# Structural Load Paths on Beams and Columns How a steel beam hands its weight down through columns and footings into the soil, and how moving loads and wind change that path\. By merry\-otter\-e6277932 Topic: Engineering [Open the interactive Jig](https://ask.jig.com/j/i6Fu20arVHF5) ## Transcript ### 1. Follow the weight Every building is a chain of parts that hands its weight down to the ground\. Here a steel beam rests on two columns, and each column stands on a concrete footing buried in soil\. The glowing beads show the load path: the route the weight takes\. Drag anywhere to look around\. A load is any force a structure has to carry: its own weight, people, snow or wind\. ### 2. Dead load Even with nothing on it, the beam has to carry its own weight\. Engineers call this the dead load: the weight of the building itself, which never goes away\. It is spread evenly along the beam, so the two columns share it equally\. Concrete weighs about 2,400 kilograms per cubic metre, so a building's own weight is usually its biggest load\. ### 3. Live load People, furniture and boxes come and go, so engineers call them live loads\. Set a heavy weight down in the middle of the beam\. The beam bends a little, and each column takes exactly half of the weight\. The bending is exaggerated here so you can see it\. Offices are designed for about 240 kilograms of people and furniture on every square metre\. ### 4. Slide the load Drag the orange weight along the beam, or use the slider\. The column nearer the weight carries more of it, and the farther column carries less\. The two shares always add up to the whole weight\. The numbers on each column show its share of the weight in per cent\. ### 5. Spread it out Snow on a roof or a crowd on a floor spreads along the whole beam\. This is called a distributed load\. Compare the same total weight in one spot and spread out: spreading it makes the beam bend much less\. For the same total weight, the bending effort at the middle of the beam is halved when the load is spread evenly\. ### 6. Inside the beam When a beam bends, its top is squeezed and its bottom is stretched\. Blue shows the squeeze, called compression, and red shows the stretch, called tension\. That is why steel beams have an I shape: most of the metal sits at the top and bottom, where it works hardest\. Halfway down the beam there is a line that is neither squeezed nor stretched\. ### 7. Columns and footings Columns carry the weight straight down by being squeezed\. At the bottom, a steel base plate and a wide concrete footing spread it out over the soil\. Spreading it over a bigger area keeps the soil from sinking\. The pressure on the soil is the load divided by the footing's area, so a footing with four times the area presses four times more gently\. ### 8. Wind pushes sideways Wind pushes on a building from the side, and that push needs a path to the ground too\. Without help, the frame leans over\. Add a cross brace: the diagonal steel carries the push down to a footing, and the frame stays nearly upright\. Wind can also lift a roof, so every joint needs to hold parts together, not just rest them on each other\. ### 9. Every part has a job Load travels from the beam, into the columns, through the footings and into the ground\. If any link in that chain is missing, the load has nowhere to go\. Drag the weight once more and follow its path all the way down\. Engineers check every load path in a building before it is built, from the roof all the way down to the soil\.