Building a Bridge – How Bridges are Fabricated
Building steel bridges is the foundation of my professional career. While many folks out there only see the end product, if it’s not covered up by concrete, or the on-site construction during the installation, the fabrication process that takes place years in advance is somewhat alien to the general public.
In this post, it’s my goal to explain how the bridge makes its way from what is commonly referred to as “the steel mill” to what allows you to make your way across a river or save time spanning mountain tops.
While this can be a very complicated process, I will attempt to break it down into the basics that are used in the industry to create the bridges you typically see. While some may vary from facility to facility based on the slant of that particular company’s efficiencies, the blocking and tackling is the same.
I will focus on the fabrication of plate girders since this is the most common type used throughout the US, although there are quite a few styles out there including major signature bridges.
Disclaimer: These are the basics and not intended to give any Larry, Mo, or Curly the impression that they can build a bridge in their garage. Go ahead and start with popsicle sticks. On the other hand, hopefully the next time you drive under a bridge or see someone installing one in the field, you can have a deeper understanding of what it took to get there!
Table of Contents
Type of Bridges
Before learning how a bridge is built, you need to understand the different types of bridges and their specific uses. In the US, there are a few different types of common bridges that the public sees. In this section, I’ll do my best to explain their characteristics so you can identify each when you are traveling. Within each, they can vary in complexity depending on their intended use, location, owner preference, and budget.
It’s most common that a bridge is owned by a municipal owner such as a DOT or local government. The main outliers are railroad bridges. This is due to the design requirements that each specific railroad entity has developed to ensure their safety of their responsibilities.
The main types of bridges are listed below:
Plate Girder Bridges
- Plate girder bridges are the most common bridge fabricated in the US. These are what many refer to as “I” beam sections. These are generally the most simple structures that can be designed. However, they can be designed in extremely complicated scenarios.
Arch Bridges
- Arches are typically designed for the “Signature Bridge” use and can be used for vehicular traffic or pedestrian traffic. These are commonly built from box sections due to the inherent structural flexibility they allow in the design as well as maintenance access. Bridges such as the Fredrick Douglas Memorial Bridge in Washington DC as well as the Pensacola Bay Bridge in Pensacola Florida, and soon to be Skillman Arch in Dallas Texas.
Cable Stayed Bridges
- Cable stayed bridges are used for signature bridges but are also designed for long span structures that require longer distances between bearing locations. The most famous example of this is the Golden Gate Bridge in San Francisco.
Truss Bridges
- Truss bridges are used similar purposes as a cable stayed bridge. These structures are more rigid and can span longer distances without an attachment to the ground. The truss structures can be above the deck of the bridge or below. Either way, the structure transfers the loads in the same fashion.
Tub Girder Bridges
- Tub girders are typically used in bridges that transfer traffic from one highway to another in a super elevated position. These are generally on and off ramps that carry a lot of traffic. We see these commonly in areas such as I–4 direct connect lanes in Orlando or in areas south of Jacksonville on I-95.
Moveable Bridges
- Moveable bridges are those that are not static structures and have a portion of the bridge that moves to allow for specific traffic to pass under them. Generally this is taller boat traffic. Although they are more expensive, in tight areas they can be much more economical as taller fly-over bridges can take up a lot of real estate.
Lift Bridges
- Lift bridges are bridges that raise an entire span level in the air. Examples would be the Main Street Bridge in Jacksonville, FL or the Chelsea Street Bridge in Boston MA.
- Moveable bridges are those that are not static structures and have a portion of the bridge that moves to allow for specific traffic to pass under them. Generally this is taller boat traffic. Although they are more expensive, in tight areas they can be much more economical as taller fly-over bridges can take up a lot of real estate.
Bascule (Draw Bridges)
- Bascule bridges are those that are commonly referred to as “draw bridges”. These can be simple one leaf bridge where only one side raises or they can be multiple leaf structures. The Bridge of Lions in St Augustine, FL is an example of a small bascule bridge. The Brightman Street Bascule in Fall River, MA is an example of a large 4 leaf bascule bridge.
Swing Span
- Swing span bridges are usually limited to railroad use bridges. These are seen in locations where a railroad may be crossing a river in lower elevations where lifting vertically isn’t economical such as the bayous in Lousiana.
Pedestrian Bridges
- Pedestrian bridges are just as the name implies, These are designed only to support foot traffic and no vehicular traffic. They can definitely vary in complexity from small plate girders, simple prefabricated trusses, to extremely complicated arch structures.
The Workforce
To understand HOW the bridge is built, you need to understand WHO is building it. It takes a diverse team of craftsmen working towards one end goal to ensure the bridge structure is fabricated to the specified quality and scope requirements.
In this process, the workforce needs to contain heavy equipment operators, laborers, CNC equipment operators, welders, fitters, QC personnel, and a dedicated office staff to run seamlessly.
