9.1 Structural Steel, Connections, and Welding/Bolting

9.1 Structural Steel, Connections, and Welding/Bolting

Structural steel is CSI Division 05 10 00 (Structural Metal Framing). On the open-book exam your authorities are the International Building Code (IBC) Chapter 22, which adopts the American Institute of Steel Construction (AISC) 360 specification, plus OSHA 29 CFR 1926 Subpart R (Steel Erection). Know which book answers which question: IBC/AISC for design and material grades, OSHA Subpart R for erection safety and sequencing.

Material Grades and Shapes

The default wide-flange (W-shape) grade is ASTM A992, minimum yield strength Fy = 50 ksi (kips per square inch), tensile Fu = 65 ksi. Hollow structural sections (HSS) use ASTM A500 Grade C. Plate and bar default to ASTM A36 (Fy = 36 ksi).

High-strength anchor rods are ASTM F1554 in Grades 36, 55, and 105. Channels (C-shapes), angles (L-shapes), and WT tees round out the AISC shape families. The AISC Steel Construction Manual (the 'green book') tabulates section properties such as area, moment of inertia (I), and section modulus (S) — your go-to open-book reference for any beam or column design lookup.

A W-shape callout reads W12x26: nominal 12-inch depth, 26 pounds per linear foot. Takeoff trap: the second number is weight per foot, NOT a dimension. For a 30-foot W12x26 beam the steel weight is 30 ft x 26 lb/ft = 780 lb. Mill pricing and crane-pick weights both depend on getting this right.

Bolted Connections

The modern high-strength structural bolt standard is ASTM F3125, consolidating the older A325 (Grade A325, ~120 ksi) and A490 (~150 ksi) bolts. Two installation/design categories you must distinguish:

Slip-critical joints resist load by friction and are required where slip would be unacceptable (fatigue, oversized holes, reversal). Pretension is typically 70% of minimum tensile strength.

Installation/inspection methods follow the Research Council on Structural Connections (RCSC) specification: turn-of-nut, calibrated wrench, twist-off tension-control (TC) bolts, and direct-tension-indicator (DTI) washers. Turn-of-nut for a typical bolt = snug + 1/3 turn (120 degrees); verify in the RCSC table by bolt length and grade. Inspection per IBC Chapter 17 is special inspection.

Standard hole types matter for capacity: standard (STD) holes are bolt diameter + 1/16 in; oversized (OVS), short-slotted (SSL), and long-slotted (LSL) holes reduce slip resistance and usually force a slip-critical design. Bolt minimum spacing is 2-2/3 d (preferably 3d) center-to-center, and minimum edge distance is tabulated by bolt size and edge condition (sheared vs. rolled). Specifying too-tight spacing is a common plan-review rejection.

Welded Connections

Welding follows AWS D1.1 (American Welding Society, Structural Welding Code-Steel). Welders must hold a current Welder Performance Qualification (WPQ), and procedures need a Welding Procedure Specification (WPS). Filler metal is matched to base metal; E70XX electrodes (70 ksi) match A992/A36.

Fillet weld strength depends on throat. A 1/4-inch fillet using E70XX develops roughly 1.39 kips per inch of weld at allowable load. Worked example: a connection needing 16 kips with a 1/4-inch fillet requires about 16 / 1.39 = 11.5 inches of total weld.

The minimum fillet size is set by the thickness of the thicker part joined (AISC Table J2.4): 1/8 in for material up to 1/4 in thick, rising to 5/16 in for material over 3/4 in. Weld types include fillet (the workhorse), complete-joint-penetration (CJP) groove welds (develop full base-metal strength at moment connections), and partial-joint-penetration (PJP) grooves. Nondestructive testing — ultrasonic (UT) or magnetic particle (MT) — is required on CJP welds in moment frames, verified by a special inspector under IBC Chapter 17.

OSHA Steel Erection (Subpart R) Traps

• Anchor bolts: Columns must be set on a minimum of four anchor rods (1926.755) and designed to resist a 300-lb eccentric load.
• Connectors / decking: Controlled Decking Zone (CDZ) rules apply when leading-edge work is done.
• Fall protection trigger: Steel erection has its own trigger of more than 15 feet for most workers (1926.760), but connectors get a choice between 15 and 30 feet, and a CDZ is limited to 90 feet wide x 90 feet deep. This 15-ft trigger is the classic trap versus the general-industry/construction 6-ft rule under Subpart M.

Common Exam Traps

Watch for the Subpart R 15-foot trigger being confused with the Subpart M 6-foot general-construction trigger. Remember A992 is the default W-shape (not A36, which is plate/bar). Distinguish slip-critical (friction) from bearing (snug-tight) connections. And read W-shape callouts correctly: depth then pounds per foot, never a length.

Bolt Grades, Welds, and Inspection

High-strength structural bolts are ASTM F3125 (formerly A325/A490); connections are snug-tight, pretensioned, or slip-critical depending on the load path. Welds come in two main forms — fillet (most common, sized by leg dimension) and groove/full-penetration (for full-strength splices). AWS D1.1 governs structural welding; welders must be certified (WPQ) to a procedure (WPS). Quality is verified by visual, then NDT — UT (ultrasonic) and RT (radiography) for groove welds, MT/PT for surface cracks.

Common Exam Traps

• Trap: All bolts are tightened the same. Slip-critical joints require pretensioning, not snug-tight.
• Trap: A fillet weld can replace a full-penetration weld at a tension splice. Use a groove (CJP) weld for full strength.
• Trap: Any worker can weld structural steel — only a certified welder to a qualified WPS.
• Trap: Forgetting that high-strength bolting and welding are special inspection items under IBC Ch. 17.