ACI 318R19 Long term deflection factor and ultimate total deflection

[concepto.ed]

Hello!

When calculating the total deflection of a RC beam according to ACI 318R19, I've followed this way: Obtain the inmediate deflection due to sustained loads (D1) and the incremental inmediate deflection due to that portion of the live load not sustained (D2). Then, to compute the ultimate total deflection of the beam do I use: Dt= Lamda (D1)+D2 or Dt= Lamda (D1)+ D1 + D2?


I understand the lamda factor as the ratio of the ultimate sustained deflection (including long term effects) to the inmediate sustained deflection. Is my code interpretation ok?. I've not been able to found a good reference regarding the use of that factor in total deflections.

Your comments will be valuable.

 

[IDS]

I work outside USA and rarely use ACI318 for real design work, but I note:

1. The wording seems unambiguous to me (especially in the commentary); it is the addittional deflection, not the total deflection.
2. The lambda factor can be less than one, so it can't be the total deflection.
3. Simplified estimates of long term creep and shrinkage effects can be very unconservative, especially for lightly reinforced sections where the section is uncracked under initial sustained loads, but cracks due to live load and/or shrinkage effects.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Agent666]

Yeah it is definitely the additional deflection. So if that factor comes out at 2 for example, the total deflection accounting for long term effects would be (1+2) times the initial deflection.

NZ code has similar provisions and that's the intent.

https://engineervsheep.com

 

[concepto.ed]

@ IDS and Agent666

Thank you very much for the clarification.