File Name: difference between footing and foundation .zip
Footing and foundation both are some basic terminologies used in civil engineering. If you are one of them then this is the topic for you.
- Foundation (engineering)
- Types of Foundation for Buildings and their Uses [PDF]
- Tall building foundations: design methods and applications
Shallow foundations sometimes called 'spread footings' include pads 'isolated footings' , strip footings and rafts. Deep foundations include piles, pile walls, diaphragm walls and caissons. Types of foundation Shallow foundations Pad foundations Strip foundations Raft foundations Shallow foundations are those founded near to the finished ground surface; generally where the founding depth D f is less than the width of the footing and less than 3m.
The foundation distributes the load over a large area. So that pressure on the soil does not exceed its allowable bearing capacity and restricts the settlement of the structure within the permissible limits. Foundation increases the stability of the structure. The settlement of the structure should be as uniform as possible and it should be within the tolerable limits.
This paper will review some of the challenges faced by designers of foundations for very tall buildings, primarily from a geotechnical viewpoint.
Some characteristic features of such buildings will be reviewed and then the options for foundation systems will be discussed. A three-stage process of foundation design and verification will be described, and the importance of proper ground characterization and assessment of geotechnical parameters will be emphasised. The application of the foundation design principles will be illustrated via four projects, each of which has presented a different challenge to the designers:.
A large number of these buildings are in the Middle East or in China. Many of the traditional design methods cannot be applied with any confidence since they require extrapolation well beyond the realms of prior experience, and accordingly, structural and geotechnical designers are being forced to utilise more sophisticated methods of analysis and design.
In particular, geotechnical engineers involved in the design of foundations for super-tall buildings are leaving behind empirical methods and are employing state-of-the art methods increasingly. This paper will review some of the challenges that face designers of foundations for very tall buildings, primarily from a geotechnical viewpoint.
The process of foundation design and verification will be described, and then the application of these principles will be illustrated via four projects, each of which has presented a different challenge to the foundation designers:.
There are a number of characteristics of tall buildings that can have a significant influence on foundation design, including the following:. The building weight, and thus the vertical load to be supported by the foundation, can be substantial.
Moreover, the building weight increases non-linearly with height, and so both ultimate bearing capacity and settlement need to be considered carefully. High-rise buildings are often surrounded by low-rise podium structures which are subjected to much smaller loadings. Thus, differential settlements between the high- and low-rise portions need to be controlled. The lateral forces imposed by wind loading, and the consequent moments on the foundation system, can be very high.
These moments can impose increased vertical loads on the foundation, especially on the outer piles within the foundation system. The structural design of the piles needs to take account of these increased loads that act in conjunction with the lateral forces and moments.
The wind-induced lateral loads and moments are cyclic in nature. Thus, consideration needs to be given to the influence of cyclic vertical and lateral loading on the foundation system, as cyclic loading has the potential to degrade foundation capacity and cause increased settlements. Seismic action will induce additional lateral forces in the structure and also induce lateral motions in the ground supporting the structure.
Thus, additional lateral forces and moments can be induced in the foundation system via two mechanisms:. Kinematic forces and moments induced in the foundation piles by the action of ground movements acting against the piles.
The wind-induced and seismically induced loads are dynamic in nature, and as such, their potential to give rise to resonance within the structure needs to be assessed. The risk of dynamic resonance depends on a number of factors, including the predominant period of the dynamic loading, the natural period of the structure and the stiffness and damping of the foundation system.
The dynamic response of tall buildings poses some interesting structural and foundation design challenges. However, some of the higher modes of vibration will have significantly lower natural periods and may well be excited by wind or seismic action. These higher periods will depend primarily on the structural characteristics but may also be influenced by the foundation response characteristics.
The factors that may influence the type of foundation selected to support a tall building include the following:. If high-rise developments contain a multi-level basement, the base of the development may be founded close to, or even embedded into, competent rock. A raft mat foundation to support the entire structure may be feasible for buildings of moderate height. However, for very tall buildings, such a shallow foundation may not be able to develop adequate resistance to horizontal and moment loadings.
