BMD : Intro : Page 4
To build a simple structure, like a dog house, you would go to the lumber store and purchase a few of 2x4's, some nails, a bundle of shingles, and a couple sheets of plywood for the outside. After bringing these materials back, you would cut the 2x4's to the right length for the walls and roof, and the plywood to cover the walls and roof. After nailing the sides, roof, and covering together, you might put shingles on the top to protect the wood from the rain, but you wouldn't be very worried about whether your dog house would support a lot of weight, because it is quite small.
But, in houses, stores, industrial facilities, and bridges, it becomes more and more important to make sure that the design of the beams and supports is done right. You certainly wouldn't want your bed or couch to fall through the floor because it wasn't strong enough to hold the weight! And when you're driving across a bridge, it would be nice to know that it won't collapse under your car.