L2 Types of Loads

Ajouté : 2026-05-30

[00:00:00]hello and welcome to lesson two of structures two in this group of lessons we will be giving an overview of the many different types of loads on a structure and we will also be looking at how to combine these loads in various load combinations as defined in the IBC today we will also be focusing specifically on one group of loads known as gravity loads next week we will be jumping into lateral loads finally we will conclude our discussion for the second set of lessons with some calculation on tributary widths and areas so let's begin what is a load this might seem like a very basic question to you but providing an exact answer and a precise definition can be a little tricky I'm sure most of you probably already know what a load is or you can at least provide examples of different types of loads but it's important that we all have the same understanding of what exactly a load is so let's see if we can find an exact definition of what a load is according to a structural engineer and for that we need to look at this document this document is written by the American Society of civil engineers and it's called the minimum design loads for buildings and other structures and it's one of the reference standards in the IBC and if we go to the definitions page of this document we see that they do provide a definition for what a load is and according to the American Society of civil engineers loads are forces or other actions that result from the weight of all building materials occupants and their possessions environmental effects differential movement and restrained dimensional changes so this definition is also pretty much just providing examples of what loads are but regardless this is the definition that we have for what a load is so let's start to break this apart first we have the weight of all building materials so that would include the walls doors windows mechanical equipment Lights Plumbing even the structure itself has weight and Structural Engineers need to design for all of that next we have occupants and their possessions occupants of course means people in the building and possessions are anything that they bring along with them next we have environmental effects and of course up here in the Northeast the first thing we probably think about when it comes to environmental effects is snow or rain water is actually quite heavy about 62.4 pounds per cubic foot in density so if we have a lot of snow or rain up on a roof we're going to need a structure that can support that load all of these loads that we've discussed up until this point have to do with weight due to gravity if there were no gravity none of these objects would have weight and there would be no load imposed on the structure and so we'll call these gravity loads but of course there are also environmental effects that don't relate directly to gravity for example wind loads are caused by changes in fluid pressure and when that pressure hits the side of the building or poles on other parts of the building that creates a load earthquakes also create loads on a structure but earthquakes are fundamentally very different from wind loads and we'll talk about that more next week soil pressure on a retaining wall or hydrostatic load is another environmental effect on a structure these loads all push and pull on the building horizontally and so we'll call them lateral loads and again we'll talk more about these lateral loads next week now let's talk briefly about these last two categories differential movement and restrained dimensional changes well differential movement is primarily referring to the foundation if you have a building a long building like this on level ground and let's say that the soil on this side of the building is much stiffer than the soil on this side of the building and so this side of the building may only settle an inch while the right side of the building May settle much more and so that's going to cause stresses in the structure of the building lastly we have restrained dimensional changes what they're referring to here is thermal expansion and contraction if a brick veneer for example is in direct sunlight all day it's going to heat up to a very high temperature and it's going to want to expand then later at night as the brick cools it's going to shrink if there are enough control joints along that veneer wall the brick can expand and contract freely but if the dimensions of the wall are restrained then there will be stresses that develop inside the brick and potentially cracking so now let's continue to look through this document and look through what the definitions of each of these different types of loads are and we'll start with chapter 3 which is dead loads soil loads and hydrostatic pressure and we can see here the definition for dead loads states that dead loads consist of the weight of all materials of Construction Incorporated into the building and they provide examples the main point here is that dead loads are permanent loads so mainly that's talking about the structural components and so if we have a wall in the building that is an architectural partition and is not part of the structural system then that would not be considered a dead load mechanical equipment can be a little bit of a gray area and I've actually heard Arguments for both sides but in general most mechanical equipment is not going to move during the lifespan of the building and so most Structural Engineers consider mechanical equipment to be dead load and the justification for that in the code would be this statement right here fixed service equipment soil loads and hydrostatic pressure we are going to save toward the end of the course for when we talk about concrete and Foundations now I'd like to point out that this chapter chapter 3 is relatively short and that's because due to their nature of being permanent loads these loads are static in other words if we plot on a graph with time on the x-axis and the force the magnitude of the force and the y-axis that line is going to be perfectly flat so we know that the load is not going to change ever during the lifespan of the