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Last month, we analyzed the floor structure of an 18x20-foot living room. The floor system has an 18-foot-long central girder supporting 10-foot-long 2x8 joists running to the outside walls. The joists are #2 Doug fir 2x8s 16 inches on-center. Using a design floor load of 40 pounds per square foot, we calculated the load on the girder to be 33.3 pounds per inch. Through analysis of bending strength, we arrived at a beam with enough raw strength to handle this load — a quadruple 2x12 girder built up from select Doug fir. But raw strength is only one of the elements we have to check out in designing beams. In this column, we'll look at two other ways