From afar, a girder used for a highway and a girder used for a freight rail line could be similar. The differences are significant when you are closer. The logic in making precast concrete bridge girders for railway and highway bridges is the same, yet the differences in design assumptions, load standards, and detailing are significant when comparing a semi-truck to a fully loaded freight train. This guide explains the difference in the applications, the load that engineers design for in each, and the practical aspects that guide decisions on the type of girder used in each application. No frills standards, no frills numbers.
Why Railway and Highway Girders Aren’t Interchangeable (Why Engineering Can’t Mix and Match)
The main manufacturing process is unchanged, with the differences being the tensioning of strands, pouring concrete, steam curing, detensioning and the placement of stays, which will be under a highway or a railway. What has changed at everything upstream of that: the load that the girder must support, the frequency of that load, and the amount of deflection the girder can take?
The load on each train is heavier, concentrated, and occurs thousands of times per year with little variation in impact as compared to highway traffic. That difference alone changes the entire conversation about design.
The HL-93 Standard for Highway Girder Design
Under the AASHTO LRFD Bridge Design Specifications, the live load model applied to the design of modern highway bridges in the United States is known as HL-93, ConTech Services’ Bridge Load Overview explains. In HL-93, the design truck or design tandem combined with uniformly distributed design lane load are analysed, and the results of these two are compared to determine the governing one.
The design tandem has a front axle load of 8 kips and a rear axle load of 32 kips (per a published example from WSDOT of a precast girder design), based on three axles for the design truck. To compensate for the jolting and bouncing impact of a moving vehicle traveling over expansion joints and irregularities in the pavement surface, a dynamic load allowance is applied to the design example, in this case, 33% as per WSDOT.
The model replaced the previous H and HS trucks from the AASHTO Standard Specifications, the “H” in the model indicating the gross tonnage of the vehicle, ConTech Services says. HL-93 was designed to represent a wide variety of actual highway traffic, from delivery vans to loaded semis, in a single, standard case design.
Railway Girder Design: The Cooper E80 Standard
According to the railroad bridges chapter of Bridge Engineering Handbook, the railway bridges have an entirely different load philosophy, and the Cooper E80 load model is currently the most widely used in North America for the design of freight railroad bridges. According to the same source in the Bridge Engineering Handbook, the “80” in E80 indicates the 80-kip weight of each of the locomotives drive axles.
The difference between the two standards is apparent when compared. Based on a technical comparison, ConTech Services’ load analysis concludes that live load pressures created under Cooper E80 loading are significantly greater than what is created by AASHTO’s highway design trucks. Not a minor point of engineering detail, that is why railway girders are deeper, more heavily reinforced and have a very different fatigue probability design than highway girders.
Some older bridges were originally designed for the lighter model E60 which has the same axle spacing as the E80, but loads reduced by 60/80 as per Bridge Engineering Handbook. Many of these older structures are now carrying loads far below what would be demanded by the current generation of freight trains, which are rated at E80, and represent a real engineering challenge, as evidenced by the on-going research into the assessment of railway bridges which has been undertaken at the University of Bristol.Many of these older structures now have loads significantly lower than those which are currently demanded from them by the more modern generation of freight trains rated E80, and this represents a real engineering challenge which is documented in ongoing railway bridge assessment research which has taken place at Bristol University.
The First Is Fatigue, the Second Is Repetition: The Silent Difference That Matters Most
The volume of highway traffic is constantly changing. The load pattern traveling along the highway girder changes from one vehicle to another, whether it be a car, van, truck, empty trailer, loaded trailer. Rail traffic isn’t like that. This kind of train layout is used on the same trains on the same routes, and it imposes almost the same load cycles many times during the structure’s use life.
That is because repetition is changing the way engineers think about fatigue. The loads are broadly and rather randomly distributed on a highway girder. A railway girder experiences a much more limited and repetitive load spectrum and therefore is a higher priority on the design list for fatigue analysis. This is one of the reasons the railway bridge assessment often relies on AREMA-based load rating methodologies especially created for railway applications, along with AASHTO Load and Resistance Factor Rating methodologies adapted to railroad applications, from the same railway bridge assessment research.
