Tube Form Calculation Principles
Concrete tube forms, typically fiberboard tubes, create cylindrical piers that are ideal for foundations like deck footings or columns. The volume calculation is based on the formula for a **cylinder**.
Volume Formula
The calculation requires converting the diameter to a radius in feet, and then applying the cylinder volume formula, summed for all forms: **1. Radius (ft):** $$\text{Radius (ft)} = \frac{\text{Diameter (in)}}{24}$$ **2. Volume of One Form ($\mathbf{V_{single}}$):** $$\text{Volume (ft}^3) = \pi \times \text{Radius (ft)}^2 \times \text{Height (ft)}$$ **3. Total Volume ($\mathbf{V_{total}}$):** $$\text{Volume (yd}^3) = \frac{\text{V}_{single} \times \text{Number of Forms}}{27}$$ Final orders for concrete are always placed in cubic yards ($\text{yd}^3$), while pre-mixed bags are based on the volume in cubic feet ($\text{ft}^3$).
Pier Construction Details
Footing vs. Pier
- **Pier (Shaft):** The vertical part calculated above. Transmits load down.
- **Footing (Base):** A wider, often square or bell-shaped base poured at the bottom to distribute the load over a larger soil area. **This calculator does not include footing volume.**
- **Depth:** Piers must extend below the frost line in your region to prevent heave.
Pouring and Materials
- **Reinforcement:** Structural piers require steel reinforcement (rebar cages) to handle lateral forces and uplift.
- **Anchor:** A metal anchor bolt or post base connector must be set into the top of the wet concrete pier to attach the post or beam.
- **Vibration:** The concrete should be tamped or vibrated to ensure no air pockets (voids) are left, which could compromise the pier's strength.
Critical Note on Footings:
For most structural applications (decks, additions), a proper **footing** is required beneath the tube form pier. You must calculate the volume for the footing separately and add it to this total for the complete concrete order.