Design of Neck Welds of Welded I-Sections and Box Sections

The RF‑/STEEL EC3 add-on module can perform the design of fillet welds for all parametric, welded cross-sections of the cross-section library. For this, the option must be activated in the detail settings of the module. As an alternative, you can also use a surface model for the design.

Loading of Neck Fillet Welds

The neck fillet welds of welded I-sections or the flank fillet welds of welded box girders are subjected to a shear force parallel to the weld axis when the beam is bent. The cause of this longitudinal shear force is that the welds prevent the flanges and the web from being displaced against each other. Only in this way can the cross-section act as a composite cross-section.

The resulting shear stress in the weld can be determined from the shear force distribution. The following formula is used for this:

τ_{∥, Vz} = - \frac{V_{z} \cdot S_{y}}{I_{y} \cdot \sum a}

τ_∥,Vz	Longitudinal shear stress in the neck seams from shear force
V_z	Shear force
S_y	Static moment of the connected cross-section part
I_y	Second moment of area
∑a	Sum of all weld thicknesses connected to the considered part of the cross-section

Longitudinal Shear Stress in Neck Seams from Shear Force

In the case of box girder cross-sections, the flank fillet welds are additionally stressed by an acting torsional moment. Here, the shear stress in the welds is determined as follows:

τ_{∥, M_{x}} = \frac{M_{x}}{2 \cdot A_{m} \cdot a_{w}}

τ_∥,Mx	Longitudinal shear stress in the flank fillet welds from torsional moment
M_x	Torsional moment
A_m	Core area
a_w	Thickness of the flank fillet weld

Longitudinal Shear Stress in Flank Fillet Welds from Torsional Moment

Activating Weld Design in RF-/STEEL EC3

Checking on Surface Model in RFEM

Another method is the design of the neck welds on a surface model in RFEM. By using line releases, the shear flow in the connection can be read efficiently and then used for the evaluation. As an alternative, you can evaluate the shear stress on the web.

In the following example, the neck welds of the welded I‑beam are designed under bending. For this, the stresses determined on the surface model are compared to the results of the calculation in RF‑STEEL EC3.

Shear Flow in Top Weld

The neck seams are each selected with 5 mm; thus, a maximum longitudinal shear stress of τ_∥ = 266 kN/m / (2 · 5 mm) = 2.66 kN/cm² results from the surface model.

An alternative evaluation results from the shear stresses τ_xy in the web. In this case, it is necessary to convert the stresses from the web thickness of 8 mm to the effective weld thickness of 10 mm. In the example, the distribution of the shear stress is evaluated over a section at the bottom weld.