Engineering Slip-Resistant Connections for Long-Life Infrastructure

Slip-resistant bolted connections are among the most critical and sensitive elements in steel structures. Their performance depends not only on design assumptions, but on execution quality — surface condition, preload reliability, and installation control — parameters that can only be validated through engineering-driven testing.

Ion Metal supports infrastructure projects as both a materials testing laboratory and a third-party engineering management partner, providing qualification and verification of material and process procedures across design, fabrication, construction, and operation stages. Our multidisciplinary team, experienced in materials engineering, manufacturing, welding, and instrumentation, bridges the gap between design intent and site reality.

This capability has been built through direct involvement in major bridge projects, including Osmangazi and 1915 Çanakkale Bridges, where Ion Metal performed slip, creep, and extended creep tests for friction-based bolted connections, supporting design and execution decisions.

This white paper presents an engineering perspective on slip-critical connections — showing how testing, when combined with engineering judgement, becomes a decision-making tool rather than a compliance exercise.

Why EN 1090-2 exists?

From Design Rules to Execution Control

Modern steel structures are designed using EuroCodes (EN 199#), but they are built using EN 1090. This distinction is fundamental. While EuroCodes define what a structure must resist, EN 1090 defines how that structure must be executed so that the design intent is actually achieved on site.

The EN 1090 family was developed to close the historical gap between structural design and fabrication quality. Before its introduction, execution quality was largely governed by national practices, contractor experience, and project-specific specifications — leading to inconsistent performance, especially in safety-critical details such as welded joints and preloaded bolted connections.

EN 1090 aligns execution with design by integrating:

• quality management principles (ISO 9001),

• material traceability and qualification,

• process control and verification,

• and conformity assessment under the Construction Products Regulation (CPR).

Within this framework, EN 1090-2 defines the technical rules for executing steel structures — not only in terms of workmanship, but in terms of engineering control of critical processes. Bolting and welding are treated not as assembly operations, but as performance-determining processes that must be qualified, verified, and documented.

This execution-driven philosophy is the foundation on which slip-critical connection verification, described later in this paper, is built.

Execution Is Where Design Becomes Reality

Structural performance is not achieved on drawings, but through execution. For this reason, EN 1090-2 does not simply prescribe workmanship rules — it defines a system of controlled execution, where critical processes are planned, verified, and documented to ensure that design assumptions remain valid in the built structure.

At the core of EN 1090-2 lies the concept of the Execution Specification. This document translates design intent into executable requirements by defining materials, processes, inspection levels, acceptance criteria, and documentation. Without a clear execution specification, even the most sophisticated structural design remains vulnerable to site variability.

Execution requirements are further structured through Execution Classes (EXC1–EXC4), which reflect the consequence of failure, structural complexity, and reliability demands. As the execution class increases, so does the level of qualification, inspection, and verification required for welding, bolting, surface preparation, and dimensional control.

EN 1090-2 therefore establishes a common language between designers, fabricators, contractors, and inspectors. It ensures that execution quality is not left to individual interpretation, but is managed through a documented and auditable engineering process. In this framework, bolted connections — especially preloaded and slip-resistant assemblies — become controlled engineering operations rather than routine site activities.

This shift from “doing” to “verifying” execution is essential for understanding why slip factor testing, preload control, and site verification are treated as engineering tasks rather than optional checks.

Structural Bolting Assemblies in Steel Structures

In steel structures, load transfer between components is achieved primarily through welded and bolted assemblies. While welding is often perceived as a fabrication process, bolting is frequently treated as a site operation. EN 1090-2 challenges this perception by recognizing both as structural processes whose quality directly determines performance and reliability.

Bolted connections are not generic components; they are engineered assemblies composed of bolts, nuts, washers, surfaces, and installation procedures. Their behavior depends on the interaction of these elements rather than on any single component alone. This is particularly true for preloaded connections, where the structural response is governed by clamp force and friction rather than by bearing or shear.

EN 1090-2 therefore requires that structural bolting assemblies be:

• selected based on structural function,

• installed using qualified procedures,

• and verified to ensure that design assumptions are achieved in the built structure.

In this framework, bolting is elevated from a fastening activity to a controlled structural operation, comparable in importance to welding. This perspective is essential for understanding why slip-resistant connections require dedicated verification beyond visual inspection or torque control.

Design Philosophy: Shear vs Slip-Resistant Connections

Bolted connections in steel structures can be designed to transfer loads through bearing and shear, or through friction generated by bolt preload. While both approaches may appear similar in drawings, they represent fundamentally different structural philosophies with distinct implications for application, inspection, and long-term performance.

In shear-type connections, load is transferred after relative movement occurs between connected plates. The bolts are subjected to shear forces, and bearing stresses develop in the plate holes. This approach is robust, tolerant to installation variability, and suitable for many secondary or static applications. Small slips are allowed and generally inconsequential.

