For example, Bridges are structures that subjected to big static and dynamic loads will be need to special foundations has a large dimension. Thus, we must consider superstructure type while design foundation.
3-3-3-2-The superstructure loads acting on foundation:
There are multi load forms that acting on superstructure such as wind loads, earthquake loads and superstructure own weight. Superstructure transform these loads to foundation and therefore to soil finally. Superstructure loads that are transformed to foundation control in value of stress on soil so that foundation area, dimensions and safety against bearing capacity of soil depend on the value of these loads. Thus, we must take these loads effect in consideration.
3-3-3-3- Superstructure material properties:
Superstructure material properties such as concrete ultimate strength, steel ultimate stress and any addition components improve structure quality increase structure resisting against any deformations. So whenever the superstructure will be more rigid and resistance against deformation, the stress between soil and foundation reduces as a result. So, superstructure material properties is an important factor in foundation analysis.
3-3-4- Dynamic loads:
Dynamic loads were the ghost that impended engineers for a long time and this for many reasons. Non-uniform value, suddenly happened and unknown direction of dynamic loads are the most important factors that made dynamic loads as a big danger on structure and had to be taken in consideration during design. So it is important to know the nature of dynamic loads and how it happen to understand the behavior of foundation under these loads. So it is important to explain some points as the following:
i. Dynamic load types.
ii. Difference between dynamic and static loads.
iii. Earthquake as a type of dynamic loading.
iv. Causes of earthquakes.
v. Types of earthquake waves.
vi. Measurement of earthquakes.
vii. Effects of earthquake.
viii. Methods used for earthquake analysis.
ix. Time-History method ECPLF 2012.
3-3-4-1- Dynamic load types Rajasekaran, S. 2009:
Dynamic loads have many types such as earthquake loads, wind loads, machine vibration loads, offshore wave loads and explosion loads, and each one of these types has a special effect on soil or foundation. By knowing the natural of structural behavior under dynamic loading we can guess how to design our structure to be safe and stability against any possible damage. Dynamic loads may be classified as ‘deterministic’ and ‘non-deterministic’. If the magnitude, point of application of the load and the variation of the load with respect to time are known, the loading is said to be deterministic and the analysis of a system to such loads is defined as deterministic analysis. On the other hand, if the variation of load with respect to time is not known, the loading is referred to as random or stochastic loading and the corresponding analysis is termed as non-deterministic analysis. Dynamic loads may also be classified as periodic and non-periodic loadings. When a loading repeats itself at equal time intervals then it is called periodic loading. A single form of periodic loading is either a sine or cosine function as shown in Figure. (3-2a). A vibration induced due to rotating mass is a periodic motion. This type of periodic loading is called simple harmonic motion as shown in Figure. (3-1a). The type of loading shown in Figure. (3-2b) is a periodic loading but non-harmonic. Later on we will see that most periodic loads can be represented by summing sufficient number of harmonic terms in a Fourier series. Any loading which does not come under the category of periodic loading is termed as non-periodic. Blast loading shown in Figure. (3-2c) and earthquake ground motion as shown in Figure. (3-2d) are the examples of non-periodic loads.
Figure 3-2 Different types of dynamic loads: (a) simple harmonic; (b) non-harmonic (periodic); (c) non-periodic (short duration); (d) non-periodic (long duration). Rajasekaran, S. (2009).
3-3-4-2- Difference between dynamic and static loads Rajasekaran, S. 2009:
Dynamic and static loads have different effect on structure so the behavior of structure changes for each case. Static load is stable and has a linear relation with time. But dynamic load is antithesis not stable and has a variable relation with time. A dynamic load is different from a static load from two aspects. The first and most obvious difference is the time-varying load and the time-varying response also. This needs analysis over a specific interval of time. Hence dynamic analysis is complex and computationally extensive and expensive compared with static analysis. The other difference between dynamic and static problems is the major occurrence of inertia forces when the loading is dynamically applied. Consider a water tank as shown in Figure. (3-3) subjected to load F at the top.
Figure 3-3 Water tank subjected to static and dynamic loads: (a) Static load;
(b) Dynamic load. Rajasekaran, S. (2009).
The resulting deflection, shear force and bending moment can be calculated on the basis of static structural analysis principles as shown in Figure. (3-3a). On the other hand, if the time-varying load F(t) is applied at the top, the structure is set to motion or vibration and experiences accelerations. According to Newton’s second law, inertia force is proportional to acceleration. Inertia forces are proportional to the mass and they develop in the structure that resists these accelerations. Depending on the contribution made by inertia force to shear and bending moment will determine whether dynamic analysis is warranted as shown in figure. (3-3b). The most famous type of dynamic loads is earthquake loads and this type which we will talk about its effect on our life.
3-3-4-3- Earthquakes as a type of dynamic loading:
Earthquakes are the Earth’s natural means of releasing stress. Earthquakes are the most famous type of dynamic loads and they in the fact are the motion or trembling of the ground produced by sudden displacement of rock in the earth’s crust, volcanism, landslides, rock bursts, and man-made explosions. Of these, naturally occurring tectonic-related earthquakes are the largest and most important. These are caused by the fracture and sliding of rock along faults within the Earth’s crust. A fault is a zone of the earth’s crust within which the two sides have moved – faults may be hundreds of miles long, from one to over one hundred miles deep, and are sometimes not readily apparent on the ground surface. Earthquakes initiate a number of phenomena or agents, termed seismic hazards, which can cause significant damage to the built environment – these include fault rupture, vibratory ground motion, inundation and various kinds of permanent ground failure, fire, or hazardous materials release. In a particular earthquake event, any particular hazard can dominate, and historically each has caused major damage and great loss of life in particular earthquakes. For most earthquakes, shaking is the dominant and most widespread agent of damage. Shaking near the actual earthquake rupture lasts only during the time when the fault ruptures, a process that takes seconds or at most a few minutes. The seismic waves generated by the rupture propagate long after the movement on the fault has stopped. Typically, earthquake ground motions are powerful enough to cause damage only in the near field – in a few instances, long period motions have caused significant damage at great distances, to selected lightly damped structures. Earthquake cause a lot of damages to structure and these due to many reasons. These reasons may be because of soil or structure itself.
