Structural Fasteners

structural fasteners

When it comes to engineered structural steel projects, very often the nut-and-bolt details of greatest importance on the job are exactly the nuts and bolts themselves. The realm of structural fasteners is a world unto itself, with specifications and quality requirements set by multiple engineering councils and institutions around the globe. Purchasing structural fasteners can be a complicated process, in making sure that all of the specs set by the engineering team are fully met or exceeded. To help your purchasing efforts of structural fasteners, let’s review the most common specifications, key details, and procurement nuances that come into play.

Specifications

A great place to start in getting familiar with structural fasteners is to understand where the requirements over these fasteners originate. Steel construction in the United States is governed by the AISC – American Institute of Steel Construction. Structural engineering and design are taught based on AISC standards, used in everything from large public projects (EG, highway bridges) down to small home structures (EG, a backyard light post). AISC standards cover how fasteners are employed on all sorts of structural assemblies, and point to separate ASTM standards to define the fasteners themselves.

Here are the most common AISC-required standards for structural fasteners.

High-strength BoltsASTM A325/A325M and ASTM A490/A490M After 2016, F3125/F3125M*
NutsASTM A563/A563M and ASTM A194/A194M
WashersASTM F436/A436M
Direct Tension IndicatorsASTM F959/F959M
Tension Control AssembliesASTM F1852/F2280 After 2016, F3125/F3125M*

The “M” character in the above specifications denotes the Metric variant. In other words, ASTM A325 covers the Imperial Inch units, and A325M covers the SI Metric units.

*Note: the ASTM A325, A490, F1852, F2280, and their “M” variant standards, were all withdrawn in 2016 and consolidated into a single new standard F3125/F3125M.

E&T Fasteners stocks and distributes structural fasteners across all specification ranges, including kitted tension control assemblies sourced from single manufacturers. For more information on our available range of engineered hardware, see our product selection here.

Key Features

In short, the above ASTM standards set four key details as unique for structural fasteners: material, connection type, mechanical properties, and inherent quality assurance. Let’s take the bolt standard A325 (now F3125) as an example.

Material – the standard defines three types of allowable materials, classified as Grade or Type 1, 2, and 3, which describe a small selection of specific carbon alloys. The standard further defines specific chemical compositions allowable for each material grade.

Connection Type – the standard defines several connection types, classified as SC, N, and X, which describe slip and shear sensitivity of the connection in use.

Mechanical Properties – the standard defines acceptable engineering values, ranging across fastener sizes, such as tensile strength, yield strength, and proof strength.

Inherent Quality – structural fasteners that conform to ASTM specifications are inherently of higher quality, in that they are required to be tested and assured to meet the design properties of the applicable standard. Common fasteners do not have the same level of requirements, and cannot be substituted for structural fasteners.

In the same way that common fasteners should never be substituted for structural fasteners, it’s also largely the case that structural fasteners should never be substituted across different standards or specifications. For example, any given two specifications might cover identical material properties, but have important minor difference such as thread length or pitch details. Such differences, no matter how seemingly small, might be significant to the long-term safety and reliability of the structural design.

E&T Fasteners will always work to meet your specifications to the letter. Our staff is fully trained on the importance of confirming spec details, and we understand that all substitutions must be reviewed and approved by the project’s structural engineers first. For trustworthy help in fulfilling structural fastener orders with no surprise substitutions and triple-confirmed specs, speak with an E&T sales representative today.

Procurement Nuances

Purchasing structural fasteners involves a few extra steps beyond purchasing normal commodity fasteners, in order to make sure you‚Äôre getting hardware of the exact form and function as intended by the project’s engineers. The Research Council on Structural Connections (RCSC) is a non-profit organization supported by AISC to further refine and promote their shared goals in fastener engineering. The RCSC’s Educational Bulletin #3 describes four key details that must be included on any structural fastener purchase order, as follows:

1 – The vendor to provide certification that the bolts, nuts and washers furnished conform to all requirements of the referenced ASTM specification.

2 – That certified manufacturer’s mill test reports be supplied that clearly show the applicable ASTM mechanical and chemical requirements together with the actual test results for the supplied fasteners.

3 – That the bolt heads and the nuts of the supplied fasteners must be marked with the manufacturer’s identification mark, the strength grade and type as specified by ASTM specifications.

4 – That, for projects requiring slip-critical connections, the lubricated bolt, nut and washer be preassembled to assure proper fit of the bolt and nut and the assembly tested for strength to meet 1.05 times the requirements of Table 8.1 of the Specification for Structural Joint Using ASTM A325 or A490 Bolts prior to shipment to the purchaser.

For more information on purchasing, receiving, verifying, and storing structural fasteners, the RCSC’s Educational Bulletin #3 can be downloaded here.

Further, there are some cases where hardware must be ordered as assemblies from a single manufacturer. For example, an ultra-critical bolt and nut pair may need to come as a machined solution from a single manufacturer such that the tolerance for any galvanizing treatment applied is taking into account and reflected in the machining of both parts. For more guidance on bolting connection procurement, see AISC online FAQ section #6 here.

About E&T

With sales and support teams spanning the Continental US, E&T Fasteners is here to serve your hardware, fastening, and component needs both domestically and internationally. We are a stocking distributor of metallic, plastic, and exotic alloy fasteners, backed by our technical staff directly experienced in your unique applications. We can assist your specifying and selection efforts, providing engineered solutions, record-fast lead times, and stellar customer service from quote through delivery. Please let us know how we can help!