December 21, 2021

Why does my slab deflection increase using ACI 318-19?

Since the release of RISAFloor v15 and ADAPT-Builder v20, both which include the latest update to the ACI code (ACI 318-19), some customers have asked our support team why they're experiencing increased slab deflections. These increased deflections are the result of changes in the calculation of the effective moment of inertia, Ie, for slabs that was made in ACI 318-19 code, compared to the previous ACI 318-14 code.

ACI 318-14 Code

In the ACI 318-14 Building Code Requirements for Structural Concrete code (as well as previous versions of the ACI code) the calculation for the effective moment of inertia, Ie for nonprestressed members was as follows:

The use of the effective moment of inertia, Ie, was developed to provide a transition between the upper and lower bounds of the gross moment of inertia, Ig and the cracked moment of inertia, Icr as a function of the ratio of Mcr/Ma.

Updates to the ACI 318-19 Code

By contrast, the ACI 318-19 code incorporates a new calculation of the effective moment of inertia, Ie, which is dependent on how the applied moment Ma compares to a ratio of 2/3Mcr (the cracking moment). The complete calculation is as shown below:

These new equations will be utilized to calculate the effective moment of inertia when the long-term deflection option is enabled for user defined slabs in RISAFloor ES.

To better understand this change, the graph below shows a plot of the ratio of Ie/Ig vs. the ratio of Ma/Mcr for both the ACI 318-14 and the ACI 318-19.

As shown above, the value of Ie/Ig drops quickly at 2/3Mcr and continues until roughly Mcr. For slabs that fall into this category, a larger deflection will be present when utilizing the ACI 318-19 code.

Evaluating in RISAFloor ES

Using a simple elevated slab model in RISAFloor ES, we can see the difference in the ratio between Ie/Ig in the ACI 318-14 and ACI 318-19 codes in the image below:

For the ACI 318-14 code, the ratio is roughly 0.58 while the ratio in the ACI 318-19 code is approximately 0.25. Comparing these values to the graph plotted above, we see the ACI 318-19 code renders a larger value of Ma/Mcr, which ultimately leads to a larger deflection (0.719" vs. 1.86").

For more information about long-term deflection calculations in RISAFloor ES, search Long Term Deflections in the RISAFloor Help file.

RISA Help File

Tags: RISAFloor ES, Concrete, Slab, New, Deflection, Codes

May 15, 2013 | RISA-3D, Plates, New, Deflection

How can I find the deflection of plates?

Below is a model of a flat concrete slab made of plates. In order to ensure adequate serviceability of the slab, you may need to check the deflection of the slab between columns. However, RISA-3D only reports Member and Joint Deflections. There is no such thing as a “Plate Deflection”. If you would like to plot the deflection between columns you can use a dummy member. Open the Section Sets spreadsheet and go to the General tab. There you can create a dummy member defined as a rectangular 1” x 1” beam. Next, draw the dummy member in your model along the line which you would like the deflection plotted. Because this member has such a small area and moment of inertia its stiffness will have a negligible impact on the model’s results. Because it is a Physical Member it will automatically connect to all of the plate corners which it passes (see Plate-Member Interaction) Link: Plate-Member Interaction After solving you can look at a detail report for the member to see a detailed deflection diagram…

October 3, 2014 | RISAFloor ES, Concrete, Slab, New

How to Model Support Lines with Parallel Walls

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August 8, 2015 | RISAFloor ES, Concrete, New

How do you add Shear Caps to columns?

Per the ACI, you can add a Shear Cap to your column to increase the shear strength of the slab. In RISAFloor ES two-way slabs, adding a shear cap will thicken the plates around the column automatically. The Shear Cap will affect the Punching Shear and the One-Way shear calculations. As you draw the columns in RISAFloorES, there is an option to include a Shear Cap. The thickness is measured from below the bottom of the slab. The Depth and Width are based on the column definition and local axis. It is common to model without Shear Caps and only add Shear Caps as they are needed. You can add Shear Caps to the columns at any time from the Windows menu- Modify – Columns. This allows you to create a Shear Cap on an existing model and shift the Shear Cap eccentrically. You can also modify the Shear Cap in information is stored in the Columns spreadsheet. The eccentricity is defined based on the column local axis. The picture below shows a Shear Cap that is shifted eccentrically by 12 inches in the ey and ez directions. Tags:…