Frame Fabrication: Transverse Frames and Floors

What Transverse Frames and Floors Do

A ship's hull is a pressurized shell that must resist sea pressure, cargo loads, and the twisting forces of waves. Transverse frames are the ribs that run athwartship (side to side) and give the hull its cross-sectional shape. A transverse web frame is a deeper, heavier frame—typically spaced at every third or fourth frame bay—that carries concentrated loads from decks, bulkheads, and machinery. Floors are the transverse structural members that span across the bottom of the hull, connecting the port and starboard bottom shell plates and tying into the keel. Together, frames and floors form the internal skeleton that keeps the shell plating in shape under load.

Reading the Frame Drawing

Before touching any metal, read the frame drawing completely. Locate the following:

• Scantling dimensions: web height, web thickness, flange (face bar or flat-bar) width and thickness.
• Weld symbols: size, type (fillet, full-penetration), and which side of the joint.
• Lightening holes: diameter, location measured from the baseline or reference edge. Lightening holes reduce weight without significantly reducing strength.
• Limber holes and drain holes on floors: size and location at the lowest point of each floor bay so bilge water can drain to the pump suction.
• Camber or curvature: some frames are pre-bent to match hull form; the drawing will show offsets.
• Tolerances: fabrication drawings for ship frames commonly specify ±1/8 inch on critical dimensions. Never assume—read the tolerance block.

Mark your cutting list from the drawing before you pick up a grinder.

Setting Up the Strongback

A strongback (also called a layout table or assembly jig) is a heavy, flat, rigid surface—usually built from heavy I-beams welded to a concrete floor—that holds components in correct position while you tack-weld them. It is the reference datum for the entire assembly.

1. Clean and check the strongback. Remove slag, spatter, and debris. Verify the surface is flat with a straightedge and level.

2. Strike layout lines. Use soapstone or a scribe to mark the frame centerline, web height line, and flange reference line directly on the strongback (or on sacrificial steel strips tacked to it).

3. Set web plate. Stand the web plate on edge along the centerline. Use angle blocks (90-degree steel stops tacked to the strongback) to hold it vertical. Check plumb with a plumb bob and a torpedo level on the web face.

4. Set flange. Lay the flat-bar or face-bar flange against the web tip. Clamp with C-clamps or dogs and wedges (drift dogs driven against clips tacked to the strongback) to pull tight. Confirm the web-to-flange angle is 90° with a combination square.

5. Check squareness. Measure both diagonals of the assembled frame. They must be equal within tolerance. A difference in diagonals means the frame is racked (out of square)—adjust before tacking.

Tack Welding the Frame

Tack welds hold the assembly in position for full welding later. They must be strong enough to hold alignment but not so large that they cause distortion or are difficult for the production welder to incorporate.

• Use the process and filler specified on the weld procedure (MIG/GMAW or SMAW).
• Place tacks at intervals shown on the drawing or per your shop standard—typically every 12 to 18 inches along a long seam, or at each end plus mid-span for short joints.
• Each tack must be visually acceptable: good fusion at both ends, no cracks, no porosity visible, proper size (usually equal to or slightly less than the final fillet size).
• Cracked tacks must be ground out completely and re-welded—never leave a cracked tack in place.

Floor Plate Fabrication

The floor plate is a flat or slightly curved plate that forms the bottom of a frame bay. Procedure:

1. Cut the floor plate to the shape and dimensions on the drawing (straight cutter, plasma, or pre-cut blanks).

2. Mark and cut the limber hole—a notched cutout at the lowest inboard corner—so bilge water can flow freely to the centerline suction. The limber hole must not remove so much material that the floor loses structural integrity; size and location are specified on the drawing.

3. Mark and cut the drain hole if shown; some drawings call for a separate drilled or burned hole at a specific location.

4. Fit the floor plate to the bottom shell plate mock-up on the strongback. Use a straightedge across the shell plate surface and a level to confirm the floor stands plumb and its top edge is at the correct height.

5. Clamp and tack. Check alignment again after the first tacks—heat can shift parts slightly.

Safety

Personal protective equipment (PPE) is mandatory throughout this lab:

• Welding helmet with correct lens shade for the process in use; safety glasses under the helmet.
• Flame-resistant (FR) clothing, welding gloves, and leather boots with no exposed laces.
• Hearing protection when grinding.

Lockout/tagout (LOTO): Before adjusting or clearing a jammed wire feeder or any powered equipment, follow your facility's LOTO procedure—de-energize, lock, tag, and verify zero energy.

Angle grinder safety: Inspect the disc for cracks before mounting. Never remove the guard. Keep bystanders out of the plane of the disc.

Hot metal: Freshly tacked steel holds heat. Use a slag hammer with a chip screen, and stamp or chalk-mark hot pieces.

Housekeeping: Keep the strongback area clear of tripping hazards. Secure cables and hoses so they do not cross walkways.

Inspection and Sign-Off

When your assembly is complete, request an instructor inspection before moving the frame. The instructor will:

• Measure web height, flange width, and overall frame length against the drawing.
• Check squareness by measuring diagonals.
• Examine all tack welds visually.
• Sign the frame inspection report—your permanent record for the course portfolio and your NCCER credential documentation.