structuralengineering

Figure 5.1.1Welded Joint Types

A Beginner's Guide to the Steel Construction Manual, 13th ed. (old)

Chapter 5 - Welded Connections 2006, 2007, 2008, 2009 T. Bartlett Quimby

Introduction toWelding

Finding Forcesin Welded

Connections

Effective Areasand Size

Limitations ofWelds

Effective Areasof Base Metal

Strength LimitState

Designing Welds

ChapterSummary

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Section 5.1

Introduction to WeldingLast Rev ised: 11/04/2014

In the modern world of structural steel, welding is the process of joining two steel pieces (the base metal) togetherby heating them to the point that molten filler material mixes with the base metal to form one continuous piece. Webster's defines welding as "to unite (metallic parts) by heating and allowing the metals to flow together...". Theprocess of welding is quite complex and the strength of welds is highly dependent on metallurgy, weldingprocedure, and the skill of the welder.

The welding process has been around for thousands of years.

There are multiple processes and methods for accomplishing this complex task. There are a couple of points toemphasize.

Welding Processes

There are many welding processes, however we will focus on the two most common processes used in structuralsteel fabrication:

Shielded Metal Arc Welding (SMAW). A manual process that is typically used when welding in the field. It is also used frequently when welding in a fabrication shop.Submerged Arc Welding (SAW). An automated welding process that frequently used when welding in afabrication shop.

The SMAW process is highly dependent on the skill of the welder while the SAW process is not. The SAW processresults in more consistent weld an a strength bonus is given to some welds created with the SAW process.

The materials and the processes used in structural welding are governed by the American Welding Society (AWS)Specification D1.1. This specification is particularly important to welders for determining how to accomplish weldsdesigned by engineers. Engineers need to have some familiarity with the material requirements of the AWS D1.1. The SCM specification is strongly linked to AWS D1.1 and has most of the information that you need to designwelds.

Weldability of Metals

Metallurgy has a strong influence on the ability to weld different types of steel. It is important to match weldmaterials to the base metals that are being connected. The primary reference for matching filler materials to basemetals is AWS D1.1 Table 4.4.1. This table gives matching electrode materials for different base metals and thevarious welding processes. The table is important for engineers when they specify the weld electrodes to be usedfor the connections that they design. In this basic text, we will use the following electrodes indicated in Table5.1.1. While this table is basically adequate for most typical projects, for real projects, you should match therequirements of the AWS.

Table 5.1.1Matching Filler Material for BGSCM Problems

Base Metal SMAW SAWFy < 50 ksi E60XX or E70XX F6XX or F7XX

50 ksi < Fy < 60 ksi E70XX F7XX60 ksi < Fy < 70 ksi E80XX F8XX

Types of Joints

There are five basic types of welded joints. The joints are depicted in Figure 5.1.1. They are:

Butt JointsLap JointsTee JointsCorner JointsEdge joints

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Butt Joints: Butt joints are formed whentwo plates are butted together. Theconnection is normally made with a full orpartial penetration weld. The edges of theplate are often prepared so that the weldcan penetrate deeper into the butt joint. Some times the plates are held apartslightly for the same reason.

Lap Joints: These common joints aremade when two members with flat surfacesover lap each other. The connection isnormally made with fillet welds along theedges of the connected parts.

Tee Joints: In this type of connectionone plate element "T"'s into another. Thejoint can be made with fillet, partialpenetration, or full penetration welds.

Corner Joints: Corner joints are a specialtype of Tee joint. this connection occurs atthe edges of two plates.

Edge joints: This type of connectionjoins the edges of two plate elements laidtogether has show in Figure 5.1.1. Theconnection is made with partial penetrationwelds. The edges are often times preparedwith grooves so that the weld can penetrate deeper.

Types of Welds

The basic weld types are groove welds, fillet welds, and slot & plug welds.

