A Primer on Elevated Slabs for Mixed-Use Projects

A Primer on Elevated Slabs for Mixed-Use Projects

Over the last few years, our team has been involved with several mixed-use projects using composite slab construction. Working together, we’ve been able to formulate some solid practices with concrete slabs. I asked Heath Cather, a senior project manager in our office, to share on the flatness and levelness of these slabs and have included his thoughts below.

  • To get started, flatness relates to how bumpy a slab is and levelness describes the pitch of the slab. The higher the F number the better. An overall Ff 25 is a standard most of the time and meets the mark for an elevated slab. It can be achieved without significant cost burden.
  • In most cases only the Ff number is specified on elevated slabs because they deflect. During the design phase, it’s important that the design professional calculate the proper amount of camber for the structural beams based on the dead and live load being supported.
  • There always seems to be a difference of opinion between structural engineers on whether saw cut control joints are necessary for slabs poured on composite decking. I tend to agree they are necessary to help control where the slab will crack from shrinkage. Control joints are only meant to control the location of cracks because eventually the slabs will crack, but it’s better to choose where this will occur. There are obviously no saw cut control joints in a structural slab, but it’s generally acceptable for slabs poured on composite deck.
  • To aid in the reduction of shrinkage cracks on elevated decks (and for that matter all slabs) it is important to have a proper concrete mix design and adhere to the design by not adding water at the jobsite for flow ability.

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Merrill Stewart is Founder and President of the Stewart Perry Company, a commercial building contractor based in Birmingham, Ala. Contact him via email.

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