Methods of design and combinations

There has not been a consensus amongst geotechnical engineers over the application of limit state principles to geotechnical design. Therefore, to allow for these differences of opinion, Eurocode 7 provides for three Design Approaches to be used for the ULS. The decision on which approach to use for a particular country is given in its National Annex. In the UK Design Approach 1 will be specified in the National Annex. For this Design Approach (excluding pile and anchorage design) there are two sets of combinations to use for the STR and GEO ultimate limit states. The values for the partial factors

Table 1

Geotechnical categories of structures to be applied to the actions for these combinations of partial factors are given in Table 2 and the partial factors for the geotechnical material properties are given in Table 3. Combination 1 will generally govern the structural resistance, and Combination 2 will generally govern the sizing of the foundations.

The partial factors for soil resistance to sliding and bearing should be taken as 1.0 for both combinations.

The partial factors to be applied to the actions at the EQU limit state are given in Table 4; the geotechnical material partial factors being the same as for Combination 2 in Table 3.

For the SLS, Eurocode 7 does not give any advice on whether the characteristic, frequent or quasi-permanent combination should be used. Where the prescriptive method is used for spread foundations (see page 3) then the characteristic values should be adopted. For

Table 1

Geotechnical categories of structures

Category

Description

Risk of geotechnical failure

Examples from Eurocode 7

1

Small and relatively simple structures

Negligible

None given

2

Conventional types of structure and foundation with no difficult ground or loading conditions

No exceptional risk

Spread foundations

3

All other structures

Abnormal risks

Large or unusual structures Exceptional ground conditions

Table 2

Design values of actions derived for UK design, STR/GEO ultimate limit state - persistent and transient design situations

Table 2

Design values of actions derived for UK design, STR/GEO ultimate limit state - persistent and transient design situations

Combination Expression reference

Permanent actions

Leading variable action

Accompanying variable actions

from BS EN 1990

Unfavourable

Favourable

Main (if any)

Others

Combination 1 (Application of combination 1 (BS EN 1997) to set B (BS EN 1990))

Exp. (6.10)

1.35 Gka

1.0 Gka

1.5b Qk

-

1.5b Co,ic Qk,i

Exp. (6.10a)

1.35 Gka

1.0 Gka

-

1.5 Co,1c Qk

1.5b Co,ic Qk,i

Exp. (6.10b)

0.925d x 1.35 Gka

1.0 Gka

1.5b Qk

-

1.5b Co,ic Qk,i

Combination 2 (Application of combination 2 (BS EN 1997) to set C (BS EN 1990))

Exp. (6.10)

1.0 Gka

1.0 Gka

1.3b Qk,i

-

1.3b Co,ib Qk,i

a Where the variation in permanent action is not considered significant Gk,j,sup and Gk,j,inf may be taken as Gk b Where the action is favourable, gq, = 0 and the variable actions should be ignored c The value of C can be obtained from Table NA.A1.1 of the UK NA to BS EN 1990 (or see Table 3 of Chapter 1) d The value of J in the UK NA to BS EN 1990 is 0.925

Table 3

Partial factors for geotechnical material properties

Table 3

Partial factors for geotechnical material properties

Angle of shearing resistance (apply to tan h)

Effective cohesion

Undrained shear strength

Unconfined strength

Bulk density

Symbol

Gh

Gc'

Gcu

Gqu

Gg

Combination 1

1.0

1.0

1.0

1.0

1.0

Combination 2

1.25

1.25

1.4

1.4

1.0

direct methods of calculation the frequent combination can be used for sizing of foundations and the quasi-permanent combination can be used for settlement calculations.

Further information on design combinations can be found in Chapter 1, originally published as Introduction to Eurocodes5.

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