Views: 221 Author: Site Editor Publish Time: 2026-03-02 Origin: Site
Content Menu
● Metallurgical Foundations: Understanding the Material Science
>> Ordinary Bolts: The Versatile Workhorses
>> High-Strength Bolts: Precision-Engineered Alloy Fasteners
● Mechanical Performance & Technical Comparison
● Load Transfer Mechanics: Why Friction Matters
>> The Bearing-Type Connection (Standard Practice)
>> The Slip-Critical Connection (Evercross Bridge Standard)
● Advanced Installation Protocols: The "Art" of Tensioning
>> A. Surface Preparation: The "Slip Factor"
● Quality Control and NDT (Non-Destructive Testing)
>> Hydrogen Embrittlement: The Silent Killer
>> On-Site Verification: The Skidmore-Wilhelm Test
● Environmental Adaptability: Selection for Longevity
● Engineering Excellence in Every Bolt
● Frequently Asked and Questions regarding Structural Bolting
>> 1. How can I visually distinguish between a high-strength bolt and an ordinary bolt on-site?
>> 2. Can high-strength bolts be reused if they were previously tightened?
>> 3. Why are high-strength bolts preferred over welding in some bridge sections?
>> 4. What is "Hydrogen Embrittlement," and why is it a risk for 10.9S bolts?
>> 5. Does a "Tight" bolt always mean a "Properly Tensioned" bolt?
In the sophisticated field of steel bridge fabrication, where structural integrity is a matter of public safety and multi-decade longevity, the selection of fasteners is a critical engineering decision. For a leading manufacturer like EVERCROSS BRIDGE, with an annual production capacity exceeding 10,000 tons and a history of partnership with global titans such as CCCC (China Communications Construction), CREC (China Railway Group), and PowerChina, we understand that the "small" details define the "big" projects.
This comprehensive guide explores the technical chasm between high-strength bolts and ordinary bolts. We will dissect their metallurgical properties, mechanical behaviors, and the rigorous installation protocols required for world-class infrastructure projects.
The distinction between a high-strength bolt and a standard fastener begins long before they reach the construction site. It starts in the furnace and the tempering bath.
Ordinary bolts, typically classified under Grades 4.8, 5.6, or 8.8, are primarily manufactured from low-to-medium carbon steels, such as Q235 or A3 steel. These materials are chosen for their excellent ductility and ease of machining.
●Production Process: Most ordinary bolts are cold-headed or hot-forged without the need for complex secondary heat treatments.
●Characteristics: They possess a lower yield point, meaning they will deform permanently under relatively lower stress compared to their high-strength counterparts. However, this ductility is an advantage in non-critical secondary structures where some "give" is acceptable.
●Application Scope: In bridge projects, ordinary bolts are reserved for temporary bracing, guardrails, inspection platforms, and non-load-bearing architectural elements.
High-strength bolts (HSB) represent the pinnacle of fastener technology. Manufactured from high-quality alloy steels such as 20MnTiB, 40Cr, or 35VB, these components undergo a sophisticated Quenching and Tempering (Q&T) process.
●The Q&T Process: By heating the steel to an austenitic state and then rapidly cooling it, we transform the molecular structure into tempered martensite. This provides the bolt with an incredible balance of extreme hardness and sufficient toughness to prevent brittle failure.
●Material Grades: In international standards, these are typically 8.8S, 10.9S, or 12.9S (the "S" denotes structural high-strength). In North American projects, we adhere to ASTM F3125 (Grades A325 and A490).
●Fatigue Resistance: Unlike ordinary bolts, HSBs are designed to withstand millions of cycles of vibration—a common occurrence in railway and highway bridges.
To the untrained eye, two bolts might look identical. However, the internal "stored energy" and load-bearing capacity differ by several orders of magnitude.
Ordinary bolts rely on the material's inherent strength to resist being pulled apart. High-strength bolts, however, are designed to be "pre-stretched." When an HSB is tightened to its design tension, it acts like a very stiff spring, clamping the steel plates together with forces that can exceed dozens of tons per square inch.
Performance Metric | Ordinary Bolts (Grade 4.8) | High-Strength (Grade 10.9S) | ASTM A490 Heavy Hex |
Nominal Tensile Strength | 400 MPa | 1040 MPa | 1035 - 1205 MPa |
Yield Strength Ratio | 0.8 | 0.9 | 0.9 |
Core Hardness (Rockwell) | < B95 | C33 - C39 | C33 - C38 |
Primary Load Transfer | Shank Shear / Hole Bearing | Friction / Clamping Force | Friction / Clamping Force |
Ductility (Elongation) | High (>20%) | Moderate (approx. 12%) | Low (requires precision) |
This is the most crucial takeaway for bridge engineers. The way a joint "works" changes completely depending on the bolt type used.
When using ordinary bolts, the joint is a "Bearing-Type" connection.
●The load is applied to the steel plates.
●The plates slip slightly until the side of the bolt hole hits the shank of the bolt.
●The bolt resists the load through Shear Stress (trying to cut the bolt in half) and Bearing Stress (the plate pushing against the bolt).
◆The Risk: Because there is physical movement (slip), these joints are prone to loosening over time under the rhythmic vibration of traffic. This is why ordinary bolts are rarely found in the primary trusses of a bridge.
For the high-performance projects we undertake for China Railway Group, we utilize "Slip-Critical" joints.
●The high-strength bolt is tightened to a massive Pre-tension Load.
●This creates a powerful Clamping Force between the steel plates.
●The load is transferred via the Friction between the surfaces of the plates.
◆The Benefit: The plates never actually move or touch the bolt shank. This "Zero-Slip" environment eliminates the risk of hole elongation and ensures the bridge remains rigid for a 50-to-100-year service life.