In contrast, slip-resistant connections are designed to prevent any relative movement under service loads. Load transfer occurs through friction at the contact surfaces, generated by the clamp force of preloaded bolts. Here, slip is not a tolerated event but a limit state that must not be exceeded during normal operation.

Eurocode 3 (EN 1993-1-8) explicitly recognizes this distinction by defining separate design checks for shear and slip-resistant connections, each associated with different assumptions and verification requirements. When slip-resistant behavior is required — such as in fatigue-sensitive, dynamically loaded, or geometrically critical structures — execution quality becomes as important as design calculations.

This design philosophy directly explains why slip-resistant connections demand controlled surface preparation, verified preload, and dedicated slip factor testing. Without these, the assumed load transfer mechanism cannot be guaranteed in the completed structure.

Slip Resistance Is Not a Property — It Is a System Response

Slip resistance is often perceived as a material property expressed by a single number: the slip factor μ. In reality, slip resistance is the result of a system-level interaction between several mechanical and procedural parameters. Treating it as a fixed material constant is one of the most common sources of mismatch between design assumptions and site performance.

At the most fundamental level, slip resistance is generated by friction at the contact surfaces between connected plates. This friction is mobilized by the clamp force of preloaded bolts, and its effectiveness depends on how reliably that clamp force is created and maintained throughout the service life of the connection.

However, preload alone is not sufficient to guarantee slip resistance. The actual performance of a slip-resistant connection is governed by a combination of:

• surface roughness and coating characteristics,

• achieved and retained preload,

• stiffness of the bolt–plate assembly,

• embedment and relaxation effects,

• installation method and sequence,

• and verification and inspection practices.

Each of these factors may vary independently during fabrication and site installation, even when nominally identical components are used. For this reason, slip resistance must be understood as a behavioral outcome, not a guaranteed material attribute.

EN 1090-2 recognizes this reality by requiring slip factor testing under controlled conditions (Annex G) and by linking laboratory verification to site application through installation control and verification (Annex H). Together, these provisions transform slip resistance from a design assumption into a verified execution parameter.

This system-based understanding is essential for selecting surface preparation methods, tightening procedures, and inspection strategies that are technically sound, economically viable, and consistent with design intent.

Designing for Slip Resistance Means Designing for Surfaces

In slip-resistant connections, the friction surface is not a passive interface — it is a structural component that directly governs performance. For this reason, Eurocode 3 and EN 1090-2 classify friction surfaces based on their expected slip behavior, allowing designers to define clear and verifiable design targets.

According to EN 1993-1-8, friction surfaces are grouped into four slip classes, each associated with a characteristic slip factor (μ):

• Class A: μ = 0.50

  Surfaces blasted with shot or grit with loose rust removed

• Class B: μ = 0.40

  Blasted surfaces with thin inorganic coatings

• Class C: μ = 0.30

  Surfaces cleaned by wire-brushing or flame cleaning, with loose rust removed

• Class D: μ = 0.20

  Surfaces as rolled

These values represent design targets, not guaranteed properties. They are only valid when surface preparation, bolt preload, and installation procedures are executed and verified consistently with the assumptions of the design model.

In practice, selecting a friction class is not purely a technical decision. It involves a trade-off between:

• achievable surface quality,

• constructability and site constraints,

• durability and corrosion protection,

• inspection complexity,

• and overall project cost.

For this reason, EN 1090-2 requires that when slip resistance is critical, surface performance must be verified by testing, rather than assumed. Slip factor tests (Annex G) provide project-specific confirmation that the selected surface preparation method can reliably achieve the intended design class under controlled conditions.

This verification-based approach enables designers and contractors to make informed decisions — not only about what is theoretically optimal, but about what is practically achievable and economically sustainable for a given project.

From Assumption to Verification

Why Slip Factor Tests Are Needed (Annex G Logic)

Design standards define slip factors as characteristic values associated with surface classes. However, these values are valid only when the actual contact surfaces, bolt assemblies, and installation procedures reproduce the assumptions under which those values were established. In real projects, this condition is rarely guaranteed without verification.

Surface preparation methods, coating systems, handling, storage, and site contamination can all significantly alter friction behavior. Even when nominally identical procedures are specified, the resulting surface characteristics may vary between fabricators, batches, or site locations. As a result, relying solely on nominal friction classes introduces uncertainty into slip-resistant design.

Annex G of EN 1090-2 addresses this uncertainty by defining a standardized method to measure slip resistance under controlled conditions. The test isolates the effect of surface condition by using:

• standardized specimen geometry,

• controlled bolt type and preload,

• defined loading protocol,

• and repeatable measurement of slip.

The outcome of Annex G testing is a project-specific slip factor, derived statistically from multiple specimens and directly usable in design verification. This transforms friction from an assumed parameter into a measured and traceable engineering value.

