14
Welding
14.1 Introduction General information on welds and welding is given in the Steel Construction Handbook (Ref. 5), which covers welding processes, electrode classifications and strengths, plate edge preparations, etc. A more detailed treatment of the structural aspects of welding is included in the book Structural Steelwork Connections – Limit States Design (Ref. 7).
14.2 Code of practice and standard welding symbols The code of practice used in South Africa with regard to all aspects of welding processes, materials, procedures, etc is the AWS D1.1 Structural Welding Code – Steel (Ref. 13). The standard symbols used on production drawings specifying the type, size, length and other particulars of welds are reproduced in the Steel Construction Handbook. These symbols should be used consistently and accurately on all workshop drawings so as to maintain an acceptable level of information transmittal.
14.3 Electrodes Electrodes are classified in accordance with American Welding Society (AWS) notation. The first and second digit (after the letter 'E') indicate the ultimate tensile strength of the deposited weld metal, in kilopounds per square inch (ksi). The commonly used grades of electrode produce the following weld metal yield stresses and ultimate strengths:
Electrode classification
Yield stress (MPa)
Ultimate tensile strength, fuw (MPa)
E60XX
345
410
E70XX
413
480
E80XX
–
550
14.1
14.4 Cost of welding The main components in the cost of making a welded joint are the volume of weld metal deposited and the time taken to deposit it. In order to achieve economy in welded joints it is therefore necessary to select a joint configuration that uses a minimum amount of weld metal and that is easy to carry out. Both these cost components are largely influenced by the type of welds specified by the designer. The time taken to lay a weld can be optimised by following certain guidelines, viz.: •
Using welds of minimum volume.
•
Using electrodes that have high deposition rates.
•
Using downhand welding in preference to overhead or vertical welding wherever possible.
•
Using single-run welds rather than multi-run welds where strength permits.
•
Avoiding excessive lengths of welds, for example by welding opposite sides only of gusset plates, angle cleats, etc, instead of all four sides.
The efficiency of a welding operation is measured by the ratio of actual arc time to total time, i.e. by the time that welding is actually taking place against the overall time to make a joint, including fitting up, tack welding, final welding, turning over, cleaning and inspection. Some of these aspects are discussed in greater detail below.
14.5 Types of joints and welds The two main types of weld used in structural connections are the fillet weld and the groove weld. The former is sometimes referred to as a projection weld because it is located outside the profile (as seen in cross section) of the parts connected, and the latter as a flush weld because it is contained within the profile. Fillet welds Fillet welds are by far the most commonly used welds because they are easy to lay, require no special plate edge preparation and do not call for a very accurate fit-up of the parts. Wherever the loading permits, they should be laid as single-run welds up to 8 mm, if the manual metal arc process is used. The laying of additional runs of weld adds considerably to the cost of a joint. The strength of a fillet weld is directly proportional to its size (i.e., its throat dimension), but its volume varies as the square of its size. Thus, an 8 mm fillet has 1,33 times the strength of a 6 mm weld, but its volume is 1,78 times as much. Its efficiency in terms of weld metal deposited is therefore only 0,75 that of the smaller weld. A further drawback of a large weld is the amount of distortion it produces in the connected parts. Where this is beyond acceptable limits it will be necessary either to preset the parts before welding or to straighten them afterwards both operations obviously being costly. It is thus of the greatest importance not to overspecify the size of fillet welds.
14.2
The leg size of a fillet weld can be reduced by using an automatic process instead of a manual procedure because of the deeper penetration that can be achieved. There will be a saving in both weld metal and welding time. In the case of manual fillet welds considerable savings can be made by using intermittent welds, but the length and spacing of the welds should be according to the designer's specification. Intermittent welds should not be used in members subject to dynamic loading because of potential fatigue failure, nor in corrosive situations where moisture might enter between the unwelded contact surfaces of the components. Examples of intermittent-welded components are intermediate stiffeners on plate girders, flange plates or built-up I-columns, and column web stiffeners at beam-to-column moment connections. Groove welds These welds, sometimes referred to as butt welds, may be used to connect two plates or other elements lying either in the same plane or at an angle (usually 90º) to each other. As shown in Fig 14.1, the welds in a butt joint may be square butt, single or double V, single or double U, or partial penetration single or double V welds. The square butt can be used for plate thicknesses up to about 6 mm without a backing strip and is obviously much cheaper than a weld requiring edge preparation to the plates. Up to about 12 mm thickness the single V is used without a backing strip and beyond this the double V. In the larger plate thicknesses the double V is the obvious choice because it uses about one half of the weld metal that a single V would and produces considerably less contraction of the plates along their length. Only in the case of much thicker plates are the U preparations used – above 20 mm for the single and above 30 or 40 mm for the double weld. The U preparation, although efficient in terms of weld metal deposited, is more difficult to execute and this limits the number of fabricators able to undertake the work.
