Bundled Bars in Risa

[jayrod12]

How does Risa deal with reinforcement?

I know that I need to bundle my bars to make the spacing work (5 bundles of 2 as opposed to 10 individuals).

My question is when I am inputting the reinforcing into RISA does it check the spacing and cover or does it just assume that whatever reinforcing I specify in a layer will fit?

Thanks,

 

[jayrod12]

And another thing, RISA is telling me that I have too much reinforcement for a single reinforced beam, but mine are doubly reinforced, am I missing a check box somewhere to tell it that it's doubly reinforced?

 

[JoshPlumSE]

Are you using a beam or column because RISA treats them differently.

Are you letting RISA choose the number of bars for you, or are you using a "custom rebar layout" to specifically tell the program where to put the bars? In your case, I believe you should use a custom layout because that will allow you to bundle bars and will account for your doubly reinforced section.

If you let RISA layout the bars, the program will always assume a capacity based on singly reinforced sections... even when there are bars on both ends. It's more conservative that way and much easier to create the rebar placement / optimization logic.

 

[jayrod12]

I knew it would be you responding Josh,

Thanks for responding,

I have used the beams and column label appropriately, and custom rebar layouts. and I am still getting those errors for all of my concrete beams.

I have entered the bottom bars as measured from the bottom of the beam and the top bars as measured from the top of the beam in all cases. Same with the column rebar. I left the shear reinforcing as optimized as I am currently only concerned with the flexural side of things right now.

 

[JoshPlumSE]

If its a small model, I'd post it (or send it into tech support). My guess is that you've created a custom rebar layout, but that you haven't actually assigned it to your member. If you look at the member's double click dialog, then on the design tab you want to see what the Flexural bar layout is listed as.

The custom rebar layout does not pass through the rebar optimization routines. So, any errors or messages that come from that routine would not get automatically triggered. I believe we have added some of this in as specific checks on the custom rebar code. But, they would be more like warnings than errors. They should not prevent RISA from calculating code checks or displaying capacity calculations and such.

 

[jayrod12]

If you've got the time to quickly check go ahead.

I think I've got them assigned correctly.

 

[jayrod12]

and upon closer inspection you are correct. They are warnings not errors. So I guess it's not an issue afterall.

 

[JoshPlumSE]

I didn't realize we were talking about the Canadian Code....

My guess is the CSA codes have something akin to the old "0.75*rho_balanced" limit from ACI which is also meant to address the overly reinforced concrete members which might be subject to brittle failure. One of the main problems with that limit is that it was based on a singly reinforced section and could not be easily extrapolated into multiple bar layers. Since non-ductile failures can be problematic we wanted to test this even for multiple bar layers, but wanted to let the users know that this section of the code was not fully applicable to their situation.

To test this out I ran your model using the ACI codes (with which I am more familiar).

In the old ACI code (1999 which still had the same type of 0.75*rho balanced limit), I get the exact same warning messages. When I run it in the latest version of ACI, I get messages about the tensile strain at capacity being less than 0.004 per ACI 10.3.5. I'm much more familiar with this other error message. Essentially it is saying that you have an overly reinforced beam which may be subject to brittle failure. Thankfully, this new ACI method (based on maximum bar strain in the outer bars) is applicable in concept to any reinforced section, not just singly reinforced beams. Therefore, the program was right to flag these members in this way.

What you do now is less clear to me. It may mean that you need to reinforce it more like a column with more closely spaced ties and such. It may mean that you want to increase member depth. Maybe you decrease the reinforcing.

 

[jayrod12]

Thanks for taking the time to look at it.

Unfortunately increasing member depth is not really an option (I think), Decreasing reinforcing will probably not work either as the spans are set. Unless I can finesse the loads down further to reduce the applied loading.

My hand calculations result in the outer layers yielding but the inner layers in general do not for all of the members.

This models dimensions are accurate, same with beam sizes. The loading was run fairly conservatively. The reinforcing that was entered has been specified on previous jobs that had been designed fully by hand making some simplifying assumptions (mainly pinned connections). I felt that the connections of this much reinforcing will cause almost ideal fixed conditions.

With fixed conditions everywhere I was seeing if we could reduce the reinforcing by taking some of the moment down into the foundation through the columns instead of having to resolve it all in the beams. That does not appear to be the case.

I am also looking at attempting to model it as partially fixed so I can find a happy medium between positive and negative moments governing the design.