Live Loads Explained for Structural DesignOct 01 2013 · 0 comments · NISHKIAN DEAN, Technical notes ·0
During the design process, it’s essential to consider the anticipated structural load of a project.
Loads are commonly understood as forces that cause stresses, deformations, or accelerations. These loads are applied to a structure or its components that cause stress or displacement.
There are many types of structural loads that you need to account for during the design process. And some – like live loads – present specific challenges that require a deeper understanding to conceptualize.
What exactly are live loads?
Live loads include any temporary or transient forces that act on a building or structural element. Typically, they include people, furniture, vehicles, and almost everything else that can be moved throughout a building.
Live loads can be prescribed to any structural element (floors, columns, beams, even roofs) and will ultimately be factored into a calculation of gravity loads, which we’ll explain below.
We measure uniform live loads as pounds per square foot (psf). The acceptable live load will vary considerably based on the occupancy and expected use of a structure or structural element. For example, the live load for a room in a single family residence will be significantly smaller than the live load for an area of equal size at a movie theatre or sporting event.
How live load codes work
To account for the inconsistencies among live loads, the American Society of Civil Engineers (ASCE), Standard 7 prescribes minimum live load requirements for buildings categorized by occupancy or use. In order to secure approval for construction, you must prove that you’ve accounted for these loads in your design and structural plans.
The ASCE’s prescribed minimum load requirements are actually based on the expected maximum load the building will see over its lifetime. As such, design load requirements will exceed the true live load that a structure will actually bear.
To understand this, consider a typical single family residence. A dining room is often lightly furnished, and the majority of the live load imposed upon the structural elements will be attributable to people. In order to reach the upper limit of 40psf live load that the ASCE prescribes, the dining room would have to hold 30 average-sized adult males at once. Assuming a more common family of 5 sitting down to eat dinner, the live load would fall somewhere between 5psf and 10psf, a mere fraction of the prescribed minimum. There are other factors of safety built into the loads and the capacity of structures that provide additional reserve in case loads are exceeded.
Live loads for occupancy types
Below, we’ve listed several prescribed live loads for different occupancy types as published in the ASCE 7-05. You’ll see their prescribed design load compared to the average load actually realized. The last column shows the average as a percentage of the minimum code:
|Occupancy||Prescribed Design Load (psf)||Average Survey Load (psf)||% of Prescribed Load Realized|
|Single family residence||40||6.0||15%|
How to design for live loads
While actual observed live loads are often lower than the prescribed minimum code, it may be beneficial to plan for a greater load. This allows the structure to accommodate unique conditions and/or provide a greater level of safety.
Sometimes the prescribed minimum load is not large enough to account for the live load concentrated in a specific area. Take, for example, the parking garage of a residential building. Though the residential structure is prescribed a 40psf minimum live load, the parking garage must be capable of supporting a cargo truck weighing 3 tons. In cases like this, building codes account for the concentrated load by prescribing a single load that those specific areas must be designed to resist.
As the designer, it’s always up to you to use your own judgment based on the facility. Be careful to consider the use of the structure and what its common occupancy will be throughout its lifetime. Is there reason to believe that the live load will spike? If so, will the prescribed minimum load cover the range of possibilities?
Other structural loads to consider
Live loads are just one of many loads to consider during the structural design process.
For instance, dead loads account for the non-dynamic forces that place continuous and permanent force on a structure. They consist of the building and all of its fixed components – both structural and non-structural. The total dead load plus the total live load make up the “gravity load” for any given structure. This “gravity load,” as the name implies, is the total load to a structure that acts in the direction of gravity.
Conversely, lateral loads can occur in all directions. However, they’re primarily measured by the horizontal forces induced upon a structure, including wind and seismic loads. Because they’re dynamic by nature, lateral loads technically qualify as live loads, but are accounted for separately in the structural design process.
If you’re interested in learning more about structural loads and what they mean for your design process, set up a meeting with one of our team members.