Column Base Plate Welding: Fillet vs CJP, Anchor Rods, and Field Techniques

Column base plate connections transfer the column load into the concrete foundation. The welding requirements vary dramatically based on what the connection needs to do. A gravity-only interior column might need simple fillet welds. A moment frame column in a seismic zone needs CJP groove welds with full penetration and notch-tough filler metals. Knowing which detail applies and executing it correctly in field conditions is where the skill comes in.

Base plate welding is frequently done in the field, which means overhead and vertical welding in congested conditions, limited access for inspection, and coordination with concrete and anchor rod placement that may not be perfect.

Types of Column Base Plate Connections

Gravity-Only (Compression)

The simplest case. The column carries only compressive (downward) load. The base plate sits on grout on the concrete pedestal.

For compression-only connections, the weld must transfer the compression load from the column into the base plate. Since the column bears on the plate, the weld primarily needs to keep the column centered on the plate and resist any incidental moment. Fillet welds around the column profile are sufficient.

Moment-Resisting (Seismic)

When the column is part of a moment frame or must resist uplift:

AISC 341 requires CJP groove welds at the column-to-base plate interface for columns in Special Moment Frames. These welds are demand-critical and require notch-tough filler metals, the same as moment connection welds at beam-to-column connections.

Shear-Only (Braced Frame)

Columns at the base of braced frames may need to transfer significant horizontal (shear) forces:

• Fillet welds sized for the shear demand
• Shear lugs (steel bars welded to the bottom of the base plate that bear against concrete in a keyway)
• Heavy anchor rods designed for combined shear and tension

Fillet Weld Details

Standard Column-to-Base Plate Fillet

For W-shape columns, the fillet weld runs around the column profile where it contacts the base plate:

• Flanges: Fillet weld along each flange edge (4 welds on a W-shape)
• Web: Fillet weld along both sides of the web where it meets the plate
• Clips/returns: The fillet should wrap around the flange tips for 2 times the weld size (minimum) at each return

Fillet Weld Sizing

The engineer specifies the fillet size based on the connection demand. AWS D1.1 Table 5.7 sets minimum fillet sizes based on the thicker material:

Base plates are typically 3/4 inch to 3 inches thick, so the minimum fillet is 5/16 inch. Design requirements often exceed this minimum.

Fillet Weld Technique on Base Plates

Shop welding (plate welded to column before erection):

• The assembly can be positioned for flat or horizontal welding
• Multi-pass fillets are standard for larger sizes
• Access to all sides is straightforward

Field welding (column erected, then welded to plate):

• Most welds are vertical or overhead depending on how the base plate is oriented
• Access is often limited by anchor rods, leveling nuts, and adjacent foundation elements
• SMAW is common in field because of portability and wind tolerance

CJP Groove Weld Details

When CJP Is Required

CJP groove welds at column bases are required when:

• The column is in a Special or Intermediate Moment Frame (AISC 341)
• The connection must resist significant tension (uplift from wind or seismic)
• The engineer’s design specifies CJP for the load condition
• Seismic forces create combined compression and moment that exceed fillet weld capacity

Joint Design for CJP at Base Plate

The joint is typically a bevel groove with the bevel on the column flange (or web). The base plate serves as the backing.

Column flange to base plate CJP:

• Bevel the column flange end (typically 45-degree bevel)
• Land (root face) of 0 to 1/8 inch
• Root opening per the WPS (typically 1/4 inch with backing, less without)
• The base plate surface acts as the backing for the root pass

Access challenges:

• The inside of the column flanges is difficult to reach, especially on deep sections
• Weld access holes in the column web (at the base plate level) may be needed to allow torch access to the web-to-plate CJP weld
• Anchor rods adjacent to the column obstruct access

CJP Welding Procedure

1. Fit the column to the base plate with the specified root opening
2. Preheat the base plate and column flange to the required temperature (base plates are thick, requiring preheat in most cases)
3. Root pass: Achieve full penetration to the base plate surface. The base plate acts as backing. Verify fusion to the plate
4. Fill passes: Build up the groove with controlled interpass temperature
5. Cap pass: Full width, proper profile, no undercut
6. Inspect: VT on 100%, UT on demand-critical welds

Thick Base Plate Preheat

Base plates 1-1/2 inches and thicker require significant preheat (225-300F for Group I steels). The thermal mass of a thick base plate on a concrete pedestal makes maintaining preheat challenging because the concrete acts as a heat sink.

