Department of Civil Engineering, N-W.F.P UET, Peshawar
Concepts of Capacity & Demand
Demand: It is an action on the structure which can cause internal disturbance(s) in the structure in the form of stresses. A demand on the structure may be in the form of Gravity, Wind, Earthquake, Snow or other loads. Capacity: The overall ability of a structure to carry an imposed demand. Internal stresses due to an imposed demand (Load effects): Compression, Tension, Bending, Shear, and Torsion. Failure: It occurs when capacity is less than demand. To avoid failure, capacity to demand ratio should be greater than one, or at least equal to one. It is, however, intuitive to have some margin of safety i.e., to have capacity to demand ratio more than one. How much? It depends. Example 1.1: Calculate demand in the form of stresses or load effects on the given concrete pad of size 12″ × 12″. Solution: Demand = 50 tons Load (50 tons) Concrete pad
12
"
12"
Figure 1.1: Concrete pad. Based on convenience either the loads or the load effects as demand are compared to the load carrying capacity of the structure in the relevant units. In the given example, load of 50 tons is demand. Its effect on the pad will be a compressive stress equal to load divided by the area of the pad. Or: (50 × 2204)/ (12 × 12) = 765.27 psi Therefore compressive stress of 765.27 psi is demand in the form of load effect on the pad. Capacity of the pad in the form of resistance should be able to carry a stress of 765.27 psi. In other words, the compressive strength of concrete pad (capacity) should be more than 765.27 psi (demand). Prof Dr. Qaisar Ali (http://www.eec.edu.pk)
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