Both Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD) are available in the Weld Capacity Calculator. The ASD method considers the maximum allowable stress for a welded joint based on joint type and base materials, stresses are then checked to ensure they do not exceed the allowable values. In comparison, the LRFD method does not set a fixed allowable stress value but instead involves calculating a welded joint based on factors like the weld strength, material properties, and loading.

The Australian Standard for steel (AS 4100) uses a limit state design approach. With this approach loads are factored up into design actions and then resolved into equivalent shear stresses on weld groups. The weld shear capacity is then calculated and compared to the maximum shear stress on the weld group.

Generally welding length is calculated by measuring the linear distance along the joint between the two members that need to be welded. Other factors also need to be considered such as the thickness and type materials being joined. The geometry of sections can also physically limit a welders ability to reach spaces and may mean that certain sides of a section can not be welded.

Weld strength is typically calculated as the shear strength of the weld in failing along the smallest possible shear plane.

For a fillet weld the shear area of the weld can be calculated by multiplying by the throat thickness of the weld. For an equal angle fillet weld this is the size of the weld divided by √2 . For an unequal fillet weld the calculation would some trigonometric calculations.

The shear strength is typically 0.6 x the ultimate tensile strength.

Calculating weld strength depends on the type of weld but the general principal remains the same that we are checking for failure through the weakest plane. For a fillet weld we calculate the throat thickness of the weld using trigonometry but for other welds like a full penetration butt weld the failure plane will be different. If the butt weld is stronger than the steel material and the butt weld has a critical cross-sectional area greater than or equal to that of the base material we might not even need to calculate the strength of the weld at all and can just check the capacity of the base metal since it will be more critical.

The total capacity of a weld per unit of length is generally determined as:

0.6 * f_u * t_t

where:

• f_u is the ultimate tensile strength of the weld
• t_t is the throat thickness of the weld

Fillet weld strength is calculated the same as the weld strength is above.

The AISC 360-16 calculates weld strength with the following equations:

LRFD: ϕ Rn = ϕ * F_nw * A_we

ASD: R_n / Ω = F_nw * A_we / Ω

where:

• Fnw is the nominal stress of the weld metal
  • = 0.60 * F_EXX * (1.0 + 0.50 * sin^1.5(θ))
  • where θ is the angle between the line of action of the required force and the weld longitudinal axis
  • F_EXX is the filler metal classification strength
• Awe is the effective area of the weld
• ϕ = 0.75
• Ω = 2

The Eurocode 3 (EC3) calculates weld strength with the following equation for the simplified method:

F_w,Rd = a * f_u / (0.577 * β_w * γ_M2)

where:

• f_u =nominal ultimate tensile strength of the weaker part joined
• a = throat thickness of weld
  • for equal angle fillet welds = t_w / √2
  • where t_w is leg length
• β_w = Correlation factor for fillet welds
  • 0.80 for S 235 steel grade
  • 0.85 for S 275 steel grade
  • 0.90 for S 355 steel grade
  • 1.00 for S 420 steel grade
  • 1.00 for S 460 steel grade
• γ_M2 = 1.25

Where the above calculation is a simplified weld strength approach, Eurocode also has a directional method for calculating weld strength requirements. The directional approach considers the normal stresses on the weld throat and the shear stresses on the weld throat differently allowing for higher strength in the normal direction. The formulation for assessing directional weld strength is:

[σ_⊥² + 3 (τ_⊥² + τ_||² )]^0.5 ≤ f_u / (β_w * γ_M2)

and

σ_⊥² ≤ 0.9 * fu / γM2

where:

• σ_⊥ is the normal stress perpendicular to the throat of the weld
• τ_⊥ is the shear stress perpendicular to the axis of the weld
• τ_|| is the shear stress parallel to the axis of the weld

In the case that we had no normal stresses on the weld we can see that if we rearrange the direct method formula we will get the same formulation as the simplified method.