(a)
(d)
(b)
(e)
(c)
(f)
(g)
Fig 14.1: Butt joints
Table 6.16 of the Steel Construction Handbook lists a range of full-penetration weld preparations as applicable to manual groove welds, including single and double V, J and U preparations, and gives the dimensions and preparation angles applicable to various plate thicknesses.
14.3
T-joints Fig 14.2 shows how plates forming a T-joint may be either fillet or groove welded. The fillet weld is usually much cheaper, but also more prone to fatigue failure under repetitive loading. The bevel and J welds may require backing runs as shown, or these may be omitted if the outstanding plates do not require to be restrained laterally. The bevel and J welds shown in details (b) and (c) could be specified as partial penetration if the full shear strength of the vertical plate does not have to be developed. An exception to the preference for fillet welds to full-penetration groove welds is when large plate thicknesses are involved; once it becomes necessary to use multi-run fillet welds to develop the full strength of the vertical plate the extra cost should be weighed against the cost of the groove weld.
(a)
(b)
(c)
Fig 14.2: T-joints
Corner joints Fig 14.3 shows three ways of making a corner joint. These would be applicable, for example, to the corners of a box column, where neither full-strength welds nor backing runs would normally be required. Detail (c) is obviously somewhat more costly and would only be used where a flush profile to the corner is required. It is important not to overspecify the amount of welding required in these applications.
(a)
(b)
Fig 14.3: Corner joints
14.4
(c)
14.6 Welding positions The position in which a weld has to be laid has a direct influence on the cost of the weld. The usual positions are shown in Fig 14.4 and it will be seen that the downhand or flat position is by far the most favourable because the molten weld metal is retained in position by gravity. The other positions are shown in order of increasing operator skill requirement and cost.
Downhand
Horizontal
Vertical
Overhead
Fig 14.4: Welding positions
It will be clear from this that structures should be designed and detailed so that the components can be laid out in the shop in a manner conducive to downhand welding for all major welds. In addition, welds should be located on one side of the assembly only to
14.5
avoid having to turn the components over. A turning operation, especially in the case of heavy plate girders or large lattice girders or trusses, is very time consuming and disruptive to the free flow of work through a shop. Examples of members designed to meet this requirement are given in Fig 14.5, where they are depicted in their flat position as resting on the assembly bench.
(a)
(e) (b)
(c)
(d)
Fig 14.5: Assemblies welded from one side only
The plate girder in detail (a) has the stiffeners located on one side of the web only. This detail not only avoids the need to turn the girder over, but also results in significant savings in the direct cost of the stiffeners and their welding as compared with stiffeners positioned on each side of the web. This detail can be applied only to intermediate stiffeners or to stiffeners carrying load from incoming beams on one side of the web. Bearing stiffeners loaded through the top flange would have to be symmetrical about the web, i.e. be in pairs. In detail (b) the plated I-section has the top plate narrower and the bottom plate wider than the flanges to allow welding to be done from the top. This is especially beneficial when an automatic welding process is used. The top plate would need to be thicker than the bottom one to maintain equal flange areas. Latticed trusses and girders can be designed with the web diagonals all on one side of the chords, as shown in details (c) and (d), provided single angles can be used and eccentricity can be tolerated at the node points (in elevation). (see Section 9.5 of Chapter 9). Where the chords are made from T-sections, as in detail (d), they require to be supported on blocks, as shown, which makes assembly slightly more difficult. The double-angle web member shown in detail (e) requires very much more labour input – in most cases it would be cheaper to use a heavier single angle, thus saving in cutting and welding cost and turning-over time. A further point to be considered is the accessibility of a weld. Fig 14.6 gives examples of assemblies where adequate electrode access is not available. They are shown as positioned for downhand welding.
