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Eurocode limits Discussion on the article by M Stocker 'Eurocode 7 - all problems solved?' European Foundations issue No 1 by B Schuppener, B Walz, A Weienbach, K Hock-Berghaus, Bundesanstalt fr Wasse

The proposal by Dr Manfred Stocker can solve some of the problems in the application of Eurocode 7 part 1 (EC7-1). In particular, we appreciate the differentiation of the partial factors according to the required level. However, some problems have still not been solved and new problems have been added.

For the ultimate limit state (ULS) two separate calculations usually have to be performed, in the case of water even four. For case 1 it is assumed that the material will fail, for case 2 failure in the ground is assumed. The correct assumption for failure according to EC 1 is that the char- acteristic actions Sk can be greater (therefore multiplied by s) and the characteristic resistances Rk can be smaller (therefore divided by R). The basic inequality must then be checked:

Sk . Rk /R.

The two (or four) calculations proposed by Stocker will result in dimensions for geotechnical structures which will differ more or less for each case. These differences are not a result of different modes of failure, but of the two different safety definitions. The decision to take the more unfavourable case as relevant for design will often lead to uneconomical solutions.

It is not possible to calculate an earth pressure coefficient for slope angles red (with the reduced angel of friction red). This does not accord with practice. Moreover, failure planes calculated with red and Cred will be different from those which will most likely be relevant in the real situation. This can lead to a mis- leading interpretation of the situation.

In the design of fixed sheet pile walls in case 1 the internal forces are determined using the characteristic passive earth resistance although the latter cannot occur due to the necessary large displacements even if over- loading of the wall is assumed. Consequently the fixing moment will be grossly overestimated and the wall will have an insufficient cross- section.

Apart from the two calculations for case 1 and case 2 an additional calculation has to be performed to verify the serviceability limit state (especially for flexible retaining walls)

The introduction of a partial factor of G = 1.0 for permanent and variable actions is incom- patible with the procedure used in structural civil engineering.

Setting G = 1.0 means com- bining the partial factors G and M to form one factor in Case 1 (and Case 3). This will bring back the old global safety concept in geo- technical engineering which con- flicts with the philosophy of EC 1.

In Case 2 (and Case 4) only the partial factor M is relevant. Thus there are two different partial factors for the design of structural elements.

These problems will not occur if a proposal is accepted which has been laid down in the German comments on EC 7 developed after more than 10 year work on the concept of partial factors of safety. With regard to the design of geotechnical structural elements its main features are:

a single calculation based on the characteristic values of the actions and the resistances,

application of safety factors to neither nor c, nor directly to the actions, but to the characteristic internal forces and bending moments in the last step of the verification of the ultimate limit state (see Table 1),

load cases (LC) to account for different probabilities of failure and the need for different safety levels: Load case 1 for permanent situations, load case 2 for the stage of construction or transient struc- tures, load case 3 for accidental sit- uations concerning both actions and resistances (Table 1 and 2).

It must be clear that the partial safety factors given in Eurocode 2, Eurocode 3 and Eurocode 5 apply to design of structural elements.

The steps of the design proc- edure proposed in this concept are very similar to those put forward by structural engineers as in the verification the ground is treated in the same way as any other structural member or material:

1.Selection of the dimensions and the design system of the geotech- nical structure (footing, strutted sheet pile wall, piles etc).

2.Determination of the charact- eristic actions of the structure and of the soil, ie the most realistic and probable actions.

3.Determination of the char- acteristic internal forces Ski in structural elements (eg strut-, anchor- or supporting-forces, bending moments and stresses) and in the ground (eg the resultant characteristic forces in the base level of a footing or in the earth pressure support of a wall etc).

4.Determination of the charact- eristic resistances Rki eg:

for structural elements: the characteristic bending moment or the characteristic compressive strength according to the stan- dards for the considered material;

for soil: the characteristic bearing capacity of shallow foundations, the characteristic passive earth pressure or the characteristic bearing capacity of piles, anchors and nails determined by calculations, tests or comparable experience.

5.Verification of the ultimate limit state in every relevant cross- section of the structure and in the soil:

to obtain the internal design forces Sdi: the characteristic internal forces are factored by safety factors eg for permanent structures G = 1.35 and Q = 1.50 (see Table 1).

to obtain the internal design resistances Rdi:: the characteristic values are divided by their corresponding safety values eg for permanent structures M = 1.10 for steel (see EC2 part 1) and R = 1.40 for soil (see Table 2).

In the last step of the ultimate limit state analyses the basic equation:

Rdi Sdi is verified.

The merits of this concept are:

1. The procedure corresponds to the concept of EC2 and EC 3 for structural engineering. Thus geotechnical engineering does not need a separate concept as proposed in the EFFC's proposal. The procedure can easily be under- stood and adopted by students and practising engineers, which makes it very user-friendly.

2.As this calculation works with characteristic values of actions, which are also used for the verification of the serviceability limit state, no separate calcul- ation is necessary for the input of the determination of the displacements.

3.The concept is open for all analytical methods of verif- ication. Steps 3 and 4 allow for the classical methods, the theory of elasticity, ultimate load method, spring models, the finite element method and cinematic element method.

The proposed con- cept for geotechnical design is simpler and better than that put forward by the EFFC. It guarantees a reason- able mean safety level both for the structural material and the ground when designing struct- ural elements as the global safety factor is equal to 1.5 for steel and 2.0 for the ground for permanent situations.

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