Bell-Bottom vs. Straight Pier Footings: When You Need a Wider Base
A straight-shaft pier works fine on soil that can bear the load at a standard diameter. Once the soil can’t, a bell-bottom footing spreads that load over more area without pouring a full-width pier the whole way up.
On this page13 sections
- Bell-Bottom vs. Straight at a Glance
- Why Belling the Base Works
- Bell-Bottom Terminology
- Estimating Bell Volume
- How to Determine If You Need a Bell
- Added Concrete Volume by Bell Size
- Alternative: Wider Uniform-Diameter Pier
- Common Mistakes
- Bell-Bottom vs. Straight vs. Footing Pad: Decision Table
- Other Factors to Weigh
- Deciding on a Bell-Bottom: Step by Step
- Frequently Asked Questions
- Related Calculators
Whichever shape you use, get your shaft volume from the concrete pier calculator, then add the bell’s extra volume separately if applicable. This guide covers when the added complexity of a bell-bottom actually pays off.
Bell-Bottom vs. Straight at a Glance
| Straight Pier | Bell-Bottom Pier | |
|---|---|---|
| Shape | Uniform diameter, top to bottom | Standard shaft diameter, flared wider at the base |
| Best soil fit | Firm soil with adequate bearing capacity at standard diameters | Marginal bearing soil needing more footprint at depth |
| Forming method | Sonotube or auger hole, no special technique | Hand-augered bell attachment, or hand-dug flare at hole bottom |
| Labor | Standard, straightforward digging | Additional step to flare the base, more labor-intensive |
| Concrete usage | Shaft volume only | Shaft volume plus the additional bell volume at the base |
Why Belling the Base Works
Bearing capacity is a function of how much load the soil under a footing has to support per unit of area — spread the same load over more area and the pressure on the soil drops. A bell-bottom achieves this by widening only the base, where the load actually transfers into the soil, rather than pouring the entire pier at a larger diameter from top to bottom. This keeps the shaft (and most of the concrete volume) at a standard, economical diameter while still meeting a wider bearing-area requirement where it actually matters.
Bell-Bottom Terminology
| Term | What It Means |
|---|---|
| Bell (or belled footing) | The widened, flared portion at the bottom of an otherwise standard-diameter pier shaft. |
| Bell diameter | The width of the flared base, distinct from the shaft diameter above it. |
| Bell height | The vertical distance over which the shaft transitions to the full bell diameter. |
| Bearing pressure | Load per unit of footing area transferred into the soil — the value a bell reduces by increasing area. |
Estimating Bell Volume
Bell Volume ≈ Frustum Volume = (π × Bell Height ÷ 3) × (Shaft Radius² + Shaft Radius × Bell Radius + Bell Radius²)
This treats the bell as a truncated cone (frustum) between the shaft diameter and the wider bell diameter — add this volume to your straight shaft volume from the concrete pier calculator for a full total.
How to Determine If You Need a Bell
- Get a soil bearing capacity assessment. A geotechnical report or local soil survey gives the number needed to check whether a standard pier diameter meets the load requirement.
- Compare required bearing area against your planned pier diameter. If the load divided by your soil’s bearing capacity exceeds the area a standard-diameter pier provides, you need either a larger diameter shaft or a bell-bottom.
- Check whether a hand or power auger with a belling attachment is available locally. Not every rental yard carries a belling bucket attachment — availability may influence whether hand-flaring or a wider straight pier is more practical.
- Ask an engineer for anything beyond a standard residential deck load. Bell sizing for anything beyond typical residential loads should be calculated, not estimated.
Example 1: Firm Soil, Standard Load
- Soil bearing capacity: Adequate for a 12 in. diameter pier at the design load
- Shape chosen: Straight, 12 in. diameter throughout
- Reasoning: No bearing-area shortfall to address
A standard straight pier is the simpler, lower-labor choice whenever soil comfortably supports the load at a normal diameter.
Example 2: Marginal Bearing Capacity
- Soil bearing capacity: Requires roughly double the bearing area of a 12 in. straight pier
- Shape chosen: 12 in. shaft with a 20 in. bell base
- Reasoning: Meets the bearing area requirement without pouring a 17 in. diameter pier the full depth
Belling only the base achieves the needed bearing area with meaningfully less added concrete than upsizing the entire shaft to match.
Example 3: Belling Attachment Unavailable
- Soil: Same marginal bearing capacity as Example 2
- Situation: No belling auger attachment available locally
- Shape chosen: Straight pier upsized to a wider uniform diameter instead
Where hand-flaring isn’t practical and a belling attachment isn’t accessible, a uniformly wider straight pier is a reasonable, if less concrete-efficient, alternative to meet the same bearing area requirement.
Calculate your shaft volume with the concrete pier calculator, then add your bell volume separately if your soil requires the wider base.
