DESIGN OF WELDED CONNECTIONS
A welded connection consists of two or more pieces of base metal joined
by weld metal. Design engineers determine joint type and generally
specify weld type and the required throat dimension. Fabricators select
the specific joint details to be used.
Joint Types
When pieces of steel are brought together to form a joint, they will
assume one of the five configurations presented in Fig. 13.3.15. Joint
types are descriptions of the relative positions of the materials to be
joined and do not imply a specific type of weld.
Weld Types
Welds fall into three categories: fillet welds, groove welds, and plug and
slot welds (Fig. 13.3.16). Plug and slot welds are used for connections
that transfer small loads.
Butt Corner
Tee
Lap Edge
Fig. 13.3.15 Joint types.
Many engineers will see or have occasion to use standard welding
symbols. A detailed discussion of their proper use is found in AWS
documents. A few are shown in Fig. 13.3.17.
Fillet Welds Fillet welds have a triangular cross section and are
applied to the surface of the materials they join. By themselves, fillet
welds do not fully fuse the cross-sectional areas of parts they join,
although it is still possible to develop full-strength connections with
fillet welds. The size of a fillet weld is usually determined by measuring
the leg, even though the weld is designed by specifying the required
throat. For equal-legged, flat-faced fillet welds applied to plates that are
oriented 90° apart, the throat dimension is found by multiplying the leg
size by 0.707 (for example, sin 45°). Groove Welds Groove welds comprise two subcategories: complete
joint penetration (CJP) groove welds and partial joint penetration
(PJP) groove welds (Fig. 13.3.18). By definition, CJP groove welds
have a throat dimension equal to the thickness of the material they join;
a PJP groove weld is one with a throat dimension less than the thickness
of the materials joined.
An effective throat is associated with a PJP groove weld. This term is
used to differentiate between the depth of groove preparation and the
probable depth of fusion that will be achieved. The effective throat on a
PJP groove weld is abbreviated by E. The required depth of groove
preparation is designated by a capital S. Since the designer may not
Complete joint
penetration groove welds
Partial joint penetration
groove welds
Fig. 13.3.18 Types of groove welds.
know which welding process a fabricator will select, it is necessary only
to specify the dimension for E. The fabricator then selects the welding
process, determines the position of welding, and applies the appropriate
S dimension, which will be shown on the shop drawings. In most cases,
both the S and E dimensions will appear on the welding symbols of shop
drawings, with the effective throat dimension shown in parentheses.
Sizing of Welds
Overwelding is one of the major factors of welding cost. Specifying the
correct size of weld is the first step in obtaining low-cost welding. It is
important, then, to have a simple method to figure the proper amount of
weld to provide adequate strength for all types of connections.
In terms of their application, welds fall into two general types: primary
and secondary. Primary welds are critical welds that directly
transfer the full applied load at the point at which they are located.
These welds must develop the full strength of the members they join.
Complete joint penetration groove welds are often used for these connections.
Secondary welds are those that merely hold the parts together to
form a built-up member. The forces on these welds are relatively low,
and fillet welds are generally utilized in these connections.
Filler Metal Strength Filler metal strength may be classified as
matching, undermatching, or overmatching. Matching filler metal has the
same, or slightly higher, minimum specified yield and tensile strength
as the base metal. CJP groove welds in tension require the use of matching
weld metal—otherwise, the strength of the welded connection will
be lower than that of the base metal. Undermatching filler metal deposits
welds of a strength lower than that of the base metal. Undermatching
filler metal may be deposited in fillet welds and PJP groove welds as
long as the designer specifies a throat size that will compensate for the
reduction in weld metal strength. An overmatching filler metal deposits
weld metal that is stronger than the base metal; this is undesirable
unless, for practical reasons, lower-strength filler metal is unavailable
for the application. When overmatching filler metal is used, if the weld
is stressed to its maximum allowable level, the base metal can be overstressed,
resulting in failure in the fusion zone. Designers must ensure
that connection strength, including the fusion zone, meets the application
requirements.
In welding high-strength steel, it is generally desirable to utilize undermatching
filler metal for secondary welds. High-strength steel may
require additional preheat and greater care in welding because there is
an increased tendency to crack, especially if the joint is restrained.
