Deflections 2

[astructurale]

I am designing a precast beam that meets ACI 318 deflection criteria, and the beam meets it fine, but I seem to have a mental block in allowing a beam to have 3/4"-1" of deflection (20ft span) under full loading. It should be noted that my live load is very small in comparision to my dead load so just under dead load I have 3/4" of deflection, and cracking is allowed but very controlled (CW=0.008). There will also be a drop ceiling beneath it so unless increase my depth increases to get rid of the deflection you will never see it. Does anyone have an opinion on this?

I'm so accostumed to designing prestressed members where the loads rarely remove all of the camber. I'm sure that in cast-in-place work it's more common to have deflections, but then those beams are considered shored until all of the slab is poured, right?

 

[UcfSE]

Did you magnify the dead load deflection to account for long term effects such as creep? Are you supporting anything that would be damaged by 1" of deflection?

 

[astructurale]

Yes, the dead loads are magnified. No, it's just a roof.

 

[WillisV]

Did you account for the cracked moment of intertia (either typically by using approximately 0.5 of the gross moment of inertia or explicitly calculating it using Mcr per 9.5.2.3)? If so and the dead load deflection including long term effects is 0.75" then it is fine.

 

[astructurale]

Yes, I've explicitly accounted for the cracked moment of intertia.

Thanks for the feedback.