What is the long term deflection?

It can be divided in to categories:

1- Non sustained portion

consider only the non sustained portion of live load

2- Sustained portion

consider the sustained loads from Dead,SDead, and the balance portion of Live load + Creep and shrinkage effection

There are basically two type of deflections:

1. Short term deflection 

2. Long term deflection

Short term deflection: One which is caused immediately when the structure is loaded and that is the one that we calculate using deflection equations for various types of members with various types of loading. It includes all the dead loads and live loads considered for the design of that structural element.

Long term deflection: It is the one to be checked for sustained loading, loads which are going to be their on the structure during life span of the structure. Long term deflection is calculated for full dead load and part of live loads which are going to be there life long on the structure. Taking part of live load or full live load or totally ignoring it depends on the engineering judgement, means the engineer needs to make a judgement about how much percentage of the live load is going to sustain throughout the life span of the building. If project engineer wants to totally ignore live load it can be done depending on the judgement and the occupancy of the building. Genrally 25-50% of live load is considered as sustained loading. Now this deflection because is for sustained loading so we assume that structure is going to crack and thats why we use cracked moment of inertia to calculate the deflection for sustained loading only(not for short term deflection). After finding out the long term deflection using cracked moment of inertia, sum up the values from short term deflection and check it against allowable limits per applicable building codes.

deflection due to creep and shrinkage are classified as long term deflections.