--- title: "Factors for selection; strap and combined footings" subject: "Foundation Engineering" chapter: 1 key: "semester_5/foundation_engineering:factors-strap-combined" source: https://www.bctengineer.com/q/semester_5/foundation_engineering/factors-strap-combined site: BCT Engineering --- # Factors for selection; strap and combined footings Foundation Engineering · Chapter 1: Introduction - Asked in: 2079 Bhadra - Video: [1.7.3 Strap Footing | CE402 |](https://www.youtube.com/watch?v=9ynsrSrbBmw) — RGPV Official - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/factors-strap-combined - Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-1 ## Question What are the factors that influence the selection of a foundation? Under what circumstances are strap footings and combined footings adopted? ## Short answer Factors: soil bearing, load, settlement, groundwater, seismic, cost. Combined footings support two or more columns when they are close or an exterior column is at the property line. Strap footings join an exterior footing at the property line to an interior footing with a rigid beam, used when column spacing is large. ## Answer #### Factors influencing selection of foundation - Bearing capacity and compressibility of soil, and depth of the firm layer. - Magnitude and type of loads, and the allowable settlement of the structure. - Groundwater level, frost, scour and seasonal moisture change. - Seismic condition, adjacent structures and site restrictions. - Availability of materials, equipment and skilled labour, and total cost. #### Combined footing A single footing supporting two or more columns. It is used when: - Columns are so close that their separate footings would overlap or touch. - An exterior column is at the property line, so an isolated footing cannot be centred under it. - The soil is weak and a larger area is needed. - Differential settlement between neighbouring columns must be reduced. Rectangular combined footing is used when the loads are similar; trapezoidal when the loads differ much or the space is limited at one end. #### Strap (cantilever) footing Two isolated footings joined by a rigid beam (strap). It is used when: - The exterior column is on the property line and its footing cannot extend outside. - The distance between columns is large, so a combined footing would be uneconomical. - The strap takes the eccentric moment of the edge footing and transfers it to the inner footing, so both footings get uniform pressure. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A combined footing is adopted when A. columns are so close that separate footings would overlap B. columns are far apart with a large gap C. the soil is very strong and loads are light D. only a single column is present Answer: A. A single footing supporting two or more columns is used when isolated footings would touch or overlap. (Key point: Combined footing use) 2. A trapezoidal combined footing is used when A. the loads are equal B. the column loads differ much or space is limited at one end C. the soil is rock D. the columns are very far apart Answer: B. Trapezoidal shape lets the centroid of the base coincide with the resultant of unequal loads. (Key point: Rectangular vs trapezoidal) 3. A strap (cantilever) footing is preferred over a combined footing when A. columns are very close together B. only one column exists C. the exterior column is on the property line and the column spacing is large D. the soil is strong everywhere Answer: C. A combined footing over a large distance would be uneconomical; a strap beam is cheaper. (Key point: Strap footing use) 4. What does the strap beam do? A. It carries the column load directly to the soil B. It prevents frost action C. It increases the bearing capacity of soil D. It takes the eccentric moment of the edge footing and transfers it to the inner footing for uniform pressure Answer: D. The strap balances eccentricity so both footings get roughly uniform soil pressure. 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