How Are Steel Structures Held Together? Bolted vs. Welded Connections

Steel structures depend on much more than just the strength of their beams and columns; structural connections have to transfer all the loads, safely, to allow the entire steel structure to function exactly as designed. 

There are many different ways that these structural connections can be made. One of the most common ways to connect structural steel members is either through bolting or welding.

There are several reasons why both methods provide very similar results. It's really up to the design of the connection itself, the process of making and installing the structural steel, and what the site conditions are like at the time the structural connection needs to be made.

What Is a Bolted Structural Connection?

A bolted structural connection connects structural steel elements using steel bolts. They usually include one or more plates or angles. The holes and connecting pieces are usually developed during fabrication to allow crews to assemble the structural steel in the field.

Advantages of Bolted Connections

An advantage to this method of structural connection development is installation speed. When the components are complete and positioned correctly, crews can easily and quickly install a number of bolted structural connections. 

This is especially true if no welding is required for the majority of the joints in the field. Another advantage of using bolted structural connections is that they typically make future repairs or modifications more practical because a bolted structural connection can be completely disassembled.

Limitations of Bolted Connections

It's worth noting that there are several limitations to developing structural connections via bolting. To achieve successful installation of a bolted structural connection, the fabrication of the structural component(s), as well as the installation of those same structural component(s) must be done accurately. 

Additionally, the specification of the type of bolts to be used, where the holes are located, and how tight they should be tightened, etc., must be met by the project specifications. 

Finally, since bolted structural connections are "visible" and often expose various parts of the fastening system, they can impact how a finished structure appears aesthetically.

What Is a Welded Structural Connection?

A welded structural connection combines structural steel elements through fusion of the metal at the point of joining. 

Like bolted structural connections, welded structural connections can be created in a fabrication shop, prior to delivery to the construction site for final installation. 

Advantages of Welded Connections

Similar to bolted connections, welded connections can be beneficial when a design requires a single, continuous structural connection. 

In addition, welding allows for creation of unique shapes and configurations not possible through bolted connections. Moreover, because welds create single-piece joints, certain types of plates and fasteners used in bolted connections can be eliminated entirely.

Considerations for Field Welding

While welding can occur in a fabrication shop prior to delivery of the components to the construction site for final installation, it can also take place at the construction site. However, when compared to fabrication-shop welding, field welding involves much more planning and coordination. 

First, sufficient access and equipment must be available for welding activities. Weather and site conditions may also affect the feasibility of performing welding activities at the site. 

Second, once a weld is completed, it must meet the applicable inspection criteria specified within the project documents. 

Third, while removal of bolts is relatively straightforward, modifying or repairing welded connections is generally far more complicated than simply unfastening bolts.

Which Method Is Better?

Neither bolting nor welding is inherently superior for every project. That being said, when properly designed and constructed both bolted and welded structural connections can support large loads. 

Ultimately performance will depend upon such factors as loading, details of the connection, material selection, and quality of installation.

Ultimately, for many construction projects the question becomes:

Where will this connection be made? If you have access to suitable facilities for welding components together (shop welding), it would likely be faster than assembling them in the field using bolts. On the other hand, bolting often provides greater ease of assembly in the field.