ASTM A500 Grade A is a specification for cold-formed welded and seamless carbon steel structural tubing, commonly referred to as hollow structural sections (HSS). These sections are available in round, square, rectangular, or special shapes and are designed for welded, riveted, or bolted construction applications. While the query specifies "hollow section Steel pipe," this typically refers to round HSS under ASTM A500, which functions similarly to structural pipe but is optimized for load-bearing roles rather than fluid transport (unlike ASTM A53 pipe). Grade A is the entry-level grade in the A500 family, offering moderate strength suitable for general structural uses where high ductility is prioritized over maximum load capacity.
This material is produced from low-carbon steel (primarily iron with additions of carbon, manganese, phosphorus, sulfur, and copper) using electric-resistance welding (ERW) for welded sections or seamless processes. It features a high strength-to-weight ratio, excellent torsional resistance, and compression support, making it predictable for engineering designs. Round HSS under A500 Grade A has tighter tolerances and a superior surface finish compared to standard pipe like A53, without a lacquer coating, which suits painted or exposed applications.
Applications in Bridge Construction
ASTM A500 Grade A hollow sections are used in bridge construction for non-critical structural components, such as secondary supports, railings, frames, and lightweight girders, where moderate strength and cost-effectiveness are key. Bridges demand materials with reliable performance under dynamic loads, environmental exposure (e.g., moisture, temperature extremes), and corrosion risks, and A500 Grade A meets these needs for less demanding elements. It provides uniformity in size, shape, and strength, which is essential for bolted or welded assemblies in infrastructure projects. However, for primary load-bearing bridge components like main girders or high-stress columns, higher grades (e.g., A500 Grade B or C) are often preferred due to their superior yield strength. In bridge applications, these sections can be galvanized or coated for enhanced corrosion resistance, especially in marine or humid environments.
Examples of use include:
Support columns and braces in pedestrian or small vehicular bridges.
Highway sign supports and auxiliary structures integrated with bridges.
Frames for bridge maintenance equipment or temporary scaffolding during construction.
Its recyclability and low maintenance (resistant to warping, rotting, or mildew) align with sustainable infrastructure goals.
Key Specifications and Properties
ASTM A500 covers tubing with a maximum perimeter of 88 inches (e.g., up to 22-inch square or 28-inch round). Production involves forming flat-rolled steel into shape and welding the longitudinal butt joint to ensure structural integrity, with inside flash typically not removed. Grade A tubing does not require heat treatment, unlike Grade D.
Chemical Composition (Weight %)
The composition ensures weldability and structural performance. Limits are similar across grades but tailored for Grade A's lower strength.
| Element | Grade A (Round & Shaped) | Maximum |
|---|---|---|
| Carbon (C) | 0.26 | - |
| Manganese (Mn) | 0.90 (round), 1.35 (shaped) | - |
| Phosphorus (P) | 0.035 | - |
| Sulfur (S) | 0.045 | - |
| Copper (Cu), when specified | 0.20 (min for weathering) | 0.60 (max) |
Mechanical Properties
Grade A emphasizes ductility (higher elongation) over peak strength, with a minimum tensile strength of 58,000 psi (400 MPa) across all shapes. Yield strength varies slightly by shape due to forming differences.
| Property | Grade A (Round) | Grade A (Shaped: Square/Rectangular) |
|---|---|---|
| Yield Strength (min, ksi/MPa) | 39 / 270 | 39 / 270 |
| Tensile Strength (min, ksi/MPa) | 58 / 400 | 58 / 400 |
| Elongation (min, in 2") | 23% | 25% (for wall < 0.15"), 23% (≥ 0.15") |
| Hardness (Brinell, typical) | ~63 | ~63 |
These values ensure the material can deform without fracturing under moderate loads, ideal for bridge elements experiencing vibration or flexing. For comparison with other A500 grades (relevant for bridge design choices):
| Grade | Yield Strength (Round/Shaped, ksi) | Tensile Strength (ksi) | Elongation (min, %) | Typical Use in Bridges |
|---|---|---|---|---|
| A | 39 / 39 | 58 | 23-25 | Secondary supports, railings |
| B | 42 / 46 | 58 | 23 | General framing, moderate loads |
| C | 46 / 50 | 62 | 21 | Primary girders, high-strength needs |
| D | 36 / 36 (heat-treated) | 58 | 23 | Demanding, low-temp applications |
Grade A has the highest elongation (ductility) but lowest yield strength, making it less suitable for heavy-duty bridge spans compared to Grade C.
Dimensional Tolerances
Tolerances ensure fit-for-purpose in construction. Measurements exclude ends.
Outside Diameter (Round, OD ≤ 1.9"): ±0.5%, rounded to nearest 0.005".
Outside Diameter (Round, OD > 1.9"): ±0.75%.
Wall Thickness: -10% (min) to +10% (max, excluding weld); applies to flat centers for shaped sections.
Length: Typically 20-40 ft, with ±1/8" tolerance; custom lengths available.
Twist (Square/Rectangular): Max 0.25" per 3 ft of length.
Corner Radius (Square/Rectangular): ≤3 × wall thickness.
Common Sizes for Round HSS (Pipe-like): OD 2" to 24", wall 0.120" to 0.500"; e.g., 4" OD × 0.237" wall for bridge supports.
For bridge projects, sections are often specified with a 0.93 factor on nominal thickness for design calculations per AISC standards, accounting for tolerances.
Manufacturing and Quality
Process: ERW welding from flat-rolled steel; seamless for specialized needs. No inside flash removal standard.
Testing: Includes tensile, yield, elongation, flattening (for round), and chemical analysis per ASTM A370/A751. Non-destructive electric testing for welds.
Finishes: Uncoated (black), galvanized (for corrosion resistance in bridges), or painted. Galvanizing adds 550 g/m² zinc typical.
Packaging: Bundled, with marking for grade, size, and heat number; compliant with ASTM A700.
Advantages and Limitations for Bridge Use
Advantages: Cost-effective for moderate loads; lightweight (reduces foundation needs); easy to weld/cut/form; fire-resistant and recyclable. Excellent for multi-axis loading in bridge frames.
Limitations: Lower yield strength than Grades B/C limits use in high-stress areas; not ideal for pressure/ fluid applications (use A53 instead). Heat intolerance above certain thresholds can cause brittleness.
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