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Cast-in items such as weld plates, frequently have reinforcement bars welded to them to transfer loads into the body of the concrete. This is commonly used in precast concrete for connections. Traditionally these were ‘designed’ by experience and welds were generally over specified, with failures being very rare indeed. The common type of weld used to attach a nominally round bar to a flat section is a flare bevel fillet weld as shown below.

Useful guidance on designing welds is hard to come by, and the guidance that exists is not very helpful. Until 2006, the relevant standard was BS 7123:1989 ‘Metal arc welding of steel for concrete reinforcement’. Clause 10.9.2 of this standard contained the following equation for the length of flare bevel fillet weld to carry the full tensile load of a reinforcing bar:

The same standard limits the throat thickness to a maximum of 0.3 x bar dia.  Thus, by taking 0.87 fy = 435 N/mm2 and fw =160 N/mm2, the required weld length to give the bar capacity can be calculated from the equation above.

Column 5 gives the required length of a single weld.  However, most bars are welded both sides to avoid eccentricity, and column 6 gives the required length of ‘full area’ double welds.  Welds however are not square ended, and an allowance (col 7) must be made for non-effective weld of smaller area, usually called ‘run-off’.  Typically this is (φ / 2), giving final values as shown in column 8.

The current standard, BS EN ISO 17660-1:2006 ‘Welding of reinforcing steel – Part 1: Load bearing welded joints’ simplifies this design further by simply stating that a weld length of ‘four x bar dia’ (col 9) will give full bar capacity.  Comparison of cols 8 & 9 show the close similarity of the results.   The current standard refers to these welds as ‘side lap welds’, rather than using the traditional terminology.    Unfortunately the standard gives no guidance at all on designing welds that either do not require full capacity, or cannot accommodate the full weld length.   

The standard makes no distinction over the direction of loading.  In practice, if the load is along the axis of the weld, then the weld is in shear only.  If the load is perpendicular to the axis then there is also a tension component as the weld tries to ‘roll’.  Since the weld is stronger in tension than shear, then the strength actually increases by as much as 50%, and detailed analysis can make use of this enhancement.  There is a corresponding reduction in deformation before failure, and this would need to be taken into account for situations where overload may occur.  It is though normal to use the conservative method and design on the basis of shear only.

Whilst giving basic general guidance on weld design, this guidance is not authoritative and, as with most Key2concrete data sheets, does not constitute a design.  Weld specification/design should only be carried out by competent people using the appropriate standards.  Equally important is making sure that the welding is carried out only by qualified and experienced welders using the correct materials and techniques.