Deflection of PT Flat Plates

[mrengineer]

This is a survey: How are you calculating long-term deflections for post-tensioned flat plate floors? Use uncracked properties? Assume a portion or assume all of the live load is permanent? Have you had any problems with deflections of this type of construction?

(I am currently doing design of condo type construction using RAM Concept).

 

[swivel63]

section 9.5.3 in ACI i believe. i've used half the live and all the dead then multiplied it by lambda factor. i think it explains it in the commentary.

 

[rapt]

mrengineer,

You can only consider it to be uncracked if the stress are less then the tensile strength of the concrete. In areas where the stresses are higher you should assume theose areas cracked and determine a reduced stiffness in those areas. The areas looked at should not be based on a full panel width. The local area around the column should be considered with a much smaller width as there is a local zone which will likely be cracked.

For permanent load for a condo, 40 to 50% live load to determine the long term deflection and 70% live load to determine the total long term deflection.

 

[mrengineer]

rapt:
How are you differentiating between long term deflection and total long term deflection? Did you mean to say 70% total load rather than 70% live load?

 

[apsix]

Just thinking out aloud here;
Perhaps total long term deflection is due to 40 - 50% long term live + 30 - 20% short term live; resulting in 70% of total live.

 

[rapt]

To calculate a total long term deflection you need to calculate

1 the extra deflection due to long term effects under permanent load at say 30 years. This deflection allows for creep and shrinkage effects. The live load applied is about 40 to 50% to the total. The easiest way to calculate this is to calculate the deflection unde this load case using long term properties and subtract from this the deflection calculated for this load using short term perperties.

2 The deflection due to the full loading using short term properties. More modern codes such as Eurocode 2 or As3600 use a reduced live load for this. About 50 to 70% of full live load for offices and residential.

Add these 2 togehter to get the total long term deflection.

 

[ash060]

I do not remember which committee paper it is, but ACI has a paper that you can find recommendations for deflections in prestressed concrete members. If you have access to a Manual of Concrete Practice it will be in there. Ram Concept got the factors they use for LT deflection from this paper. Usually I assume that live loads have no effect on LT deflections.