The effects of lateral and moment loading should be incorporated into the assessment of ultimate bearing pressure. It is often good practice to look at an upper and lower bound set of soil stiffness values to evaluate raft performance. The soil and rock parameters adopted for design should be carefully chosen considering the variation in the ground conditions both vertical and horizontal across the relatively wide foundation area.
The possible effect of future construction activity should also be considered in the estimation of bearing capacity. For rafts founded on rock, the bearing capacity is highly dependent on factors such as the intensity and orientation of joints, degree of weathering and other local or general defects.
For a weak rock mass having very closely spaced discontinuities or heavily weathered rock materials, it is common practice to consider the conventional bearing capacity equations for soil mechanics for the foundation design.
For more accurate evaluation of the bearing capacity, the geotechnical strength parameters can be obtained from large-scale field tests in conjunction with an in situ test program, which will also provide the deformation characteristics of the ground.
In such cases, the construction of the raft involves excavation of the soil prior to construction of the foundation and the superstructure. Because of the stress reduction in the underlying ground caused by excavation, the net increase in ground stress due to the structure will be decreased, and hence it may be expected that the settlement and differential settlement of the foundation will also be decreased.
The resulting foundation is termed a compensated or buoyancy raft, and can be very beneficial when constructing buildings on soft clay or loose sand, as the settlements that occur can be significantly less than those if the foundation was located at or near the ground surface. In these circumstances, it is necessary to support the building loads on piles, either single piles or pile groups, generally located beneath columns and load bearing walls.
A piled foundation for high-rise structures often comprises a large numbers of piles and, therefore, the challenge in the design is capturing the effects of the group interaction. It is well recognised that the settlement of a pile group can differ significantly from that of a single pile at the same average load level due to group effects.
Also, the ultimate load that can be supported by a group of piles may not be equal to the sum of the ultimate load which can be carried by each pile within the group, and so consideration must be given to the pile group efficiency. Many high-rise buildings are constructed with thick basement slabs. When piles are used in the foundation it is generally assumed that the basement slab does not carry any of the foundation loads.
In some cases, it is possible to utilise the basement slab, in conjunction with the piles, to obtain a foundation that satisfies both bearing capacity and settlement criteria. A piled raft foundation is a composite system in which both the piles and the raft share the applied structural loadings.
Within a conventional piled foundation, it may be possible for the number of piles to be reduced significantly by considering the contribution of the raft to the overall foundation capacity. In such cases, the piles provide the majority of the foundation stiffness while the raft provides a reserve of load capacity. In situations where a raft foundation alone might be used, but does not satisfy the design requirements in particular the total and differential settlement requirements , it may be possible to enhance the performance of the raft by the addition of piles.
In such cases, the use of a limited number of piles, strategically located, may improve both the ultimate load capacity and the settlement and differential settlement performance of the raft and allows the design requirements to be met. As piles need not be designed to carry all the load, there is the potential for substantial savings in the cost of the foundations.
Piles may be located strategically beneath the raft so that differential settlements can be controlled. Piles can be designed to carry a load approaching or equal to their ultimate geotechnical load, provided that the raft can develop an adequate proportion of the required ultimate load capacity. Poulos [ 56 ] has examined a number of idealised soil profiles and found that the following situations may be favourable:.
In both circumstances, the raft can provide a significant proportion of the required load capacity and also contribute to the foundation stiffness, especially after the pile capacity has been fully mobilised.
It has also been found that the performance of a piled raft foundation can be optimised by selecting suitable locations for the piles below the raft. In general, the piles should be concentrated in the most heavily loaded areas, while the number of piles can be reduced, or even eliminated, in less heavily loaded areas [ 31 ].
There are soil profiles in which piled rafts may not provide much, if any, advantage over a conventional piled foundation as follows:. Profiles with very soft clays at or near the surface of the raft, where the raft can contribute only a relatively small proportion of the required ultimate load capacity. Profiles which may be subjected to long-term consolidation settlement; in this case, the soil may lose contact with the raft and transfer all the load to the piles.
Profiles which may be subjected to expansive upward movements; in this case, the soil movements will result in increased contact pressures on the raft and the consequent development of tensile forces in the piles. There is a reluctance on the part of many foundation designers to consider the use of piled raft foundations in soft clays, for at least two reasons:. The soft clay often provides only a modest bearing capacity and stiffness for the raft, with the piles having to carry the vast majority of load.
