Tuesday, December 11, 2012

Structures: Settlement.

If the building settles excessively, particularly differentially – e.g. adjacent columns settling by different amounts – the settlement may be serious enough to endanger the stability of the structure, and would be likely to cause serious serviceability problems.

Less serious settlement may still be sufficient to cause cracking which could affect the building’s weathertightness, thermal and sound insulation, fire resistance, damage finishes and services, affect the operation of plant such as overhead cranes, and other  serviceability factors. Further-more, settlement, even relatively minor, which causes the building to tilt, can render it visually unacceptable. (Old Tudor buildings, for example, may look charming and quaint with their tilts and leaning, but clients and owners of modern buildings are unlikely to accept similar tilts.)

Differential settlement, sagging, hogging and relative rotation are shown in Fig. 1.1.

In general terms it should be remembered that foundations are no different from other structural members and deflection criteria similar to those for superstructure members would also apply to foundation members.

From experience it has been found that the magnitude of  relative rotation – sometimes referred to as angular
distortion – is critical in framed structures, and the magnitude of the deflection ratio, ∆/L, is critical for load-bearing walls. Empirical criteria have been established to minimize cracking, or other damage, by limiting the movement, as shown in Table 1.2.

The length-to-height ratio is important since according to some researchers the greater the length-to-height ratio the greater the limiting value of ∆/L. It should be noted that cracking due to hogging occurs at half the deflection ratio of that for sagging. Sagging problems appear to occur more frequently than hogging in practice.

Since separate serviceability and ultimate limit state analyses are not at present carried out for the soil – it is current practice to adjust the factor of safety which is applied to the soil’s ultimate bearing capacity, in order to obtain the allowable bearing pressure.

Similarly, the partial safety factor applied to the characteristic structural loads will be affected by the usual superstructure design factors and then adjusted depending on the structure (its sensitivity to movement, design life, damaging effects of movement), and the type of imposed loading. For example, full imposed load occurs infrequently in theatres and almost permanently in grain stores.

Overlooking this permanence of loading in design has caused foundation failure in some grain stores. A number of failures due to such loading conditions have been  investigated by the authors’ practice. A typical example is an existing grain store whose foundations performed satisfactorily until a new grain store was built alongside. The ground pressure from the new store increased the pressure in the soil below the existing store – which settled and tilted.

Similarly, any bending moments transferred to the ground (by, for example, fixing moments at the base of fixed portal frames) must be considered in the design, since they will affect the structure’s contact pressure on the soil.

There is a rough correlation between bearing capacity and settlement. Soils of high bearing capacity tend to settle less than soils of low bearing capacity. It is therefore even more advisable to check the likely settlement of structures founded on weak soils. As a guide, care is required when the safe bearing capacity (i.e. ultimate bearing capacity divided by a factor of safety) falls below 125 kN/m2 ; each site, and each structure, must however be judged on its own merits.

Fig. 1.1 Settlement definitions.

 Table 1.2 Typical values of angular distortion to limit cracking (Ground Subsidence, Table 1, Institution of Civil Engineers, 1977) (2)

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