Groove Welds

Groove welds are generally used to fill the gap between the two pieces being connected. They are called groovewelds because the edges of the materials being joined are prepared so that there is a groove of some shape formedwhen the pieces are first laid together. The weld metal fills the groove.

Groove welds are considered to be either "complete joint penetration" (CJP) or "partial joint penetration"(PJP).

A CJP weld completely fills the gap between the two pieces. Parts A, B, and C of Figure 5.1.2 illustrate CJP welds. CJP welds made with appropriate filler material are stronger than the base metals that they connect, so strengthcalculations are not necessary.

A PJP weld only fills a portion of the gap as seen in Figure 5.1.2 parts D, E, F, and G. PJP welds are used when itis not required to develop the full strength of the connected parts to transfer the load.

Figure 5.1.2Groove Weld Examples

Click on image for larger v iew

Fillet Welds

Fillet welds do not penetrate the gap between the parts being connected. A fillet weld generally has a triangularcross section with one leg of the triangle being attached to each piece being connected.


Fillet welds are very common and are used for a variety of connections. A typical fillet weld is shown in Figure5.1.3.

Figure 5.1.3Typical Fillet Welded T Joint


Slot & Plug Welds

Slot & Plug welds are similar to fillet welds in that they do not penetrate the gap between the parts beingconnected. These welds fill a slot or hole in one of the pieces being connected with the connection being betweenthe edge of the slot or hole on the one piece and the surface of the other piece. The welds can be made inconjunction with fillet welds to shorten the lap of two pieces where space is limited.

Prequalified Welded Joints

The AWS specification defines a number of "prequalified" joints that can be made. Before a welded joint can bemade on a project, it must be proven that the weld can be made using the desired materials and attain the requiredstrength and ductility. Once the joint has been proven, a welding procedure that details how the weld is to bemade is published and the procedure is considered to be prequalified. If the engineer specifies a joint or weld thathas not been prequalified it is necessary for the welders to go through the qualification process to develop a newqualified welding procedure.

Before a welder is allowed to make a particular joint he/she must be CERTIFIED to make that weld. Thecertification process requires the welder to create the weld on a sample using the materials, procedure, andposition that will be used for making the final connection. The sample is tested to insure that it meetsspecifications. Once a welder demonstrates that they can consistently create a weld that meets performancespecifications then they are certified to make that particular weld.

The SCM Table 8-2 (SCM pages 8-34 through 8-64) presents that design parameters for the most commonprequalified welds used for structural building connections.

Weld Symbols

A means for communicating the intent of the designer to the welder through standard weld symbols has beendeveloped by the AWS. A table defining the weld symbols for prequalified welded joints is included in the SCM onpage 8-35. You should take some time to examine this table. Pay particular notice to the notes at the bottom ofthe table. As an engineer you need to understand the language of the symbols or you may not get the weld thatyou are expecting.

Some things to notice:

1. The basic weld symbol consists of an arrow that points to the faying surface (i.e. the surface of contactbetween the pieces being connected) of the weld and a horizontal line where symbols are placed to describethe type of weld to be made.

Figure 5.1.4 shows an example a common mistake. The left example is from a drawing where thedesigner desired four welds on the outer side of the HSS section, but actually specified the welds asshown. The appropriate symbol is shown on the right side of the figure.

2. The arrow may be placed at either end of the horizontal line and may have one or two corners in the leader. The line segments are always straight lines.

3. The "field weld" flag and "weld all around" symbols always appear at the intersection of the horizontal lineand the arrow leader.

The flag of the "field weld" always point towards the tail end of the horizontal line as shown in Figure5.1.5.

4. The basic symbols are graphically similar to the type of weld or edge preparation that needs to be made.Figure 5.1.5 illustrates this concept. Notice that the fillet weld symbol has it's "back" (i.e. the verticalline) on the left side of the triangle regardless of which side the arrow is on. The weld information is


the same on both weld symbols.5. The arrangement of the symbols and notes on the horizontal line are exactly as shown regardless of which

end of the horizontal line the indicating arrow is located.

