1、UDC 624.1 51.5.042 DEUTSCHE NORMEN September 1974 iiI 1 I Subsoil Calculation of the Bearing Pressure Distribution under Spread Foundations DIN 401 8 3 Term 1 4.1 General information 1 Data 1 - a c Ul - c o II a 4.2 Indication and representation of the subsoil . 1 4.3 Parameters of the subsoil . 2 4
2、.4 Decisive calculation values 2 6 Calculating procedures 2 6.1 General . 2 6.2 Existing bearing pressure distributions. 2 .- $1 5 Rigidity of the structure . 2 1 0 c E P. 5 ._ .- 1 Scope The Standard deals with the principles and calculating procedures for the determination of the bearing pressure
3、distribution under spread foundations on cohesive and non-cohesive soils in the case of dwellings and commer- cial buildings, industrial buildings, warehouses, tanks, etc., subjected to predominantly perdendicular loads. - P 8 5 5 5 % E i g .E i .$ c 2 2 n d r P The following calculating procedures
4、may be used as approximation methods in cases where the boundaries between the strata under the spread foundation are roughly horizontal and plane. Even in such cases, how- ever, it is to be expected that the subsoil will behave differently from one point to another. For this reason, the effects of
5、fault zones must be taken into account, either when projecting the calculation or - should this not be possible - in the result. In unfavourable cases, it is to be assumed that there is, as it were, support on only certain points of the more resistant zones of the subsoil. 2 Purpose The purpose of t
6、he Standard is to make it possible to design spread foundations on a uniform basis. For this purpose, simplified assumptions and the introduction of average values are necessary. In consequence of the sensitivity of the bending moments of spread foun- dations to small changes in the bearing pressure
7、 distri- bution which can be only approximately determined and in view of the influence of opposed vertical displacements of columns and walls, such calculations cannot be made with the same accuracy as is the case in other sectors of civil engineering. Even so, it is hoped that the static system an
8、d deformations therein are treated with the greatest possible approximation to actuality. Page 6.2.1 Linearly limited bearing pressures . 2 to Boussinesq 2 6.2.2.1 General 2 6.2.2.2 Biaxially stiffened structures 2 6.2.2.3 Uniaxially stiffened structures . 2 6.2.3 Distribution corresponding to loads
9、 . . 2 6.2.2 Bearing pressure distribution according 6.3 Bearing pressure distribution 6.3.1 6.3.2 Consistency module method 6.3.3 Combined method (halfspace and dependent on deformation . 2 Bedding module method (spring model) 3 (halfspace model) 3 spring model) 3 3 Term Spread foundations, within
10、the meaning of this Standard, are mat foundations and strip footings for which an analysis of the bending moments is required. 4 Data The following data must be available for the calculation of spread foundations. 4.1 General information Information on the foundation depth, the overall design of the
11、 structures, the magnitude and nature of loads imposed on the foundation for the various loading cases and the location in respect of neighbouring structures. The forces acting also include the loading on the foun- dation due to uplift. 4.2 Indication and representation of the subsoil DIN 4021 Part
12、1 Subsoil; investigation by excavation and boreholes and by extraction of samples, indications in the soil DIN 4022 Part 1 Subsoil and ground water; desig- nation and description of types of soil and rock; layer diagram for investigation and boring without continuous extraction of cored samples DIN
13、4023 Subsoil boring and water boring; graphic representation of the results (Subsequent edition, at present still in draft form) DIN 4094 Part 1 Subsoil; pile driving and pressure probe equipment, dimensions and mode of operation of the equipment Continued on pages 2 and 3 Sole sale rights of German
14、 Standards (DIN-Normen) are with Beuth Verlag GmbH, Berlin 30 and Kln 1 DIN 40 18 engl. Prekgr. Vertr.-Nr. 0104 01.81 Page 2 DIN 4018 DIN 4094 Part 2 (Preliminary Standard) Subsoil; pile driving and pressure probe equipment, notes on use DIN 4107 -; settlement observations on structures either compl
15、ete or under construction 4.3 Parameters of the subsoil Information on the modules of consistency or bedding. The consistency module can be ascertained from com- pression tests or empirically (see DIN 4019 Part 1). The mean bedding module beneath the structure can be deter- mined by means of a settl
16、ement computation in accord- ance with DIN 4019 Part 1 for a rigid body of represen- tative dimensions by dividing the mean bearing pressure causing the settlement by the settlement, or it can be obtained empirically, e. g., by measurements on the structure. 4.4 Decisive calculation values For purpo
17、ses of approximation, it is sufficient, in the case of homogeneous soil, to assume a constant module of bedding or consistency. It is obtained by averaging the parameters (see Section 4.3) which can normally only bedetermined at relatively few points in the subsoil. The accuracy with which the calcu
18、lation values reflect the actual behaviour of the soil is of paramount import- ance to the reliability of the calculation. In some cases, differing values beneath the ground plan must also be reckoned with. It is recommended that, when fixing the calculation values in difficult cases, a testing cent
19、re should be consulted. 5 Rigidity of the structure Because deformation of the structure exerts a major influence on the bearing pressure distribution, the rigidity of the structure as a whole (slab and load- bearing superstructure) is to be approximately estimated in both axial directions and used
20、as the basis of the bearing pressure computation. If the structure is adequately stiffened in three dimen- sions by external walls and load-bearing internal walls or by other statically effective structural members, the structure as a whole can be deemed to be rigid. Depend- ing on its distribution,
