January 16, 2024

How To: Understand Long Term Deflection Results in RISAFloor ES

RISAFloor ES allows users to analyze Long Term Deflection for Concrete Floor Slabs using "User Defined" as the design method. For more information on how to model Long Term Deflection in RISAFloor ES, follow our other article link below. In this article, you'll learn how to:

Understand Deflection Methods
Understand Your Deflection Results

Link: Long Term Deflection in RISAFloor ES

Understanding Deflection Methods

In the Model Settings, there are two Concrete Deflection methods available. The deflection method options allow you to choose from I Cracked/Elastic or Long Term.

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I Cracked/Elastic

Selecting a Deflection Method of I Cracked/Elastic in the Model Settings controls the stiffness reduction based on the Icr factor in the Slabs spreadsheet .

The Icr factor default is set to 0.25 per ACI 318-14 Section 6.6.3.1.1 (ACI 318-11 Section 10.10.4.1). You can modify this factor with any value between zero and 1.0 by typing in this field. The Icr factor adjusts the stiffness of the plate elements comprising the slab by altering the thickness during solution.

Long Term
Selecting a Deflection Method of Long Term, the program will automatically adjust the plate stiffness by adjusting the Moment of Inertia to be I_e (ACI 318-14 24.2.3.5). This stiffness adjustment is an iterative analysis per plate based on the global axis. The slab is modeled as plates, and based on the finite element analysis, the program is obtaining a moment demand for each plate, then the program will adjust the plate moment of inertia by the equation below based on the Mcr/Ma.

Note: The Icr in the equation for Ie is not using the user input from the Slabs spreadsheet, but actually calculating Icr from the following table: Table 10-2 Gross and Cracked Moment of Inertia of Rectangular Section in the PCA Notes on ACI 318-11.

Understanding Your Deflection Results

There are internal Load Combinations included when the Long Term Deflection analysis is turned on. These internal Load Combinations can be shown in the Slab-Contours- Displacements as well as the Slab Point Deflections spreadsheet.

Slab Point Deflections Spreadsheet

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The Sustained LL Factor and the Creep Shrinkage Coefficient gets used in the Load Combinations listed below.

Service LC: Uses the Icr factor on 1.43*Ig (ACI 318-14 Section 6.6.3.2.2)
Sustained Load: DL + (Sustained LL Factor)*LL
Total Load: DL + LL Total Load tab uses Ie described in the Deflection Methods section of this article
Long Term Sustained: Sustained Load * (1 + Creep Shrinkage Coeff)
Long Term: Long Term Sustained + ( Total Load - Sustained Load )

Slab-Contours- Displacements Graphically

The Long Term Deflection can also be viewed graphically using the Slab Contours-Displacement.

Open your Model Display Options.
Navigate to the Slabs tab to change the display of the slab contours. The internal Load Combinations are listed in the Results Display.

More Questions on Modeling Long Term Deflection in RISAFloor ES? Reach out to our support engineers by clicking the button below.

RISA Support

Tags: RISAFloor ES, Concrete, RISAFloorES

June 28, 2018 | RISA-3D, New, Analysis, Deflection

A Deeper Understanding of Deflection and Member Deflection Ratios

There are many different types deflection values calculated by RISA-3D. Let’s take a deeper dive into each. Absolute deflection is the maximum absolute value for deflection in the member local lateral y- and z-axis. In engineering terms, this value is also commonly referred to as global deflection since it encompasses the entire movement of a member as the entire model moves. The term “global deflection” is not used in RISA-3D since this can be confusing when also talking about global and local axis for direction. The values for absolute deflection are reported in the Service and Strength tabs of the Member Deflections spreadsheet. The deflection value used in the member deflection ratios L/y’ and L/z’ is the relative deflection: y’ and z’. This is the deflection of a member relative to its own non-deflected shape. This deflection only captures the effects of loading specific to the member itself and does not include any effects from a neighboring member’s deflection. The deflection ratios L/y’ and L/z’ are applicable to all member types (beams, columns, and braces). It is important to understand that the relative deflection used in this ratio is not equal to the absolute deflection that is reported in the…

October 8, 2024 | Featured, RISA-3D

How to Troubleshoot Large Deflections in RISA-3D?

Deflection is a key tool for verifying whether your model was built correctly in RISA-3D. After solving, you can view the deflected shape to confirm if the deflections are within the expected range. If you notice abnormal deflections, use the following steps to troubleshoot the issue. Steps for Investigating Large Deflections Check Member Material Properties: Ensure that the correct material properties are assigned to your members. Incorrect materials can result in inaccurate stiffness and deflections. Review Member Sizes: Verify that your members are sized appropriately for the loads they are designed to carry. Oversized or undersized members can affect deflections significantly. Examine Member Connectivity: End Releases: Incorrectly applied end releases can result in overly flexible members. Analysis Offsets: Verify that any analysis offsets are accurate, as these can influence the effective stiffness of the members. Plate Connectivity: Remember that plates only connect to other elements at their corner nodes, not along edges. Verify all plates are supported at their corners. Inspect Boundary Conditions: Ensure that your supports and restraints are correctly defined. Improper boundary conditions can lead to unexpected deflections. Review Applied Loading: Pay close attention to the magnitude and direction of your applied loads. Misapplied loads may cause unexpected…

June 21, 2018 | RISA-3D, New, Analysis, Deflection

Beam Deflection Ratio Options

RISA-3D v16.0.4 introduces an enhancement that will allow for more control over the beam deflection ratio through Deflection Ratio Options. As part of the member properties, you can now designate the ends of single and multi-span beams as a cantilever or supported. This will determine whether the beam deflection ratio is calculated as L’/y’ (supported) or 2L’/y’ (cantilever). Modifying Members From the Members spreadsheet, go to the Advanced tab, where you can designate the Defl Ratio Options. Note: These options are only applicable to members designated as beams. By default, the deflection ratio will be calculated automatically, either as a supported member or as a cantilever. An example of when the deflection ratio option can be useful is if the beam deflection ratio of a member is calculated by the program as a cantilever, but you consider it to actually be supported. You can then set the deflection ratio options to explicitly specify that it is supported and the beam deflection ratio will be adjusted accordingly. Single span beams can be designated as supported on both ends, or cantilevered from either end. In addition, multi-span beams have the option to be designated as a double cantilever, where both ends are…