The calculation of concrete floor deflections is complicated. There are many issues to consider, and even with adequate consideration and calculation, any deflection prediction should only be considered an estimate.

Deflections are affected by:

• member size (section properties)
• concrete modulus of elasticity
• reinforcement (unstressed and post-tensioning)
• applied loads
• load history
• cracking
• shrinkage
• creep
• dynamic effects (vibrations)

Historically, designers have usually calculated deflections of concrete members using elastic methods with modifications made for some of the factors listed above.

Most post-tensioned floors have been designed to minimize cracking and so elastic deflection calculation methods have generally been acceptable. The increasing usage of partial prestress methods has made deflection calculations more important for post-tensioned design. Reinforced floors usually undergo more extensive cracking than post-tensioned floors, which is one reason why they need more concrete section, and more consideration of deflection issues.

More aggressive designs (that is, with shallower or thinner concrete sections) require a more rigorous analysis to determine that the deflection limit states are satisfactory. The aggressive designer must, however, have a thorough understanding of the methods and issues to ensure satisfactory deflections.

In particular, if the designer pushes the floor design "to the limit" then problems may occur due to issues that are unforeseen or not considered by RAM Concept . These include:

• poor placement of reinforcement (less effective depth resulting in more cracking)
• vibrations (dynamic effects are not considered by RAM Concept )