Eurocode 1993-1-8:2005 gives a designer a choice between two methods for the design of fillet welds: the simplified and the directional methods.

There is no specific guidance on which method to use and, in practice, the choice is usually made on practical grounds.

In the directional method, the force acting on the fillet weld is resolved into components parallel and transverse to the plane of the weld throat. As a result, there are uniformly distributed normal stresses and shear stresses.

** Figure 1.**

In the case from figure 1 for the directional method:

(as no longitudinal shear force is present in this case).

The design resistance of the fillet weld will be sufficient if the following are both satisfied:

where:

fu - is the nominal ultimate tensile strength of the weaker joined part;

βw - is the appropriate correlation factor taken from Table 4.1;

- is the normal stress perpendicular to the throat;

- is the shear stress (in the plane of the throat) perpendicular to the axis of the weld;

- is the shear stress (in the plane of the throat) parallel to the axis of the weld;

F - is the total force;

L - is the length of weld;

a - is the throat thickness.

The simplified method does not require determination of the stress components in the weld. The stress is taken as the vector sum of all force components and the weld strength is equal to the shear strength, independent of the direction of the force acting on it. As the weld is weakest in pure shear, this method always gives more conservative results than does the directional method.

Calculations for the Figure 1 case using the simplified method are as follows:

The design resistance of the fillet weld will be sufficient if the following is satisfied:

Where:

fu - is the nominal ultimate tensile strength of the weaker joined part;

βw - is the appropriate correlation factor taken from Table 4.1;

- is the resultant stress acting on the weld;

F - is the total force;

L - is the length of weld;

a - is the throat thickness.

Please note that the root of three on the resistance side for the simplified method treats the resultant stress as the shear stress.