As a structural engineer, I am always dealing with loads and how a structure can resist those loads. A load is pretty much a force that a building or structure needs to be able to resist. Loads cause stresses and deformations to a structure and it is my job to make sure that a structure or part of the structure does not fail when these loads are applied. Loads can be applied vertically or laterally on a structure.
Types of Loads on Structures
Let’s start with vertical loads. Vertical loads, or gravity loads, are those forces that are applied perpendicular to the roof or floor system. These are separated into two categories: Dead Loads and Live Loads. Dead load consists of the weight of the structural members that make up the structure, in addition to all of the finishes that make the structure look nice and pretty! We call these dead loads because they never change. Live loads are loads imposed on a structure that are made up by the people who use the structure and what they decide to place in the structure (furniture/storage etc.).
As a structural engineer, I take into account all of the materials when figuring out the dead load of a structure. This could include the buildings insulation, drywall, wood studs, flooring, brick veneer etc. Individually these items may seem fairly light, but when their weights are all added together it can account for a significant amount of weight applied to the structure. These loads are in addition to the self weight of the structure which could include the weight of the floor/roof decking and joists, beams, bearing walls, columns, bracing etc. Dead loads are always present throughout the lifetime of a structure, compared to live loads which can come and go.
Live loads are harder to predict than dead loads, as it’s never possible to predict exactly how many people will be using a space at a given time or how they will layout furniture and store materials in a given space. When it comes to live loads, I use the National Building Code of Canada to figure out the magnitude of load I should be using based on the type of occupancy in the space being used. If you are not in Canada, a list of building codes used around the world can be found here https://www.fmglobal.com/~/media/Files/FMGlobal/Resilience%20Index/P15105.pdf
That will hopefully help you figure out what live load to use. The live loads used when designing a structure can vary between rooms in a building. For example, a mechanical room in the office building you may work at will have a higher live load than your office since there are often very heavy mechanical equipment in these rooms compared to a few people and some furniture found in your office.
Other vertical loads that are taken into account when I design a structure are the ones caused by the elements, snow and rain loads. The weight of snow and rain should not be ignored as it its weight after an extreme storm can often be heavier than the weight of the roof structure that supports it! A heavy snowstorm actually caused the roof of my parents garage to collapse a few years back. Take a look!
Poor engineering or poor workmanship? Who knows, but thankfully they had insurance and no one got hurt!
The lateral loads that are applied to structures include wind, seismic and earth loads. These loads act in the direction perpendicular to the buildings wall and roof systems. Lateral loads on a building are usually resisted by walls and bracing. When you see large steel X’s in the windows or exposed elsewhere in a building, this is often one of the elements used to resist lateral loads imposed on the structure.
Wind loads can be applied towards a surface of a building/structure but it can also be applied away from the building causing a suction force. These are called positive and negative pressures. Wind loads on a structure get greater the higher they are applied to a structure. On a high rise building, the wind pressures are significantly higher at the peak of the structure compared to at the ground level. If you have ever been outside during an intense windstorm you can understand how large these wind forces can be and why it is so important to design a structure to resist these loads.
Earthquakes are what cause seismic loading on a structure. Seismic loads used in designing structures vary depending on where the structure is relative to seismic zones and the potential for earthquakes. I live in Winnipeg where we currently don’t have to worry about designing structures for seismic loads as the chances of earthquakes occurring here are very low. In a place like California, seismic loading is much more of a concern and the additional structural elements required to resist these loads can be extensive. The magnitude of seismic loads when an earthquake occurs is directly related to the weight of the building. Buildings with heavy materials such as concrete will have to be designed for greater seismic loading compared to a light framed steel structure.
Earth loads occur when soil is built up against a wall causing lateral earth pressures. These loads can be seen on basement foundation walls, retaining walls and tunnels. The magnitude of this lateral load is dependant on the type of soil built up against the structure and the depth of the soil. A house with a very high basement would likely have foundations walls that would have to resist high lateral loading from the soil built up against it if the basement was fully underground. This can be one of the causes of cracking seen in basement walls if the wall was not built strong enough to resist these lateral loads. If water is allowed to build up against a wall, lateral loads from hydrostatic pressure would need to be designed for. Installing a weeping tile system is a way to prevent water from building up against a basement wall.
Hopefully this blog post helps you get a good understanding of what loads are considered when designing a structure. If you want to know more about loads and how structural engineers use them to design structures, there is lots of other great content on this site. If you are interested in taking our course on structural engineering basics you will learn lots more as well! Do you have any stories to share similar to what happened to my parents and their garage? Feel free to share it in the comments section below.
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