Gusset Weld – Purpose, Design Considerations, and Applications

David
Etukudo

A gusset weld connects a gusset (piece of plate) to multiple structures such as beams or columns. Thus, it adds strength to a connection. Moreover, in industry, it is common to refer to the piece of plate as a gusset plate.

KT gusset plate welded to column and bolted to braces
Courtesy: Wikiwand

In this article, you will learn the purpose of a gusset weld, its design considerations, and common applications.

Purpose of a Gusset Weld

Generally, a gusset weld serves to fasten a gusset plate to a permanent load-bearing member. Moreover, in the construction industry, it connects these plates to beams or columns. Thus, it serves as an alternative to bolts and rivets. Ideally, fillet welds join gusset plates to members because it ensures the max resistance of the entire plate area to buckling. This contrasts with bolts, which limit the effective area of the plate to only what is within the bolts. However, in most applications, it is common to find the deployment of both methods. Often bolts join gusset plates to braces or trusses, while weld connections affix between the plates and columns or beams.

Gusset weld (left) and combination of gusset weld and bolt (right)
Courtesy: ResearchGate

As a result, the load-bearing capacity of the gusset weld is a critical part of its design. This especially holds true as the gusset plate provides support against buckling and other types of loading of the main member.

Hence, there are recommended practices that guide the design of both gusset welds and plates. However, in some complex applications, the requirement to avoid failure is difficult to predict without the use of finite element analysis.

Gusset Weld Design Considerations

Generally, fillet welds are the ideal method for welding gusset plates to beams and columns. This is due to their ease and low cost of fabrication. Also, spot welds may provide an option for flat gusset plates, but their effect on the adjoining materials could be adverse to strength of the materials. Moreover, the factors that affect the loading on the gusset weld include the gusset plate size, the eccentricity of the brace, and the brace angle. Changes to these factors result in a variety of loading that can lead to tear-out of the welds either by tension or shear.

So, the goal of every gusset weld design is to find out the minimum weld length requirement to avoid failure from the design loads. Although the approach of designing a gusset weld varies by case, there are several standards that provide guidance such as the AWS D1.1, ANSI/AISC 360, and IS 800. Thus, using these design codes makes it straightforward to determine a gusset weld’s subject loads.

Strength in Tension

When a gusset plate is in tension, the critical values to review are the design strength (ϕRn) and allowable strength (Rn), according to the limit states of tensile yield and tensile rupture. So, for tensile yielding, the value of Rn is the product of the gusset’s specified minimum yield stress (Fy) and the gross area (Ag).

    \[ R_{n}=F_{y}\times A_{g} \]

Where:

    \[ \phi =0.90 (LFRD) \]

    \[ \Omega =1.60  (ASD) \]

While for tensile rupture, the value of Rn is the product of the gusset’s specified minimum tensile strength (Fu) and the effective net area (Ae).

    \[ R_{n}=F_{u}\times A_{e} \]

Where:

    \[ \phi =0.75 (LFRD) \]

    \[ \Omega =2.00  (ASD) \]

According to ANSI/AISC 360, the lower of either the rupture or yield value should serve as the design value available to resist the tensile loading. Hence, this value is what will serve as the design tension load on the gusset weld.

Strength in Shear

Similarly, the available shear strength of the gusset is the lower value from the limit states of shear yielding and shear rupture. So, for shear yielding, the resistance of the gusset Rn is the product of the gusset’s specified minimum yield stress (Fy) and the gross area subject to shear (Agv).

    \[ R_{n}=0.6F_{y}\times A_{gv} \]

    \[ \phi =1.00 (LFRD) \]

    \[     \Omega =1.50  (ASD) \]

While for shear rupture, the net area subject to shear (Anv) replaces the gross area in estimating the resistance.

    \[ R_{n}=0.6F_{u}\times A_{nv} \]

    \[ \phi =0.75 (LFRD) \]

    \[ \Omega =2.00  (ASD) \]

Block Shear Strength

Failure by block shear is a limit state that combines tension failure on one plane, and shear failure on a perpendicular plane. Moreover, the available strength of the gusset weld to resist this failure method (Rn) is a function of the net area subject to tension (Ant), a reduction coefficient (Ubs), and other variables as follows:

    \[ R_{n}=0.6F_{u}A_{nv}+0.6U_{bs}F_{u}A_{nt}\leq 0.6F_{y}A_{gv}+0.6U_{bs}F_{u}A_{nt} \]

When tension stress is uniform, the Ubs value is 1. But for non-uniform tension, a value of 0.5 is suitable.

Minimum Weld Length

Finally, evaluating the minimum weld length that can withstand the design loads is aim of the gusset weld design. Moreover, the IS 800 is clear on this process. Also, it gives the design strength of a fillet weld (fwd) as a function of a partial safety factor for welds (γmw) and the smaller value between the ultimate stress of the weld and the parent material (fu).

    \[ f_{wd}=\frac{f_{u}}{\gamma _{mw}\sqrt{3}} \]

Before calculating the minimum gusset weld length, it is necessary to evaluate the throat thickness (tt) of the weld. Moreover, the throat thickness is a product of the size of weld (s) and a constant (K).

    \[ t_{t}=K\times s \]

The constant, K, depends on the angle between the faces of the gusset and member as Table 22 of the IS 800 highlights. So, after determining the throat thickness, calculate the minimum acceptable length of weld (lw). This value is a function of the design strength of the fillet weld (fwd), the design load (P), and the throat thickness (tt).

    \[ l_{w}=\frac{P}{f_{wd}\times t_{t}} \]

The procedure above is a simple approach for designing a gusset weld when considering a single type of loading. Also, the emphasis of this procedure is only on the design loads, but in practice, the design engineer should consider other factors such as fatigue and corrosion. Furthermore, in dynamic applications, the use of simulations is advisable as it will reveal scenarios that design codes do not address.

Common Applications of Gusset Welds

Gusset welds offer a more robust method of attaching gussets to load-bearing members of a structure or machine. Although the shape of most gusset plates are rectangular, it is common to deploy different shapes and sizes to match the adjoining members and the strength requirements. Typically, the higher the loading on the structure, the larger the size of the gusset and welding. The use of gusset welds is predominant in bridges, buildings and several other structures that have beams and columns. Also, gusset welds serve in retrofitting and providing load support for older structures.