Slip factor testing therefore serves a dual purpose:

1. It confirms that a selected surface preparation method can meet the design target.

2. It provides a technical basis for adjusting design or execution strategies if the target is not met.

In this sense, Annex G is not a testing requirement imposed by the standard, but a risk management tool that aligns design intent with achievable execution quality.

Preload – The Hidden Variable

Slip Resistance Starts with Clamp Force

In slip-resistant connections, friction can only be mobilized if sufficient bolt preload is achieved and maintained. While design calculations often assume a defined preload level, the actual preload in installed bolts is highly sensitive to installation method, surface condition, and assembly details. This makes preload the most influential — and least visible — variable in slip performance.

Unlike geometric dimensions or material grades, preload cannot be verified by visual inspection. Torque values, commonly used in site practice, are only indirect indicators of bolt tension and are strongly affected by friction in the threads, under-head surfaces, washers, and coatings. As a result, two bolts tightened to the same torque may carry significantly different preload levels.

Preload variability directly translates into slip resistance variability. Even when friction surfaces perform as intended, insufficient or inconsistent preload reduces the effective clamp force and lowers the available slip resistance. Conversely, excessive preload may introduce risks such as bolt yielding, relaxation, or long-term preload loss — particularly in coated or layered assemblies.

EN 1090-2 acknowledges this hidden variability by:

• defining minimum preload levels for preloaded bolts,

• requiring controlled tightening methods,

• and linking laboratory slip tests to verified preload application.

Annex G requires that preload during testing be measured and maintained within strict tolerances, precisely because slip factor results are only meaningful when the applied clamp force is known and controlled. This requirement highlights a key principle: slip resistance cannot be verified without preload verification.

Understanding preload as a controlled engineering parameter — rather than an installation detail — is essential for achieving reliable slip-resistant performance in real structures.

Verification Does Not End in the Lab

Annex G and Annex H Together: Closing the Laboratory–Site Gap

Slip factor testing under Annex G provides a controlled and traceable measurement of friction performance. However, this verification is only meaningful if the same preload and installation conditions can be reproduced on site. Without this continuity, the slip factor remains a laboratory result rather than a structural reality.

This is where Annex H becomes essential. While Annex G verifies what is achievable, Annex H verifies what is actually achieved during installation. It establishes procedures for site tightening control, calibration of tightening methods, and verification of preload application using the same bolt assemblies, surface conditions, and lubrication states defined in the execution specification.

Annex G and Annex H are therefore not alternative routes, but complementary parts of a single engineering workflow:

• Annex G confirms that a surface preparation method and assembly system can meet the design target.

• Annex H ensures that this verified system is reliably reproduced during construction.

When used together, they create a closed verification loop:

This integrated approach transforms slip-resistant connections from a design assumption into a managed performance system, where risk is controlled rather than merely specified.

From Test Results to Engineering Choices

Engineering Decision-Making for Slip-Critical Surfaces

In slip-resistant design, the objective is not simply to achieve the highest possible slip factor, but to select a surface preparation and installation strategy that can be executed reliably, verified consistently, and maintained throughout the service life of the structure.

Different surface conditions may produce similar slip factors in laboratory tests, yet behave very differently in practice when exposed to handling, environmental conditions, and site variability. Conversely, surfaces with lower nominal friction may offer superior reliability and repeatability under real construction constraints.

Slip factor testing therefore enables engineers to move beyond “best-case” assumptions and adopt a decision-based approach:

• What slip class is required for structural performance?

• What surface condition can be reproduced consistently on site?

• What level of inspection and control is feasible?

• How does surface preparation affect corrosion protection and durability?

• What is the total cost of achieving and maintaining the required performance?

By answering these questions, slip testing becomes a tool for balancing technical performance, constructability, and economics — not merely a compliance exercise.

EN 1090-2 supports this approach by allowing slip factor verification to inform both design confirmation and execution strategy selection. When used in this way, testing reduces uncertainty, prevents over-specification, and enables informed trade-offs that align design intent with project reality.

This engineering decision-making framework is what transforms slip-resistant connections from a theoretical design concept into a robust and buildable structural solution.

From Testing to Engineering Partnership

Slip-resistant connections cannot be reduced to a single test result or a nominal friction value. Their performance emerges from the interaction of design assumptions, surface preparation, preload control, and site execution. Managing this interaction requires engineering judgement — not only testing capacity.

Ion Metal approaches slip factor testing as part of a broader engineering verification framework, where laboratory results are connected to execution realities and design decisions. By integrating testing, preload verification, and installation control into a single workflow, we help project teams reduce uncertainty, avoid over-specification, and achieve reliable structural performance.

This approach reflects a broader philosophy: testing is most valuable when it informs decisions. In slip-critical connections, it becomes a bridge between design intent and built reality — and between compliance and performance.