The Selection of Girder Shapes and Spans Precast Concrete Bridge Girders
The precast shapes used on both the railway and highway projects are similar, known as AASHTO I-girders, bulb-tees and box girders, but the sizes and details of these shapes vary depending on the load case that governs the structure.
- AASHTO Type girders, especially deep sections 72″ or greater, are still frequently used in highways and provide an efficient design that is well understood and has a wide variety of span lengths.
- Bulb-tee sections provide increased span length in highway applications by placing material in the flanges, which are structurally optimal.
- Box girders (hollow, torsionally stiff section) are found on railways and highways, especially under curved alignments or skewed crossings, where a standard I-shaped section becomes difficult to use.
The Bridge Engineering Handbook says that prefabricated Bridge elements which can be lifted by track mounted equipment provide real practical benefits for railway applications, where construction time for a closing of a rail line is measured in hours and not weeks. That constraint influences the design of girder size and weight for highway projects, which is usually not as urgent.
Construction Timing: A Bigger Deal for Railways Than Highways Is a Blog About Time Management in the Construction Industry
When traffic is live, it can be diverted to alternate routes or lanes closed during construction with temporary closure, usually without much trouble. Railways are much less flexible. In the case of a freight or passenger line, service interruptions due to outages are a significant operational and financial challenge if the outage is extended beyond a brief period.
It is in this context that precast construction justifies its use for railway projects in particular. The time required to install the speed between trains becomes a factor and prefabricated elements that can be delivered cured, tested and ready-to-place are more attractive choices, as described in the railroad bridges chapter of the Bridge Engineering Handbook. Precasting also has its advantages for highway projects, but the rush to make the right choice is not as dire as it can be for an active rail corridor.
Maintenance and Inspection: Different Rhythms, Different Stakes
Highway girders are inspected on a regular schedule, usually according to state DOT schedules, and are checked for cracking, spalling and bearing condition. The same basic inspection logic applies to railway girders, but the repercussions of deferred maintenance are different. A highway bridge can have a load limit lowered for a short period of time while repairs are scheduled if there is a developing issue. A railway bridge with freight loads that are already on the limit of or exceed the original design is not going to have a lot of headroom.
This is one of the reasons railway owners invest in more frequent structural assessment sometimes with the help of finite element modeling to simulate the situation and see what’s really left. Similar tools are used by highway agencies, except that they typically are given greater time flexibility.
Selecting the Appropriate Strategy for Your Project Precast Concrete Bridge Girders
To help determine what factors are most important in a particular project, a few questions are addressed here.
- So what is the determinant load that the building is required to resist? Verify that your project is eligible for HL-93 highway criteria, Cooper E80 railway criteria or, in a few transit cases, a dedicated load model for that specific transit system.
- What is the realistic time for outage? For railway projects, the narrow closure window, and the weight and lift logistics are very critical for shape selection.
- How long is the service life for the structure’s fatigue life? The situation for loading on railways is generally a more conservative one because of the repetitive loading on a rail rather than on a highway.
- Are curves or skew used in the alignment? Whatever it is, if it’s a rail project or a highway project, box girder sections might have advantages here.
Final Thoughts on Precast Concrete Bridge Girders for Railway and Highway Bridges
The manufacturing process is the same for precast concrete bridge girders used in railway and highway bridges, but the engineering is very different. Not the same numbers in a spec sheet, HL-93 is a load philosophy based on a variety of highway traffic, and Cooper E80 is a load philosophy based on the concentrated and repetitive weight of a freight train.
Knowing the standard that applies to your project and why will determine the depth of the girder along with the sequencing of construction. Once you have the foundation correct, the rest of the design process follows naturally in terms of design whether it is a car or freight car that is conveyed.
