P-M interaction diagram of Concrete section

[YunJW]

Hello all,

I have some doubt a P-M interaction diagram of concrete section.
I got a result as attached figure.
I think "It's OK", however any others told "It's NG".

I think it is OK, because Mr > phi Mn.
Any others told design force is not placed inside P-M line, because it's NG.

I want to hear your opinions.
Thank you!

Section properties (Uni-axial)
H = 800 mm
B = 1000 mm
f'c = 33.2 MPa
fy = 400.0 MPa
As'1 = As'2 = As = 3216.8 mm2
ds'1 = 116.4 mm
ds'2 = 154.4 mm
ds = 683.6 mm
Pu = -3089 kN (tension)
Mu = -91 kN.m (tension of top surface)

 

[hokie66]

I've never used an interaction diagram for a tension member. The tension has to be taken solely by the reinforcement, and it doesn't look like you have enough.

What type member is this in your structure?

 

[YunJW]

I have been performed a analysis of well foundations by shell elements.
And it is a result from vertical wall of well foundations.
Thus, sections are subjected high tensile force locally.

 

[IDS]

Area x yield stress x 0.9 = 2316 kN, which is much less than your applied tensile load, so clearly the section is inadequate for the loading.

What you were told is correct; the applied force and moment combination must be inside the interaction diagram line.

Also the interaction diagram seems to be wrong, as it shows a maximum tensile capacity well over 3000 kN.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[IDS]

Just re-read the data and I see I missed the bottom steel (As). With that added the tensile capacity is greater than the applied load, so the interaction diagram looks right, but you are still outside the interaction diagram envelope, so you need more bottom steel.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

As'1=As'2=As'=3216? So how much total steel? The nomenclature is unclear to me.

 

[YunJW]

Thanks IDS, I got your point.

Dear, hokie66
The reinforcing steel is consist of total 3 layers.
1st(As'1) and 2nd(As'2) is top layer and each area is 3216,
3rd(As) is bottom layer and area is 3216.
Thus, total reinforcing steel area is 3216*3 = 9648mm2.
Thanks for your attention.