6.5 Bolting and Welding: Structural Steel Connections

Bolting and Welding: Structural Steel Connections

The structural integrity of a steel-framed building relies entirely on the strength of its connections. Connections are generally categorized into bolted connections and welded connections. The PE Construction exam tests your knowledge of installation procedures, verification methods, and inspection criteria for these structural connections.

High-Strength Bolting

Structural steel connections primarily use high-strength bolts (such as ASTM F3125, Grades A325 and A490). The design of the connection dictates the installation method and the required tension in the bolt.

Connection Types

• Bearing-Type Connections: The bolts act like pins. The load pushes the plates sideways until they bear directly against the body of the bolt. The bolt resists the load through shear strength, and the plate resists it through bearing strength.
• Slip-Critical Connections: These connections are designed so that the plates do not slip under load. The bolts are tensioned to a very high, specific clamping force. This immense pressure creates friction between the steel plates (the faying surfaces). The load is transferred entirely by this friction, not by the bolt bearing against the hole. These are used in joints subjected to fatigue, vibration, or load reversal.

Installation and Tensioning Methods

All high-strength bolts must first be brought to a snug-tight condition. Snug-tight is defined as the tightness attained by a few impacts of an impact wrench or the full effort of an ironworker with an ordinary spud wrench. This ensures all the steel plates are in firm contact.

If the connection is a bearing-type connection, snug-tight is usually sufficient. However, if it is a slip-critical connection, the bolts must be fully tensioned using one of the following approved methods:

1. Turn-of-Nut Method: After the joint is snugged, the nut is turned an additional, specific fraction of a rotation (e.g., 1/3 turn, 1/2 turn) depending on the bolt length and diameter. The rotation physically stretches the bolt to achieve the required tension. It is reliable and easy to inspect visually if the ironworker uses match-marks.
2. Direct Tension Indicators (DTIs): DTIs are special washers with small protrusions (bumps) stamped into them. They are placed under the bolt head or nut. As the bolt is tightened, the protrusions are crushed. The inspector uses a feeler gauge to ensure the gap has been reduced to a specific tolerance, verifying the correct tension was achieved.
3. Calibrated Wrench Method: An impact wrench is calibrated on site daily (or when conditions change) using a Skidmore-Wilhelm tension calibrator. The wrench is set to stop or "stall" when it reaches the torque corresponding to the required tension.
4. Twist-Off-Type Tension Control Bolts (TC Bolts): These bolts have a splined extension on the tail end. A special installation tool holds the spline while turning the nut. When the required tension is reached, the spline physically twists off and breaks. This provides an immediate, visual confirmation of tension.

Welding Principles and Types

Welding fuses two pieces of steel together by melting the base metal and adding a filler metal. The quality of a weld depends on the welder's skill, the electrode used, and adherence to the Welding Procedure Specification (WPS).

Common Weld Types

• Fillet Welds: The most common and economical weld. They are triangular in cross-section and are used to join two surfaces at approximately right angles (e.g., a lap joint or T-joint). They require little to no edge preparation.
• Groove Welds: Used in butt joints where two plates align in the same plane. The edges of the plates must be beveled or prepared to allow the weld metal to penetrate the joint.
  • Complete Joint Penetration (CJP): The weld metal penetrates the entire thickness of the plates, creating a joint as strong as the base metal.
  • Partial Joint Penetration (PJP): The weld metal only penetrates a portion of the plate thickness.

Welding Symbols

Engineers communicate welding requirements using standard symbols on shop drawings.

• The Reference Line is horizontal. The Arrow points to the joint to be welded.
• A symbol placed below the reference line means the weld is on the "arrow side" of the joint.
• A symbol placed above the reference line means the weld is on the "other side" of the joint.
• The symbol itself indicates the type (e.g., a triangle for a fillet weld).
• Numbers indicate the size, length, and pitch (spacing) of the weld.
• A flag at the intersection indicates a "field weld" to be done on site, rather than in the shop.

Weld Inspection and Testing

Quality assurance for welding involves Non-Destructive Examination (NDE) methods.

• Visual Testing (VT): The most fundamental and widely used method. An inspector visually checks the weld size, profile, and checks for surface defects like undercut, porosity, or cracks. All structural welds require VT.
• Ultrasonic Testing (UT): High-frequency sound waves are transmitted into the steel. Flaws inside the weld reflect the sound waves back to a receiver. UT is the primary method used to detect internal defects (like lack of fusion or internal cracks) in thick CJP groove welds.
• Radiographic Testing (RT): X-rays or gamma rays penetrate the weld to expose film on the opposite side, similar to medical x-rays. It provides a permanent picture of the internal weld structure but poses radiation safety hazards.
• Magnetic Particle Testing (MT) and Dye Penetrant Testing (PT): Both are used strictly to detect surface or near-surface cracks that are too small to be seen with the naked eye. PT uses a brightly colored liquid dye that seeps into cracks via capillary action.