bridge gusset plate

What is Gusset Plate?

A gusset plate is a thin metal plate for connecting beams and girders to columns, which fastened to a permanent member, either by bolts, rivets, welding, or a combination of the three. They are used in structures, buildings, and bridges. It helps to transfer the stresses between connected members such as beam, column, or another truss member and strengthen them. Generally, the material of the gusset plate is metal or wood.

Purpose of gusset plate

A gusset plate toughens the connection of joint of the intersection of two or more beams, columns, or other members. So, this member has less chance to bend at the intersection when load or force applies to it. A gusset plate has a short brace at the intersection of the joint, and it gives more area to welding or bolting, which helps to tighten the entire section. So, this gusset plate helps to provide strong support to the complete structure.

Uses of gusset plate

A gusset plate is using in between the intersection of two columns or beams. It gives support to the entire structure by sharing the load. Generally, gusset plates are triangular, square, or rectangular, depending upon the type of application.

The size and thickness of the gusset plate depend upon function and type of application. If the structural load is high, then the thickness of the gusset plate is more. If structural weight is low, then a low thickness gusset is using.

A gusset plate is mainly used in bridges, buildings, and other structures. The thickness of the gusset plate for bridges is thick compare to others. While for the different fabrications, the thickness of the gusset plate is slightly low.

Triangular shape gusset plate is common in the application of bridges because both bolting and welding are possible to achieve good strength.

Materials

Depending upon the type of application, a gusset plate is made up of cold-rolled steel and galvanized steel. Galvanized steel is widely using in gusset because of anti-corrosive property. Due to this, there is no chance of corrosion. Paint is using to get an extra layer of protection against corrosion.

Sometimes for small applications, a gusset plate is made up of aluminum or copper. It gives a good and shiny look to the entire structure.

Basic consideration while designing gusset plate.

The shape, size, and thickness of the gusset plate mainly depend upon the direction of members (beams, columns, or other members) that meeting to the joint.

Gusset plate can be square, rectangular, triangular, or customized as per application and space for welding or bolting feasibility.

The plate size is decided to meet the specification of pitch and edge distance of the bolt to connect the member meeting at joint.

The purpose of the gusset plate is to withstand shear stress, direct and bending stresses meeting at the same section.

Generally, the thickness of the gusset plate is the same or greater than the thickness of the member, which connects the joint.

Causes of corrosion in gusset plate

The main cause of gusset plate failure is corrosion. It may cause the accumulation of water droplets in the interface or a small portion of fastening areas of the gusset plate. Sulfur compounds, chloride, and dust particles make the corrosion process faster. 

In industries, these sulfur compounds burn and form sulfur dioxide (SO2) and other harmful gases which emit into the environment. These gases react with the water accumulated in the gusset plate area and form sulfuric acid(H2SO4). This acid increases the speed of corrosion very fast. 

Chloride particles present in the sea and sea area get driven by the wind and contaminate the gusset plate area and structures. These particles cause corrosion. 

In the rainy seasons or snowfall, water particles spread by the vehicle on the structure of the bridge and get contains in the area of the gusset plate. This water, humidity, and environmental factors like sun rays, dust cause the corrosion process.

Ways to prevent gusset plate corrosion

Use corrosion-resistant material

The material used for gusset plate manufacturing contains nickel, silicon, copper, chromium, and phosphorus, which offers less resistance to corrosion. These are not suitable for complex environmental conditions.

In some materials, adding to the chromium up to 12.5% increases resistance to corrosion to some extent. 

The titanium alloy gusset plate gives high resistance to corrosion. But the cost of titanium alloy is high. These are using for critical applications only. It is necessary to use non-conductive material in between titanium alloy and steel to avoid galvanic corrosion.

Perform surface preparation

Fixing of gusset plate requires more bolts and welding for large structures. It is more challenging and expensive to cleaning, inspection, and surface preparation of gusset areas than coating the main structure.

After carrying out welding at the site, it is necessary to inspect the weld area to avoid any defects. If defects are present, then processes like grinding and others are using at the site to correct them. 

For the bolted connection, it is necessary to use a suitable lubricant that can prevent corrosion. If HDG surfaces of the bolt cause corrosion, then wire brushing is using to remove it.

Inspect coating integrity

It is necessary to check the coating integrity of the gusset plate. Visual inspection is a common inspection method to check the overall condition of the gusset plate and joints. Ultrasonic testing requires during testing of the thickness of the gusset plate. It helps to check the deterioration of thickness. In some cases, the new coating is using for prevention.

Applications

  1. Vehicle and pedestrian bridges.
  2. Joining steel while constructing a building.
  3. Small gusset plate used in small truss connection.
  4. Used in the construction of large mechanical equipment.