Many of these craftsmen work years to hone their specific trade to ensure the quality that they are personally responsible for is on point. In addition, the entire workforce MUST maintain the utmost attention to safety. In this industry, even the smallest plates can weigh 100’s of pounds. This can break bones easily if they fall on you. This isn’t even close to the damage that could happen if a full sized plate girder weighing 10 tons to more than 75 tons fell on you.
This is a dangerous environment. However, if the risks are managed and proper education is in place, the risks are drastically reduced and generally eliminated. Your employees are your #1 asset and keeping them safe is your #1 responsibility.
The Process
The moment we’ve all been waiting for. How are these pieces fabricated? As noted above, I’m only going to focus on a plate girder bridge as it’s not as complicated to explain as many others. Those are better to learn by doing and working with experienced fabricators. Plate girder fabrication is where we begin to develop our institutional knowledge base.
Let’s start at the top once a contract is awarded to your company.
Detailing
- Detailing is the process that starts the development of the shop drawings. Shop drawings are the foundation of how each bridge is to be fabricated. They explain where each piece goes, how each weld is to be made, and what is to be incorporated into the member. In addition, the detailer creates drawings for the field to install the pieces once they are delivered.
Mill Orders
- In order for the material for the bridge to be purchased and delivered to the fabrication facility, the detailer must create the mill orders or Advanced Bill of Materials so the fabricator can purchase the raw materials from the steel mill. When the mill orders are issued, the fabricator will start to received the pieces of the bridge by rail car or truck. Typically, raw material is delivered in flat plates or rolled shapes that are then cut into the pieces needed.
Material Preparation
- Material preparation starts once the raw material is received. This is when the miscellaneous members are processed into smaller, identifiable pieces that make up larger pieces such as crossframes.
Burning
- The burning process is where you start to see the members of the plate girder taking shape. This process is completed using CNC operated equipment that will cut the webs and flanges to the shape needed before being assembled. This usually includes specific holes that need to be drilled for connection. This usually includes cutting any sweep (horizontal curve) into the flanges and camber (vertical curve) into the webs.
Buttwelding
- During the butt welding process, shorter pieces of flanges and webs are butt welded (end to end) to create the final length of the girder. Butt welding is a critical part of the process as each weld is tested by X-Ray to ensure that the weld is 100% free of any defect that could compromise the structural integrity of the bridge.
Flanging
- Once all of the webs and flanges of a specific girder have been cut and butt welded, they can then be assembled into the “I” shape everyone is familiar with. This is called flanging. Newer technologies have provided CNC systems for flanging but the process is still the same. The web is laid flat on a table, then the flanges are fit to the proper dimensions of the web. Once these are fit in place, the flanges are squeezed onto the web with pressure where they are then welded together. Once the flanges are welded to the girder, it can move onto the next step.
Assembly
- During the assembly, the bridge is built to ensure it fits properly when it’s delivered. In a plate girder bridge, this is where field splices are drilled and test fit. This is generally a specification requirement. In addition, it can allow the fabricator to find and resolve any flaws prior to delivery.
Painting
- Bridges can be painted, while others utilize weather steel. Before a bridge is shipped, it has to be either sand blasted or coated with the required paint noted by the specific owners requirements. Weathering steel only requires the steel to be blasted to ensure a uniform finish. When a bridge is required to be coated, it will receive between 1 to 4 coats of the specified paint. These are complicated coatings that are driven by the location the bridge will ultimately be shipped to. Generally they receive a zinc rich primer, while others may receive a primer and the manufacturers recommended system if the customer wants the top coat applied in the shop.
Shipping
- When the time comes to deliver the steel, the plate girders are loaded on a truck, or sometimes a barge, to be shipped to the jobsite. This is where the erector and general contractor take over to install the pieces on site and ultimately finish the bridge so you can drive on it. Generally, plate girders are delivered over the road on trucks and loaded with an overhead crane. These loads can vary in size and length depending on the member. Some are shipped on legal flatbed loads. Typically these are permitted over sized loads and sometimes super loads depending on the weight and length.
In Conclusion
Keep in mind, this is a very simplistic overview of the steps that go into building a plate girder bridge. There are many different ways to skin a cat. In this case, there are many different ways to achieve the end goal of a perfect bridge.
However, the basics of bridge fabrication are still in place. Many of which are dictated by owners specifications and code mandated requirements.
It takes years of experience to learn all of the requirements and “whys” of these processes. In addition, it also requires the flexibility of a business model to walk the fine line between being a manufacturer or a contractor. While it may not seem important at first, this requires you to think differently and to understand the motivation behind each decision you make.
I have to give credit to all of my brothers and sisters in the industry for keeping the country safe, looking good, and mobile. Let’s keep moving forward.
