The connections in steel structures are essential to ensure strength, stability, and durability. Among the commonly used methods for connecting steel members are welding, riveting, and bolting. This article focuses on the preparatory steps involved in fabricating and assembling steel members connected by riveting or bolting, highlighting the critical processes of laying out, fabrication, hole making, and assembling.
Fabrication of Riveted or Bolted Steel Members
Laying Out of Steel Members
The fabrication process begins with laying out steel members according to full-scale drawings on a level platform. This layout may be completed in full or in parts as specified in engineering drawings. Accurate measurements using a steel tape are vital to ensure that the components fit together precisely in later stages.
Fabrication of Steel Members
Before actual fabrication, detailed shop drawings must be prepared. These drawings specify all necessary information, including the type, size, and location of rivets, bolts, or welds. Clear markings or stenciling on steel members help in easy identification and tracking throughout the fabrication and assembly process.
Accuracy is paramount in fabrication to prevent distortion such as twists, kinks, or open joints that could complicate assembly. To achieve this, wooden or metal templates are created for each member, which are used to mark the positions of rivet and bolt holes precisely. The steel members are then cut square to the required dimensions by shearing or sawing and are straightened if necessary. It is important that no two pieces are welded or joined to make up the required length of a member.
The base positions of columns and anchor bolts are carefully set to ensure proper alignment during erection.
Making Holes for Riveting or Bolting
Hole preparation is critical to ensure secure and precise connections. Holes larger than the thickness of steel members should be drilled only after the members are fully assembled and clamped together tightly. If holes are punched prior to assembly, they must be undersized by approximately 3 mm and then reamed to the full diameter after assembly. It is also important that the thickness of steel for punched holes does not exceed 16 mm.
Rivet Hole Specifications
The diameter of rivet holes must accommodate the nominal rivet diameter plus an additional allowance—1.5 mm for rivets up to 25 mm diameter and 2 mm for larger rivets unless otherwise specified. Bolt holes for fitted and turned bolts require reaming or drilling to a size slightly larger than the bolt diameter, typically ranging from 0.2 to 8 mm depending on bolt size.
To maintain strength and fit, holes must be drilled or reamed with axes perpendicular to the member surfaces and must be free from burrs or irregularities. The use of gas cutting for forming these holes is prohibited due to the potential for damage and inaccuracies. For countersunk bolts, the hole must allow the bolt or rivet head to sit flush with the surface after installation.
Assembling of Riveted or Bolted Steel Members
Before drilling holes in individual members, the steel components are assembled and tightly clamped to ensure that the surfaces abut or lap closely. This pre-assembly guarantees proper alignment and prevents gaps that could weaken the connection.
Stiffeners, which provide reinforcement, are secured at both top and bottom but must not be drawn or caulked, which could introduce stress. Joints must be cut or dressed true and fitted closely, with web plates flush to adjoining angles or tops unless otherwise directed.
Splices on web plates should have a maximum clearance of 5 mm, while erection clearance at cleated ends should not exceed 1.5 mm to maintain tight connections. Column splices and compression member joints are machined carefully to ensure that contacting surfaces are close butted for effective stress transmission.
In column caps and bases, all components including gussets, angles, and channels are riveted together and then machined so their contacting surfaces fit precisely over the entire area. Connecting angles and channels must be fabricated and placed with accuracy to avoid reducing their thickness through machining. Bearing stiffeners are also machined or ground at their ends to fit tightly both at the top and bottom, further ensuring the structural integrity of the assembly.