HL-93 Live Load Calculator

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The American Association of State Highway and Transportation Officials (AASHTO) sets the standard live load for bridge designs in the U.S. They introduced the HL-93 live load in 1993, making it the most used model. This model considers a design truck and a design lane load, showing how bridges are affected the most.

Before HL-93, AASHTO used the HS20 vehicle in its guidelines. Established in 1914, AASHTO gives advice for building highway structures. The newest guidebook for bridge design, the 17th edition, came out in 2002.

Key Takeaways

The HL-93 live load is the current AASHTO standard for bridge design, introduced in 1993.
It combines a design truck or design tandem with a design lane load to create the most critical force effects.
The HL-93 live load model replaced the previous HS20 vehicle used in earlier AASHTO specifications.
AASHTO has been providing guidelines for highway structure design since its creation in 1914.
The most recent edition of the AASHTO "Standard Specifications for Highway Bridges" is the 17th edition, published in 2002.

Introduction to AASHTO Live Load Models

In the early 1870s, America's bridge design history took shape. This started with the first written rules by railroad companies. As more highway bridges were needed, groups like the American Railway Engineering Association (AREA) and the American Association of State Highway Officials (now AASHTO) set common rules.

History of Bridge Design Specifications

In 1928, the Specifications for Steel Highway Bridges set out the first "H" and "HS" truck loads. These rules evolved through AASHTO's editions over the decades. Since 1994, the AASHTO LRFD standards have stayed mostly the same.

Development of Early AASHTO Live Loads

The first AASHTO bridge specs came out in 1931. AASHTO started using its current live loads in 1996. The HL-93 live load model appeared in 1993. It includes an HS20 truck or a design tandem, plus lane loads.

Live Load Model	Key Characteristics
HS20 Truck	Total load of 72,000 pounds with specific axle loads
Design Tandem	Pair of 25-kip axles spaced 4 feet apart
Lane Loading	Uniform load of 0.64 kips per linear foot distributed over a 10-foot width

AASHTO added more than HS20 and HS25 trucks to its live load models. This includes off-highway trucks like the U80 and U102.

Understanding the HL-93 Live Load

The HL-93 live load model sets the standard for designing bridges, following the AASHTO Bridge Design Specifications. It involves a design truck or tandem and a design lane load. These live loads were first set in 1996 and updated to match modern traffic and design trends.

Components of HL-93 Loading

The design truck in HL-93 is similar to the old HS20 truck, weighing 72 kips. It can have any axle spacing between 14 and 30 feet. The design tandem has two 25-kip axles that are 4 feet apart, totaling 50 kips. The lane load is a 0.64 klf uniform load spread over a 10-foot width. These loads combine to show the most critical impacts on a bridge from truck/tandem and lane load interactions.

Design Truck and Design Tandem

The HL-93 live load vehicle has a 72 kips total weight with specific axle and spacing set for trucks and tandems. States have different Permit Vehicles up to 110 kips, affecting bridge designs. Multi-lane structures are considered, and different factors are used along with Average Daily Truck Traffic (ADTT) criteria.

The provided links have data showing how important the HL-93 model is for bridge designs. Modern bridges use special software to look at thousands of load cases to find the worst scenarios. The Federal Highway Administration (FHWA) is studying if the current live load models fit today's traffic needs.

Parameter	Design Truck	Design Tandem
Total Weight	72 kips	50 kips
Axle Spacing	14 to 30 feet	4 feet
Axle Weights	Variable	25 kips each

"The HL-93 live load model, which serves as the industry standard for bridge design as per the AASHTO Bridge Design Specifications, consists of two primary components: the design truck or design tandem, and the design lane load."

hl-93 live load Application in Bridge Design

Multi-Lane Loading and Traffic Factors

When designing a bridge with the HL-93 live load, engineers think about many lanes and how many trucks might drive there. They know that bridges won't usually have every spot full of trucks, so they use special factors. These factors change based on how many lanes there are, helping to spread the load out evenly.

