Hybrid Fabrication: Combining Rolled Sections and Built-Up Members in Modern Angle Steel Towers

Quick Answer

Hybrid fabrication in angle steel tower design is the strategic combination of standard hot-rolled angle sections and built-up members (fabricated from steel plates) within a single tower structure. This approach allows engineers to use cost-effective, readily available rolled sections for lower-stress members, while deploying higher-capacity built-up sections—such as cruciform, T-shaped, and star-battened configurations—for heavily loaded components where standard angles are insufficient. The result is a tower that meets all structural requirements with optimized material usage, reduced fabrication complexity, and lower overall cost. By selecting the most efficient section type for each structural role, hybrid fabrication delivers weight savings of 10–20% compared to designs relying exclusively on either rolled or built-up members.

Key Takeaways

1. Rolled sections are the economical baseline: Standard hot-rolled angles (Q235B, Q345B, Q420) offer predictable mechanical properties, proven design rules, and cost-effective availability for the majority of tower members

2. Built-up members address capacity gaps: When single angles cannot provide sufficient load capacity, built-up sections (cruciform, T-shaped, star-battened) deliver the required strength without requiring custom rolling of oversized sections

3. Hybrid design optimizes cost-to-performance: Applying the right section type at the right location—rolled angles for bracing and secondary members, built-up sections for heavily loaded legs and primary diagonals—achieves the optimal balance of cost and structural efficiency

4. High-strength steel amplifies the advantage: Using Q420 or higher grades in built-up members can further reduce weight, with studies showing up to 20% weight reduction when replacing S355 with S460 in comparable designs

5. Design rules are maturing: Recent research projects like ANGELHY have developed new, economic design rules for both single angles and built-up members, providing engineers with reliable guidance for hybrid designs

6. Fabrication considerations differ: Rolled sections require minimal processing; built-up members demand additional welding, drilling, and assembly operations—costs that must be justified by performance gains

1. Understanding the Two Member Types

1.1 Hot-Rolled Angle Sections

Standard hot-rolled equal-leg angles—with leg lengths up to 300 mm for tall towers—are the traditional building blocks of lattice towers. These sections are produced in steel mills under controlled conditions, offering:

1. Predictable mechanical properties: Consistent yield strength, ductility, and toughness

2. Proven design rules: Well-established calculation methods per Eurocode 3, GB 50017, and ASCE/SEI 10-15

3. Cost efficiency: Economies of scale in production make rolled angles the most economical option for most members

4. Availability: Standard sections are readily available from multiple suppliers

Common steel grades include Q235B, Q355B, and Q420, with high-strength variants increasingly used in transmission and telecommunication towers.

1.2 Built-Up Members

Built-up members are fabricated from steel plates or multiple angle sections joined together to form a composite section with greater load capacity than any single rolled section. Common configurations include:

Built-up members are particularly valuable when:

1. Required section sizes exceed available hot-rolled dimensions

2. Very high compression loads demand greater capacity

3. Existing towers need strengthening without replacing entire members

4. Customized section properties are needed for specific load conditions

2. The Hybrid Design Philosophy

2.1 Matching Member Type to Structural Demand

The fundamental principle of hybrid fabrication is selecting the most efficient section type for each structural role. In a typical lattice tower:

This graduated approach—heavier built-up sections at the base where forces are highest, transitioning to lighter rolled sections toward the top—mirrors the natural load distribution of a cantilever tower and optimizes material usage throughout.

2.2 When to Choose Built-Up Over Rolled

The decision to specify a built-up member instead of a rolled angle is driven by several factors:

1. · Load capacity: When the required compression or tension force exceeds the capacity of the largest available rolled angle (typically 300 mm leg length)

2. · Section availability: When the required section size is not economically available as a hot-rolled product

3. · Buckling considerations: When the effective slenderness of a single angle is too high, and a built-up section provides better stability

4. · Strengthening existing towers: When adding capacity to an in-service tower without replacing primary members

2.3 Economic Considerations

The economic trade-off between rolled and built-up members is not always straightforward:

1. · Rolled sections have lower fabrication costs but may require more material if the section must be oversized to meet capacity requirements

2. · Built-up members have higher fabrication costs (welding, drilling, assembly) but can achieve the required capacity with less material

Research has shown that the factor with the highest contribution to decision-making between built-up and hot-rolled proposals is the base price of raw material for each option in the market. When steel prices are high, the material savings of built-up members become more attractive; when fabrication costs dominate, rolled sections may be preferred.

3. High-Strength Steel and Hybrid Design

The use of high-strength steel grades—particularly Q420—has transformed the economics of hybrid fabrication. In extra-high-voltage transmission towers, Q420 large-size angle steel can often substitute for double-spliced or multi-spliced common angle steel members, reducing fabrication by multiples and significantly decreasing the number of connection bolts and filler plates.

The ANGELHY research project demonstrated that using S460 high-strength steel (comparable to Q420) in a typical transmission tower reduced total weight from 66 tonnes to 56 tonnes—a saving of over 15%. More broadly, high-strength steel can reduce the weight of standardized towers by up to 20% compared to normal steel grades (e.g., S355/Q345), resulting in lighter structures that are easier and faster to erect.

