Reactions: 1. Load Path Uniform Concentrated Loads

So in this video I'd like to discuss load path, uniform loads and concentrated loads from the roof down to the foundation. And of course this discussion applies to wood, steel or concrete structures. Excellent. So a deck, for example, is attached to the top flange of a beam here in this picture. And a deck does not carry a lot of load. It's relatively a light load. And it's attached quite frequently. So we're going to consider this to be a uniform load. And we're going to represent it as such. So a uniform load is basically in pound per foot. Or maybe kilopound per foot. And after so many feet, you get a total resultant. So a uniform load in all of your structures materials is going to be represented with little w, pound per foot. And then you add up the load and you get a resultant, big W, a total for the uniform load. And that one will be in pounds or kilopounds, also known as kips, kilopounds. Very good. So the load from the deck to the beam is pretty light, and it's frequent. The same with open web joists. They're spaced close to one another, and they don't carry a lot of load. Open web joists are typically for roof, not for floor. Floor is a larger live load, and so it warrants more of a beam or a girder or something a little bit heftier than an open web joist. But with OpenWebJoyce, the spacing is pretty tight and the load is light. So again, we consider OpenWebJoyce to be a uniform load. If we look at this example here, we have a beam, this one, and it's carrying deck uniformly. And then the beam is spaced farther apart than Joyce, and it takes its load and sends it to the girder as a concentrated load. That's a pretty big load, and it's at one location, and so that's a concentrated load onto the girder. The girder is receiving these two beams, and there's another concentrated load from this other beam. So there's an example of a concentrated load versus a uniform load. Looking at this next image, I see a 4x4 over here carrying a pretty hefty roof load, And I see a beam here. Maybe it's a 2 by 10. I'm not sure. And I see a reaction here from this lower column. So this is another example of a concentrated load. It's a pretty large load, not distributed over a large area, but concentrated at one location. Very good. So let's backtrack and say deck, floor, and slab are all planar. structures, and they carry small loads, relatively, and they spread it over a large area, so they carry a uniform load. And a uniform load is typically represented on a free body diagram with a uniform load coefficient, little w, and a uniform resultant, or a total, big W. And so the units here are pound per foot or kilopound per foot and pounds or kilopounds. So this one is sometimes represented as PLF or pound per linear foot. So the load goes from the deck floor slab as a uniform load and it's supported on a joist or a beam or a larger beam called a girder. So that's the first part of the load transfer. The next part is what does the joist or the purlin or the rafter. All these guys are linear members that receive load from a planar member, which is the deck, the floor, the slab, and they're closely spaced. Joist purlins, rafters are closely spaced and they carry small loads. So we're going to say again this is a uniform load. and it is represented with little w, big w, in pound per foot for little w and pounds for big w. So the load goes from the joist, purlin, and the rafter, and it lands on a beam, or a big beam, a girder, or maybe a truss. I don't know. So let me add here open web joist for steel. Open web joist, open web joist. That's another repetitive member that is closely spaced that is not carrying a large load. Very good. These guys are larger and spaced farther apart, the beam, the girder, the truss. So let's come down here and say a beam is carrying a concentrated load. versus a uniform load for the previous members, a concentrated load, and a concentrated load is represented on a free body diagram with an arrow, and it gets the symbol P in all of your structures material, including the ARE. And this one is in pounds or kilopounds versus the previous uniform load. Okay. So a beam will sit on a girder, which is a big beam basically, and maybe a column or a wall. So let's look at this image, and then we'll come back to our table. Looking at this framing here, we have a deck. The deck is deploying a uniform load on that beam, and the open web joists on the roof are also deploying a uniform load. They're evenly spaced, and a roof live load is small compared to a floor live load. That's why we see open web joists on the roof framing, and then we see beams and girders. On the floor framing, the live load is greater. And this is the girder. The girder is the beam that carries other beams. Okay, so the load comes from the roof down this column, and it continues down. Now, where did the beam load go? This beam here is sitting on a girder on one end and another girder on the other end, and the beam has accumulated quite a bit of load. It's accumulated that much load from the deck, and that's pretty hefty, and the beams are spaced farther apart than the joists. So that is considered to be a concentrated load here and another concentrated load here. Let me erase a little bit. I'm starting to get dizzy myself. Okay, so this goes away, and now this girder here, oops, this girder is carrying a concentrated load from this beam, and it sends it down to a column, and the column has to respond with a reaction. and then the column takes its load and sends it downward. Now, if we look at this girder here, this girder is receiving a concentrated load from the column. On the ARE, you probably won't see a beam landing on a girder, but in the real world, it happens. So, looking at this, the center line of the girder is somewhere over here, And this load is slightly to the left, which tells me that the column on the left is carrying a little bit more load from this girder than the column on the right. The closer the load gets to the left column, the greater the reaction from the left column and the less the reaction from the right column. If this column were in the dead middle, then the two reactions would have been equal. If this column were on this side, then the right reaction becomes larger than the left reaction. Very good. So back here, the beam is sending its load to a girder, or a column, or a wall, And then the girder, in turn, sends its load to a column or to a wall. And this, of course, is a concentrated load. And it is called P, and its units are pounds or kips, not pound per foot. That's little w, the coefficient of the uniform load. Very good. So P and capital W are similar. They're total loads. One is the resultant of a uniform load, and P is the symbol for a concentrated load. Then the column takes the load from the girder and delivers it to another column below it, on the floor below it, or maybe, if not on a column, then maybe it's on a wall or on a footing. So this, again, the column delivers a concentrated load, and its units are pounds and kips, and its symbol is P. Now, when the column reaches the foundation, the foundation takes the load and spreads it to the soil. And this one is a uniform load. So what happens here is I have a column, and it's carrying a concentrated load, and it takes its load and spreads it to the soil. So I have a concentrated load from the column, and then we're going to spread it over a larger area of the footing over here before we re-deliver it to the soil. So the soil is receiving a uniform load that went from the column to a footing and then spread to the soil. And this one is also called little w. But its units are pound. This time we spread it over an area. So pound per square foot or kilopound per square foot, also known as PSF and KSF. Now we're talking about a stress, pounds per square foot. Very good.