Slot Weld Calculation

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. 1) The area of the fillet weld A u. (unit thickness) is calculated assuming the weld is one unit thick. 2) The (unit) Moment of Inertia I u is calculated assuming the weld is one unit thick. 3) The maximum shear stress due to bending is determined. Τ b = M.y/I u 4) The maximum shear stress due to direct shear is determined.

• Fillet Weld Calculation
• Plug And Slot Welds
• Slot Weld Calculations
• Weld Time Calculator
• Aluminum Weld Calculation
• Plug Weld Design Calculation

Section 5.3

Effective Areas and Size Limitations of Welds

Last Revised:11/04/2014

This section investigates the mechanics of load transfer between connected members by means of welds. We will look at this problem for each of the basic weld types. The strength of welds is determined by multiplying an effective area by the shear or tensile stress capacity of the weld material.

The effective area is generally determined by multiplying the length of the weld by and EFFECTIVE THROAT, t_e. The effective throat is a function of the WELD SIZE. The determination of effective throat size is a bit different for each type of weld.

The goal of the design process is to determine the required length and effective throat thickness for the connection being designed.

The specification requires a minimum effective throat thickness for most types of welds. The intent of the minimum effective throat thickness requirement is to ensure that the weld is of sufficient size to generate enough heat to form a good bond with the base metal. During the welding process, the base metal acts as a heat sink which draws heat away from the connection. If it draws the heat away too quickly, the poor bond is created between the base metal and weld metal.

The effective throat chosen for any weld design will be the LARGER of:

• the minimum requirement for ensuring a good bond with the base metal and
• the minimum required to develop the strength required to transfer the design forces.

Groove Welds

The effective area for the two main types of groove welds is shown in Figure 5.3.1 as specified in SCM specification section J2.1.

For CJP welds, the effective throat equals the thickness of the thinner part joined.

For PJP welds, the effective throat equals the depth of the groove. There is a 1/8' reduction of the depth for the SMAW process if a 45^o bevel is used. (See SCM Table J2.1).

Table J2.3 gives minimum required effective throat to ensure a good bond for PJP welds.

Fillet Welds

The effective throat, t_e, for a fillet weld is taken as the least distance from the root of the weld (i.e. where the two connected pieces meet) to the outer surface of the weld as shown in Figure 5.3.2.

Figure 5.3.2
Fillet Weld Sectional Dimensions
Click on image for larger view

The effective area of your typical fillet weld equals the effective throat times the length of the weld as depicted in Figure 5.3.3. For clarity purposes, the figure shows the effective areas of two fillet welds, one on each side of the vertical piece.

Figure 5.3.3
Effective Area of Fillet Welds
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It is important to note that fillet welds are specified by their leg size, a, instead of by the effective throat, t_e. The vast majority of fillet welds are designed to have equal leg size, which makes t_e = a*sin(45^o) = 0.707a. If the leg sizes are not equal, you will need to determine t_e using trigonometry.

A change in the 13^th edition SCM is the elimination of an explicit allowance for an increase in t_e when using the SAW process. The prior editions of the specification allowed an increase in t_e for the SAW process because the process produced welds that had a consistent penetration beyond the root of the weld. The 13^th edition allows an unspecified increase based on test results that prove this condition. Since the increase is not explicitly stated, we will take the conservative approach of not taking an increase in t_e for the SAW process.

The specification places a number of limits on the size of fillet welds. These are found in SCM J2b.

The MINIMUM ALLOWED SIZE is specified in SCM section J2.2b, first paragraph which states that 'the minimum size of fillet welds shall not be less than the size required to transmit calculated forces, nor the size shown in Table J2.4' (Emphasis added!) The commentary on this section states that the 'requirements are not based on strength considerations, but on the quench effect of thick material on small welds.' (SCM pg 16.1-331) This means that the weld needs to be big enough to heat the base material sufficient to create a good bond between the base metal and the weld metal. Table J2.4 lists the minimum sized fillet welds. As you look at the table, you see that as the material thickness increases so does the minimum weld size.

A MAXIMUM ALLOWED SIZE is also discussed in the specification. This particular requirement causes some confusion among first time users of the specification. This requirement is based on the AVAILABLE space for the weld. The limit applies only at the EDGES of plates. For example in Figure 5.3.4(A) the weld is not applied to an edge of either connected plate so the available space for the fillet weld is very large. In other words, t_pl is large on both legs of the weld. In Figure 5.3.4(B) one of the legs of the weld is along the edge of a plate so t_pl is limited.

