Conexiones estructurales con tornillos de apoyo frente a conexiones resistentes al deslizamiento: qué debe especificar el comprador

En un bearing-type connection, the design relies on bolt shear strength and bearing resistance at the bolt holes rather than on a specified frictional resistance between the plates. In a slip-critical connection, pretensioned bolts clamp prepared contact surfaces—called faying surfaces—together so that friction resists the specified shear load before slip occurs. The joint must also satisfy the applicable bolt shear, tension and connected-material bearing limit states.

For buyers, the distinction affects more than the bolt quotation. It determines which connection designation, bolt assembly, hole arrangement, surface condition, installation procedure and inspection requirements must be transferred from the approved structural design into the procurement documents. The Engineer of Record (EOR), not the buyer or bolt supplier, is responsible for the engineering decisions.

How Bearing-Type and Slip-Critical Connections Transfer Load

Consider two steel plates connected by high-strength bolts and subjected to an in-plane shear force.

In a bearing-type joint, friction may initially resist movement, especially if the bolts have been pretensioned. However, that friction is not the basis of the specified shear resistance. Relative movement can bring the bolt shank into bearing against the hole wall, after which load is transferred through bolt shear and bearing of the connected material. The designer checks the relevant strength and deformation limit states rather than relying on an assumed slip coefficient.

A slip-critical joint is designed differently. The bolt pretension generates clamping force normal to the faying surfaces. Their frictional resistance then opposes movement parallel to those surfaces. If slip would compromise the required performance of the connection, the design must provide adequate slip resistance at the applicable load level.

Bearing-type does not necessarily mean snug-tightened

One important distinction is often lost when purchase orders use the terms bearing-type y snug-tight interchangeably.

RCSC 2025 recognizes three joint types for the relevant structural bolting applications:

Comparison based on RCSC 2025 Section 4 and Table 4.1 for the relevant shear or combined shear-and-tension applications. Tension-only joints have separate provisions.

A pretensioned joint can therefore remain a bearing-type joint. For example, pretension may be required because of a particular fatigue or load condition even when the design does not require the joint to resist shear through qualified faying-surface friction.

That distinction has a direct procurement consequence. Ordering an ASTM F3125 structural bolt and requesting a pretensioned installation does not, by itself, establish a slip-critical connection. The approved design must also define the necessary slip-resistance requirements.

Conversely, specifying a slip-critical connection does not simply mean purchasing a stronger bolt. Bolt strength, minimum pretension, surface condition and hole arrangement contribute to different parts of the engineering assessment. None can automatically replace the others.

When Does the Engineer Require a Slip-Critical Joint?

The practical question is whether slip at the connection can be tolerated under the applicable design requirements.

For a bearing-type joint, some relative movement may occur before the bolts bear against the holes. Whether that movement is acceptable depends on the connection geometry, loading and structural performance requirements. In many applications, the designer can provide adequate strength without requiring slip-critical behavior.

In other situations, even limited movement may be unacceptable. The concern may be structural displacement, fatigue performance, connection stability or the assumptions used in the analysis.

RCSC 2025 Section 4.3 identifies circumstances requiring slip-critical joints for connections involving shear or combined shear and tension. These include fatigue loading with reversal of the loading direction, oversized holes, certain slotted-hole arrangements, and cases where slip at the faying surfaces would adversely affect structural performance.

The details matter.

Fatigue loading is not one uniform condition. RCSC distinguishes fatigue with load-direction reversal from fatigue without reversal. For the latter, the commentary recognizes design alternatives involving pretensioned or slip-critical joints, with different fatigue design bases. A buyer cannot infer the required joint type simply because a drawing mentions cyclic loading.

Slotted holes also require a geometric distinction. RCSC 2025 Section 4.3 contains an exception to its slotted-hole slip-critical requirement where the applied load is approximately perpendicular—within 80° to 100°—to the long dimension of the slot. That exception does not eliminate other applicable design requirements or the need for engineering approval of certain hole configurations.

This matters because a slotted hole aligned with the direction of loading can permit substantially different movement from one loaded across the slot. A purchasing description such as “slotted holes permitted” is insufficient to establish how the joint is intended to behave.

A similar distinction applies to vibration. The presence of vibrating equipment does not, by itself, define the connection as slip-critical. The actual loading, potential reversal, required pretension and consequences of slip must be evaluated against the governing design provisions.

What the EOR must decide

Under RCSC 2025 Section 1.6, the EOR is responsible for specifying the joint type when it is other than snug-tightened. For slip-critical joints, the required class of slip resistance must also be identified. The specification separately identifies required bolting group and type information, optional design selections and conditions that need written engineering approval.

That division of responsibility is important when a quotation contains a missing field.

If a drawing identifies a connection as slip-critical but does not identify the required slip-resistance class, the buyer should request clarification. Selecting Class A or Class B from a supplier catalog would amount to making an engineering decision that belongs to the approved design.