building we can also calculate these dead loads very easily because we know the weights of all the materials we know how much steelways we know how much concrete weighs Etc and so there isn't really much guesswork or need for too much research when it comes to dead loads but as we'll see as we continue onward that's not true of all the loads that we will be talking about the next type of load we're going to be talking about are live loads and to refer back to the definition this is where we talked about occupants and their possessions occupants of course includes the owner and the owner's possessions can include chairs desks tables other pieces of furniture art installations what have you all of that would be live load anything the owner might pick up and move around or replace all of that is considered live load and here the exact definition of a live load is stated as a load produced by the use and occupancy of the building or other structure that does not include construction or environmental loads so basically they're defining live load as anything that's not any of the other loads like I said before everything in Structural Engineering is very cyclical and interlinked and it can be hard to know where to start exactly to Define things but nevertheless a live load is essentially anything that can be found inside the building that has weight that is not permanent to the structure now when we talked about dead loads I mentioned that dead loads are basically static loads they don't ever change and as you can imagine based on our discussion of live loads so far if I draw just a basic elevation of a building like this and I draw just a little person here so that person the weight of this person is considered live load and this person of course is not going to stay in one spot for their entire life for their entire existence they're going to move around so the load on this member is going to change and so this is clearly not a static load and instead it's a dynamic load meaning that the load changes over time but of course the structural engineer is not going to sit down and analyze every single Moment In Time throughout the entire lifespan of a building instead they're going to look for the worst case scenario and consider that that brief moment in time where a member is experiencing its worst case stress and design for that load and so Structural Engineers take what is a dynamic load and turn it into a worst case scenario static load a worst case single instant in time and design for that and if you design the member for that single worst case instant in time then of course the member will be adequate for all other moments in time and it's important to note that the worst case moment in time may not be the same for every member in the building so for example one might assume that the single worst case moment in time might be for when the building is completely full but in fact if we were to draw a cantilever out here and if we were to design that cantilever for the maximum possible occupancy that we're expecting over the lifespan of the building both in this room and in the room behind you can see that this would this Arrangement would not be the worst case scenario for this cantilever because of course we're placing more weight on the backspan which is helping that cantilever so actually for this cantilever the worst case scenario would be when we have full live loading on the cantilevered member and no live load in the adjacent space so each component has a different worst case scenario in time but we're designing each of them according to one specific instant or one quote-unquote static load condition and so we call this method of analysis pattern live loading and basically we're just putting different live loads in different locations throughout the building and picking the worst possible arrangement of those live loads for each component next we have flood loads so we're getting into our environmental loads here and note that not every building is going to be designed for flood loads according to this General section here the provisions of this section apply to buildings and other structures located in areas prone to flooding as defined on a hazard map and so for this course we won't be focusing very much on flood loads next we have snow loads and snow again is somewhat dynamic in that it will build up over time and it may shift around on the roof however we're only going to look for that single worst case scenario for when we design the members on the roof and so we're going to take a dynamic load and analyze it as a static condition next is rain loads and the one thing I want to point out here is this ponding instability and ponding refers to the retention of water due solely to the deflection of relatively flat roofs so it's actually easier to think about this when you see pictures of it but you can imagine that as it rains the water is going to add load to the roof due to its weight and that's going to cause the roof to deflect and as the roof deflects more rain and more water is now going to collect in that dip that's caused by the deflection of the initial rain load and so as it deflects more load collects and then it deflects more and then gathers more load etc etc and sometimes this can lead to collapse and lastly in our group of gravity loads we have ice loads again ice loads are not something that we typically include in design although if you are designing somewhere up in Alaska then you'll definitely need to consider ice loads and so you can see we're less than a sixth of the way through this document and we've already covered all of the Gravity loads which means that the rest of the document is all of our lateral loads and just so you can see here all of our gravity loads were covered in the first 100 pages of this document the remaining 558 pages are all going to cover just two more loads seismic and wind and so we'll be looking more at lateral loads and the remaining 558 pages of this document next week don't worry we definitely aren't going to look at every single page we're just going to pick out the ones that are most relevant for you to know as an architect