Sloped or Trapezoidal Footings

Design and Execution Considerations for Sloped or Trapezoidal Footings

Introduction

Sloped or trapezoidal footings are often used in building construction to economize concrete volume. In these footings, the top surface is laid with a slope to match structural or architectural needs. However, concerns arise regarding the compaction and structural integrity of such footings, especially on small sites without adequate equipment.

Figure 1: Sloped Footing with top surface inclined

Alternative: Stepped Footings

Some engineers avoid sloped top surfaces due to vibration issues and prefer stepped footings, though it requires more shuttering.

Figure 2: Stepped Footing (requires more formwork)

Note: Stepped footings being cast in layers with horizontal construction joints can create planes of weakness.

Figure 3: Improper construction joint in stepped footing

Concrete Strength and Load Resolution

In sloped footings, compressive stress acts along the slope. It is resolved into components:

• Horizontal component → contributes to bending
• Vertical component → contributes to shear

Figure 4: Force resolution in sloped footing concrete

Execution Guidance

• Float tamping must be supervised where top shuttering is absent
• Consider top concrete to be M5 lower grade if doubtful
• Use surface vibrators or roller screeds in large-scale pours
• Design using conservative section ignoring side flare if unsure

Concerns in Sloped Footings

Engineers often raise concerns that due to absence of top shuttering and difficult access, vibration of concrete on sloped surfaces becomes inadequate. As a result, there is a chance that concrete at the top may not achieve full strength. To overcome this, some opt for:

• Flat Pad Footings – leading to more concrete consumption
• Stepped Footings – requiring additional formwork

Note: Stepped footings are often being cast in horizontal layers, introducing construction joints between steps. This is structurally undesirable unless proper joint treatment is done.

Concrete Quality on Sloped Surfaces

For small works, where needle vibrators may not be effective on slopes, proper tamping using wooden floats and rodding is recommended. Supervised manual compaction can ensure sufficient concrete quality. A conservative assumption may be made by considering the top region of the concrete to be one grade lower (e.g., M25 considered as M20 for top sloped surface).

IS Code Recommendations

• IS 456:2000 – Plain and Reinforced Concrete – Code of Practice:
  • Clause 13 – Detailing of Footings
  • Clause 5.3 – Concrete grades and strength assumptions
  • Clause 12.3 – Compaction and finishing of concrete
• IS 3370 (Part 2):2009 – For placing concrete on inclined planes (used by analogy for slope finishes)
• IS 383:2016 – Aggregate quality affecting workability and compaction

Structural Design Considerations

In flexural design of sloped footings, the compression face is inclined. Therefore, the compressive force can be resolved into horizontal (participating in bending) and vertical components (resisting shear). This is supported in RCC design literature.

Design Practice: Use the horizontal section for flexural design. The additional side concrete due to slope can be conservatively ignored to simplify calculations without underestimating strength.

Execution Issues with Stepped Footings

A common malpractice is laying stepped footings in horizontal layers with cold construction joints. As per IS 456:2000, Clause 13.4, footings should be cast monolithically to avoid weakness at joints. Poorly treated joints may lead to water ingress, corrosion, and poor load transfer.

Recommendations

1. For small sloped footings, allow with proper compaction by float/tamping, and reduce design strength for top layer by 5 MPa conservatively.
2. For large pours, adopt proper mix design (IS 10262:2019), ensure vibration using immersion and surface vibrators.
3. Do not prohibit sloped footings; instead, improve execution standards and supervision.
4. Do not allow stepped footings to be poured in stages with untreated joints.
5. Where workability is a concern, use plasticizers as per IS 9103:1999 to aid compaction without excess water.

Conclusion

Sloped or trapezoidal footings offer economic advantage in material usage. With proper care in execution, especially compaction and supervision, they can perform adequately. IS codes support their use, and banning them is not technically justified. Instead, good site practices, monitoring, and awareness must be promoted.

Field Tip: For top sloped surface finishing, use a wooden float with consistent tamping, and check by penetration rod or small core if feasible.

References

• IS 456:2000 – Plain and Reinforced Concrete – Code of Practice
• IS 383:2016 – Aggregates for Concrete
• IS 9103:1999 – Concrete Admixtures
• IS 10262:2019 – Concrete Mix Proportioning
• RCC Texts: S.N. Sinha, Pillai & Menon – for design of inclined surfaces