AD171 Partial Penetration Butt Welds

8/15/2019 AD171 Partial Penetration Butt Welds

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SCI ADVISORY DESK

AD 171: Partial penetration butt welds

Questions arise from time to time about weld design that indicate some confusion betweenwhat are called in BS 5950 “partial penetration butt welds” and what BS 5950 refers to as a

“partial penetration butt weld with a superimposed fillet weld”.

BS 449 recognises only the single-sided type of partial penetration butt weld, and then only

with very specific geometry, see Figure 1. It actually calls it an “incomplete penetration butt

weld” and it requires an actual penetration of at least 7/8 of the thickness of the material,

then takes account of the eccentricity of the throat relevant to the centreline of the material

by requiring it to be treated as effectively only 5/8 as strong as the material, see clause 54(b).

t 7/8 t t 7/8 t

a) V preparation b) U preparation

Fig. 1. “Incomplete” (partial) penetration butt weld to BS 449.

Both clause 6.6.6.2 of BS 5950: Part 1, and the first paragraph of clause 6.6.6.3, refer to

the types of weld shown in Figure 2. Unlike BS 449, double-sided partial penetration butt

welds are covered in BS 5950. They are referred to as “welded from both sides” and are

illustrated in Figure 2(a).

The single-sided type shown in Figure 2(b) is referred to as welded from one side. It differsfrom that in Figure 1 only to the extent that the degree of penetration is not rigidly fixed as in

BS 449. Clause 6.6.6.3 calls for allowance to be made in design for the actual eccentricity of

the throat relative to the centreline of the material. However, this provision is not limited to

single-sided welds and would also apply, for example, in the unlikely case of a double-sided

partial penetration butt weld having two unequal throats.

Advisory Desk Notes - AD 171: Partial penetration butt welds

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SCI ADVISORY DESK

a) Double-sided partial penetration butt welds

b) Single-sided partial penetration but welds

c) Single-sided partial penetration corner welds

Fig. 2. Partial penetration butt welds to BS 5950

A different single-sided type is illustrated in Figure 2(c). This represents the corner weld of a

welded four-plate “box” section, as sometimes used for heavy columns. An interesting

feature of such welds is that, whereas they are clearly needed to hold the plates together,

normal design calculations for an axially loaded column do not predict any forces in the

welds. Even when account is taken of the second order effects involved in “strut action”, the

forces in the welds are small, which would leave designers puzzled as to what sizes to

specify for them. To avoid this, 6.6.6.2 gives a rule for a minimum throat size of t 2 ,

where t is the smaller plate thickness (and must be in millimetres). This rule was originally

introduced in American specifications to avoid welds that were so small compared to the

plate that they would cool too rapidly, with a consequent risk of cracking.

The same rule is applied to the minimum throat for the case shown in Figure 2(b) and for

each throat as shown in Figure 2(c). The rule is useful for these other cases to the extent

that it avoids designers specifying small welds that may cause problems, but in modern

conditions welding engineers can apply procedures, such as preheat, that do not need this

minimum size limit. Thus, current American specifications no longer include such a rule, so

the only real justification is to avoid “zero-size” welds at the corners of box columns.

Clause 6.6.6.2 also discusses how the depth of penetration to be used in calculations should

be determined. In the case of J or U weld preparation (dimensioned properly - as

recommended in welding specifications), it is reasonable to expect that the welder will have

no problem in producing a weld with at least the specified penetration to the root of the weld

penetration, and so this depth can be used in calculations. On the other hand, if a V or bevel

weld preparation is used, it is wiser to allow for the possibility that the weld penetration

actually achieved may be less than the depth of penetration, and BS 5950: Part 1

recommends deducting 3 mm from the depth of preparation.


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SCI ADVISORY DESK

Fig. 3. T-butt joint – partial penetration butt weld with a superimposed fillet weld.

Where appropriate tests are done to verify the actual penetration, logically this could be used

instead. BS 5950: Part 1 Clause 6.6.6.2 recognizes this in the case of J or U weld

preparations, but as written does not apply it to V or bevel preparations. This appears illogical

and it seems possible that this exclusion was unintentional, or that it may have been due to a

desire to promote the wider use of J and U preparations. This latter might also explain why

the recommended deduction from the preparation depth is 3 mm compared to 2 mm inEurocode 3 Part 1.1.

Confusion sometimes seems to arise in the case of the type of weld shown in Figure 3. This

is sometimes called a T-butt joint (for example in Eurocode 3: Part 1.1) but it is referred to in

BS 5950: Part 1 as “partial penetration butt weld with a superimposed fillet weld” in Clause

6.6.5.5, which classes it as modified fillet weld. It is referred to in Clause 6.6.6 “Design of

butt welds” only in the second paragraph of 6.6.6.3, which recommends that its capacity

should be calculated as for a fillet weld, as given in 6.6.5.

It is not really clear, and perhaps open to interpretation, whether or not the rules in 6.6.6.2

for relating the depth of penetration to the depth of preparation should be applied in a T-butt joint. Provided that the angle between the fusion faces complies with 6.6.5.4, and the throat

size is determined in accordance with 6.6.5.3, there is no obvious reason why such a weld

should be treated differently from a standard fillet weld. Certainly, the provision for a

minimum specified penetration of t 2 has no relevance to this type of weld.

There are other anomalies between clauses 6.6.5 and 6.6.6, not least the inclusion of a

provision to allow for deep penetration (where this is demonstrated to be consistently

achieved) for a partial penetration butt weld but not for a fillet weld, where it is more likely to

be relevant. It is particularly relevant to the T-butt case of a “partial penetration butt weld

with a superimposed fillet weld”, and this may be why there is a tendency to try to apply

Clause 6.6.6.2 to such welds. Amendment is clearly needed to improve the clarity andconsistency of these clauses.


Discuss me ...

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AD171 Partial Penetration Butt Welds