Introduction
Weld design plays a critical role in ensuring the strength and stability of structural connections. One common scenario is the connection of a hollow circular section (HSS) to an end plate or base plate using a circular fillet weld. This type of weld is widely used in structural applications due to its ability to transmit both axial and shear forces effectively, while also accommodating torsional loads. In this post, we’ll walk through the design of a circular fillet weld following the guidelines in AISC 360-22, paying special attention to transverse, longitudinal, and resultant stresses. We'll conclude with an example problem to illustrate the design process in detail.
Factors to Consider During Weld Design
Several factors should be considered when designing a circular fillet weld, particularly for connecting a hollow circular section to a plate:
Geometry of the Weld and Connection
The geometry of the connected components directly influences the distribution of forces within the weld. For circular fillet welds, the radial distribution of stress must be considered. A hollow circular section (HSS) creates a uniform weld profile along the perimeter, but the stress distribution may vary depending on the type and magnitude of applied loads (axial, shear, or torsional).
Material Properties
Both the base metal and the weld metal’s properties impact the overall strength of the connection. It is essential to ensure that the selected weld metal has compatible properties with the HSS and base plate materials. The weld metal's yield and tensile strength must be considered when calculating capacity, as they govern the weld’s resistance to failure.
Loading Conditions
Welds are often subject to a combination of axial, shear, and torsional forces. In the case of a circular fillet weld, it is critical to evaluate both transverse and longitudinal stresses that develop due to these loads. The resultant stresses from different load directions need to be combined appropriately to ensure the weld capacity is not exceeded.
Weld Size and Length
The size of the weld directly impacts its capacity. For a fillet weld, the leg size determines the throat dimension, which is used in capacity calculations. In circular welds, the length is typically defined by the perimeter of the HSS section, and increasing the weld size or length can improve the connection’s capacity.
Connection Ductility
The ductility of the welded connection ensures it can deform under load without fracturing. For fillet welds, maintaining a balance between adequate strength and ductility is crucial to avoid brittle failure, particularly under dynamic or cyclic loading conditions.
Weld Quality and Inspection
Ensuring high-quality welds with minimal defects is essential to prevent premature failure. Proper inspection and testing, such as visual inspections or non-destructive testing methods (e.g., ultrasonic testing), should be employed to verify the integrity of the weld.
Directional Strength Increase Factor ('kds')
The 'kds' factor accounts for the directional strength increase in the weld. For certain load directions, such as longitudinal shear parallel to the weld axis, a 50% increase in the weld’s strength is permitted per AISC 360-22. This factor must be carefully applied based on the orientation of the forces relative to the weld to ensure the design maximizes allowable weld strength when appropriate. If you are unsure of which value to use, it is typically conservative to prescribe a value of 1 for the directional strength increase factor (ie. no strength increase).
Important Details to Be Aware of in Weld Design
While weld design largely depends on calculations for strength and geometry, there are several additional details that must be kept in mind to ensure safe and effective connections:
Fatigue Considerations: In connections subject to cyclic or dynamic loads, fatigue becomes a significant concern. Welds can develop cracks over time due to repeated stress fluctuations, and careful consideration should be given to designing welds that minimize stress concentrations and potential fatigue failure.
Corrosion Protection: Welds, especially in exposed environments, are prone to corrosion. Proper coatings, galvanizing, or cathodic protection can help prolong the lifespan of welded connections by reducing the impact of environmental factors.
Heat-Affected Zone (HAZ): The area around the weld, known as the heat-affected zone, can experience changes in material properties due to the high temperatures involved in welding. This can lead to localized reductions in toughness or strength, making it crucial to consider HAZ behavior in design.
Residual Stresses and Distortion: Welding induces residual stresses and may cause distortion, particularly in large or complex connections. Preheating, post-weld heat treatment, and proper fixturing can help manage these effects, ensuring the connection remains dimensionally accurate and free from excessive residual stresses.
Weld Orientation and Positioning: The orientation of the weld, such as flat, horizontal, vertical, or overhead, affects both the ease of welding and the quality of the final weld. For critical applications, it may be beneficial to use position welds that minimize the likelihood of defects.
These considerations are essential to producing high-performance welded connections that meet the required safety and performance standards.
Example Problem (Solutions Provided Using CalcBook): Problem Statement:
Step 1: Design Inputs
Step 2: Demand on Weld
Step 3: Capacity of Weld
Step 4: Determine Controlling Demand/Capacity Ratio:
Conclusion:
In this post, we’ve covered the essential factors in designing a circular fillet weld for connecting a hollow circular section to a base plate, as outlined in AISC 360-22. The example problem demonstrated how to calculate transverse, longitudinal, and resultant stresses, along with verifying the weld capacity. By considering geometry, material properties, and important weld details, engineers can ensure safe and effective welded connections.