The same principle applies when a supplier proposes a different hole arrangement, bolt assembly configuration or installation approach. Some substitutions require the EOR’s written approval under the applicable specification. They should not be accepted solely because the proposed components appear mechanically similar.

Consider an illustrative connection that moves in service after bolts with a higher grade are installed. If the design requires slip resistance, bolt grade alone has not established the necessary faying-surface and pretension conditions. Revisit the approved joint classification, inspect the specified surfaces and resolve the installation evidence. Do not redesign the connection from a fastener catalog label.

Why Hole Geometry and Faying Surfaces Change the Specification

A correct bolt grade cannot compensate for a faying surface that does not satisfy the approved slip-critical design requirements.

That is because the slip-resistance mechanism depends on the interaction of bolt clamping force, the friction characteristics of the contact surfaces and the actual connection geometry.

Under RCSC 2025 Section 5.4, the nominal slip-resistance calculation accounts for more than a nominal bolt strength. Relevant terms include the specified minimum bolt pretension, mean slip coefficient, number of slip planes, applicable filler effects, hole configuration and other design factors. When combined shear and tension acts on the joint, additional adjustments may be necessary.

The commonly referenced surface classes under RCSC 2025 have the following design values:

These are specification-defined reference values, not measured results for an unidentified supplier’s coating. They must be applied with the surface definitions, qualification requirements and design conditions in the governing specification. A coating described commercially as “high friction” cannot be assigned Class B merely from that description.

Hole type also affects the engineering calculation. The design provisions distinguish standard, oversized and slotted-hole configurations rather than treating all bolt holes as interchangeable. The buyer therefore needs the approved hole type and any relevant orientation or thread-position requirements, not merely the nominal bolt diameter.

A shop primer is not automatically a qualified slip-critical coating

For coated slip-critical faying surfaces, RCSC 2025 Section 3.2.2 generally requires the appropriate blast-cleaned substrate and a coating qualified under Appendix A. The qualification relates to defined coating and application conditions, including the relevant slip-resistance performance.

Coating cure is particularly important. The specification requires the coating to reach the applicable curing condition before the joint is assembled. A surface assembled too early may behave differently from the surface used during qualification.

Unqualified coating or overspray is also controlled within designated faying-surface areas. This means that an otherwise ordinary shop-painting operation can create a connection acceptance issue if paint reaches an area that must remain uncoated or otherwise qualified for the specified joint.

Hot-dip galvanized faying surfaces require a separate reading of the specification. RCSC 2025 designates applicable surfaces galvanized in accordance with ASTM A123 as Class A and prohibits hand or power wire brushing of those surfaces. This should not be confused with the coating specification for the bolts themselves, which is a separate component procurement issue.

The distinction between these surfaces is easy to miss: a bolt may have an approved corrosion-protective finish while the steel plates it connects have a different, separately controlled surface preparation. One coating specification does not establish compliance for both.

Hypothetical procurement example: correct bolts, unresolved surfaces

Assume an approved steel splice drawing identifies a slip-critical joint with oversized holes and Class B faying surfaces. The fastener quotation offers the specified structural bolt assembly, including its corresponding nuts and washers.

The steel fabrication proposal, however, identifies a general shop primer without documentation establishing that the primer and its application conditions qualify for the required Class B slip resistance.

The bolt quotation alone does not resolve the discrepancy. Even if the supplied fasteners meet their applicable product specifications, the available information does not establish that the proposed faying surfaces meet the approved joint design.

The buyer should request the relevant coating qualification and fabrication details or seek an EOR-approved change. The proper response is not to assume that the primer is acceptable, substitute a higher-strength bolt, or classify the joint as bearing-type.

This is an illustrative specification conflict, not a recorded customer project or tested connection. Its purpose is to demonstrate why bolt procurement and steelwork surface preparation must be coordinated.

What Changes in Bolting Assemblies, Installation and Inspection?

The connection type determines installation and verification obligations, but it does not automatically identify every component that should appear on a purchase order.

A structural bolt must first be specified as a product. For projects using the ASTM system, ASTM F3125/F3125M addresses high-strength pernos estructurales and assemblies within its stated product scope. Its publicly listed 2026 edition establishes the current publisher catalog identity, but a project may contractually reference a different edition. In fact, RCSC 2025 cites ASTM F3125/F3125M-25 among its referenced standards.

Buyers must therefore distinguish the published edition, the edition referenced by the joint specification, and the edition governing the actual contract. These are not automatically identical.

The purchase description should identify the required bolting group and type, the applicable grade and style where specified, nominal diameter, length, thread configuration, finish and compatible components. Group, grade, type and style describe different aspects of the bolting system; one term should not silently substitute for another.

Heavy-hex assemblies and spline-end systems, for example, can involve different component configurations and installation procedures. Similarly, nuts and washers must be selected as part of an applicable assembly rather than assumed interchangeable because their nominal thread sizes match.

