Foundations eccentricity

If the plant engineer is a specialist in anything, it is in his/her own plant or facility. Plant engineers must learn to know their own plants thoroughly, from the geology underlying its foundations and the topology of the rainwater runoff to the distribution of its electricity and the eccentricities of its production machinery. They must ensure the quality of the environment both inside... 

There are two forces acting on foundations of the type under consideration the dead load, acting in a vertical direction and the wind load, acting in a horizontal direction. The combined action of these two forces has the same effect as an eccentric vertical load. It is not necessary to calculate the eccentricity to determine the stability of the foundation. However, because there are definite relationships between eccentricity and stability, they will be explained as a matter of interest. 

For octagonal bases, usually used for tower foundations, the maximum allowable eccentricity becomes... 

The stability of this foundation will now be calculated on the basis of the eccentricity for the purpose of comparing the two methods. 

Because the actual eccentricity (1.59) is less than the maximum permissible eccentricity (1.65), the foundation is stable. This confirms the conclusion reached in Example 1. 

In order to allow the eccentricity e of the resultant load R on the base of the foundation of width B, an effective foundation width B given by B = B - 2e is used. For a double eccentricity of the load, an effective contact area can be determined in such a way that its center of gravity coincides with the resultant of the load. Both effective length and width are calculated from the following relationships ... 

Owing to shear stresses induced in the foundation soils by the vertical eccentric foxm-dation load, the magnitude of horizontal load that the foundation soils can sustain may decrease witii an increase in vertical load. This fact must be recognized when computing and evaluating sliding resistance by the simplified procedures described below. 

Foundation tilt may develop during installation owing to uneven skirt penetration resistance or sloping seafloor. It is desirable to keep the platform level, and for most platforms this has been attempted by applying eccentric ballasting during installation. 

Mats, which generally cover the total area of the platform, are usually A-shaped. The thickness is about 3 m. The performance of mat-supported rigs has been described by Hirst et al. (1976), and by Young et al. (1981). They have the disadvantages that they require a relatively level bottom and that they are affected by differential settlements caused by eccentric or cyclic loading and by scour. However, the advantage is that a mat foundation can be used in very soft soil because of the large surface area. 

Each column anchor rod (anchor bolt) assembly, including the column-to-base plate weld and the column foundation, shall be designed to resist a minimum eccentric gravity load of 300 pounds (136.2 kg) located 18 inches (.46m) from the extreme outer face of the column in each direction at the top of the column shaft. 

As depicted in Figure 8.2, eccentrically loaded shallow foundation experiences rotation (a) about the central axes of the contact surface. This rotation occurs as a result of nonuniform settlement along the transverse direction due to nonuniform contact stress caused hy unbalanced moment about the longitudinal central axis of the bottom surface of the foundation. This type of foundation rotation results in the titling of the supported structure. Tilt is defined as... 

A review of the literature to trace back the original basis for the limiting eccentricity requirements for shallow foundations will assist in the understanding of this issue. 

On the basis of the above discussion, stabihty against both traditional bearing capacity failure and rotational failure will be prevented provided shallow foundations are designed to achieve adequate FS against soil bearing failures evaluated by considering all possible combinations of the total vertical loads and the eccentricities of the resultant loads. 

In summary, the requirements that the eccentricity (e) of B/6 and B/4 are appropriate for use in the WSD and LRFD service limit state design of shallow foundations in competent soils with low compressibility and rock, respectively. The rotational stabihty evaluation of shallow foundations for seismic and LRFD strength and extreme limit state design should be performed based on the evaluation of the soil bearing stabihty under the extreme eccentricity and total vertical load combinations. The design must ensure that the maximum factored uniform bearing stress is less than or equal to the factored... 

Shear buckling of the vertical wall Where a squat silo (low aspect ratio) is either eccentrically filled (unsymmetrical top pile producing different heights of solid-wall contact) or is subjected to seismic excitation, the wall can buckle in shear near the foundation. These buckles have a characteristic diagonal stripe shape, but these load cases are relatively rare. 

Normally, frictional resistance and the shear capacity of the anchors used in a foundation adequately resist column base shear forces. In some cases, however, the engineer may find the shear force too great and may be required to transfer the excess shear force to the foundation by another means such as shear lugs. If the total factored shear loads are transmitted through fiiction plus shear lugs, the anchors need not be designed for shear, but the eccentricity induced by the couple of the applied shear and the shear lug resultant force should be taken into account when designing the anchor for tension. 

This formula is based on a certain failure mode of the soil underneath the foundation as shown in Figure 8.14. The hterature presents different failure modes and formulas for the above-mentioned factors. Reference is made to Bowles (1997) or Eurocode 7, Part 1, Annex D (2004) for specific formulas. Special consideration is required in case of eccentric and inclined loading conditions. These specific situations will not be discussed in this manual. Reference is made to relevant geotechnical textbooks. 

Meyerhof G. G. The bearing capacity offoundations under eccentric and inclined loads, 3rd International conference on soil mechanics and foundation engineering, Zurich, Switzerland, 440-445,1953. 

Step 2 Calculate the equivalent dimensions for the footing to account for the load eccentricity, which is caused by the seismic moments applied to the foundation in both directions. The vertical load can be transferred to an eccentric position defined by e = MhlQ and ei = MilQ, where Q is the central vertical load due to seismic load plus other service loads ... 

It is known from basic principles of strength of materials that if the eccentricity in one direction is less than 1/6 of the foundation s length in that direction, the footing is in compression throughout. As eccentricity exceeds this value, a loss of contact occurs. The concept of effective width was introduced by Meyerhof (1953) who proposed that at the ultimate bearing capacity of the foundation, it could be assumed that the contact pressure is identical to that for a centrally loaded foundation but of reduced width. The reduced... 

Besides, interdisciplinarity in this field is more desirable. For instance, a structural engineer may commit a considerable mistake if they neglect the soil nonlinearity, due to which the rotation center of the foundation of an added wall may lay in a quite eccentric position (see section Introductory Example ). 

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