Monthly Mechanics: Design Loads

Last month we learned how to figure out the reactions on both ends of a beam, given some loads. But we’re missing an important piece of the puzzle: how do we know what those loads will be? They keep changing, of course: when you host a party, or jump on the bed, or move your furniture, you temporarily or permanently increase the load on some beams. For what load should you design them?

Engineers have a great way to simplify this unknown-load business: they assume the worst case and design for that. OK, but what’s the worst case? The American Society of Civil Engineers (ASCE) publishes standards for the loads to use when planning various types of rooms – the design loads. They always exceed the weight you’d expect to act most of the time, which is why they are considered worst case. For example, according to one ASCE table, the minimum design load for “habitable attics and sleeping areas” is 30 pounds per square foot. This weight is about what you’d get from a queen-size bed with four adults on it (1000 pounds over 35 square feet), which clearly is an unusually heavy case.

Did you ever hear a story about a library that sank into the ground because some engineer forgot to account for the weight of books? Well, that story is apocryphal – it never actually happened. The ASCE table gives plenty of respect to the weight of books… minimum design load for library stacks is a whopping 150 pounds per square foot.

Design live loads, from ASCE.

Design live loads, from ASCE.

The loads I just described are known as live loads, because they move and change over time like living organisms. Another live load is the weight of snow on a roof, which ASCE specifies depending on your location – see the contour map at the beginning of this entry. Wind and earthquakes round out the list of live loads you commonly design for. (Earthquakes don’t just happen in California. At my old company, I spent weeks designing seismic controls for a bridge located in… Rhode Island.)

What’s the opposite of live loads? Dead loads. Basically, dead loads are the weights that don’t move: the beams themselves, the subfloor and finish floor, the roofing materials. They can be a pain to add up, but at least you know them with certainty.

Many types of loads may act on a surface, but they don’t all act simultaneously. A roof for instance might sustain a heavy live load (say, a construction crew with 70-pound bags of shingles) or a heavy snow load, but not both together. For this reason, ASCE specifies load combinations. Basically you multiply each type of load by a decimal indicating its probability (with a built-in factor of safety), then add them all together. See an example below.

Comparing three load combinations.

Comparing three load combinations.

When you compare every load combination, the highest total is said to govern. That’s your worst case!

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