Foundations allowable stress

Crete used. As a matter of convenience, Table 11-3 is included to show allowable stresses and miscellaneous constants applying to two grades of concrete quite generally used for foundations. It is strongly recommended that the 1 2 4 mixture be used in practice. The figures for the 1 2 5 mixture are shown primarily as a matter of interest. 

The structural engineer will need this load data to properly design their foundations and structures to accommodate these loads. The vessel engineer will need this data to determine the local stresses imposed by the installed transfer lines. The piping engineer will need the loads, forces and expansion data to determine if expansion joints are needed in the system. The skin temperatures are needed to determine the allowable stresses of the steel jackets. 

To fully appreciate this, we must explore the skin-doubler specimen beyond its fundamental value, i.e., that of providing the decisive shear at the doubler tip. This foundation allows us to predict the adhesive shear displacement Aa (Figure 5) at the doubler tip. We speak of this as a shear stress, because we can relate displacement to stress with thick adherend data. This has value as a practical simplification, but in reality, the adhesive at the doubler tip sees secondary and tertiary deformations. These are implicit in Aa but not precisely defined or measured. They cannot be ignored, because they must be decisive in initiating the fatigue failure. To calculate them is formidable at best, and fruitless at worst, if they are reproduced faithfully in a test specimen. We suggest that the skin-doubler specimen may faithfully reproduce these deformations, and do so in terms of correct proportions for actual structure. 

Having selected a foundation of such size and shape as to fulfill the requirements of the problem from the standpoint of stability and soil loading, it becomes necessary to calculate the stresses in the foundation itself, to see that they do not exceed the allowable limits. 

In special cases, values d > 1.0 and d > 1.0 may still be used, provided that the foundation installation procedure and other critical aspects allow for the mobilization of resisting shear stresses in the soil above the foundation level. In such cases the following expressions for d, valid for d upper limit of this contribution ... 

Geophysical crosshole tests (CHT) may be conducted in parallel cased boreholes to evaluate the profiles of compression wave (Vp) and shear wave (Vs) velocities (Wightman et al. 2003). The shear wave data allow the direct assessment of the small-strain shear modulus (Go = Pt V where pt = total mass density). The fundamental stiffness Go serves as the initial stiffness of soils, thus the beginning of all shear stress vs. shear strain curves, applicable to both monotonic and dynamic problems (Atkinson, 2000 Clayton 2011). In fact, this well-known fact is also missing from many textbooks, even though Go has been shown relevant to practical foundation problems for over 2 decades (e.g., Burland 1989). 

Using the permissible stress approach, both the demanded stresses from loading and the ultimate stress capacity of the foundation are evaluated. The foundation is considered to be safe as long as the demanded stresses are less than the ultimate stress capacity of the foundation. A factor of safety (FS) of 2-3 is usually applied to the ultimate capacity to obtain various allowable levels of loading in order to limit the displacements of a foundation. A separate displacement analysis is usually performed to determine the allowable displacements for a foundation, and for the bridge structures. Design based on the permissible concept is still the most popular practice in foundation design. 

---