Washer requirements deserve particular attention. RCSC Section 6 distinguishes washer provisions according to the connection, bolting components, installation method, hole geometry and other relevant conditions. Neither “always include one washer” nor “washers are unnecessary for high-strength bolts” is a reliable universal procurement instruction.

Coating also affects assembly compatibility. The surface treatment, mating nut condition and lubrication can influence installation behavior. A supplier’s quotation should clearly identify the approved coated or uncoated component configuration instead of combining components with unverified compatibility.

Product test reports do not establish installed-joint compliance

RCSC 2025 Section 2.9 requires test reports documenting conformance of the applicable bolting components and matched assemblies to be available to the EOR and inspector before assembly or erection.

These reports serve a different purpose from field installation records.

A manufacturer’s product report can support identification and conformity of supplied components. It does not prove that a contractor used the required installation procedure, achieved the required pretension, or inspected the faying surfaces before assembling a slip-critical joint.

For snug-tightened joints, RCSC Sections 8.1 and 9.1 govern the corresponding installation and inspection requirements. Pretensioned joints bring in the applicable pre-installation verification, pretensioning and inspection provisions of Sections 7, 8.2 and 9.2.

Slip-critical joints require the applicable pretensioned-joint procedures plus the additional faying-surface verification defined in Section 9.3. The surfaces must be visually verified against Section 3.2.2 before assembly, when those surfaces are still accessible.

An inspection conducted only after the splice has been fully assembled cannot be treated as an equivalent substitute for the specified preassembly surface check.

Do not specify installation by a generic torque value

A fixed tightening torque is not a reliable standalone proof that a structural bolting assembly has achieved the required pretension. The torque–pretension relationship depends on the actual assembly, friction, coating, lubrication and installation conditions.

RCSC 2025 recognizes turn-of-nut, twist-off tension-control, direct-tension-indicator and combined methods in its pretensioning provisions. The 2025 revision withdrew the calibrated-wrench method as an independent pretensioning method. That change does not prohibit every use of a calibrated torque tool; torque is still involved in specified procedures, including the combined method and certain arbitration provisions.

For procurement, the necessary distinction is that the selected bolting assemblies must support the project’s specified and permitted installation approach. The contractor remains responsible for the applicable procedures, pre-installation verification and installation inspection.

A bolt supplier should not be asked to certify field pretension or final joint compliance when the supplier has not performed and controlled those activities.

How to Write an RFQ Without Changing the Approved Joint Design

A useful structural bolt RFQ translates the approved connection requirements into purchasable components while keeping design approval, manufacturing evidence and site installation responsibilities separate.

In a hypothetical RFQ, the buyer specifies a slip-critical connection but supplies no surface preparation or installation requirements. The bolt supplier cannot infer those project controls from diameter and grade. Obtain the connection specification, separate steelwork and fastener responsibilities and define the required inspection and traceability records before comparing offers.

The following matrix is a practical procurement aid derived from the responsibility structure in RCSC 2025. It is not a replacement for the governing specification or an official RCSC order form.

The table deliberately separates bolt assembly coating de faying-surface coating. These items may be purchased from different organizations and verified through different documents.

Likewise, a supplier can quote a complete matched bolting assembly without having responsibility for preparing the connected steel surfaces. The buyer should coordinate that interface with the steelwork package rather than invent a universal fastener-supplier acceptance certificate.

What if the quotation and approved drawing do not agree?

A quotation that matches the nominal bolt diameter but changes the grade, type, style, coating or mating components is not automatically equivalent. The appropriate response depends on whether the change is permitted by the governing documents or requires an engineering approval.

If the joint is marked slip-critical but the surface class is missing, the buyer should return the issue for design clarification before making an assumption. If the surface class is known but the fabricator cannot demonstrate that the proposed treatment meets it, the discrepancy belongs in the steelwork and coating approval process.

Where RCSC 2025 Section 1.6.3 requires written EOR approval, a verbal assurance or supplier catalog comparison does not replace it. The buyer should retain the relevant approval reference with the final purchasing documents.

For the bolt package itself, an RFQ should request the approved specification, group/type and any specified grade/style, dimensions, compatible nuts and washers, finish, quantities, and the applicable conformance reports. It should also identify any proposed deviations so they can be resolved before supply rather than discovered during receiving inspection or erection.

The central purchasing decision is straightforward: do not replace a missing connection-design requirement with a stronger bolt, an assumed friction value or an unapproved coating. Obtain the design clarification, then purchase and verify the components against that approved basis.

Technical basis: The engineering distinctions and responsibility boundaries in this article are based primarily on the RCSC Specification for Structural Joints Using High-Strength Bolts, June 5, 2025, particularly Sections 1.6–1.7, 2.9, 3.2–3.3, 4–5 and 7–9. Product-standard identity is referenced against the ASTM F3125/F3125M-26 publisher catalog. Contractual edition, design and acceptance requirements must be confirmed for the individual project.