How to Design Floor Slabs on Grade

Written by cassandra tribe
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How to Design Floor Slabs on Grade
Slabs on grade must be constructed to meet all required building codes. (Jupiterimages/Comstock/Getty Images)

To design floor slabs on grade, you must have a complete understanding of the static loads (like tanks and boilers), moving load (vehicles, occupancy) and the horizontal and vertical load-bearing weights that will be dispersed through the slab from the walls and columns supporting the upper shell. In addition, the designer must also take into account all mechanical and electrical penetrations, drainage issues and concrete reinforcement while meeting all the applicable building code requirements and regulations. Using a systemic approach to the design prevents forgetting to include a needed element.

Skill level:

Things you need

  • Soil test results
  • Mechanical prints (site/structure) specific
  • Structural and architectural plans
  • Building Code
  • Site specific code and regulation requirements
  • CAD or manual drawing system

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  1. 1

    Read the soil test results for the site to determine the federal and state requirements for preparing the subgrade for the slab. Some soil types require that the soil be removed to a certain depth and replaced with construction grade sand. Other types of soil determine the weight and complexity of the rebar and/or post-tension reinforcement that is required.

  2. 2

    Look over the mechanicals for the slab area. The mechanicals are the design drawings that specify the construction of HVAC, plumbing and electrical for the site. Check the notes on the drawing to determine if the mechanicals penetrate into the shell of the building on a horizontal level above the finish grade of the slab or if they require vertical penetrations through the slab itself. For the latter, check the building codes and requirements for what type of reinforcement is needed around each penetration.

  3. 3

    Read through the structural and architectural drawings for the building where the slab is going to be located. Note the static loads detailed to be placed on the slab when finished (such as boilers and tanks). Do not overlook checking the architecturals as it is not unusual for a load item that should have been drawn into the structural plans to be moved to the architecturals to avoid over cluttering a page.

  4. 4

    Read the building codes and requirements for the specifics regarding the type of concrete to be used in the slab on grade and the type of reinforcing to be placed in the concrete. Typically, rebar or iron mesh is placed in the slab as reinforcement with the spacing and weight of both determined by the load placed on the slab. The code details spacing, weight and any other special requirements. Some jobs require the additional placement of post-tension cables. Note the specifics regarding the placement of the cables and post-tension stressing requirements.

  5. 5

    Note the required thickness of the slab. The thickness of the slab may change in areas around penetrations or areas that are supporting a greater static weight and in areas where the grade of the top of the slab slopes to a drainage point.

  6. 6

    Use a computer-aided design (CAD) program (or draw by hand) to make the preliminary drawing incorporating all of the mechanical penetrations and reinforcement requirements for the slab. Specify the height that the layer of reinforcement must be above the subgrade (ground) of the slab. Most layers are placed at the midpoint of the slab thickness.

  7. 7

    Add control joints (places where the reinforcement and concrete are cut to allow for expansion) to the plan by following the building code requirements for control or expansion joints in the slab.

  8. 8

    Locate all drainage points and add the degree of the slope to the finished grade of the slab needed to achieve this in the construction. Mark the slope for the finished grade (top) of the slab and not the degree as measured from the bottom of the grade (the ground) or the finished slope will be incorrect.

  9. 9

    Proof the drawing. Check that all measurements required for construction are in place and all special elements (like reinforcement type, spacing and slab-specific code requirements) are in place. Have a helper read through and match the notes taken from the codes and references with the details on the drawing to verify nothing has been missed.

Tips and warnings

  • Make sure any changes made in the field -- during the actual construction of the slab on grade -- are approved of by the Engineer of Record in writing.
  • Leaving any part of the slab on grade design out or not following the plans exactly can lead to the slab cracking; the company responsible for placing and finishing the concrete is held accountable for any needed repairs.

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