21、 the bearing pressure then exerts greater or lesser supplementary strains on the stiffening structural members. 6 Calculating procedures 6.1 General For practical purposes, there are at present four methods: a) The statically determined trapezoidal stress method, b) the bedding module method, c) the
22、 consistency module method and d) the combined method. Methods b) to dl are statically indeterminate. None of the methods listed provides an exact reproduction of the actual situation in regard to forces and deformation. However, if the method and the calculation values are judiciously selected, the
23、y will suffice as a basis for design. The calculations presuppose that, apart from limited local edge disturbances, the mean contact pressures do not stress the subsoil to the point of flow or failure. This can always be assumed provided that the bearing pressures permissible in accordance with DIN
24、1054 are not exceeded. Because of the occurrence of plastic deformations of the subsoil, the calculated peak stresses, e. g., at the edge of the foundation, must be reduced to correspond approxi- mately to the prevailing circumstances. In so doing, the conditions of equilibrium must be preserved. Ca
25、re is also to be taken that tensile stresses do not arise in the foundation bottom. 6.2 Existing bearing pressure distributions 6.2.1 Linearly limited bearing pressures This simple assumption suffices in the case of light structures with adequately uniform load distribution. It can, however, lead to
26、 unnecessarily large dimensions of the foundation, although this is not so if the loads are concentrated in the central region of a very rigid spread foundation (see Section 6.2.2). 6.2.2 Bearing pressure distribution according to Boussinesq 6.2.2.1 General The bearing pressure distribution beneath
27、very rigid structures may be calculated according to Boussinesqs equations provided that it can be at least approximately assumed that, immediately beneath the foundation bottom, there is a stratum extending deep down which has a uniform and stable consistency module E, and the thickness of which is
28、 greater than the width of the foundation under investigation. As the thickness of the stratum diminishes, and provided that the compressibility of the deeper strata can be dis- regarded, the pressure distribution gradually approaches an even contact pressure. This applies in particular to extended
29、ground areas. 6.2.2.2 Biaxially stiffened structures The bearing pressure distribution is obtained in accord- ance with Section 6.2.2.1. An averaged bearing pressure distribution can be assumed for the individual mat areas and the calculation effected as for a flexible mat. 6.2.2.3 Un i ax i a I I y
30、 stiff en ed structures The bearing pressure distribution in the direction of the stiffening is roughly in accordance with Section 6.2.2.1. It gives rise to supplementary stresses in the stiffening members. The bearing pressure distributions at right angles to the stiffened direction can be determin
31、ed by one of the methods described in Section 6.3. If, for this purpose, the calculation of settlements is necessary, it is sufficient to determine them for one characteristic cross-section (Fig. 1). This presupposes that bending of the mat parallel to the stiffening members can be disregarded. It i
32、s, however, possible to take it subsequently into account on the basis that the calculated mean reinforcement for the characteristic cross-section is distributed to correspond to the bearing pressure distribution in the stiffened axis. 6.2.3 Distribution corresponding to loads Very yielding parts of
33、 a structure approach the border- line case of a structure which is slack in comparison with the subsoil and where the area loads and the bearing pressure are of equal magnitude. In this case, the deflec- tion of the spread foundations is equal to thesettlements occurring beneath the existing area l
34、oads. 6.3 Bearing pressure distribution Should the simplified methods described above not be applicable, the following possibilities are at present available: dependent an deformation DIN 4018 Page 3 Section K - K I Calculated bearing pressure distribution (flexible axis) p- b -_- 1 I Characteristic
35、 cross-section , I p- 627 / Bearing pressure distribution under the longitudinal stiffening (rigid axis) Figure 1. Bearing pressure distribution under a uniaxially stiffened structure If, ,If2 Points of intersection of the changes in shape (settlement troughs) S1 , S2 Points of intersection of the c
36、ontact pressures 6.3.1 Bedding module method (spring model) The basis of the bedding module method is that the bearing pressure is assumed to be proportional to the related sinking of the foundation. It provides adequately accurate results in the case of long flexible foundation beams and extended f
37、lexible mat foundations having only few individual loads the points of application of which are movable in terms of their relative elevation, and also in the case of a consistency module which increases linearly from zero with increasing depth; it also applies to thin, soft strata on a hard base. 6.
38、3.2 Consistency module method (halfspace model) The basis of the consistency module method is that that particular bearing pressure distribution is sought at which the shape of the deflection area of the foundation conforms as closely as possible to the shape of the settlement trough of the subsoil.
39、 It is applicable to any stratification of the subsoil. In the case of simple calculations, it is usually assumed that the subsoil has a constant consistency module. This is most likely to be the case for cohesive soils with very thick strata. 6.3.3 Combined method (halfspace and spring model) Combined solutions, incorporating the bedding module and the consistency module methods, have been devel- oped for a consistency module which increases linearly, but not from zero, with increasing depth.
