# Checking Long-Term Deflection (LTD) using CSI SAFE

CSI SAFE (Slab Analysis by the Finite Element Method) is one of the best tools available for analyzing and designing concrete slabs and/or foundations. SAFE was developed by Computer and Structures, Inc. In this article, we will walk you through the process of checking the long-term deflection of a simple slab using SAFE.

## What is long-term deflection?

There are two types of deflection. First, the short-term deflection or sometimes called immediate deflection, which is the deformation that happens in slab or any element once the load is applied. Usually, the immediate deflection will give an indication on the performance of the element under load.

On the other side, long-term deflection in a slab or beam in the deformation that happens over a longer period of time. It considers the effect of shrinkage, cracking, and creep.

Codes most often specifies limitations for the long-term deflection.

This article focuses mainly on the long-term deflection of slabs according to ACI, in an easy way without the confusion that usually accompany this process.

Once you have drawn the geometry of the slab on SAFE, you are ready to start inputting the load cases and combinations in the software that will allow you to calculate the long-term deflection.

and,

This means:

Thus, we are going to create three load cases as following:

Don’t forget to check nonlinear (cracked) from the analysis type on the right.

This time select nonlinear (long-term cracked) in the analysis type. You can keep the creep coefficient and shrinkage strain as the default values.

Choose nonlinear (cracked) for the load case.

Once you’ve defined all the load cases, you can start defining the load combinations:

Two cases are defined: One for the immediate (elastic) deflection considering the service loads dead, and live loads