A709 Steel Grades & Resistance Factors

1. Structural Steel for Bridges: ASTM A709

When designing a steel bridge in the United States, the material properties are not arbitrary; they are governed by a specific standard: ASTM A709, "Standard Specification for Structural Steel for Bridges." This specification is harmonized with the AASHTO material standard M270. ASTM A709 is critical because it ensures that the steel possesses not only the required strength but also the weldability and, most importantly, the fracture toughness necessary for the demanding environment of a bridge structure.

The grades within A709 are designated by their minimum specified yield strength ($F_y$) in kips per square inch (ksi). For your work in tub girder design, you will most frequently encounter the following:

• Grade 36: $F_y=36$ ksi. Often used for secondary members or stiffeners.
• Grade 50 & 50S: $F_y=50$ ksi. The workhorse for modern steel bridges, commonly used for webs and flanges.
• Grade 50W: $F_y=50$ ksi. A "weathering" steel that forms a stable rust-like patina, eliminating the need for painting in many environments.
• High-Performance Steels (HPS): Such as HPS 50W, HPS 70W, and HPS 100W. These steels offer enhanced weldability and superior toughness, and are often used strategically in hybrid girders (e.g., HPS 70W flanges with Grade 50 webs) to optimize the design for high-stress regions.

To get a formal introduction to these materials, please review the following sections from the NSBA Steel Bridge Design Handbook.

A Note on Fracture Toughness

While yield strength ($F_y$) and tensile strength ($F_u$) are primary properties, they don't tell the whole story. For a structure like a bridge, which is subject to cyclic loading and potential flaws, fracture toughness is paramount. This property measures a material's resistance to brittle fracture in the presence of a crack.

AASHTO addresses this through Charpy V-Notch (CVN) testing requirements, which are an integral part of the ASTM A709 specification. The required CVN toughness depends on the steel grade, plate thickness, service temperature zone, and whether the member is classified as fracture-critical. A fracture-critical member (FCM) is one whose failure could lead to the collapse of the bridge. This designation, made by the design engineer, imposes the most stringent material and fabrication requirements.

Given your extensive experience, you can appreciate the importance of material behavior beyond simple strength. The following reading provides a brief but essential overview of this concept in the AASHTO framework.

2. Resistance Factors ($\varphi$) per AASHTO 6.5.4

Now that we've identified the materials, we turn to the resistance factor, $\varphi$. This factor is a key component of the LRFD philosophy. It reduces the calculated nominal resistance ($R_n$) to arrive at the factored resistance ($\varphi R_n$). The value of $\varphi$ is always less than or equal to 1.0 and is calibrated based on the specific limit state being checked. It accounts for:

• Uncertainty in material properties ($F_y$, $F_u$).
• Variability in member dimensions and fabrication tolerances.
• Inaccuracies in the theoretical equations used to predict resistance ($R_n$).

The value of $\varphi$ varies depending on the type of failure mode being prevented. More ductile and predictable failure modes are assigned higher $\varphi$ factors, while more brittle and uncertain modes receive lower, more conservative factors.

AASHTO LRFD Article 6.5.4 specifies the resistance factors for steel structures. Let's examine the ones most relevant to our initial design checks.

Flexure and Shear

For flexural and shear resistance of I-shaped members (and by extension, the flanges and webs of tub girders), AASHTO specifies:

• Flexure: ${\varphi }_f=1.00$
• Shear: ${\varphi }_v=1.00$

A factor of 1.0 might seem surprisingly non-conservative. However, for flexure and shear in steel girders, the conservatism is built into the equations for the nominal resistance ($R_n$) itself. These failure modes are well-understood and typically exhibit significant ductility before failure, providing ample warning.

You can verify these values in the following design guide.

Axially Loaded Members and Connections

The design of connections (like bolted splices) and bracing members often involves checking members for axial tension and compression. Here, the resistance factors directly reflect the nature of the failure mode.

• Compression Members: ${\varphi }_c=0.90$

  • This applies to members failing by buckling. Buckling can be a less ductile failure mode than yielding and is sensitive to geometric imperfections, justifying a lower $\varphi$ factor.

• Tension Members: This limit state is split into two distinct checks:

  • Yielding on the Gross Section: ${\varphi }_y=0.95$
    • This checks for the onset of yielding across the entire cross-section. Since steel yielding is a very ductile and predictable phenomenon, the resistance factor is high.
  • Fracture on the Net Section: ${\varphi }_u=0.80$
    • This checks for rupture at a cross-section reduced by bolt holes. Fracture is a non-ductile failure mode with more uncertainty, especially around stress concentrations. Therefore, AASHTO applies a significantly lower and more conservative resistance factor.

This distinction between yielding and fracture is one of the most important concepts in steel design. The following reading specifies these factors.

3. Summary of Resistance Factors

Here is a summary of the key resistance factors we will use throughout this course. This table will serve as a quick reference as we begin performing design calculations.

(Note: We will cover the specific factors for bolts in detail in Module 9.)

Conclusion

In this lesson, we have populated the resistance side of the LRFD equation with its foundational elements. You now know where to find the material specifications for steel bridges and have been introduced to the key resistance factors that ensure a safe design.

Key Takeaways:

• ASTM A709 is the governing specification for bridge steels, ensuring minimum strength, weldability, and fracture toughness.
• Common grades include 36, 50, 50W, and HPS grades, designated by their yield strength in ksi.
• The resistance factor ($\varphi$) reduces the nominal resistance to account for uncertainties and reflects the ductility of the failure mode.
• For ductile flexural and shear failures, $\varphi =1.00$.
• For less ductile failures like buckling, ${\varphi }_c=0.90$.
• For brittle fracture at net sections, AASHTO uses a more conservative ${\varphi }_u=0.80$, contrasted with ${\varphi }_y=0.95$ for ductile yielding.

With our understanding of both factored loads ($\gamma Q$) and the principles of factored resistance ($\varphi R_n$), we are ready to compute the nominal resistance, $R_n$. The very first step in calculating the flexural or compressive resistance of a girder is to determine how its cross-sectional elements will behave under compression. In our next lesson, we will do just that by learning how to classify cross-section elements as compact, noncompact, or slender according to AASHTO's width-to-thickness limits.