What this module covers
Pre-installation Verification is 8% of the S1 outline. It sits between delivery and installation: before the crew pretensions a lot in the structure, a few assemblies from that lot are tightened in a calibration device by the method the crew will use, to prove the method and the assemblies together reach the required tension. The same stage covers the faying surfaces that slip-critical joints depend on.
Faying surfaces and slip-critical joints
What to know
- Faying surfaces are the contact surfaces between the plies of a joint.
- Why they matter. A slip-critical joint is designed so the plies do not slip under load. Friction between faying surfaces, clamped by pretensioned bolts, carries the load. The surface condition sets the friction.
- Classes. The bolting specification sorts faying surfaces into classes by their slip resistance. Clean mill scale and blast-cleaned steel fall in different classes. Coated faying surfaces count only when the coating has been qualified by testing for the class shown.
- What the drawings say. Slip-critical joints are marked on the drawings, often with the surface class. The inspector checks before assembly that the surfaces match it.
- Before assembly. Faying surfaces are free of loose scale, dirt, oil, paint not qualified for the class, and burrs that would keep the plies from seating. Masking keeps shop paint off faying surfaces that are to stay bare.
- Snug-tight and pretensioned bearing joints do not depend on faying surface friction, though the plies still must be clean enough to come together.
How to apply it
Invented example. A crane-runway splice on a fictional mill building is marked slip-critical, with blast-cleaned bare faying surfaces. At the splice you find shop primer on the faying surfaces. The project has no test record qualifying that primer for the class shown. Report it before the splice is bolted.
Common traps
- Accepting primer on a faying surface because "paint protects the steel."
- Inspecting faying surfaces after the joint is bolted, when they can no longer be seen.
Pre-installation verification testing with a tension calibrator
What to know
- The device. A tension calibrator is a hydraulic load cell with a gauge, sometimes called a bolt tension calibrator. A bolt passes through it, and the gauge reads the tension as the nut is tightened. It must be in calibration: check the calibration record and date against the interval the agency or project sets.
- Who and when. The installing crew does the test, at the start of work, before a lot is installed, with the special inspector observing. Each combination of diameter, length, grade and lot gets its own test. A new lot means a new test.
- How many. Your reference sets the number of complete assemblies tested per combination. Every one must pass; one failure fails the combination.
- Same as the field. Each test assembly is installed the way it will be in the structure: the same washers in the same places, the same element turned, the same tool, the same lubrication.
- Snug first. The assembly is first brought to snug-tight in the device. Your reference sets a snug tension band for the test; a low snug reading shifts every result after it.
- The target. The bolting specification sets a minimum pretension for each bolt diameter and group, and requires the verification test to reach a target a little higher than that minimum. Read both from your book.
- By method:
- Turn-of-nut: after snug, apply the rotation required for that bolt length and diameter, then read the gauge.
- Calibrated wrench: find the wrench setting that reaches the target, then confirm it on further assemblies. The setting applies only to that combination and only for the period your reference allows.
- Twist-off-type assemblies: tighten until the spline shears, then read the gauge.
- Direct tension indicators: tighten to the target in the device and check the indicator gaps with a feeler gauge against the manufacturer's instructions.
- Short bolts. Some assemblies are too short to fit the calibrator. Your reference gives an alternative procedure for them; do not skip verification.
How to apply it
Invented example (exercise values only). For this exercise, the project specification gives a minimum pretension of 41 kips for the assembly in this exercise and sets the verification target at 6% above it.
- Target: 41 × 1.06 = 43.46, which you round to 43.5 kips.
- The crew snugs three twist-off assemblies from lot T-55 in the calibrator, then tightens each until the spline shears.
- Readings: 45.2, 44.0 and 43.9 kips. Each is at least 43.5 kips.
- Lot T-55 passes. Record the lot, the device and its calibration date, the readings and the result.
Common traps
- Comparing the readings with the minimum pretension instead of the higher verification target.
- Averaging the readings. Each assembly must reach the target on its own.
- Testing in the device with a different washer arrangement than the field connection.
Working a verification test from given readings
What to know
Every verification question boils down to four steps: find the target, read each assembly, compare each one, and decide what the result means for the lot or the method.
How to apply it
Invented example 1, turn-of-nut. For this exercise: target 30.0 kips for a 3/4 in. assembly, required rotation 1/2 turn past snug. Readings after rotation: 31.8, 29.4 and 32.6 kips. The second assembly is under 30.0 kips, so the combination does not pass. The crew and inspector look for the cause (dry threads, wrong snug, wrong rotation) before any of that lot is installed with that method.
Invented example 2, calibrated wrench. For this exercise: target 55.0 kips for a 1 in. assembly. At wrench setting 6, three assemblies read 53.1, 54.0 and 54.6 kips. All are under 55.0. Raise the setting and test again. At setting 7: 56.2, 55.8 and 57.0 kips. All pass, so setting 7 is used for that lot, diameter, length and grade, subject to your reference's rule on how long a setting stays valid.
Invented example 3, units. The calibrator gauge reads in pounds: 44,600 lb. That is 44,600 ÷ 1,000 = 44.6 kips, which meets a 43.5 kip target.
Common traps
- Reading a gauge in pounds as kips, or the reverse.
- Applying one lot's wrench setting to another lot "because it is the same size."
- Accepting a turn-of-nut result that fails because it still beats the minimum pretension.
Check yourself
Try the three questions at the end of this module before moving on.
Where to look in your own books
- The RCSC bolting specification: faying surfaces and slip-critical joints, pre-installation verification, the tension calibrator, and each pretensioning method's verification requirements
- The AISC steel specification's quality assurance provisions for bolting, if your reference list includes it
- The manufacturer's instructions for twist-off assemblies and direct tension indicators used on the project
- The project specification for any added verification requirements
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