- Minimum Fillet Weld Chart
- Minimum Throat Thickness Of Fillet Weld Wire
- Fillet Weld Throat Size Calculation
How many fillet welds do you specify on any given job? Are you confident that you have a complete and thorough understanding of the intent behind the code provisions as they apply to the maximum size fillet weld allowed? The use of fillet welds can get so repetitive and instinctive that we often specify them without thought to determining whether their design thickness aligns with the intent of the code provisions.
In the February 2017 SEU Core Session, Duane Miller, PE, from The Lincoln Electric Company, presented Welding Myths that Structural Engineers Need to Know About. Duane covered a variety of topics that are often misunderstood by structural engineers regarding the design and detail of welded connections. As he pointed out, many of these myths have some roots in truth, but some are only partly true or can be completely false.
But according to clause 4.5.2(2), the effective throat thickness of a fillet weld should not be less than 3mm, therefore, adopt 4mm effective throat thickness. By Simplified Method The design resistance of a fillet weld may be assumed to be adequate if, at every point along its length, the resultant of all the forces per unit length transmitted. = Throat area of fillet weld mm 2, in 2 B = Width of weld mm, in s = Web thickness mm, in L = Length of weld mm, in r = Weld radius mm, in t = Flange thickness mm, in H = Height of fillet weld mm, in a = Fillet weld cross sectional area.
The eighth myth that Duane addressed states that ‘Fillet Welds cannot be larger than the thickness of the steel on which they are placed,' or ‘Fillet welds cannot be larger than 1/16' less than the thickness of the steel on which they are placed.' These statements are rooted in truth, but cannot be applied universally.
It is true that in AISC 360-10 Section J2.2b states that 'The maximum size of fillet welds of connected parts shall be: (a) Along edges of material less than ¼' (6mm) thick; not greater than the thickness of the material, (b) Along edges of material ¼' (6mm) or more in thickness; not greater than the thickness of the material minus 1/16' (2mm), unless the weld is especially designated on the drawings to be built out to obtain full-throat thickness. In the as-welded condition, the distance between the edge of the base metal, and the toe of the weld is permitted to be less than 1/16 in. (2mm) provided the weld size is clearly verifiable.' However, Duane explained the intent of the code provision and the history behind this mandate.
In the past, it became apparent that some welders were creating subpar welds when the weld was placed at a lap joint. The welder was able to melt away the corner portion of the steel and create a weld that visually looked to be the correct size, but actually did not have the necessary required throat thickness. To address this situation, the Code was modified to require that the maximum weld thickness be 1/16' less than the thickness of the steel, to prevent these subpar welds from occurring. This provision specifically applies to only those circumstances where the weld occurs at the edge.
Duane gave an example where this provision does NOT apply, such as in the case of a tee joint connection as shown on the slide below. In this instance, the engineer can design the fillet weld to any size needed, since the weld does not occur at the edge of the steel, but rather on the surface of the steel. This illustrates a great reminder that we, as engineers, should always keep in mind the intent of the code provisions, and avoid reflexive designs from habit rather than solid understanding.
Preamble: this article has been published in the Oct 2017 issue of the Canadian Welding Association's Welding Journal.
Some cynicism is intended in the title of this article as I have often seen mistakes made or shortcuts taken in assessing the humble, everyday fillet weld. I also intend to clarify with examples how to accurately measure fillet welds with respect to their size and shape.
Weld Features to Measure
I do need to emphasize that this article will deal with fillet welds measured by 'Leg Length' which is the standard dimensional feature specified on engineering drawings, at least in North America. This refers to distance from the root to toe and should be measured in both directions. In a future article, I will review how measuring fillet weld size based on throat dimension instead of leg size can have both engineering and productivity benefits.
To clarify the features specified in the symbol below that can be inspected for, on the arrow side of the joint a fillet weld is required with a 1/4' leg size and for a finite length of 8' (location may be noted on dwg). On the other side of the joint an unequal leg length fillet weld is required measuring 1/4 x 3/8'.
