Jonathan Ochshorn's Structural Elements for Architects and Builders, Third Edition
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Chapter 1: Introduction to structural design

The study of structural behavior and structural design begins with the concept of load. We represent loads with arrows indicating direction and magnitude. The magnitude is expressed in pounds (lb), kips (1 kip = 1000 lb), or appropriate SI units of force; the direction is usually vertical (gravity) or horizontal (wind, earthquake), although wind loads on pitched roofs can be modeled as acting perpendicular to the roof surface (Figure 1.1).

direction of loads on structures
Figure 1.1: Direction of loads can be (a) vertical; (b) horizontal; or (c) inclined

Where loads are distributed over a surface, we say, for example, 100 pounds per square foot, or 100 psf. Where loads are distributed over a linear element, like a beam, we say, for example, 2 kips per linear foot, or 2 kips per foot, or 2 kips/ft (Figure 1.2). Where loads are concentrated at a point, such as the vertical load transferred to a column, we say, for example, 10 kips or 10 k.

load diagram with resultant
Figure 1.2: Distributed loads on a beam