Figure 5.1.4Arrow Side / Other Side Example


Figure 5.1.5Weld Information Location


Prequalified Welded Joint Tables

Each AWS standard prequalified joint has a table associated with it. A set of these tables is found in SCM Table 8-2(SCM pages 8-34 through 8-64). The table gives the geometrical and material parameters associated with thejoint. Typically a figure is given to define the different dimensional quantities. The associated table gives theacceptable parameters associated with each dimensional quantity. The table also assigns a joint designation toeach weld for each process. This designation directs the welder to the AWS welding procedure associated with theweld.

Weld Quality

As a design engineer you should be aware of the factors affecting weld quality, however it is not the responsibilityof the designer to check the quality of the welds.

There are quite a number of factors affecting the quality of a weld. A good quality control program will haveprocedures in place to ensure that welds are of appropriate quality. The elements of that program will include theuse of prequalified welding procedures, performed by welders that have been certified to perform the designatedweld, qualified welding inspectors present on the job, and the specification of specialized weld inspectiontechniques as required.

Some of the factors affecting weld quality are:

Proper Electrodes, Welding Apparatus, and ProceduresProper Edge PreparationControl of Distortion

Inspection of welds must be done by qualified individuals. Most engineers are not qualified to determine thequality of weld. Visual inspection is the least expensive method but cannot detect many weld defects. Visualinspection can be used to ensure proper weld size has been obtained. Ultrasonic or X-ray techniques can detecthidden defects of welds but are very expensive. Many projects will specify that these techniques be used to spotcheck the welding on a certain percentage of the welds and on all welds that are deemed to be particularly critical.


Figure 5.1.6Force < StrengthClick on image for larger v iew

There is a good discussion of inspection techniques starting on SCM pg 8-4.

Possible defects in welds include:

Incomplete FusionInadequate Joint PenetrationPorosityUndercuttingSlag InclusionCracks

Limit States

The primary objective of checking all strength based limit states to ensure that the strength of the structuralelement is strong enough to handle anticipated forces exerted on them. In the case of welds, this can be expressedas:

The FORCE on the weld < min[STRENGTH of the weld, STRENGTH of adjacent base metal]

For welds, the forces can be resolved into to tension and shear components. In the special case of fillet welds, allstresses are assumed to be shear.

Figure 5.1.6 summarizes the following discussion about determining the forces on welds and the strength of welds.

Force on the Weld

The force on any given weld is the result of the forcesbeing applied to the connection and the geometry of theconnection. Principles of Mechanics and StructuralAnalysis are used to determine the force at any particularpoint in a weld in a connection. The next sectiondiscusses several commonly used methods for computingthe forces in welds.

Strength of a Weld

Welds have one tensile limit state and one shear limitstate. Typically the SCM denotes the nominal capacitiesof each as Rn.

Tensile Limit State: Tensile Rupture

For the case of tension, the limit state is:

The TENSILE FORCE on the weld < The TENSILERUPTURE STRENGTH of the weld

Shear Limit States: Shear Rupture

For the case of shear the limit states can be stated as:

The SHEAR FORCE on the weld < The SHEAR RUPTURE STRENGTH of the weld

Strength of Base Metal

The connected parts are referred to as the "base metal". There are two base metal components associated witheach weld. The strength of both base metals in the vicinity of the weld needs to be considered. The base metalwith the least strength controls the base metal capacity. Typically the SCM denotes the nominal capacities of eachas Rn.

Tensile Limit State: Tensile Rupture

For the case of tension, the limit state is:

The TENSILE FORCE on the base metal < The TENSILE RUPTURE STRENGTH of the base metal

Shear Limit States: Shear Rupture

For the case of shear the limit states can be stated as:

The SHEAR FORCE on the base metal < The SHEAR RUPTURE STRENGTH of the base metal

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