Added Concrete Volume by Bell Size
| Shaft Diameter | Bell Diameter | Bell Height | Approx. Added Volume |
|---|---|---|---|
| 10 in. | 16 in. | 12 in. | ~0.6 cu ft |
| 12 in. | 20 in. | 14 in. | ~1.0 cu ft |
| 14 in. | 24 in. | 16 in. | ~1.6 cu ft |
These are illustrative frustum-shape estimates — actual bell dimensions should be sized to your specific soil bearing capacity and load, not assumed from a general table.
Alternative: Wider Uniform-Diameter Pier
Rather than belling the base, the same bearing-area requirement can be met by simply increasing the entire pier’s diameter to match the bell’s width. This uses more total concrete than a belled design but avoids the extra labor step of hand-flaring the hole bottom or renting a specialized belling attachment. It’s a reasonable trade to make on a small job with just one or two footings needing the extra bearing area, where the added concrete cost is minor compared to the labor savings.
Common Mistakes
- Assuming a bell is needed without checking actual soil bearing capacity. Belling adds labor and complexity that firm soil may not require at all.
- Flaring the bell unevenly by hand. An inconsistent or off-center bell can leave part of the base under-supported despite the added concrete.
- Not accounting for bell volume separately when ordering concrete. The straight-shaft calculation alone underestimates total volume once a bell is added.
- Skipping an engineer’s input on anything beyond standard residential loads. Bell sizing for larger structures or unusual loads should be calculated precisely, not estimated from a general table.
- Confusing a bell-bottom with a separate footing pad. A bell is a continuous flare integrated into the pier shape; a footing pad is a distinct, usually flat element poured beneath a pier — the two solve similar problems differently.
Bell-Bottom vs. Straight vs. Footing Pad: Decision Table
| Situation | Recommended Approach |
|---|---|
| Firm soil, standard load | Straight pier at standard diameter |
| Marginal bearing capacity, belling equipment available | Bell-bottom footing |
| Marginal bearing capacity, no belling equipment | Wider uniform-diameter straight pier |
| Significant bearing-area shortfall, easier to form flat | Separate square or round footing pad beneath a standard pier |
Other Factors to Weigh
- Soil bearing capacity at your specific depth. This is the core number driving whether a bell is needed at all.
- Equipment availability. Belling attachments for power augers aren’t universally available for rent in every area.
- Number of footings needing a bell. The labor trade-off of hand-flaring versus a wider straight pier shifts with how many footings are affected.
- Local code acceptance of bell-bottom footings. Some jurisdictions have specific requirements or limitations on belled footing designs — confirm before planning around one.
- Depth at which the bell sits. A deeper bell can be harder to inspect visually before backfilling — some inspectors want to verify bell dimensions before the pour is covered.
Deciding on a Bell-Bottom: Step by Step
- Get your soil’s bearing capacity from a geotechnical report or local soil data.
- Calculate the bearing area needed for your load at that soil’s capacity.
- Compare that area against a standard straight pier’s footprint at your planned diameter.
- If a shortfall exists, check equipment availability for belling, or plan a wider straight pier instead.
- Size the bell (or wider straight pier) to close the bearing-area gap.
- Calculate shaft volume with the concrete pier calculator, then add bell volume separately.
- Schedule an inspection before backfilling if your jurisdiction requires visual bell verification.
Frequently Asked Questions
Do most residential decks need bell-bottom footings?
No — most standard residential decks on reasonably firm soil use straight-shaft piers without a bell. Bell-bottoms are a solution for marginal bearing capacity, not a default design choice.
Can I hand-dig a bell-bottom shape without special equipment?
Yes, on a small scale — hand-flaring the base of an auger hole with a shovel or spade is a traditional method, though it’s harder to achieve a consistent, even flare than with a belling auger attachment.
How much extra concrete does a bell-bottom footing need?
It depends on the bell’s size relative to the shaft, but typically an additional fraction of a cubic foot to a few cubic feet per footing — calculate the frustum volume for your specific bell dimensions rather than estimating.
Is a bell-bottom footing stronger than a wider straight pier?
For the same bearing area at the base, they perform comparably in terms of bearing capacity — the bell’s advantage is using less total concrete by keeping the shaft narrower above the flared base.
Do inspectors need to see the bell before backfilling?
Often yes — some jurisdictions want to visually verify the bell’s dimensions before the hole is covered, since it’s difficult to confirm after backfilling. Check with your local building department before scheduling your pour.
Can rebar be used with a bell-bottom footing?
Yes, and it’s common in belled footings carrying significant load — rebar placement typically follows the shaft through the transition into the bell, check your local code or an engineer’s spec for exact placement.
More deck pier & footing guides
Shaft dimensions set? Use the concrete pier calculator to get your base concrete volume before adding any bell allowance.