures (about 65 feet or less) that are likely to be founded on shallow foundations, the calculations of the base shear can be simplified considerably over the calculations required for higher structures. In the low structures that are of primary interest here, the upper bound for the base shear is also specified by Code.

V=2.5C a I

RW (2)

Where,

Ca is an average seismic coefficient given in Table 2.1

Code gives equation (2) as an absolute upper bound on all values computed from equation (1). For the low structures of interest here, the upper bound is found to be the applicable equation for most structures up to about 50 feet high, and is only slightly in error up to about 70 feet. In all cases the error is on the “safe” side.

Adopting the upper bound for the design of routine shallow foundations provides a worthwhile simplification of the design procedure.

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Experimentation with equation (2) reveals that for a low structure founded on shallow spread footings, the maximum earthquake force will be about 10% to 13% of the vertical dead load. In areas of light earthquake intensity, the lateral load may drop to as little as 2.5% of the dead load. These percentages of the vertical dead weight are known as the “lateral g-load” or “lateral g-force” on the structure.

2.2.2 Overturning Moment Due to Earthquake

Since the base of the structure is securely anchored to the ground, the base will undergo accelerations identical to the accelerations of the ground. The top of a structure, however, can undergo accelerations as much as 2×1/2 times that of its base. Such amplification maybe attributed either to the effects of partial resonance or the effects of “whip” or a combination of both.

The overall effect of inverting the acceleration rates (zero at the base, maximum at the top) is to increase markedly the inertia forces toward the top of the structure. This “whip” effect also increases significantly the overturning moment produced by these inertia forces.

The component of the base shear to be assigned to any level x between levels from 1 to n is computed by multiplying the base shear V by the inertia factor C x for that level, or,

F x=Cx V

Where,

C x=W x hx

∑W i hi

Where,

Wi= dead load weight at the level i

hi = height of level i above the base

hn = height of the highest level

aavg= average acceleration (at hLAT)

The overturning moment Mov produced by these forces Fx is calculated as,

M ov=∑ F ihi

The location of the center of lateral inertia forces above the base, hLAT, is calculated as,

hLAT=M ov

V

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As the base shear and the overturning moments are evaluated using above equations, the total lateral and longitudinal load is obtained. The foundation is then designed based on these seismic forces and moments.

For severe seismic zones, individual spread footing or pile caps should be interconnected with ties, except when individual spread footing are directly supported on rock. All ties should be capable of carrying, in tension and in compression, an axial force equal to Ah/4 times the larger of the column or pile cap load, in addition to the otherwise computed forces.

2.3 RESPONSE CONTROL

The conventional approach to seismic design of structures relies on the ductile behavior of the structural system to dissipate the seismic energy through plastic deformation cycles. Devices proposed and in use are either to prevent an earthquake force from acting on a structure (isolators) or to absorb a portion of the earthquake energy (dampers) that is introduced to the structure.

2.3.1 Base Isolation and Isolating Devices

In order for a structure to withstand the distortions resulting from the earthquake motions, an adaptive system is designed to isolate the upper portions of a structure from destructive vibrations, by confining the severe distortions to a specially designed portion at its base. The building is detached or isolated from the ground in such a way that only a very small portion of seismic ground motion is transmitted up through the building (shown in figure 7).

A practical base isolation system should consist of the following:

A flexible mounting to increase the period of vibration of the building sufficiently to reduce forces in a structure above.

A damper or energy dissipater to reduce the relative deflection between the building and a ground to a practical level.

A method of providing rigidity to control the behavior under minor earthquakes and wind loads.

Fig.7: Base Isolation Technique

2.3.2 Rubber Pads

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Laminated rubber pads prevent ground motion from being transmitted from the building foundation in the superstructure (shown in figure 8).

Fig.8: Structure of elastomer rubber pad

2.3.3 Lead Rubber Bearings

A lead rubber bearing is made from layers of rubber sandwiched together with layers of steel. In the middle of the solid lead “plug”. On top and bottom, the bearing is fitted with steel plates which are used to attach the bearing to the building and foundation. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction (shown in figure 9).

Fig.9: Lead rubber bearing

2.3.4 Spherical Sliding Base Isolation

Spherical sliding isolation systems are another type of base isolation. The building is supported by bearing pads that have a curved surface and low friction. During an earthquake the building is free to slide on the bearings. Since the bearings have a curved surface, the building slides both horizontally and vertically. The forces needed to move the building upwards limits the horizontal or lateral forces which would otherwise cause building deformations (shown in figure 10).

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Fig.10: Spherical Sliding Base Isolation

2.4 RETROFITTING

Retrofitting is the process of modifying something after it has been manufactured. Typically this is done with the expectation of improving strength and performance of the building. The development of new technologies mean that building retrofits can allow for significant reductions in energy caused due to earthquake. Different retrofitting measures are described briefly:

Adding shear wall/infill wall Adding bracing Adding wing wall/buttresses Wall thickening Masonry foundation retrofit Supplemental damping

2.4.1 Adding Shear Wall/Infill Wall

Shear walls and infill walls are vertical elements of the horizontal force resisting system. When the sheathing is properly fastened to the stud wall framing, the shear wall can resist forces directed along the length of the wall. They have the strength and stiffness to resist the horizontal forces (shown in figure 11).

Fig.11: Shear wall and Infill wall

2.4.2 Adding Bracing

This is another way to stiffen the existing building. Adding diagonal cross bracings (shown in figure 12) to the columns on a multi-storey building stiffens the structural foundations and allows better shear resistance.

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Fig.12: Cross bracings

2.4.3 Adding Wing Wall/Buttress

These are not so popular since they require vacant area around the building. Lateral strength of the building can be increased using wing walls (shown in figure 13).

Fig.13: Wing wall/Buttress

2.4.4 Wall Thickening

This is done by making grooves in an existing masonry wall and dowelling in epoxy as shown in figure 14. A new set of reinforcements are made to anchor the grooves which is finished with reinforced concrete. It increases strength and stiffness of the structural members like slab, infill wall and shear wall.

Fig.14: Wall thickening

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2.4.5 Supplement Damping

The supplement damping devices are the viscous dampers, visco-elastic dampers and frictional dampers (shown in figure 15). These are provided between two adjacent columns like a bracing and they have the tendency to absorb shocks like a shock up which in turn reduces the earthquake response.

Fig.15: Supplement Damping

2.4.6 Masonry Foundation Retrofit

The foundation can be retrofitted by drilling a gap through the foot of the wall and add reinforcements with stirrups which contains connector keys that anchors it to the wall as shown in figure 16. The reinforcements are then finished by concreting to form and R.C. beam.

Fig.16: Strengthening of foundations

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REFERENCES

1. Samuel, E. French, (1930). “Design of Shallow Foundations” Lateral loads on foundation, Reston, Virginia 20191-4400: 31-66.

2. S.K. Duggal, (2007). “Earthquake Resistance Design of Structures”, New Delhi, 110001: 103-180.

3. Michael R. Lindeburg, (2011). “Seismic Design of Building Structures”, California, 2011934772: 7-1 – 7-11.

4. Robert E. Englekirk, (2003). “Seismic Design of Reinforced and Precast Concrete Buildings”, New Jersey, 2002008561: 738-753.

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