Why use different moment of inertia in deflection calc in ACI and PCI?
Why use different moment of inertia in deflection calc in ACI and PCI?
(OP)
ACI and PCI basiclly deal with same material, i.e. concrete, why in ACI, Ie=0.35Ig or 0.25Ig is used in calculating the member or slab deflection. While in PCI, the gross Ig is used in calculating the member or slab deflection? That makes no sense.






RE: Why use different moment of inertia in deflection calc in ACI and PCI?
RE: Why use different moment of inertia in deflection calc in ACI and PCI?
RE: Why use different moment of inertia in deflection calc in ACI and PCI?
Agreed, PCI is definitely wrong.
However, your 12 * sqrt f'c limit is not correct.
Even ACI318 says 7.5 * sqrt f'c and this is only correct if you take into account restraint effects (shrinkage restraint due to internal reinforcement as well as external effects with shrinkage and temperature movement).
And this is still wrong for flat slabs due to the complete illogic of ignoring column strip moment concentrations in ACI318.
RE: Why use different moment of inertia in deflection calc in ACI and PCI?
uncracked (bottom stress < 7.5*rootf'c
transition (7.5*rootf'c<bottom stress<12*rootf'c
cracked (bottom stress>12*rootf'c)
The 7.5*rootf'c is the modulus of rupture for traditional (non-prestressed) C.I.P concrete.
RE: Why use different moment of inertia in deflection calc in ACI and PCI?
1. Should I Add column strip deflection in one direction, say 20' spacing direction to middle strip deflection in 22' spacing direction?
2. will following approximation method to get the slab deflection be justified:
first, calculate 22' direction middle strip deflection by delta1=(w*22'*22'*20'/2)/(384*Ec*Ie), where 20'/2 is the middle strip width. Then considering that the end point of 22' direction middle strip is sitting on mid span of column strip in 20' direction, the end point itself has deflection, so I need also add 20' direction column strip deflection delta2=(w*20'*20'*22'/2)/(384*Ec*Ie).
3. If I put W12x65 beam across 22' direction cloumns, can I now consider the slab as one way slab and consider the 20' span length slab as fixed boundary condition at both ends. So the deflection is: delta=(w*20'*20'*1')?(384*Ec*Ie_at_1_foot_width)?
RE: Why use different moment of inertia in deflection calc in ACI and PCI?
I don't think you should make sweeping assertions like "PCI is definitely wrong".
As StrEIT mentioned, they deal with prestressed concrete (precast is very frequently presetressed) and cracking is usually avoided. In that context, it makes sense for them to focus on what they do, and using Ig is reasonable to them.
RE: Why use different moment of inertia in deflection calc in ACI and PCI?
UUUALLY avoided is not a good excuse. If always avoided and that is stated then ok, but USUALLY avoided means there are cases where it is allowed and must be designed for so the book should say that.
Structlin,
Yes adding the column strip deflection in one direction (allowing for any cracking in that strip) to the middle strip deflection (allowing for any cracking in that strip) in the other direction is an appropriate method. tests I saw the results of a few weeks ago showed it to be remarkebably accurate.
But the load on each strip has to be determined logically. The column strip will have 75% of the moment at the support reducing to about 60% at midspan and the middle strip will have the rest. RAPT does all of this for you automatically.
RE: Why use different moment of inertia in deflection calc in ACI and PCI?
That being said, I interned at a very large precaster for over a year and they would always design every flexural member to remain uncracked (or in transition).
RE: Why use different moment of inertia in deflection calc in ACI and PCI?
The slab load tables (section 2.5) are limited to Class U members (2.3.2) and therefore use gross section properties to determine an elastic deflection.
RE: Why use different moment of inertia in deflection calc in ACI and PCI?
There is a lengthy thread on this forum precisely about using engineering judgment.
Codes aren't meant to substitute common sense or engineering knowledge.
Personally, I don't want the codes to hold my hand every step of the way.
Designers with even a cursory knowledge of concrete design know that cracked concrete has significantly lower stiffness. I don't think it's necessary for PCI to spell this out.
RE: Why use different moment of inertia in deflection calc in ACI and PCI?
Agreed, and PCI manual is NOT a code unless I am mistaken. But any design advice in print should not be misleading. I have not read the PCI manual for about 30 years, I was just commenting on StructLin's comments. Unfortunately a lot of designers depend on books such as this as well as forums such as this one for design guidance. And some people do need to have their hand held. The problem comes when they do not realise it.
StructuralEIT,
In Transition (if you are referring to the ACI definition of it) means it is cracked for deflection calculations according to both ACI and also to logical design. The member needs to be considered cracked at all points where it is IN Transition. Ig can be used elsewhere.
RE: Why use different moment of inertia in deflection calc in ACI and PCI?