# Design of neck welds of welded I-sections and box sections

## Technical Article on the Topic Structural Analysis Using Dlubal Software

### Technical Article

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. Alternatively, a surface model can also be used for the design.

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:

Longitudinal shear stress in the neck seams from shear force

$$τ∥,Vz = - Vz · SyIy·∑a$$

 τ∥, Vz Longitudinal shear stress in the neck seams from shear force Vz Shear force Sy Static moment of the connected cross -section part Iy second moment of area ∑a Sum of all weld thicknesses connected to the considered part of the cross -section

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:

Longitudinal shear stress in the flank fillet welds from torsional moment

$$τ∥,Mx = Mx2 · Am · aw$$

 τ∥, Mx Longitudinal shear stress in the flank fillet welds from torsional moment Mx torsional moment Am Core area [LinkToImage02]w Thickness of a flank fillet weld

#### Checking on the 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 efficiently read and then used for the evaluation. Alternatively, 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 with the results of the calculation in RF-STEEL EC3.

The neck seams are each selected with 5 mm and thus a maximum longitudinalshear 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 τxy 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.

τ = 3.34 kN/cm² · 8 mm/10 mm = 2.67 kN/cm²

The design in RF-STEEL EC3 provides a comparable shear stress of τ = 2.72 kN/cm².

Unfortunately, the design of the neck welds in the RF-/STEEL EC3 add-on module is not possible for user-defined cross-sections created with SHAPE-THIN.

#### Dipl.-Ing. Oliver Metzkes

Product Engineering & Customer Support

Mr. Metzkes is responsible for the development of the add-on modules for steel structures and provides technical support for our customers.

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• Updated 12 May 2021

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