However, high-strength steels present fabrication challenges:

1. · Drilling required for Q420: Punching is generally not recommended; holes must be drilled

2. · Welding restrictions: Welding operations should be avoided where possible for Q420B high-strength steel

3. · Design rule gaps: Some standards (e.g., EN 50341) do not include high-strength steel grades, requiring reference to other codes

4. Design Standards and Research

4.1 Current Standards

Several standards govern the design of rolled and built-up members in lattice towers:

4.2 The ANGELHY Research Project

The ANGELHY project (2017–2020), funded by the European Commission, specifically addressed the design of both single angle and built-up members in lattice towers. Key outcomes include:

1. New economic design rules for single angle and built-up members

2. Improved rules for built-up sections, including innovative types composed of two angles with unequal sections

3. Testing on lattice towers with high-strength steel (S460)

4. Testing of hybrid members (steel angles reinforced with CFRP strips)

5. Design recommendations for Eurocode 3

The project confirmed that built-up members provide a cost-effective solution for strengthening existing towers and for new designs requiring higher capacity than available rolled sections.

4.3 Built-Up Member Design Considerations

Designing built-up members requires attention to several factors not present in single-angle design:

1. · Shear effects: Built-up compression members exhibit extra flexibility due to shear action, affecting overall stability

2. · Connector spacing: The distance between batten plates or filler plates must be sufficient to ensure composite action

3. · Local buckling: Plate elements in built-up sections must satisfy width-to-thickness (b/t) ratio requirements

4. · Modeling considerations: In structural analysis programs like SAP2000, built-up members are typically modeled as single member elements with double-section cross-sections

5. Application: Qingdao Altai Tower Approach

Qingdao Altai Tower Co., Ltd. is a professional manufacturer of telecommunication towers, power towers, and tower accessories, established in 2003. The company specializes in the design, manufacturing, and installation of steel towers, with products exported to more than 100 countries and regions.

Hybrid Fabrication Capabilities

Design Philosophy

Qingdao Altai Tower applies hybrid fabrication principles through:

1. · Section optimization: Selecting the most efficient section type (rolled or built-up) for each structural role based on load analysis

2. · High-strength steel integration: Using Q345B and Q420 grades to achieve weight savings in critical members

3. · CNC precision: Ensuring accurate connections for both rolled and built-up members

4. · Trial assembly: Verifying that hybrid designs assemble correctly before shipment

6. Comparison: Rolled vs. Built-Up Members

F A Q s

Q1: What is the difference between a hot-rolled angle and a built-up member?

1. A: A hot-rolled angle is a standard steel section produced in a rolling mill, with consistent dimensions and properties. A built-up member is fabricated by joining multiple steel plates or angles (e.g., into cruciform or T-shaped sections) to achieve greater load capacity than any single rolled section.

Q2: When should I specify a built-up member instead of a rolled angle?

1. A: Specify built-up members when: (1) the required load exceeds the capacity of the largest available rolled angle, (2) the required section size is not available as a hot-rolled product, (3) buckling resistance requires a more efficient section shape, or (4) strengthening an existing tower.

Q3: What are the most common built-up section configurations?

1. A: Common configurations include cruciform—two angles toe-to-toe, T-shaped—two angles back-to-back, and star-battened—two chords connected by batten plates.

Q4: How much weight can hybrid fabrication save?

1. A: Using high-strength steel in hybrid designs can reduce tower weight by 10–20% compared to designs using only standard steel grades. The exact savings depend on the specific load conditions and section choices.

Q5: What design standards apply to built-up members?

1. A: Key standards include GB 50017 (T-shaped and cruciform sections), ASCE/SEI 10-15 (latticed transmission structures), and EN 1993-3-1 (towers and masts). The ANGELHY project has also developed new design rules for built-up members.

Q6: Can existing towers be strengthened using built-up members?

1. A: Yes. Built-up members—particularly star-battened configurations—are commonly used to strengthen existing lattice towers without replacing primary members. This is a cost-effective alternative to complete tower replacement.

Q7: What steel grades are commonly used for built-up members?

1. A: Common grades include Q355B, Q420, and higher grades such as Q460 or S460 for specialized applications. High-strength steels provide greater capacity with less material.

Q8: How do built-up members affect fabrication time?

1. A: Built-up members require additional fabrication operations—welding, drilling, assembly—compared to rolled sections, increasing production time. However, the weight savings can offset this through reduced handling and erection time.

Q9: Are built-up members more expensive than rolled angles?

1. A: Generally, yes—the additional fabrication operations increase cost per unit weight. However, the material savings from using a more efficient section can offset this, making built-up members cost-competitive for high-load applications.

Q10: How does Qingdao Altai Tower approach hybrid fabrication?

1. A: Qingdao Altai Tower applies hybrid fabrication through section optimization (selecting the most efficient member type for each role), high-strength steel integration (Q345B, Q420), CNC precision fabrication, and trial assembly verification to ensure quality.

Conclusion

Hybrid fabrication—the strategic combination of standard hot-rolled angles and built-up members—represents a sophisticated evolution in lattice tower design. By applying rolled sections where they are most economical and built-up members where capacity demands it, engineers can achieve towers that are:

1. Structurally efficient: Meeting all load requirements with optimized material usage

2. Cost-effective: Minimizing both material and fabrication costs

3. Adaptable: Addressing capacity gaps where standard sections are insufficient

4. Future-proof: Capable of strengthening existing towers without complete replacement

Research projects like ANGELHY have provided the design rules and validation needed to implement hybrid designs with confidence. High-strength steels such as Q420 and S460 further amplify the benefits, enabling weight reductions of up to 20%.

For project owners and engineers, the choice is clear: a rigid reliance on either rolled or built-up members alone is no longer necessary. Hybrid fabrication offers the optimal path—using the right member type, in the right location, with the right steel grade—to deliver towers that are stronger, lighter, and more cost-effective.

Ready to optimize your tower design with hybrid fabrication? Contact Qingdao Altai Tower's engineering team today for custom section optimization, high-strength steel integration, and a detailed proposal.

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