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ACI minimum reinforcement at zero moment

ACI minimum reinforcement at zero moment

(OP)
Hi all,
I was designing a beam using RSA when w faced a huge minimum area of reinforcement at zero moment (attached screen shot)
I didn't find any clue in ACI mentioning this huge increase of As,min .
the beam dimension is 90x30 cm. Can someone clarify me why Im getting this issue?

RE: ACI minimum reinforcement at zero moment

as per ACI318-08 item#10.5.3 u don't need to apply the minimum reinforcement if u put at least one third greater than that required by analysis

RE: ACI minimum reinforcement at zero moment

(OP)
thank you for your reply,
section 10.5.3 is under minimum reinforcement of flexural member. In my case, due to axial load and zero moment, the minimum area of reinforcement is taken the same as the column (1% x section) .
my question is:
1-this section can be used in my case?
2-I am using RSA for my analysis, (http://forums.autodesk.com/t5/robot-structural-ana...)
kindly check the forum and especially the sentence "If you consider the axial force as small enough to be neglected just exclude it from the analysis and run reinforcement design for pure bending. "
what part of ACI-318 Talks about this statement?

I really appreciate your help because no one gave me a clear answer about this issue:s

RE: ACI minimum reinforcement at zero moment

sorry i don't use RSA in the analysis but , the important question how didn't u find any straining action on your element , if you don't have any vertical load the self weight will be enough to create moment , also Asmin. as per ACI318 item 10.5 will be 30 x 85 x .003323 =8.97 cm2 (4T18) it’s not huge bars

RE: ACI minimum reinforcement at zero moment

(OP)
RSA gave me a minimum area of reinforcement 21.68 (1% of section) (check attachment). when I asked about it in RSA forum the response was :
"It is due to applying Amin provisions of ACI for elements under pure tension (it is in the location where bending moment is zero and tension force exists). If you consider the axial force as small enough to be neglected just exclude it from the analysis and run reinforcement design for pure bending. "

My main question is there any ACI provision that we can use to neglect axial load?
ACI318-08 item#10.5.3 can be used in this case?

note: load applied (Self weight -Z direction , LL +Z direction)

RE: ACI minimum reinforcement at zero moment

This sounds like a software defect. There will NOT be zero flexure, despite a theoretical computation. The defining issue is whether the member (not the section) acts primarily as an axial member or a flexural member. Also, once redistribution is applied, the "zero moment" point shifts, removing any true point of zero moment.
ACI 318-14, in reference to the effects of combined axial and moment, says:

Quote:

R9.5.2.2 Beams resisting significant axial forces require consideration of the combined effects of axial forces and moments. These beams are not required to satisfy the provi- sions of Chapter 10, but are required to satisfy the additional requirements for ties or spirals defined in Table 22.4.2.1. For slender beams with significant axial loads, consider- ation should be given to slenderness effects as required for columns in 6.2.6.
I would suggest that there would be need for reinforcement top and bottom in sections subject to significant axial.

RE: ACI minimum reinforcement at zero moment

(OP)
Thank you TXSstructural,

the member resist only the loads mentioned before with a one end fixed and other pinned.
as for ACI318-14 provisions the Word "Significant" is defined by who? by what formula? what is the limit of axial load in order to consider it as significant or not?

RE: ACI minimum reinforcement at zero moment

The provision shown is commentary to go with the code provision 9.5.2.2 which refers to 22.4 (Axial strength or combined flexural and axial strength.) I think the idea is that assessment is required in all cases, and "significant" means that the axial load changes the stresses such that the basic moment computation is no longer conservative.
Obviously, "significant" would not be used in the code, only the commentary.

RE: ACI minimum reinforcement at zero moment

(OP)
What I understand from your statement above that its all about engineering judgment to neglect axial load or not.

RE: ACI minimum reinforcement at zero moment

Yes, but numerically you could calculate the combined effects for each section and each load case to confirm your judgement. This would be inefficient for members with light axial loads unless the software does it for you (which would be trivial and necessary for good software.) If the axial load is tension, you would need to examine reinforcement stresses and shear and torsion (since shear cracking is principally a tension phenomenon.) If the axial is compression, you would mostly be evaluating the added stresses when moment results in maximal concrete compression.

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