Assuming the length feature should be a straight forward measurement, let's look at how to properly measure leg length. The leg size is the length of the legs of an imaginary triangle that can be inscribed within the actual weld as shown with feature L1 and L2 in the image below for both a convex and concave fillet weld. In the case of the convex fillet weld shown on the left, the measured leg (L) is equivalent to the size (S). In the case of the concave fillet shown on the right, the measured leg (L) provides an untrue measure of weld strength which is why these welds are instead assessed based on throat dimension to determine effective weld size (S). More discussion on the various throat dimensions will follow in a future article.
Gauges
In most cases, a standard fillet weld gauge set is used to verify that the leg length matches the specified size. These gauges may not measure exact size but instead verifies that the specified size has been attained. The standard set is shown below along with some other gauges that may be used. In cases where larger or non-standard fillet sizes are specified beyond gauge capabilities, special gauges may be required.
When using the standard fillet weld set, the weld should first be visually examined and if it appears to be flat or convex, it should be assessed based on leg length only as shown below. The image below shows a fillet weld being inspected to verify the specified 3/8' leg size. If the fillet instead appears to be concave, the weld should instead be assessed using the side of the gauge with the centre tab which would need to touch the face of the weld.
Concavity
In the next scenario, the measured leg length is again 3/8' but the weld should be rejected as undersize due to its concave profile resulting in an effective weld size of only 1/4' as shown on the right below. The black lines you see on the gauge points to the effective toe of the imaginary triangular fillet.
Minimum Fillet Weld Chart
Convexity
While the above example illustrates how concave fillet profiles should be scrutinized, fillets that are convex may also be rejected due to profile or shape. At first glance, this fillet appears to be acceptable based on a specified leg length of 1/4'. In examining its profile however, noticeable convexity is evident. Listed below the image is convexity criteria from one structural welding code which should be used to assess this weld.
Convexity Criteria: Convexity, C, of a weld or individual surface bead shall not exceed 0.07 times the actual face width of the weld or individual bead, respectively, plus 1.6 mm (1/16 in).
The criteria above is particularly important for cyclic loading applications as increased convexity adds to the stress riser or notch effect at the toe of the fillet that can be a location for crack initiation. While many inspectors may make a judgement call based on experience or best guess, the image below shows how a 1/4' convex fillet should be assessed to evaluate if convexity is excessive. This will require the proper gauges to measure actual throat and yes, some mathematical calculations. As shown, the maximum measure throat dimension would be 0.264' based on this calculation. A proper gauge measuring the actual throat dimension should be used to assess if this criteria has been met.
Gaps
Minimum Throat Thickness Of Fillet Weld Wire
One last thing … that in fact, should have been the first thing the inspector did prior to welding. If there is a gap between members being joined, the measurement methods already mentioned may not state the effective weld size. Listed below is criteria from one structural welding code that places limitation on gaps between parts being joined via fillet welds. In some cases the measured gap would be subtracted from the measured fillet leg while gaps in excess of 3/16 may cause outright rejection.
Fillet Weld Assembly Criteria: The parts to be joined by fillet welds shall be brought into as close a contact as practicable. The separation between parts shall normally not exceed 5 mm (3/16 in) except in cases involving shapes and plates 75 mm (3 in) thick or greater when, after straightening and in assembly, the gap cannot be closed sufficiently to meet this tolerance. In such cases, there may be a maximum gap of 8 mm (5/16 in), provided that a sealing weld or suitable backing material* is used to prevent melt-through. If the separation is 2 mm (1/16 in) or greater, the leg of the fillet weld shall be increased by the amount of the separation.
In performing fitup and preparation inspection, inspectors noticing gaps between parts to be joined should use a proper gauge to measure the magnitude of gap.
In the case of the fillet weld measured with a 1/4' leg length as shown on the left below, the weld would be rejected as undersize based on the criteria listed above. Based on the measured gap of 5/64' a measured leg of 21/64' would be result in an effective fillet size of 1/4'.
Fillet Weld Throat Size Calculation
In my next article, I will look more closely at how fillet welds can be assessed based on throat dimension and emphasize important benefits this can provide. In the meantime, make sure you have fresh batteries in your flashlight, a proper suite of gauges in your inspection kit and a diligent mindset as you tackle your inspection chores.
