construction usually includes（Allowable Settlement）

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• Allowable Settlement
• usually的句型结构

Allowable Settlement

The allowable settlement is defined as the acceptable amount of settlement，and it usually includes a factor of safety. In many cases the allowable settlement of the project will be provided by the structural engineer. As previously mentioned，at the start of a project the structural engineer should be consulted about the anticipated loading conditions，allowable settlement of the structure，and preliminary thoughts on the type of foundation. The ideal situation would be to obtain this information directly from the structural engineer.

If this information is unavailable，then the geotechnical engineer will have to estimate the allowable settlement of the structure in order to determine an appropriate type of foundation. There is a considerable amount of data available on the allowable settlement of structures. For example，it has been stated that the allowable differential and total settlement should depend on the flexibility and complexity of the structure including the construction materials and type of connections.

In terms of the allowable settlement，Coduto stated that it depends on many factors， including the following:

·Type of construction. For example，wood-frame buildings with wood siding would be much more tolerant than unreinforced brick buildings.

·Use of the structure. Even small cracks in a house might be considered unacceptable， whereas much larger cracks in an industrial building might not even be noticed.

·Presence of sensitive finishes. Tile or other sensitive finishes are much less tolerant of movements.

·Rigidity of the structure. If a footing beneath part of a very rigid structure settles more than the others，the structure will transfer some of the load away from the footing. However， footings beneath flexible structures must settle much more before any significant load transfer occurs. Therefore，a rigid structure will have less differential settlement than a flexible one.

Coduto also stated that the allowable settlement for most structures，especially buildings， will be governed by aesthetic and serviceability requirements，not structural requirements. Unsightly cracks，jamming doors and windows，and other similar problems will develop long before the integrity of the structure is in danger. Because the determination of the allowable settlement is so complex，engineers often rely on empirical correlations between observed behavior of structures and the settlement that results in damage.

Skempton and MacDonald studied 98 buildings，where 58 had suffered no damage and 40 had been damaged in varying degrees as a consequence of settlement. From a study of these 98 buildings，Skempton and MacDonald in part concluded the following:

The cracking of the brick panels in frame buildings or load-bearing brick walls is likely to occur if the angular distortion of the foundation exceeds 1 /300. Structural damage to columns and beams is likely to occur if the angular distortion of the foundation exceeds 1 /150.

The angular distortion criteria of 1 /150 and 1 /300 were derived from an observational study of buildings of load-bearing-wall construction，and steel and reinforced-concrete-frame buildings with conventional brick panel walls but without diagonal bracing. The criteria are intended as no more than a guide for day-to-day work in designing typical foundations for such buildings. In certain cases they may be overruled by visual or other considerations.

Concerning allowable settlement， Terzaghi stated: Differential settlement must be considered inevitable for every foundation，unless the foundation is supported by solid rock. The effect of the differential settlement on the building depends to a large extent on the type of construction.

Terzaghi summarized his studies on several buildings in Europe where he found that walls18 m ( 60 ft) and 23 m ( 75 ft) long with differential settlements over 2. 5 cm ( 1 in) were all cracked，but four buildings with walls 12 － 30 m ( 40 － 100 ft) long were undamaged when the differential settlement was 2 cm ( 3 /4 in) or less. This is probably the basis for the general design guide that building foundations should be designed so that the differential settlement is 2 cm ( 3 /4 in) or less.

Settlement versus cracking damage. Table 12. 3 summarizes the severity of cracking damage versus approximate crack widths，typical values of maximum differential movement Δ，and maximum angular distortion δ / L of the foundation. The relationship between differential settlement Δ and angular distortion δ / L is based on the equation Δ = 350δ / L.

Table 12. 3 Severity of cracking damage

continued

In assessing the severity of damage for an existing structure，the damage category ( Table 12. 3) should be based on multiple factors，including crack widths，differential settlement，and the angular distortion of the foundation. Relying on only one parameter，such as crack width， can be inaccurate in cases where cracking has been hidden or patched，or in cases where other factors ( such as concrete shrinkage) contribute to crack widths.

Component of lateral movement. Foundations subjected to settlement can be damaged by a combination of both vertical and horizontal movements. For example，a common cause of foundation damage is fill settlement. Figure 12. 4 shows an illustration of the settlement of fill in a canyon environment. Over the sidewalk of the canyon，there tends to be a pulling or stretching of the ground surface ( tensional features ) ， with compression effects near the canyon centerline. This type of damage is due to two-dimensional settlement，where the fill compresses in both the vertical and horizontal directions.

Another common situation where both vertical and horizontal foundation displacement occurs is at cut-fill transitions. A cut-fill transition occurs when a building pad has some rock removed ( the cut portion) ，with a level building pad being created by filling in ( with soil) the remaining portion. If the cut side of the building pad contains nonexpansive rock that is dense and unweathered，then very little settlement would be expected for that part of the building on cut. But the fill portion could settle under its own weight and cause damage. For example，a slab crack will typically open at the location of the cut-fill transition as illustrated in Figure 12. 5. The building is damaged by both the vertical foundation movement ( settlement) and the horizontal movement，which manifests itself as a slab crack and drag effect on the structure.

Figure 12. 4 Fill settlement in a canyon environment

Figure 12. 5 Cut-fill transition lot

In the cases described above，the lateral movement is a secondary result of the primary vertical movement due to settlement of the foundation. Table 12. 3 can therefore be used as a guide to correlate damage category with Δ and δ / L. In cases where lateral movement is the predominant or critical mode of foundation displacement，Table 12. 3 may underestimate the severity of cracking damage for values of Δ and δ / L.

usually的句型结构

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