You're on the right track in terms of identifying the challenges and potential solutions. Let's break down the situation and explore some options.

Understanding the Problem

You want to use Rigid Metal Conduit (RMC) as a post on a helical pier. The biggest hurdle is finding a direct connection method. Standard helical pier caps are designed for wood posts, square steel tubing, or engineered brackets. RMC presents a unique geometry and purpose (electrical) that isn't directly addressed with readily available caps.

Why a Direct Helical Pier Cap for RMC is Unlikely to Exist (and why your approaches are good)



RMC's Primary Function

RMC is primarily for electrical conduit, not structural support. While it's strong, its diameter and intended use don't align with the structural applications for helical piers.

Liability Concerns

Manufacturers are hesitant to create components that might lead to misuse. If an RMC-based post failed, the helical pier cap manufacturer could face liability, even if the RMC itself was the weak link.

Lack of Demand

The demand for an RMC-specific helical pier cap is likely very low. The combination of helical piers (usually for more substantial structures) and RMC posts (which are generally used for lighter applications) isn't a common design requirement.

Your Proposed Solutions (and Refining Them)

Both of your proposals are reasonable and practical:1.

Bolting a Floor Flange to a Flat Helical Pier Cap



Pros

Relatively straightforward, allows for adjustability (within the bolt hole clearance), and can be disassembled if needed.

Cons

Drilling accurately can be tedious, especially if you need a precise bolt pattern.

Refinements



Drilling Jig

Create a simple jig (a piece of metal with precisely drilled holes) to guide your drill bit when making the holes in the pier cap. This will significantly improve accuracy and speed.

Grade 8 Bolts

Use high-strength Grade 8 bolts, washers, and lock washers to ensure a secure and durable connection.

Thick Washer/Spreader Plate

Consider using a thick washer or a small spreader plate between the bolt head and the floor flange. This will distribute the load more evenly and prevent the bolt head from pulling through the flange.

Consider Alignment

Before drilling, ensure the floor flange is perfectly centered on the pier cap. A slight misalignment can cause problems later.2.

Welding the Floor Flange to the Cap



Pros

Potentially stronger connection, eliminates the need for drilling.

Cons

Requires welding skills (or hiring a welder), the connection is permanent, and you'll need to ensure the weld is strong enough to handle the load. Welding can also affect the galvanization or coating of the pier cap and flange, potentially leading to rust.

Refinements



Professional Welder

If you're not an experienced welder, hire a certified welder. A bad weld is a weak point.

Proper Welding Technique

Use a welding technique appropriate for the metals being joined (likely steel to steel). Ensure full penetration and good fusion.

Rust Prevention

After welding, clean the weld area thoroughly and apply a rust-preventative coating (cold galvanizing spray) to protect the exposed metal.

Alternative Considerations (Worth Exploring)



RMC to Square Tube Adapter

Could you find an adapter that securely connects the RMC to a square steel tube? Square tubes are easier to mate with existing helical pier caps. You might need to have something custom fabricated. This allows you to use a standard square-tube helical pier cap.

Direct Burial Rated PVC Conduit

If the RMC is only being used for electrical purposes, can you substitute it with direct burial rated PVC.

Engineer Consultation

Crucially, consult with a qualified structural engineer. They can: Verify that RMC is suitable for the intended structural load. RMC may be susceptible to buckling if the load is high. Specify the appropriate size and type of helical pier for your soil conditions and structural requirements. Design the connection between the RMC and the pier cap to ensure it's structurally sound.

Important Considerations



Local Building Codes

Always comply with local building codes and regulations. Consult with your local building department to ensure your design meets their requirements.

Soil Conditions

The load-bearing capacity of the helical pier depends heavily on the soil conditions. Conduct a soil test to determine the appropriate pier size and depth.

Load Calculations

Carefully calculate the loads the post will be subjected to (vertical, lateral, wind, etc.). The engineer can help with this.

In Summary

While a direct RMC helical pier cap is unlikely to exist, your proposed solutions of bolting or welding a floor flange are viable. However,

engineering consultation is vital to ensure structural integrity, code compliance, and safety. Prioritize using a qualified structural engineer for design and approval. Flag for review