14.6
(a)
(b) min
(c)
= 300 (450 preferable)
(d)
(e)
Fig 14.6: Accessibility of welds
In detail (a) the column web cleat should perhaps be made narrower, or be welded across the top and bottom. The gusset in detail (b), say a horizontal bracing gusset on a floor beam, should be placed further down from the top flange. The combined channel and Isection in (c) is popular for crane gantry girders and crane beams and the welding would normally be intermittent; a sufficiently wide channel should be used to allow electrode access. In detail (d) the spacing of the column or beam web stiffeners would probably be determined by the location of the incoming load, but could perhaps have smaller outstands. In the haunch to rafter connection of a portal frame, as shown in detail (e), the welding along length 'a' is omitted since electrode access here is in any case impossible.
14.7 Procedure qualification The AWS D1.1 Welding Code lays down requirements in Section 5 for qualification of welding procedures and welding personnel. Exemption from tests or qualification is granted, however, to procedures conforming to the provisions of Sections 2 to 4 and the pertinent provisions of Sections 8 to 10 of the code. These are defined as prequalified procedures. Part C of Section 2 sets out the requirements for prequalified joints in considerable detail, including dimensional, welding process and welding position parameters for fillet welds, and in tabular form (Fig 2.5), for a wide range of groove weld types. The weld types detailed cover the great majority of welds used in day-to-day welding practice. As stated, welds complying with these requirements are exempted from the welding procedure qualification tests prescribed in Section 5.2. It is desirable that designers and detailers be familiar with these requirements so as to be able to prescribe welds that will fall within the 'prequalified' category.
14.7
14.8 Inspection The AWS D1.1 code requirements for inspection are given in Section 6, where two classes of inspection are defined, viz.: a) Fabrication erection inspection and tests, performed as necessary prior to assembly, during welding and after welding to ensure that materials and workmanship meet the contract document requirements. This inspection function is normally the responsibility of the contractor. b) Verification inspection and testing by an independent inspector acting for and on behalf of the owner or engineer to ensure compliance with the requirements of the contract, with the results being reported to the owner and the contractor. The owner may take responsibility for this inspection function, or he may waive independent verification, or he may stipulate that both inspection and verification be performed by the contractor. In the interest of economy it is clearly to the advantage of the contractor that a regular progress inspection routine is maintained so as to avoid the re-working that is necessary when defects are shown up by the verification inspection. On the engineer's or designer's side it is wise not to call for unnecessarily stringent inspection levels. Very few, if any, of the welds in a typical structure call for special inspection or testing procedures and it would be good policy simply to specify the use of the AWS D1.1 code since this is the accepted document within the steelwork industry. For critical regions within a structure where closer inspection might be required it would be up to the engineer to define a reasonable level of inspection.
14.9 Summary of welding economics •
Adopt simple details for welded attachments.
•
Avoid excessive weld volume, i.e. do not overspecify weld size.
•
Use fillet welds in preference to groove welds, provided the size is not too large (i.e. in excess of 8 mm for a single-run manual weld).
•
Remember that longer, smaller fillet welds are cheaper than shorter ones of larger size. However, on a lapped-end connection of a member the length should not exceed about 70 times the size.
•
In prepared butt joints use partial penetration welds in preference to full penetration welds when the force on the joint will allow.
•
Avoid expensive plate edge preparations (i.e. J and U) for groove welds, except where the fabricator is equipped for such preparations.
•
Arrange for as much welding to be done in the shop and as little as possible at site.
•
Use the downhand welding position.
•
Avoid having to turn members or assemblies over to weld the other side.
•
Ensure adequate accessibility of the electrode to all welds.
•
As a designer do not specify in too great detail the manner of making a weld; the fabricator is in a better position to decide on the most economical joint preparation and welding procedure.
14.8
•
Use bent or folded plates for corner joints in preference to welding.
•
Use pre-qualified joints and welding procedures to avoid qualification testing.
•
Do not call for unduly stringent inspection.
14.9