For the HL-93 model, the dynamic load allowance always stays at 33%. This is true for when bridges are at their strongest or at normal use limits. Yet, studies recommend using a 25% dynamic load allowance for vehicles. This advice isn't for medium bridges since they need 33% to handle the extra impact from trucks. Smaller bridges use a different approach based on the design of the tandem, with a 33% dynamic load allowance to handle these variations.

The HL-93 live load is used for MnDOT bridges. It considers various live loads like HL-93, Fatigue, Single-Unit SHVs, and others. Each load has its own dynamic factor, which is a percentage of the total load. If there's a wide sidewalk, pedestrian loads can be added in three different ways.

There's also an option for special vehicles in the design, but you must pick HL-93 to use them. If you don't choose any live loads for the Strength II case, the system won't calculate responses for it. When sidewalks are too narrow, they may not include pedestrian loads. But if they are wide enough, you must consider the weight of people and any barriers.

To sum up, using the HL-93 live load in bridge calculations means thinking about many lanes, vehicle types, and how they might stress the structure. By carefully considering these, engineers make sure the bridge is safe for everyone who crosses it. Best Clipping Software for Gaming Free

Comparison with Previous Load Models

The HL-93 live load model is now used more than the older HS20-44 model for making bridges. It includes a pair of 25-kip axles spaced 4 feet apart. The HL-93 model also features the HS20-44 lane load without extra concentrated loads.

For bridges that are not too long, the main loads come from the design tandem or design truck. The design lane load is more important for long bridges. The HL-93 model replaced an older, heavier truck model, the HTL-57, to avoid using illegal load sizes.

Surprisingly, the HL-93 model's effects on a bridge's structure are similar to using the old HTL-57 truck model. This shows the HL-93 model mirrors the stresses of very heavy vehicles for bridge design, without actually naming specific illegal loads.

Moving from the HS20-44 to the HL-93 live load model has been a journey filled with tests. Mexico's bridges used a unique system called "Tridilosa", not the common girder system. Some of these bridges faced big wear issues, leading to their switch to the usual girder bridges. Despite this, a few "Tridilosa" bridges are still being used, prompting a closer look at their weight effects to ensure they're safe.

To tackle this, experts analyzed Weigh-In-Motion (WIM) data from Guanajuato, Mexico. They looked into how trucks impact the "Tridilosa" bridges, comparing various load models from Mexico, the U.S., and Canada. The goal was to check if current live load models can correctly predict stress on 3D-truss bridges, like the "Tridilosa" ones in Mexico.

Testing load models goes beyond just special bridges. Researchers have also looked at how two methods, LRFR and LFR, fit concrete bridges using standard notional loads (HS20 and HL-93). They found that one method’s Strength-I live load factors were a bit lower, but Strength-II was the same. Additionally, the amount allowed for dynamic load by LRFR could be less, based on the road and how fast vehicles go.

The shift from the HS20-44 to the HL-93 bridge load model, along with the continuous effort to check its fit for different bridges and design styles, underscores the ongoing need to review and adjust these standards. This is crucial for the safety and good working order of our bridges.

Future of Live Load Modeling

For more than 25 years, the HL-93 live load model has been the top pick. People now wonder if it shows today's traffic and car designs well. The FHWA started to gather weigh-in-motion (WIM) data from different places across the U.S. The goal is to check how accurate the HL-93 model is by looking at real traffic, vehicle weights, and configurations. This info will help decide if the AASHTO's live load rules need an update in the future.

Weigh-in-Motion Data Analysis

Looking at WIM data gives us a good look at bridge live loads right now. Things like moment ratios for different span lengths and how they vary, the coefficient of variation for live load moments and shears, and comparing live load models all tell us a lot about the AASHTO's rules' accuracy. We can also see the HL-93 model's strengths and where it might need changes by comparing its expected loads with reality.

