Structural load
Structural load refers to the forces and stresses applied to a building or other structure, which can include its own weight, the weight of occupants or contents, and environmental influences. There are two main types of structural loads: dead loads and live loads. Dead loads encompass the weight of the structure itself—such as beams, walls, and installed systems—while live loads involve variable forces, such as people, furniture, and vehicles, which can change over time. Engineers must accurately calculate both types of loads to ensure a structure can safely support them.
In addition to these loads, structures also face environmental loads from natural forces like wind, snow, rain, and earthquakes, which can impose significant stress, sometimes unexpectedly. For example, engineers must consider how snow accumulation or wind pressure interacts with the structure. Building designs must balance strength and flexibility to withstand these forces while ensuring safety for those inside. Structures like bridges face unique challenges as they must support prescribed design loads, which account for the heaviest anticipated traffic, while also considering their own construction weight. Understanding structural load is essential for creating resilient buildings that can endure various stresses throughout their lifespan.
Subject Terms
Structural load
A structure is subject to many forces, including its own weight, its location, and its environmental conditions. Structural load refers to the forces applied to a structure or its elements. These loads apply stress to the structure and may cause shifting.
Engineers must calculate the strength and weight of the materials used in construction, the location, and many other factors in designing buildings, bridges, and other structures to ensure they can support both dead and live loads. The combined dead and live loads make up the structure's gravity load. Gravity, a vertical load, is the one force always affecting structural load.
Dead and Live Loads
Dead load is the weight of the structure itself. If an item may be moved without cutting it away from the structure—furniture, for example—it is not counted as dead load. Dead load includes the beams, columns, floors and ceilings, carpets, walls, installed cabinets, plumbing, heating and cooling systems, electrical components, elevators, windows, and doors. Materials must be chosen based on many factors, including use and weight. For example, different kinds of wood—Douglas fir, Southern pine, and so on—will bear different loads. The weight of materials such as shingles or tile roofing must be supported by the structure.
Live loads are changing forces that affect a structure. Live load often includes furniture, people, and vehicles and is measured in pounds per square foot (psf). Design live load must exceed true live load, because engineers must consider the maximum load over the lifetime of the structure.
The intended use of the structure must be known so it is designed to support the live load. For example, designers must consider the number of people who will typically use a building. The weight of the people using the dining room of a single-family home will be much different than the live load a similarly sized room in a school building must support. The American Society of Civil Engineers (ASCE) recommends an upper limit of 40psf live load for a dining room in a single-family residence. The upper limit is known as the prescribed design load. In this example, 40psf equals about thirty average adult males, far more people than will use a typical home's dining room at one time. The usual live load of a family dining room is 6psf, or the average weight of five people. This is 15 percent of the prescribed design load. The prescribed load of an office building is 50psf, while the average actual load of such structures is about 10.9psf, or 22 percent of the prescribed load.
Some structures must be designed to hold loads that can vary greatly. For example, a parking garage must support the weight of the passenger vehicles that could fill it, as well as the weight of fully loaded delivery trucks that may occasionally use the garage. Impact is the dynamic effect of a load, or the factors that affect a structure quickly, such as traffic moving onto and over a bridge. These may occur suddenly and without warning.
Environmental Loads
In addition to such live loads as traffic, a structure may simultaneously experience environmental loads such as changing weather conditions. Forces of nature may be horizontal or vertical.
Environmental loads include earthquakes, ice, rain, snow, and wind, as well as the force of water ponding on a roof. Ponding occurs when a puddle forms on a so-called flat roof; if rainwater does not drain quickly, its weight can add much stress to a roof. Wind flowing around a structure produces wind loads, which are affected by the surroundings, the slope of the roof, and other factors. Snow load design depends upon the geographic location of a structure as well as wind exposure and other factors. Though snow, ice, and rain are not always present, these loads must be calculated as if they are always going to impact the structure in a region where such conditions are expected.
Other forces, such as earthquakes, provide additional challenges for engineers. The motion of an earthquake is both horizontal and vertical. Vertical motion stresses a structure, but horizontal motion seems to cause the most structural damage. Earthquakes are so forceful that it's not possible to design buildings to survive them in all cases. Instead, engineers focus on creating structures that are flexible enough to remain standing long enough to allow people inside to escape.
Engineers must design structures to be strong enough to withstand multiple forces at once. Although engineers must calculate the load of a variety of forces, they do not build structures to take the maximum impact of all forces simultaneously. Instead, they use tables to help them determine reasonable multiple force loads.
Bridges
Short bridges are likely to have to support the prescribed design load: for example, four large, heavily loaded trucks may cross a small bridge at the same time. Long bridges are highly unlikely to bear a maximum load of only the heaviest vehicles, and it would be very expensive to build to those standards. Instead, engineers design these spans to support probable loads. Short bridges are likely to have lighter dead load weight. Longer spans must be lighter, so engineers often use beam, arch, truss, and suspension designs.
Like other structures, bridges experience vertical and horizontal stresses. They are designed to support these loads. For example, many bridges are designed to allow air to flow over them to reduce the wind load.
Bibliography
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Quimby, T. Bartlett. "Basic Design Concepts: Loads and Their Combinations." A Beginner's Guide to the Structural Engineering. T. Bartlett Quimby. 4 Nov. 2014. Web. 22 Dec. 2014. http://www.bgstructuralengineering.com/BGDesign/BGDesign06.htm
Quimby, T. Bartlett. "Chapter 3—D: Dead Loads." A Beginner's Guide to ASCE 7-05. T. Bartlett Quimby. 4 Nov. 2014. Web. 22 Dec. 2014. http://www.bgstructuralengineering.com/BGASCE7/BGASCE7003/