Figure 5.3.4
Maximum Fillet Weld Size
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The specification limits the weld size, a, to being equal to t_pl if t_pl < 1/4' and to t_pl-1/16' if t_pl> 1/4', where t_pl is the minimum of the distances available for the fillet weld leg.

SCM specification J2.2 has a number of other limitations on fillet welds that you need to be familiar with as well. The rest are fairly self explanatory in the SCM so further coverage will not be made here.

As will be seen later, the strength of a welded connection is controlled by the weaker of the base metal or the weld (i.e. filler material). The strength of the base metal and the weld are based on their effective areas. Suffice it to say right now that as you increase fillet weld size, the weld strength may, at some point, exceed the strength of the base metal. Increasing the weld size any such that the weld strength exceeds the strength of the connected parts will be a waste of resources since it will not longer increase the strength of the connection.

The fillet weld size whose strength equals the base metal strength is called the largest effective fillet weld. It is, in essence, another practical limit on fillet weld size. This is discussed in more detail later.

Plug and Slot Welds

SCM specification J2.3a specifies the effective area of a plug or slot weld to equal 'the nominal cross-sectional area of the hole or slot in the plane of the faying surface.' These areas are depicted in Figure 5.3.4.

Figure 5.3.4
Effective Area of Plug and Slot Welds
Click on image for larger view

There are limits on the minimum and maximum sizes of slot welds that can be found in the specification section J2.3b.

Plug welds are a round weld that is made inside of an existing hole most commonly in one piece of metal, welding that piece to another member. The plug weld symbol is a rectangle with a diameter symbol placed to the left of the symbol as well as the number associated with that diameter.

Some drawings will not indicate the hole in the print so the use of dimensions come in to play when locating where a plug weld will be executed. The location will be indicated by a centerline through the part.

Above is indicating a ½” plug weld offset 1” from the edge to the center of the weld.

Some plug welds may include a countersink of the hole of the plug weld. This is called the included angle of countersink. This angle is shown below the rectangle of the symbol itself or if the plug weld is to be on the other side it will be placed above the weld symbol. When figuring sizing of the hole remember that the diameter will be the narrow of the hole at the base of the weld.

Without a countersink included it will be necessary to follow shop standards and procedures to dictate what this needs to be, if any angle. Most shops have a procedure in place for tasks that will be done often. If it is needed it may be listed on a welding procedure for the plug welds that are being completed.

If a number of plug welds are needed there will be yet another element added to the symbol. This will be a number that is surrounded in parentheses, such as (6) for example.

When applying a plug weld it is important to know the depth of fill that is required. If the plug weld should fill the hole provided then the symbol will be left empty. This means there will be no dimension inside of the rectangle. If the hole should be filled only so much then this will be placed inside of the rectangle. This dimension will be in a fraction and indicates the amount in inches the hole will be filled, not the necessarily how much the hole will be filled.

Another element that can be added to this weld symbol may be the pitch (spacing) for multiple welds. This is located to the right of the symbol and is a number representing the center to center spacing for weld location.

Plug welds may have a contour symbol which will be added below the symbol or countersink angle if on the arrow side and above if it is on the other side of the reference line. There are many types of contours and finishing designations, these are covered in supplementary welding symbols.

This symbol represents:

Plug Weld

Arrow Side

½ inch in diameter

1/8” amount of fill

45 degree included angle of countersink

Flat contour

Fillet Weld Calculation

Finished by Machining

Slot Weld Symbol

The slot weld symbol is the same that is used for plug welds. The symbol will not show a diameter symbol before the size however. The size of the weld will be the slot width instead. This is shown to the left of the symbol just as it is shown in plug welds.

1/2” width slot weld

The length of the slot weld will be presented to the right of the symbol. This may also include a pitch showing the center to center spacing of the slot welds. If there is a pitch there will be a number of slot welds provided in parenthesis under the symbol on the arrow side or above the symbol on an other side weld.

The drawing must show the orientation of the slot welds as to not confuse direction along the part. The above image shows the slots with a vertical orientation to the part versus a horizontal layout as shown below.

Plug And Slot Welds

A slot weld can include any number of elements, these are very similar to the plug weld symbol that was just explained.

These can include:

Slot Weld Calculations

Arrow or other side

Size (width)

Length of slot

Pitch

Depth of fill

Number of welds required

Contour

Weld Time Calculator

Finish

Aluminum Weld Calculation

Make no mistake on the fill of a plug or slot weld fill. There is a possibility of having a fillet weld inside of a hole versus actually filling the hole for a plug weld. This could also be mistakenly done on a slot weld.

Plug Weld Design Calculation

Plug and Slot Quiz

Write down all information regarding the below Welding Symbols.