minimum reinforcement for column 1

[Phatt]

I'm going to design a 3.5' height pedestal per ACI318-11.
It has to support very small axial load.(5 kip axial, 2 kip shear, and 50 kip-ft moment)
I'm curious about minimum longitudinal reinforcement.

1. I have read ACI318-14, and it doesn't tell us to check the minimum flexure reinforcement for the column.
it only shows 1-8%Ag limits for reinforcement.
Do I have to follow ACI318-11 10.5.1, 10.5.2, and 10.5.3 parts for this column?

Or I can only check the 10.9 part?

2. If I want to apply with ACI318-11 10.5.1, 10.5.2 and 10.5.3 minimum reinforcement, how should I arrange the longitudinal reinforcement?
Assume that the minimum reinforcement for X-axis is 6#6 and 4#6 for Y-axis. Can I count the same rebar for another axis?
Is the reinforcement in the left column correct?

 

[KootK]

I probably wouldn't sweat the minimum flexural reinforcement for this. The purpose of that is to telegraph warning to folks when the structure is under distress. In this situation:

1) I doubt that there will be anyone "occupying" the area such that they'd notice the distress in the pedestal and;

2) Unless I miss my mark, this will mostly be a base plate anchorage problem. As such, you're not likely to see much of that sexy, plane sections remaining plane, Bernoulli flexure in this thing anyhow.

 

[JoshPlumSE]

I'm going to design a 3.5' height pedestal per ACI318-11.
It has to support very small axial load.(5 kip axial, 2 kip shear, and 50 kip-ft moment)
I'm curious about minimum longitudinal reinforcement.

Usually, we look at pedestals as having a 0.5% reinforcement requirement, not the 1%. That's related to the following two provisions (based on my 2014 version of ACI 318):
10.3.1.2: For columns with cross sections larger than required by considerations of loading, it shall be permitted to base gross area considered, required reinforcement and design strength on a reduced effective area not less than one-half the total area.

There was another one specifically related to pedestals saying that 50% of the reinforcement from the column above should continue into the pedestal (or something like that), but I can't find it today. Maybe I'm thinking of an old code provision.

Also, the flexural reinforcement minimum sort of got out the window when you have a member (like a column or pedestal) that is primarily a compression member. I believe we have the minimum flexural reinforcement (like KootK said) to insure that the beam's reinforced capacity is larger than it's uncracked capacity. Meaning that it won't be subjected to a high load in an uncracked state, start to crack and then immediately collapse.

For compression members, we're expecting compression failure not the preferred tensile failure we want in beams. So, ACI accounts for this by merely decreasing the Phi factor based on the bar strain at failure. That's why columns / compression members have very different reinforcement requirements.

 

[lexpatrie]

I'm not convinced this is a column, though, it's a pier/pedestal as you noted. That's where your 1/2% reinforcement is, I believe.

Regards,
Brian

 

[WesternJeb]

Note that these reductions don't apply in SDC D and higher.