Deemed to satisfy slab deflection checks example (ACI)

Tekla Structural Designer
2021
Tekla Structural Designer

Deemed to satisfy slab deflection checks example (ACI)

A simple way to assess slab deflection in Tekla Structural Designer is to run a linear analysis using adjusted analysis properties, and then check the resulting deflections by manually determining critical spans.

Download and open the tutorial model

  1. Download the tutorial models from here.
  2. Open the following tutorial model:
    • Slab deflection ACI.tsmd

Perform Linear Analysis

  1. From the Analyze toolbar, click Analyze All (Static)
  2. Open a view of the Typical floor level
  3. Switch to the Results View
  4. From the Results toolbar, review 2D deflections for the FE chase-down analysis for the load combination 1, service load results

Identify critical check locations

We can see that the maximum reported deflection is 1.1in, occurring in the middle of a corner bay. This should be assessed by taking the slab span diagonally across the bay.

Note: In 'real world' flat slabs engineering judgment will be required to assess which deflections and span lengths require checking.

The deflection at the identified location now needs to be checked against a limiting span-to-depth ratio which we will assume for this example can be taken as span /240.

Taking the diagonal dimension across the columns, the deemed-to-satisfy span / 240 rule provides a deflection of [√(26' 3"2+26' 3"2)] / 240 = 1.8in.

This compares unfavorably.

Note: Remember, the method does not predict actual deflections. The total deflection is simply expected to be less than span / 240.

Concrete properties used in the analysis

The Tekla Structural Designer deflection result is completely dependent upon the concrete elastic modulus used in the analysis which is adjusted by a modification factor to consider such things as creep, cracking and shrinkage.

The modification factor is set from the Settings dialog on the Analyze ribbon. As shown below, for the FE chase-down analysis of flat slabs this defaults to 0.25.
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