Chapter 5 - L: Live Loads
© 2007, T. Bartlett Quimby
Last Revised: 09/11/2008
Live loads, L, are a result of the occupancy of a structure. In other words, it varies with how the building is to be used. For example, a storage room is much more likely to larger loads than is a residential bedroom. Bleachers at a stadium are likely to see larger loads than what is seen on a pitched building roof.
Live loads include any type of occupancy related loading and ASCE 7-05 Chapter 4 specifies the magnitude of the loads to be considered for a wide variety of cases.
The specified live loads are generally expressed either as uniformly distributed area loads or point loads applied over small areas.
The uniformly distributed loads are applied to portions of the structure that is likely to see a fairly uniform distribution of items over large areas (areas the size of a single room or larger). They are indefinite and are designed to encompass all probable occupancy loading scenarios. ASCE 7-05 Table 4-1 lists the specified uniformly distributed area loads.
Certain occupancies, such as office space, have the potential for a larger concentrated load (such as a large copy machine) being located in a space. This space may also be designed for uniformly distributed loads, but it is not probable that both the uniformly distributed load and the large concentrated load will occupy the space at the same time. Consequently the space must be designed to accommodate, separately, the uniformly distributed load and the point load, with the point load being moved around the space so as to cause maximum effect on the supporting elements. ASCE 7-05 Table 4-1 lists the specified concentrated loads that are associated with with occupancies where the concentrated loads are likely to occur.
Other occupancies or structural elements are to be subjected to point loads of various types. Such loads include the loads to be applied to handrails, guardrails, fixed ladders and other such items.
While ASCE 7-05 is pretty comprehensive in specifying common live loads, there is always the possibility that some load has not be covered by the specification. ASCE 7-05 section 4.5 allows the designer the option to determine live loads that are not specifically addressed using a method that is acceptable to the local building enforcement authority. This leaves lots of room for personal interpretation by both the designer and the code authority. When such conditions arise, some rational method that can be justified to other design professionals should be used to determine the live load.
It is the nature of live load that it can be everywhere present or it can be distributed over only a portion of the structure. As it is the requirement that the structure be able to support any possible loading condition, care must be taken to ensure that all critical load distributions are considered. The use of influence lines is extremely useful in determining where and how live loads are to be arranged so as to cause maximum effect throughout the structure.
Many live loads are dynamic in nature. Some are sufficiently dynamic that they result in impact on the structure. Impact results in very short duration impulse loads. For example, using statics we can easily determine the forces in a elevator cable or hydraulic system, however when the elevator starts or stops suddenly, there is a temporary inertia based spike in the loads. The ASCE 7-05 approach to this is to apply a general percentage increase of the loading depending on the significance of the device and its occupancy as well as the dynamic nature of the device.
The probability of simultaneous full live load being applied to structural elements that support large area, in many occupancies is very low. ASCE 7-05 recognizes this and allows for a reduction in design live load that is based on the size of area supported. A distinction is made between normal floor surfaces and roof surfaces since the nature of the live loads for floors and roofs are different.
Finally, ASCE 7-05 addresses crane loadings. This is a specialty case that includes capacity of the crane, concentrated point loads where the wheels are in contact with crane rails, and vertical, lateral and longitudinal impacts.