Installation is where most failures occur. At EVERCROSS BRIDGE, we mandate strict adherence to technical standards during on-site assembly for our SOE partners.
Since high-strength joints rely on friction, the surface of the steel (the "faying surface") must be treated. We utilize specialized sandblasting or grit-blasting to achieve a specific Slip Coefficient (typically ≥ 0.45 or 0.55). If the surface is greasy, painted with standard primer, or rusted, the friction drops, and the joint could fail even if the bolts are tight.
Bolts must be tightened in two stages:
●Initial Tightening: Usually 60-80% of the design tension to ensure all plates are in close contact.
●Final Tightening: Bringing the bolt to 100% of its specified pre-load.
◆Method 1: Torque Control. Using calibrated electric or hydraulic wrenches. However, torque is often unreliable because 90% of the effort is spent overcoming thread friction, not stretching the bolt.
◆Method 2: Turn-of-Nut. A more reliable geometric method where the nut is turned a specific number of degrees (e.g., 180° or 120°) after a snug-fit is achieved.
As a top-tier manufacturer, our Quality Management System (QMS) involves more than just a visual check. High-strength bolts are subject to rigorous "Search and Destroy" and "Non-Destructive" testing.
For Grade 10.9S and especially A490 bolts, Hydrogen Embrittlement is a major concern. If hydrogen atoms enter the steel during the pickling or galvanizing process, the bolt can snap suddenly under load without any warning.
●Evercross Solution: We utilize specialized baking processes post-plating to drive out hydrogen, and we strictly control the acid-cleaning times in our production line.
Before any bolts are installed on a project for CNOOC or Gezhouba Group, a representative sample of each bolt lot is tested in a Skidmore-Wilhelm Calibrator. This device measures the actual tension (in Kilonewtons or Pounds) produced by the installation method. If the tension doesn't meet the minimum requirement (typically 70% of the tensile strength), the entire batch is rejected.
Bridges are exposed to the harshest elements—from the salty air of coastal ports to the industrial smog of urban centers.
●Hot-Dip Galvanizing (HDG): Best for Grade 8.8S or A325 bolts. It provides a thick sacrificial layer of zinc.
●Weathering Steel (Cor-Ten): For bridges made of weathering steel, we provide Type 3 High-Strength Bolts. These develop a stable, "self-healing" rust patina that matches the bridge and requires no painting for life.
●Dacromet/Geomet Coatings: For ultra-high-strength (Grade 10.9S), we often recommend zinc-flake coatings. These provide excellent corrosion resistance without the risk of hydrogen embrittlement associated with traditional galvanizing.
The difference between ordinary bolts and high-strength bolts is the difference between a temporary structure and a legacy monument. High-strength bolts offer the clamping force, fatigue resistance, and slip-critical performance required to keep global transport networks moving safely.
At EVERCROSS BRIDGE, our 10,000-ton annual output is backed by a deep understanding of these mechanical nuances. Whether we are fabricating for a domestic high-speed rail project with China Railway or an international procurement contract, we ensure that every fastener meets the highest global standards of excellence.
The most reliable way to identify a bolt is by checking the head markings embossed during manufacturing.
●Ordinary Bolts: Typically marked with numbers like 4.8, 5.6, or 8.8. They often have a standard hexagonal head.
●High-Strength Bolts: These are marked with 8.8S, 10.9S, or 12.9S (the "S" stands for Structural). In the North American system, look for A325 or A490. Additionally, high-strength structural bolts usually feature a Heavy Hex head, which is slightly larger than a standard hex head to provide a greater bearing surface for the massive clamping forces involved.
No. It is a strict industry standard—especially in projects involving China Railway (CREC) or CCCC—that high-strength bolts (particularly Grade 10.9S and A490) must never be reused once they have been fully tensioned. When a high-strength bolt is tightened to its design tension, the threads undergo a degree of plastic deformation (permanent stretching). Re-tightening a "stretched" bolt significantly increases the risk of sudden fracture or "thread stripping." Ordinary bolts (Grade 4.8 or 5.6) can sometimes be reused if they show no signs of damage, but for structural integrity, new fasteners are always recommended.
While welding creates a monolithic structure, bolted high-strength joints offer several unique advantages in bridge engineering:
●Fatigue Resistance: High-strength "slip-critical" joints are superior at handling the rhythmic vibrations and heavy dynamic loads of trains and trucks.
●Ease of Inspection: A bolted joint can be inspected visually or with a torque wrench, whereas weld integrity often requires expensive X-ray or Ultrasonic testing.
●Field Assembly: Bolting is faster and less weather-dependent than field welding, which requires highly controlled environments to prevent weld porosity and cracking.
Hydrogen Embrittlement is a phenomenon where high-strength steel becomes brittle and fractures unexpectedly under load. This happens when hydrogen atoms are absorbed into the metal during chemical cleaning (acid pickling) or certain plating processes. Because Grade 10.9S and A490 bolts are extremely hard, they are highly susceptible to this "silent killer." This is why EVERCROSS BRIDGE recommends that these ultra-high-strength bolts should not be Hot-Dip Galvanized using traditional methods. Instead, we use Zinc-Flake coatings (like Dacromet or Geomet), which provide excellent corrosion resistance without the risk of hydrogen-induced failure.
Not necessarily. This is a common misconception. In high-strength bolting, "tightness" (Torque) is not the same as "Tension" (Clamping Force).
The Friction Variable: If a bolt is rusty or poorly lubricated, you might reach the target "torque" on your wrench, but the friction in the threads is so high that the bolt hasn't actually stretched enough to clamp the plates together.
