The following structural engineering terms and figures are described in order to give the average person more of an understanding of the mathematical engineering jargon used by Vitruvius engineers. After reading this, you will be able to keep up in an engineering meeting or even out do your friends at a Friday night gathering. However, if you are friends with other engineers we refer you to professional engineering journals and resources to learn more about each term.
Loading of a member describes what types of forces the member will be subjected to during construction and occupation of the structure. There are many different types of loading conditions and most do not occur alone. This section describes the different types of loading conditions as well as a brief explanation of building code prescribed load combinations.
A force applied to any member of the structure or the structure itself. For structural engineering purposes a load can be categorized in a few different ways, each of these categories is listed below and used in Load Combinations.
This is the how the loads are carried through the structure, providing a continuous load path to the ground is the structural engineer’s most important job.
A load type that does not change over time, which includes loads like Dead, Live, Roof Live, Rain, Snow, as well as Equivalent Wind and Seismic
A load type that changes over time. Dynamic loads are used during a dynamic analysis of a structure. Wind and Seismic loads are dynamic but are used in both static and dynamic analysis.
A dead load is a static load applied by the permanent weight of the structure and its components.
A live load is described as any load that is not permanent to the structure. This means it could be occupancy of specific areas or specific equipment used for the building occupants.
Roof Live Load
A live load that occurs only at the roof level, typically this is around 20 psf.
A load that occurs during roof flooding.
A load that occurs due snow accumulation and drifts on the roof.
A load applied to the structure due to wind pressures. While the wind pressure itself is not a static load, we treat it as one for simplicity of analysis.
A load that is applied to the structure due to the ground movements during an earthquake. Typically, these loads are applied in the horizontal direction, as indicated in the figure.
A load type that includes the additional dead loads from the other engineering services; for example, mechanical, electrical, and plumbing. Typically, this load is about 5 psf.
A load applied perpendicular to the member which causes a twisting motion in the member.
Load combinations are used to create loading conditions with multiple loads acting on the structure or one of its’ components, but factored to account for the statistical likelihood of them happening together.
Figure 1: Free body diagram of various loading and support conditions.
The figure shows a simplified version of a structural member, which is known as a free body diagram (FBD), each of the labeled components corresponds to the terminology listed below.
The end of a member which is not supported, think swimming pool diving board. The most left span, Span 1, of Figure 1 is a cantilevered end of a beam.
A member at which one end is a pin connection and the other a roller connection. This is the most common member type. Span 2, the middle span of Figure 1, if by itself would be a simply supported beam.
This describes the amount of displacement the member undergoes during loading. Building codes allow for different amounts of deflection for different building systems.
Related to deflection a Camber is designing a member with opposing deflection so that when loaded the member has smaller or no deflection than without the camber.
Modulus of Elasticity (E)
Named Young’s Modulus, the Modulus of Elasticity is the slope of the straight line in the elastic region of the stress-strain graph shown. For structural steel used in most building applications the Modulus of Elasticity is equal to 29,000 ksi (kips per square inch).
Moment of Inertia (I)
The moment of inertia is a mathematical term defined by the member’s cross-sectional geometry, which provides the resistance to rotation during loading. Examples of how this works structural are provided here. Link to bending members article.
For the purposes of building construction structural engineering a moment is simply a force that causing bending in a member or rotation at a fixed support reaction. In general engineering moment has a little more to its definition, please see other resources for this information.
A point load is expressed in pounds (lbs) or kips (k) and is described as a single amount of force applied at a single point. Area Loads or Line Loads can be converted to a single point load by multiplying by the area or distance the load acts on. Span 3 in Figure 1 above is loading with a single point load.
A load that acts along a single distance or span of a member, typically expressed in plf (lbs/ft). A distributed load can be evenly distributed along the member or vary along it’s length. Span 1 in Fig. 1 above is loaded with a uniformly varying Distributed load, where Span 2 is loaded with a uniform Distributed load.
An area load, typically expressed in psf (lbs/ft^2), is a load that is spread over an entire region. Most of the time these loads are used for floor systems and different areas of the building.
A reaction is forces and/or moment that occurs at a support. When transferring forces from member to member the reaction force is the force that is transferred. At each support in Figure 1 above, the reactions at each different support type are shown as pink arrows.
Shear forces can most easily be described as forces acting opposite and parallel to one another. These forces cause sliding between the two materials or members, which can act in the vertical (Pin Connection) and horizontal direction (Shear Wall).
The distance from one support to the other for a single member. See the dimension-like call outs below the member in Figure 1.
The slope of a force-displacement graph, the stiffness of a member describes its resistance to deformations.
A pin connection only carries shear forces to the reaction. Pin connections are the most common connection in repetitive framing. Support B in Fig. 1 above is a pin connection with two reactions, a vertical and horizontal force.
A moment connection carries shear and moment, it can also be called a fixed connection. This connection is used in most lateral force resisting system (LFRS) frames. Support C in Fig. 1 above is a moment connection with its’ three reactions, vertical horizontal, and moment.
Units and Unit Types
A kip is equal to 1000 pounds. It is used most frequently when talking about point loads, building weights, or base shear values. The point load applied to Span 3 of Figure 1 would be expressed in Kips or pounds.
Pounds per Linear Foot (plf)
A Distributed Load acting on a member in plf (lb/ft). The two line loads shown in Figure 1 would be expressed as this unit type.
Pounds per Square Foot (psf)
An Area Load acts on an area in psf (lb/sq ft)
Continuity is used to define the transfer of internal and external loads from member to member.
The term damping is used to describe the rate at which vibration intensities decrease. For more information on damping and other terms used it structural dynamics, see “this article”. Link to structural dynamics article.