What are Special Inspections for Threaded Fasteners?
In the structural construction world, threaded fasteners take on significant importance despite being only small pieces of overall massive structures. Found most often in the form of steel and concrete fasteners in building, bridge, highway, and similar infrastructure works, threaded fasteners hold the framework of these structures together while being out of sight, out of mind. A single bolt failure will likely not bring an entire building down, however, wide-spread bolt failure usually stemming from improper installation techniques can indeed compromise overall integrity to the point of causing major issues. For this reason, the concern with structural fasteners is usually less about their individual resilience and more about their overall installation. In this vein, robust validation procedures that confirm proper fastener installation have been created and written into construction codes, known as Special Inspections.
The term Special Inspections comes from building code regulations, defined as “inspection of construction requiring the expertise of an approved special inspector in order to ensure compliance with the code and the approved construction documents”. Special inspections, or SIs, are found in a variety of trade disciplines including welding, concrete, steel, soil, masonry, and more – each intended to confirm the technical details specific to the work at hand.
Since SIs are a function of building and construction code, they are most often found in construction projects that are permitted and governed by municipal building departments (such as large building construction or renovation projects). In some cases, non-permitted projects may add special inspections for heightened safety, though not required by code. SIs usually do not apply to residential, small-scale, or otherwise low-risk projects. The determination of what SIs apply to a project is usually the domain of the structural engineer on the project, but can also be done by another registered design professional on the job.
Special Inspections are performed by a neutral third-party company, not the installing contractor or the municipal inspector. This way, SIs have no conflict of interest – they are not influenced to approve questionable conditions on behalf of the owner or contractor, nor impose overly-restrictive code interpretations on behalf of the city agency. A Special Inspector’s sole interest is in making sure that the installation meets the job’s engineering requirements, and is paid their fee whether the installation passes or fails inspection.
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Special Inspection Process
Special inspection work begins once the design engineering work on a project is complete. Engineers on the job will denote certain parts of the design as requiring special inspections where called for by applicable codes. The engineers will also determine all applicable codes and standards involved, commonly pulling from sources such as the American Institute of Steel Construction (AISC), American Concrete Institute (ACI), and the American Society for Testing and Materials (ASTM).
The requirements for a project’s SIs are submitted to the municipality during permit review submittal. During this permit review, city building officials will review, confirm and/or adjust the project’s SI requirements. Most often, building officials will request that their building inspector be present during critical special inspections, and may also adjust the quantity or duration of inspections to be performed.
With the requirements from the engineers and building officials in hand, SIs will review the engineered drawings, absorb the technical requirements, check hardware manufacturer requirements, and build a process for confirming intended results in the field during installation.
After documentation review, the SI will discuss the procedures to be employed with the contractor, making sure the contractor is prepared for the process and that the SI will be present when needed to perform their observation. A common SI process will read similar to: “Inspector will review the work process with the trade workers performing the work, confirm that all hardware to be used is correct, and visually observe workers perform installation work for 20% of the fasteners being installed”.
Next, we’ll discuss a typical inspection sequence for structural threaded fasteners.
Typical Special Inspection Steps for Fasteners
The American Institute for Steel Construction provides a nice framework for special inspections on fasteners used in steel construction, one that generally matches steps involved in other codes. Those steps are as follows:
- Preparation prior to bolting: gather docs and certifications, check fastener specs, confirm procedure paperwork, confirm installer qualifications, and discuss prep steps (such as protective storage of fasteners and site cleanliness).
For ultra-critical applications, destructive testing may be performed on samples of the hardware to be used, to make sure that they perform up to their failure point as expected by the engineers. This step can often catch issues related to unapproved foreign manufacturer supply and manufacturing defects. Believe it or not, counterfeit hardware made of low-grade, cheaper alloys from foreign suppliers is a modern-day problem.
- Inspection during bolting: confirm proper fasteners are installed per drawings, that all hardware is used as designed, that manufacturer directions are followed, and that torque / tension values are achieved.
Often, NDT (non-destructive testing) is used in this step, using tools and instruments that can check the integrity of the fastener and joint after installation, confirming that it performs as intended. A tension test is a good example, which applies and reads the holding pressure of the fastener without damaging it. An NDT check is a good way to catch fastener issues that otherwise appear correct to the naked eye and hand.
- Inspection conclusion after bolting: document the acceptance or rejection of completed connections. In some cases, physical performance confirmation is required, which may include x-ray scanning and hydraulic pull testing.
The role of Special Inspections is to confirm that technical requirements are met on a project, assuring the project owners that the integrity (and thus safety) of their structures are as intended. If an SI observes installation work that is lacking or incomplete, the inspector will direct the contractor to correct the issues, and will not approve the installation until the correction is made. The inspector can fully halt progress on a project until satisfactory corrections are complete, ensuring that corners are not cut and safety is not at risk. In extreme cases, an inspector can fail an installation and contractor entirely, requiring the project owners to bring in alternative contractors or technical installers to complete the installation work correctly. Overall, the role of the Special Inspector is to provide an added layer of risk management and technical oversight for critical installation work, so that the health and safety elements of our major infrastructure projects are all as high as we expect.
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