Undermatching filler metals such as E70 are the easiest to use and are
preferred, provided the weld is sized to impart sufficient strength to the
joint.
Allowable Strength of Welds under Steady Loads A structure, or
weldment, is as strong as its weakest point, and ‘‘allowable’’ weld
strengths are specified by the American Welding Society (AWS), the
American Institute of Steel Construction (AISC), and various other professional
organizations to ensure that a weld will deliver the mechanical
properties of the members being joined. Allowable weld strengths are
designated for various types of welds for steady and fatigue loads.
CJP groove welds are considered full-strength welds, since they are
capable of transferring the equivalent capacity of the members they
join. In calculations, such welds are allowed the same stress as the plate,
provided the proper strength level of weld metal is used (e.g., matching
filler metal). In such CJP welds, the mechanical properties of the weld
metal must at least match those of the base metal. If the plates joined are
of different strengths, the weld metal strength must at least match the
strength of the weaker plate.
Figure 13.3.19 illustrates representative applications of PJP groove
welds widely used in the economical welding of very heavy plates. PJP
groove welds in heavy material will usually result in savings in weld
metal and welding time, while providing the required joint strength. The faster cooling and increased restraint, however, justify establishment of
a minimum effective throat te (see Table 13.3.1).
Other factors must be considered in determining the allowable stress
on the throat of a PJP groove weld. Joint configuration is one. If a V, J,
or U groove is specified, it is assumed that the welder can easily reach
the bottom of the joint, and the effective weld throat te equals the depth
of the groove. If a bevel groove with an included angle of 45° or less is
specified and SMAW is used, 1⁄8 in is deducted from the depth of the
prepared groove in defining the effective throat. This does not apply to
the SAW process because of its deeper penetration capabilities. In the
case of GMAW or FCAW, the 1⁄8-in reduction in throat only applies to
bevel grooves with an included angle of 45° or less in the vertical or
overhead position.
Weld metal subjected to compression in any direction or to tension
parallel to the axis of the weld should have the same allowable strength
as the base metal. Matching weld metal must be used for compression,
but is not necessary for tension parallel loading.
The existence of tension forces transverse to the axis of the weld or
shear in any direction requires the use of weld metal allowable strengths
that are the same as those used for fillet welds. The selected weld metal
may have mechanical properties higher or lower than those of the metal
Table 13.3.1 Minimum Fillet
Weld Size v or Minimum
Throat of PJP Groove Weld te
Material thickness of v or te,
thicker part joined, in in
*To 1⁄4 incl. 1⁄8
Over 1⁄4 to 1⁄2 3⁄16
Over 1⁄2 to 3⁄4 1⁄4
†Over 3⁄4 to 11⁄2 5⁄16
Over 11⁄2 to 21⁄4 3⁄8
Over 21⁄4 to 6 1⁄2
Over 6 5⁄8
Not to exceed the thickness of the thinner part.
* Minimum size for bridge application does not
go below 3⁄16 in.
† For minimum fillet weld size, table does not
go above 5⁄16-in fillet weld for over 3⁄4-in material.
being joined. If the weld metal has lower strength, however, its allowable
strength must be used to calculate the weld size or maximum
allowable weld stress. For higher-strength weld metal, the weld allowable
strength may not exceed the shear allowable strength of the base
metal.
The AWS has established the allowable shear value for weld metal in a
fillet or PJP bevel groove weld as
t 5 0.30 3 electrode min. spec. tensile strength 5 0.30 3 EXX
and has proved it valid from a series of fillet weld tests conducted by a
special Task Committee of AISC and AWS.
Table 13.3.2 lists the allowable shear values for various weld metal
strength levels and the more common fillet weld sizes. These values are
for equal-leg fillet welds where the effective throat te equals 0.7073leg
size v. With the table, one can calculate the allowable unit force per
lineal inch f for a weld size made with a particular electrode type. For
example, the allowable unit force per lineal inch f for a 1⁄2-in fillet weld
made with an E70 electrode is
f 5 0.707vt 5 0.707(1⁄2 in)(0.30)(70 ksi) 5 7.42 kips/in in
The minimum allowable sizes for fillet welds are given in Table
13.3.1. When materials of different thickness are joined, the minimum
fillet weld size is governed by the thicker material; but this size need not
exceed the thickness of the thinner material unless it is required by the
calculated stress.

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