The AS4100:2020 calculates weld strength with the following equation:

ϕ v_w = ϕ * 0.6 * f_uf * t_t * k_r

where:

• ϕ = reduction factor
  • 0.6 for general purpose (GP) fillet welds
  • 0.8 for structural purpose (SP) fillet welds
• f_uf =tensile strength of weld metal
• t_t = throat thickness of weld
  • for equal angle fillet welds = t_w / √2
  • where tw is leg length
• k_r = reduction factor for lap splice connections
  • 1 for typical welds and lap slices less than 1.7 m in length
  • 1.10 - 0.06 lw for lap splices between 1.7 m and 8 m in length
  • 0.62 for lap splices greater than 8 m in length

The SkyCiv WeldStrength Calculator is useful to determine what the maximum load a joint between two members can safely be expected to stand before failing. Checking weld capacity is a key part of the engineering process to ensure that connections are structurally sound and meet design standard requirements.

The SkyCiv Blodgett Weld Strength Calculator is useful in a range of structural engineering projects that may include building construction, bridge construction and many other infrastructure projects. Welding also has applications in automotive, aerospace, marine (shipbuilding), railway, offshore and mining engineering. Often due to the nature and safety requirements these industries have particular welding regulations that need to be followed.

Welds have several advantages over bolted connections which are:

• No removal of steel material. Whereas welds only add material to the side of a member bolted connections will remove material from the steel section, reducing section capacity and introducing concentrated stress points in the member.
• Welds are stiffer than bolted connections. This can help if a structural engineer has designed a connection to be a fixed connection instead of a pin connection.
• Welded connections can provide higher strength since they provide a continuous joint. Bolted connections can have physical limitation on how many bolts can be provided due to spacing requirements and will only provide discrete points of restraint.
• Bolts extrude whereas welds can remain neat and hidden. Welds run along existing joints and are less visually intrusive than bolts which will be spread further away from the end of a beam and be less subtle than welds.

A weld treated as a line pattern in a method to graphically represent weeds on engineering drawings. The above weld capacity calculator has twenty-one weld patterns available based on the Blodgett Weld Type outlined in the book Design of Welded Structures. These weld patterns represent a variety of different cross section types that are commonly found in structural members.

The AS 4100 weld capacity calculator also allows importing the dimensions of typical steel sections and editing weld patterns into custom arrangements. Calculations for custom weld patterns are resolved using the parallel axis theorem and elastic theory.

There are multiple types of welding joints available that are used in structural engineering. Some common types of joints include Butt Joints, T-Joints, Lap Joints, Corner Joints, Edge Joints, Groove Joints, Plug Welds, Filleted Joints, Spot Weld and Seam Welds.

A butt joint weld is used to join two pieces of metal end-to-end that are parallel to each other.

A fillet joint weld is a weld between two perpendicular (or near perpendicular) members. They are common in baseplates and beam to column connections.

A lap joint weld is used to weld two overlapping pieces of metal that are parallel to each other and sitting flush. Fillet welds are used on one side or both sides of the connection.

Blodgett Welding is based on Omer W. Blodgett book Design of Welded Structures.

We can also design for custom arrangements by using the second moment area of line welds and then using the parallel axis theorem to find the result. Blodgett Welding has simplified different arrangements into a series of simple to apply formulas to reduce the amount of steps in calculations.

The SkyCiv Weld Capacity Calculator takes properties include the ultimate strength of the weld, the weld size and the depth of the weld.

The weld capacity calculator returns a weld capacity result as well as a utilization ratio for the welded connection. In addition to this the following results are also provided:

• Preferred Weld Size
• Member Weld Capacity
• Fillet Weld Capacity
• Maximum Effective Weld Size
• Maximum Force per Unit Weld

The SkyCiv Weld Capacity Calculator requires the applied force along the x, y and z directions as well as the moment about the x and z axis.

Currently only the imperial unit system is available for the AISC 360-16 calculator and only the metric system is available for the AS 4100 calculator and EN 1993-1-8 weld strength calculator.

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