Strap Footing Design

A strap footing, also known as a cantilever footing, is a type of combined footing used when an exterior column cannot be placed at the property line and its footing cannot extend beyond the property line. It consists of an exterior footing, an interior footing, and a connecting beam or "strap" that transfers the eccentric load from the exterior column to the interior column and helps to equalize the soil pressure under both footings.

Assumptions in Strap Footing Design

For the simplified design and analysis of strap footings, several key assumptions are typically made. These assumptions simplify the complex soil-structure interaction into a more manageable structural problem. Understanding these assumptions is crucial for proper design.

• Strap Rigidity: It is generally assumed that the connecting strap beam is perfectly rigid. This assumption implies that the strap does not deform significantly under load, and it effectively transfers moments and forces between the footings.
• No Direct Soil Pressure on Strap: The strap beam itself is assumed to be above the ground or its contact with the soil is considered negligible for load transfer. Therefore, the strap is not subjected to any direct upward soil pressure along its length. Soil pressure is considered only under the footings.
• Uniform Soil Pressure: A fundamental assumption is that the soil pressure acting on the underside of each individual footing (both the exterior and interior footings) is uniform. This simplifies the calculation of bending moments and shear forces within the footings.
• Centrally Loaded Interior Footing: The interior column footing is assumed to be subjected to a concentric load from the column. This means the column's axial load acts directly through the centroid of the interior footing, leading to uniform pressure distribution under it.
• Exterior Footing Reaction: The reaction from the soil beneath the exterior footing is assumed to pass through the centroid of the exterior footing. This assumption helps in determining the necessary dimensions and eccentricity of the exterior footing to achieve uniform soil pressure.

Analyzing Strap Footing Design Assumptions

Let's evaluate each given option based on the common assumptions made for strap footing design:

• The strap is perfectly rigid: This is a standard assumption in strap footing design. Assuming perfect rigidity simplifies the analysis by ensuring that the strap effectively distributes the load and moments without significant deformation.
• The strap is not subjected to any direct soil pressure: This is also a common assumption. It is presumed that the strap beam itself does not directly bear on the soil or that any such contact is negligible. The soil pressure is considered to act primarily on the footings.
• The soil pressure varies linearly: This assumption is NOT made for the design of a strap footing. In fact, for simplified analysis, it is assumed that the soil pressure under each individual footing is uniform, not linearly varying. The goal of a well-designed strap footing is often to achieve uniform soil pressure distribution under both footings to avoid differential settlement and excessive bending moments.
• The interior footing is centrally loaded: This is another key assumption. The load from the interior column is typically assumed to act through the centroid of the interior footing, simplifying the analysis of soil pressure under it.

Understanding Soil Pressure Distribution

In strap footing design, the primary objective related to soil pressure is to achieve a uniform pressure distribution under the individual footings. This means that the total load and moment from the eccentric exterior column are balanced by the reaction from the interior column and the geometry of the footings and strap, resulting in a constant pressure value ($\sigma_u$) under each footing. The assumption of uniform soil pressure simplifies the calculation of the dimensions of the footings and the forces in the strap beam.

Therefore, the statement that "The soil pressure varies linearly" is contrary to the fundamental assumption of uniform soil pressure used in the design of strap footings.