If the soft clay is likely to undergo settlement, for example due to reclamation filling or dewatering, the soil may settle away from the base of the raft, again leaving the piles to carry the load. Despite these reservations, piled rafts have been used successfully in the past, most notably in Mexico City, where Zeevaert [ 78 , 79 ] pioneered the use of rafts and compensated rafts with friction piles. If the raft weight is lower than the effective excavation weight, the soil will still behave as an over-consolidated soil during the first stage of raising the building structure.
For compensated pile rafts, the excavation and pile installation process must be selected to suit each case. In some buildings, with shallow excavations, the piles can be executed before the excavation, from the ground level. In others, where greater depth must be achieved, part or the whole excavation is carried out first and the piles are installed once excavation is complete. The presence of groundwater can also influence the construction process. When the piles are constructed in advance of the excavation, the piles will act as anchors, reducing the tendency for bottom soil heave.
The upward soil movement will generate tensile stresses in the piles. A preliminary design, which provides an initial basis for the development of foundation concepts and costing. A detailed design stage, in which the selected foundation concept is analysed and progressive refinements are made to the layout and details of the foundation system.
This stage is desirably undertaken collaboratively with the structural designer, as the structure and the foundation act as an interactive system. A final design phase, in which both the analysis and the parameters employed in the analysis are finalised. It should be noted that the geotechnical parameters used for each stage may change as more knowledge of the ground conditions, and the results of in situ and laboratory testing, become available.
The parameters for the final design stage should also incorporate the results of foundation load tests. The following issues will generally need to be addressed in the design of foundations for high-rise buildings:.
The influence of the cyclic nature of wind, earthquakes and wave loadings if appropriate on foundation capacity and movements. Differential settlements, both within the high-rise footprint, and between high-rise and low-rise areas. Possible effects of externally imposed ground movements on the foundation system, for example, movements arising from excavations for pile caps or adjacent facilities. Dynamic response of the structure-foundation system to wind-induced and, if appropriate, wave forces.
Structural design of the foundation system, including the load-sharing among the various components of the system for example, the piles and the supporting raft and the distribution of loads within the piles. For this, and most other components of design, it is essential that there be close cooperation and interaction between the geotechnical designers and the structural designers.
There is an increasing trend for limit state design principles to be adopted in foundation design, for example, in the Eurocode 7 requirements and those of the Australian Piling Code , In terms of limit state design using a load and resistance factor design approach LRFD , the design criteria for the ultimate limit state are as follows:. The above criteria are applied to the entire foundation system, while the structural strength criterion Eq.
It is not considered to be good practice to apply the geotechnical criterion Eq. The structural and geotechnical reduction factors are often specified in national codes or standards. If any of the design requirements are not satisfied, then the design will need to be modified accordingly to increase the strength of the overall system or of those components of the system that do not satisfy the criteria.
The required load combinations for which the structure and foundation system have to be designed will usually be dictated by an appropriate structural loading code. In some cases, a large number of combinations may need to be considered.
These may include several ultimate limit state combinations and serviceability combinations incorporating long-term and short-term loadings. In addition to the normal design criteria, as expressed by Eqs. This criterion attempts to avoid the full mobilisation of shaft friction along the piles, thus reducing the risk that cyclic loading will lead to a degradation of shaft capacity.
Types of Foundation for Buildings and their Uses [PDF]
In engineering, a foundation is the element of a structure which connects it to the ground, and transfers loads from the structure to the ground. Foundations are generally considered either shallow or deep. The design and the construction of a well-performing foundation must possess some basic requirements:. Buildings and structures have a long history of being built with wood in contact with the ground. Timber pilings were used on soft or wet ground even below stone or masonry walls.
Tall building foundations: design methods and applications
Shallow foundation is commonly accepted as foundation with founding level less than 3m from ground surface. In case surface loads or surface conditions could still affect the bearing capacity, the foundation which sits on it is called shallow foundation. Pad foundation refers to the foundation which is intended for sustaining concentrated loads from a single point load such as structural columns. Strip foundation is used to support a line of loads such as load-bearing walls. For instance, closely-spaced columns render the use of pad foundation inappropriate and strip foundation may be a better alternative.
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