Happy Thursday! This is Back of the Envelope – the place to be to get your weekly mini-splash of structural engineering sprinkles 🧁🍩.

Last week, we talked about how to preliminarily check a composite deck for fire rating. Today, we’ll riff off on the decks a little bit more. (For some reason, they don’t teach you these things in school… at least not for me when I was in college.)

If you specify composite decks and beams regularly, this will be more of a refresher than anything. Still, it is an important reminder because if you miss the two checks that I am about to tell you, the result could be detrimental.

Let’s dive in.

(Estimated read time = 1 minute and 47 seconds)

Check #1: Unshored span for decks

Before the deck and concrete can become “composite,” the deck alone must be able to support its own weight plus the weight of the wet concrete.

The increased capacity from composite action only materializes after the concrete cures.

Most manufacturers publish the “maximum unshored span” in their catalogs to ensure the bare deck has sufficient capacity (some manufacturers have calculators, such as this).

So if the design intent is to not shore the deck (common practice), you have to make sure that the beam spacing is less than the maximum unshored span. Otherwise, you may risk overloading the deck during concrete pour.

(Example of “maximum unshored clear span” from a legacy Verco catalog)

Check #2: Unshored construction for beams

Similarly, before the beam becomes composite, it must be first designed as non-composite to support its own weight plus the weight of the deck and concrete.

Most software gives you the option to choose shored or unshored construction -- meaning, whether the beams will be shored or unshored during construction.

Just make sure you consciously select one or the other and not leave it up to chance (unshored construction is most common). Otherwise, you risk overloading the beam during concrete pour.

(Example of shored vs unshored construction from RAM Structural Systems)

And that’s all for now. Hope this was a helpful reminder.

Thanks for being part of Back of the Envelope! (By the way, we’ve just reached a small milestone of 100+ subscribers last week)

Speaking of which – you might be wondering: “Andy, it's great that you are sharing these tidbits but what is the end game here?”

I actually have a vision of where I want to take this and what it means for us (people like you who are passionate about structural engineering). I will share that vision once I am able to clearly articulate that with words.

Until then, enjoy the rest of your week, and I’ll see you in the next email 😊

Andy Lin
Back of the Envelope/Structural Engineer HQ