Over time, AASHTO's live load rules have changed. They've added models like H20, HS20, H25, and HS25. These model styles have different axle loads and vehicle tonnages. The HL-93 model, part of the AASHTO LRFD rules, uses a specific design load. It picks the worst between the design truck and the design lane load. The Cooper E-80 model, for railroad bridges, uses heavier loads. It features larger live loads than the AASHTO models because of its different specifications.

By looking at WIM data and AASHTO's model history, engineers and policymakers can figure out if the live load rules need an update. This approach makes sure AASHTO's models match today's traffic and cars. It aims to make bridge designs safer and more reliable.

Live Load Model	Axle Loads	Total Vehicle Tonnage
H20	8,000 pounds	40,000 pounds
HS20	32,000 pounds	72,000 pounds
H25	10,000 pounds	50,000 pounds
HS25	40,000 pounds	72,000 pounds
HL-93	HS20 design truck or design tandem	0.64 KLF design lane load
Cooper E-80	80,000 pounds	N/A

"The analysis of weigh-in-motion data, combined with the historical development of AASHTO live load models, can help bridge engineers and policymakers determine the need for future updates to the live load specifications."

As we move forward, making sure bridge live load models are up to date is key. The FHWA's work with WIM data, and understanding AASHTO's model history, will guide the future. This effort is crucial for the safety and reliability of bridges in the U.S.

Conclusion

The AASHTO HL-93 live load model is today's bridge design standard in the US. It uses a special truck or tandem design and lane load. This shows the most critical force effects on bridges. Even though it uses sub-legal truck weights, it acts like a super-long, heavy "super-legal" vehicle. With changing traffic and vehicles, we must keep studying weigh-in-motion data. This helps decide if the HL-93 model needs updating to match real bridge loads better.

Since 1914, AASHTO has been key in setting bridge design rules, like the "Standard Specifications for Highway Bridges." It also issued the 2004 "LRFD Bridge Design Specifications". The HL-93 loading, with its specific truck and tandem design, improves how we figure out bridge force effects. This is better than earlier models such as HS20 and HS25. But, as vehicles and traffic change, we need to keep studying and analyzing data. This keeps the AASHTO bridge design standards, and the AASHTO HL-93 live load in use by structural engineering pros, up to date and useful.

In conclusion, the HL-93 live load model is vital for the AASHTO bridge design standards. It helps structural engineers make bridges that meet modern transportation needs. As the field moves forward, it's crucial to keep enhancing this model with up-to-date weigh-in-motion data analysis. This ensures our bridges are safe and reliable.

FAQ

What is the HL-93 live load?

The HL-93 live load is a key standard in bridge design. Set by the American Association of State Highway and Transportation Officials (AASHTO), it includes a design truck or tandem. It adds a design lane load. These elements create the most force on the bridge.

What is the history of bridge design specifications in the United States?

In the early 1870s, first written specs for bridges were made by railroad companies. As highway bridges demand grew, groups like the American Railway Engineering Association (AREA) and AASHTO developed solid design guidelines.

What are the main components of the HL-93 live load model?

The HL-93 model includes a design truck or tandem and a design lane load. The design truck resembles the older HS20 with 72 kips total weight and axle spacing from 14 to 30 feet. The design tandem has 25-kip axles 4 feet apart, totaling 50 kips. Also, there's a design lane load of 0.64 klf, spread over a 10-foot width.

How is the HL-93 live load applied in bridge design?

Engineers apply the HL-93 model by considering multiple lanes and truck traffic's impact. They use presence factors to adjust for different truck traffic levels in each lane. This is based on average daily truck traffic data.

How does the HL-93 live load compare to previous load models?

The HL-93 model was designed to improve from the HS20's limits in bridge design. Although it uses smaller loads, the effects on the bridge are similar to those from a larger, 6-axle truck. This change allows for more accurate evaluations.

What is the future of live load modeling?

After 25 years, experts are looking into the HL-93 model's accuracy with modern traffic and vehicle trends. The FHWA is gathering data on traffic weight and movement to see how the current model holds up. This information is key for possibly updating the AASHTO specifications in the future.

HL-93 Live Load Calculator