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When two column loads are unequal, which of the possible footing can be provided?
Strap footing
Raft footing
Trapezoidal combined footing
Mat footing
Trapezoidal combined footing
A trapezoidal combined footing is used when two column loads are unequal and there is a restriction on the property line or space available, ensuring the center of gravity of the footing area coincides with the resultant of the two column loads. This uniform pressure distribution is essential to prevent differential settlement in foundations supporting columns with asymmetric loading.
A trapezoidal combined footing is used when two column loads are unequal and there is a restriction on the property line or space available, ensuring the center of gravity of the footing area coincides with the resultant of the two column loads. This uniform pressure distribution is essential to prevent differential settlement in foundations supporting columns with asymmetric loading.
q=AP1тАЛ+P2тАЛтАЛ тАФ Uniform soil bearing pressure required for stability
x╦Й=P1тАЛ+P2тАЛP2тАЛ├ЧLтАЛ тАФ Location of the resultant load from the larger column
The principle relies on aligning the centroid of the footing area with the resultant line of action of the loads P1тАЛ and P2тАЛ. By varying the widths of the footing at the ends corresponding to the larger and smaller loads, the area is distributed such that the soil pressure remains uniform, calculated as q=AP1тАЛ+P2тАЛтАЛ, which avoids tilting or excessive bending moments in the footing.
Used specifically when column loads are unequal and space is limited.
Ensures uniform soil pressure distribution to prevent unequal settlement.
The shape is determined by the ratio of the column loads and the spacing between them.
Prevents structural damage due to tilting moments.
Prevents eccentric loading on the foundation soil.
Economical when column spacing is small.
Allows construction near property boundaries.
Complex geometry complicates shuttering and reinforcement.
Requires precise calculation of the center of gravity.
Multi-story building foundations with interior and exterior column load differences.
Foundation systems where column spacing is constrained by site boundaries.
Rectangular footings are preferred when loads are equal; trapezoidal are strictly for unequal loads.
Strap footings are used to connect an exterior column to an interior column when property line constraints exist, not necessarily due to unequal loads alone.
C is correct тАФ A trapezoidal combined footing allows for the alignment of the footing centroid with the resultant of unequal column loads, ensuring uniform pressure distribution.
Always check for 'property line' constraints in questions; if mentioned alongside unequal loads, a Strap or Trapezoidal footing is almost certainly the required design.