3-3-4-4- Causes of earthquakes Nelson, A, S. 2013:
Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can be generated by bomb blasts, volcanic eruptions, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake prone areas, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth. The causes of earthquakes are generally divided into:
ii- Tectonic causes.
iii- Volcanic causes.
iv- Surface causes.
A fault is a zone of the earth’s crust within which the two sides have moved. Faults are the physical expression of the boundaries between adjacent tectonic plates and thus may be hundreds of miles long. Generally, the longer a fault the larger the earthquake it can generate. Beyond the main tectonic plates, there are many smaller sub plates, ”platelets,” and simple blocks of crust that occasionally move and shift due to the ”jostling” of their neighbors and the major plates. The existence of these many sub plates means that smaller but still damaging earthquakes are possible almost anywhere, although often with less likelihood. Faults are typically classified according to their sense of motion. Scientists use the angle of the fault with respect to surface (known as the dip) and the direction of slip along the fault to classify faults. Faults in general are divided into three categories:
i- Normal fault.
ii- Thrust (reverse) fault.
iii- A left-lateral Strike-slip fault.
iv- A right-lateral Strike-slip fault.
A) Normal fault:
This type is a dip-slip fault in which the rock or block above the fault move downward relative to the rock or block below as in figure (3-4).
Figure 3-4 normal fault type. Nelson, A, S. (2013)
B) Thrust (reverse) fault:
This type is a dip-slip fault in which the upper rock or block, above the fault plane, move up and over the rock or block below as in figure (3-5).
Figure 3-5 thrust fault type. Nelson, A, S. (2013)
C) A left-lateral strike-slip fault:
In this type the displacement of the far block is to the left when viewed from either side as in figure (3-6).
Figure 3-6 left lateral strike-slip fault type. Nelson, A, S. (2013)
D) A right-lateral strike-slip fault:
In this type the displacement of the far block is to the right when viewed from either side as in figure (3-7).
Figure 3-7 right lateral strike-slip fault type. Nelson, A, S. (2013)
3-3-4-4-2- Tectonic causes:
The tectonic plate theory is the main cause of earthquakes. A theory of global tectonics in which the lithosphere is divided into a number of plates that act like rigid bodies and that interact with one another at their boundaries causing earthquakes. So, in this type earthquakes occur due to displacement in underground layer of soil when rocks in the Earth’s crust break resulting of geological forces created by movement of tectonic plates.
3-3-4-4-3- Volcanic causes:
Earthquakes related to volcanic activity may produce hazards which include ground cracks, ground deformation, and damage to manmade structures. There are two general categories of earthquakes that can occur at a volcano:
i- Volcano-tectonic earthquakes.
ii- Long period earthquakes.
A) Volcano-tectonic earthquakes
In volcano-tectonic type earthquakes produced by stress changes in solid rock due to the injection or withdrawal of magma are called volcano-tectonic earthquakes (Chouet, 1996). These earthquakes can cause land to subside and can produce large ground cracks. These earthquakes can occur as rock is moving to fill in spaces where magma is no longer present. Volcano-tectonic earthquakes don’t indicate that the volcano will be erupting but can occur at any time.
B) Long period earthquakes
In long period type earthquakes produced by the injection of magma into surrounding rock. These earthquakes are a result of pressure changes during the unsteady transport of the magma. When magma injection is sustained a lot of earthquakes are produced (Chouet, 1996). This type of activity indicates that a volcano is about to erupt. Scientists use seismographs to record the signal from these earthquakes. This signal is known as volcanic tremor. People living near an erupting volcano are very aware of volcanic earthquakes. Their houses will shake and windows rattle from the numerous earthquakes that occur each day before and during a volcanic eruption. To prevent damage from being done, structures should be built according to earthquake standards, building foundations should be constructed on firm ground and not unconsolidated material which may amplify earthquake intensity, and buildings should be constructed on stable slopes in areas of low hazard potential.
3-3-4-4-4- Surface causes:
There are some causes for earthquakes occur on ground surface. These causes may be from man-made or natural such as, great explosions, landslides, slip on steep coasts, dashing of sea waves, railway trains, heavy trucks and some large engineering projects cause minor tremors. These loads may be have small values but by repeating its value will be large enough to make earthquakes.
3-3-4-5- Types of earthquake waves:
During earthquake some waves are associated with it. These waves are created when stress is released as energy in earthquakes. These waves are divided into three categories as shown in figure (3-8):
iii- Surface waves
The P wave, or primary wave, is the fastest of the three waves and the first detected by seismographs. They are able to move through both liquid and solid rocks. P waves, like sound waves, are compressional waves, which mean that they compress and expand matter as they move through it.
S waves, or secondary waves, are the waves directly following the P waves. As they move, S waves shear, or cut the rock they travel through. S waves cannot travel through liquid because, while liquid can be compressed, it can’t shear. S waves are the more dangerous type of waves because they are larger than P waves and produce vertical and horizontal motion in the ground surface.
C) Surface waves
The third type of wave, and the slowest, is the surface wave. These waves move close to or on the outside surface of the ground. There are two types of surface waves: Love waves, that move like S waves but only horizontally, and Rayleigh waves, that move both horizontally and vertically in a vertical plane pointed in the direction of travel.