New Specs, New Opportunities for Press-Brake Tub Girders

Topics: News, Press Brake & Folded Plate

The American Association of State Highway and Transportation Officials recently released the 10th edition of its LRFD Bridge Design Specifications for use in the design, evaluation and rehabilitation of bridges nationwide.

Developed by the AASHTO Committee on Bridges and Structures, these specifications employ Load and Resistance Factor Design or LRFD methodology, using factors developed from current statistical knowledge of loads and structural performance.

The 10th edition includes updates to almost all sections of the specifications, with extensive revisions made to several sections, including Section 6, Steel Structures.

In December 2024, AASHTO released the 10th edition of its LRFD Bridge Design Specifications.

SSSBA member Valmont Structures developed a summary to offer information on how the updates impact the use of press-brake tub girders (U-BEAMS) for construction.

The following information was provided by SSSBA member Valmont Structures.

Bent Plates

Section 6.7.8 – Structural steel plates shall be bent to the requirements of the AASHTO LRFD Steel Bridge Fabrication Specifications, which specifies 5.0t, where t is the thickness of the plate, and the radius is measured to the concave face of the plate.

ASHTO BDS did not previously specify a bend radius
The previous recommended bend radius in the AASHTO Construction Specifications was 3.5t
Bend radius is no longer a barrier to designing or specifying PBTG

Shear Stud Spacing

Section 6.10.10.1.2 – The minimum spacing of sheer studs was reduced from six times the diameter of the stud to four times the diameter of the stud

We can now get six 7/8” diameter studs per linear foot of beam instead of four
This is a 50% increase in allowable studs on beam
This spacing reduction allows for higher fatigue cycles
We can accommodate 50% more truck traffic with a single row of studs per flange

Proportion Limits

Section 6.11.2.2 – Top flange to web proportion limits “shall only apply to built-up tub-section members” and “need not apply to tub-section members that are formed from a single plate”

PBTG did not meet the previous proportion limit requirement
AASHTO BDS did not previously recognize “tub-section members that are formed from a single plate”
This eliminates a major obstacle from designing and specifying PBTG

Bottom Flange Proportion

Section 6.11.2.2 – Bottom flange proportion limits have now been included for tub girders in AASHTO BDS-10

There were no previous bottom flange proportion requirements
ALL Valmont U-Beams currently meet the new bottom flange proportion requirements
No change
This eliminates a major obstacle from designing and specifying PBTG

Bracing Requirements

Section 6.11.3.2 – Top flange bracing systems shall extend between 20 to 25 percent of the span length from each support.

AASHTO BDS did not previous specify a bracing system length
Previously Engineers needed to follow a rigorous analysis approach to determine bracing requirements
This simple approach allows bracing design to no longer be an impediment to designing or specifying PBTG
This eliminates a major obstacle from designing and specifying PBTG

Creep Reduction Factor

Section 6.13.2.8 – A creep factor taken equal to 0.80 for Class C galvanized faying surfaces

Previously there we no AASHTO BDS creep reduction factor for galvanized surfaces
However, Michigan DOT has already applied this creep reduction factor to their design requirements
This will increase the number of bolts in a splice by about 20%
We already have splice design standards for this change due to the MDOT requirement

Download the PDF version of this summary.

Learn more about Press-Brake Tub Girders

For More Information:

Kay Jimison
National Director Business Development and Sales
North America Transportation – Structures – Bridge
Valmont Industries, Inc.
15000 Valmont Plaza
Omaha, NE 68154 USA
Phone: +1 (503) 207-6400
Mobile: +1 (971) 416-7316
kay.jimison@valmont.com
www.valmontstructures.com

What are Press Brake Tub Girders?

Press-brake-formed tub girder (PBTG) is a new technology for short span bridge applications. It consists of modular galvanized shallow trapezoidal boxes fabricated from cold-bent structural steel plate (watch fabrication video). A concrete deck is recommended to be precast on the girder and the modular unit can be shipped by truck to the bridge site.

The system utilizes standard plate widths (based on availability) and is optimized to achieve maximum structural capacity, with most of the steel in the bottom flange and increased torsional stiffness. It is a closed system, since the girder is closed at the bottom. It is versatile for multiple-deck options.

The system utilizes Accelerated Bridge Construction practices, since it:
- Can be installed in one or two days
- Is modular, allowing the use of a precast deck
- Is cost-effective―as much as 1/3 less than a standard concrete girder structure
- Is simple to fabricate, requiring very little welding.
The Short Span Steel Bridge Alliance has developed standardized plans using standard plate sizes (60”, 72” 84”, 96”, 108”, 120”). The designs were developed to achieve maximum structural capacity and are available to use in the free-to-use, web-based design tool eSPAN140.

The press-brake-formed tub girder system is open source.

Learn more about press brake tub girders→