Tensile and Shear Strength

[MiguelPenaWSE]

Hi There!

I have some basic questions regarding Rebar Design.
#22 soft metric === #7 imperial

#22 Rebar (soft metric) - 300 MPa (Grade)
Diameter - 22.225 mm
Area - 387 mm2

I am confused on how to come up with correct values for "Shear Force" and "Tesile Force"...
This is what I am doing, please let me know if I am making a mistake, thanks.

Fy (Yield Strength)
Fu (Tensile Strength)
Fv (Shear Strength)

Fy = 300 MPa Fu = 400 MPa Fv = (1/sq.rt(3))*Fy = 173 MPa

After I apply a Safety Factor of "1.5", we get the following "Allowable Strengths"...

Fy(allow) = 200 MPa Fu(allow) = 267 MPa Fv(allow) = 116 MPa


Shear Force = Fv(allow) * Area
Shear Force = (116 MPa) * (387 mm2)
Shear Force = 44.8 kN

Tensile Strength = Fu(allow) * Area
Tensile Strength = (267 MPa) * (387 mm2)
Tensile Strength = 103.3 kN

I think these values are too lrge for a simple #22 (soft metric) rebar.

Any thoughts?
Thanks

 

[jayrod12]

I don't think the tensile strength, a 20m bar here in canada (which is what I'm assuming you're attempting to replicate) is noted as 300mm^2 in our design manuals and the Fy of said size bars is always used as 400MPa unless you specifically ask for lower so the numbers should roughly balance out.

The tensile strength I would use for said bar assuming fully developed is 102kN so basically the same number you have. If you wanted to use 300MPa I get a value of 76.5kN.

However I don't have much experience with ASD so that was done using the formulas in CSA A23.3 which I am quite familiar with.

 

[MiguelPenaWSE]

Jayrod12;
Thank you very much for your response.

I am working on a Dam Stability Analysis. The dam was built in 1969, imperial units.
The drawings show #7 bars / 44ksi.

Strength
44 ksi = 300 MPa (roughly)
Area
0.60 in^2 = 387 mm^2 (roughly)

This is why I have to use these values. I am just verifying whether or not the structure is stable.

Could you please tell me how you came up with the 76.5 kN?? Maybe show your equation if its not too much to ask.
Also, what exactly are the CSA A23.3 formulas? I don't have a Canadian Concrete Design handbook, however I do have the ACI handbook.

Thank you!

 

[jayrod12]

The 76.5 would be T=Phi*fy*as, straight forward, like I said that is using the CSA code which is based on Limit states and not on ASD. I can try digging up one of CSA codes from that era later and see what kind of value it gives.

That being said, your numbers make more sense now and the only real variable is your factor of safety. So perhaps your numbers are correct?

 

[BAretired]

Where is the dam?
Which code are you using?
How does the shear strength of a bar enter into your calculations?
Why use Working Stress Design instead of Ultimate Strength Design?
Why convert to metric as opposed to staying in Imperial?

BA

 

[MiguelPenaWSE]

The dam is located in Ontario, Canada
The strength of the bar is used as an rock anchor preventing Sliding and overturning.

Sliding and Overturning results:
The self-weight (concrete dam-spillway) is sufficient for Sunny Day Conditions but not for Winter Conditions with Ice Loading.

The dam has (18) #7 rock anchors embedded into the foundation.
I need to know how to come up with allowable Shear Force and Tension Force of the rebar to be able to use those values into my calculations for Sliding and Overturning

I need to convert all imperial values to metric for report purposes. The Conservation Authority does not allow imperial values (that is my understanding).

Thanks!

 

[graybeach]

I the bars resist sliding in tension rather than shear, i.e. via shear friction.

 

[DamsInc]

Generally, you would not consider the tensile and shear strength of the rebar at the concrete-rock contact area. You need to refer to the CDA guidelines. I hope you are working under a senior person because it is not appropriate to use the strength of the rebar unless you have confirmed its capacity through detailed inspection and testing.

 

[MiguelPenaWSE]

DamsInc,

This dam has already been built. It was built in 1969.
I am just doing a Dam Stability Analysis. I figure I could use the Tension and Shear forces from the rock anchors to help with he stability.
If the rock anchors aren't use for stability purposes, then why install them? I am just curious, as you can see, I am new at this.

Under Sunny Day Condition, the dam is stable for both Sliding and Overturning.
But once I apply Ice Loading (winter conditions), the dam is no longer stable.

Thanks!

 

[DamsInc]

I am somewhat new at this as well, however I can offer some advice.

The reason why I said you cannot consider the rebar is because the effectiveness of that rebar is unknown at this point.

Do you know the as-built conditions? Do you know for a fact that the rebar is even there or that the spacing/embedment/cover depth is sufficient? If you have detailed construction records then that may help you confirm, but that is rarely the case for older structures. I am assuming you were provided with some design drawings, but there is nothing to say how the actual structure was built. Often there are discrepancies.

The structure has undergone 45 years of exposure to a harsh environment. The concrete may have significant erosion and seepage which would affect the bond to any rebar. Corrosion has probably occurred which would reduce the cross section of the bars.

These are assumptions I am making (I don't know the information you have available), but they are realistic and safe. If you want to make unconservative assumptions in your analysis, you need to have evidence that they are representative of what is really happening. This would require a more detailed investigation which would involve testing of representative samples.

Start your analysis using conservative estimates of unknown parameters. If the preliminary analysis does not work, then you need to work with the owner to "sharpen the pencil".

For gravity structures, anchors should not be used as a primary means of stability. There are a thousand reasons why the anchors may not be effective.