Practical approach:

• Use a rosebud torch to preheat the base plate and column flange
• Verify temperature with a contact pyrometer 3 inches from the joint
• Use insulating blankets on the base plate to retain heat during welding
• Maintain preheat throughout the welding operation (don’t let it cool between passes)

Anchor Rod Considerations

Hole Sizing and Placement

Anchor rod holes in the base plate are oversized to allow for placement tolerances:

These oversized holes accommodate the tolerance in anchor rod placement (which is set in concrete before the steel arrives). Washers or plate washers are used between the nut and the base plate to cover the oversized hole.

Do NOT Weld to Anchor Rods

Standard anchor rod materials (ASTM F1554) are not designed for welding:

• F1554 Grade 36: Technically weldable (low-carbon steel), but welding is not standard practice
• F1554 Grade 55: May or may not be weldable depending on chemistry
• F1554 Grade 105: Not weldable (high-strength, quenched-and-tempered). Welding destroys the heat treatment

If the design requires a welded connection to anchor rods (rare), the engineer must specify weldable rods and provide a welding procedure.

Leveling Nut System

Most modern base plates use leveling nuts:

1. Anchor rods are set in concrete with threads projecting above the foundation
2. Heavy hex nuts (leveling nuts) are threaded onto the rods at the correct elevation
3. The base plate is lowered onto the leveling nuts
4. The column is erected on the base plate
5. Top nuts are tightened against the plate
6. Non-shrink grout is pumped under the base plate to fill the gap

The grout fills the space between the concrete and the base plate, providing full bearing support. Welding isn’t involved in this leveling process, but the welder needs to understand the sequence because the base plate welds happen after the column is set and the anchor nuts are tensioned.

Shear Lug Welding

A shear lug is a short piece of steel plate or bar welded to the bottom of the base plate. It projects into a keyway in the concrete foundation to resist horizontal shear forces.

Typical Shear Lug Details

• Material: A36 or A572 Grade 50 steel plate
• Dimensions: 1 to 2 inches thick, 4 to 8 inches deep, length varies
• Weld: Fillet welds on both sides, sized per the design shear load
• Position: Centered under the base plate, aligned with the shear force direction

Welding Shear Lugs

Shear lugs are shop-welded to the base plate before erection:

• Position the lug perpendicular to the base plate bottom surface
• Run fillet welds on both sides of the lug for the specified length
• Weld size per design (typically 3/8 to 5/8 inch fillets)
• Inspect for full fusion and proper size

The lug must be positioned accurately because the keyway in the concrete is sized for it. Misplacement means the lug won’t engage the concrete keyway properly.

Field Welding Challenges

Overhead Access

When the base plate is already set on the leveling nuts and the column is erected, welding the column-to-plate connection often involves overhead work. The welder is reaching under and around the base plate, working between anchor rods, with limited space.

Tips for overhead base plate welding:

• Use short electrodes (E7018 in 12-inch lengths) for better access in tight spaces
• Smaller diameter electrodes (3/32 inch) give better control in overhead position than 1/8 inch
• Clean aggressively between passes. Overhead slag doesn’t fall off; it sticks
• Use a mirror to inspect areas you can’t directly see

Anchor Rod Obstruction

Anchor rods projecting through the base plate limit torch access to the column-to-plate joint. On connections with 4 or more anchor rods close to the column, the welder must plan the pass sequence around the rods.

Approach: Start welding in the areas with the best access. Fill the tight spots between anchor rods last, when the accessible areas are already partially welded and providing some restraint.

Inspection Access

After welding, the inspector needs to see and measure the welds. On base plates where the welds are between the column flanges and behind anchor rods, visual inspection is difficult.

• Use mirrors and flashlights for inaccessible areas
• Fillet gauges on short handles reach into tight spots
• UT inspection on CJP welds may require special transducer positions

The inspector should plan their inspection access before the grout is placed. Once grout fills the space under the base plate, many weld areas become inaccessible.

Common Base Plate Welding Mistakes

Not preheating thick base plates. A 2-inch base plate sitting on concrete is a massive heat sink. Without preheat, the root pass cools too fast and hydrogen cracking risk increases significantly.

Welding to the wrong side of the column. On base plates with non-symmetrical anchor rod patterns, the column has a specific orientation. Welding before verifying orientation means the anchor rods don’t align correctly.

Inadequate root fusion on CJP welds. The base plate backing absorbs heat from the root pass. Increase amperage slightly compared to a standard groove weld to compensate for the heat sink effect of the thick plate.

Ignoring weld sequence. Welding all four sides of a column continuously causes shrinkage that can lift or tilt the base plate. Alternate sides (weld one flange, then the opposite flange) to balance shrinkage forces.

Column base plate welding connects the steel superstructure to the concrete foundation. Every building load passes through these connections. Getting them right, with proper weld type for the design, correct preheat on thick plates, and sound execution in challenging field conditions, ensures the structure performs as the engineer intended.