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N.A. Location, doubly reinforced beam
3

N.A. Location, doubly reinforced beam

N.A. Location, doubly reinforced beam

(OP)
Any body has any step by step guide for it. I know the formula.

RE: N.A. Location, doubly reinforced beam

2
I made a spreadsheet for this.  You have to assume a NA location and it checks out if T=C.  If it doesn't, you adjust your NA location until it does.

RE: N.A. Location, doubly reinforced beam

You have to make a spreadsheet for this or you'll drive yourself crazy going through the iterations.  Are you just trying to get the moment capacity of a doubly reinforced beam?  I will typically use PCACol (but you can use any concrete column software that gives you an interaction diagram) and just pick off the pure moment value.

RE: N.A. Location, doubly reinforced beam

(OP)
I need this to find Icr. U mind uploading your spedad sheet if it is in excel. MathCad I do not know.

RE: N.A. Location, doubly reinforced beam

I'll do you one better.  Here is a spreadsheet that will find Icr and Ieff directly.  I developed this for input into RAM Advanse for the Ie/Ig factor.

I developed it for T-beams, but if you have a rectangular beam, just use b=0.  The top part of the spreadsheet is for a quick strength check.  The bottom is what you're interested in.  Only input the boxes shaded yellow.

RE: N.A. Location, doubly reinforced beam

(OP)
file has an extension xlsx.
Cannot open it MS excel 97

RE: N.A. Location, doubly reinforced beam

Iterations can be very simple when using goal seek to solve for T=C. I would ignore the extra capacity that you get for beams reinforced near the compression face. Basically all that  steel is going to do is raise your neutral axis but I would not include it in the design.

RE: N.A. Location, doubly reinforced beam

Your depth of compression has to be below the top layer of reinforcement or the top steel does nothing for moment capacity.

RE: N.A. Location, doubly reinforced beam

Same basic procedure as a concrete column in bending, effectively having both tension and compressive steel.

Mike McCann
MMC Engineering

RE: N.A. Location, doubly reinforced beam

Structaco-

It has to be a fairly good distance above the NA to do anything for the moment capacity, not just above it.  More importantly, however, is it's role in deflections, specifically long-term deflections.  I've rarely had a situation where I needed compression steel for strength, but often need it for deflections.

RE: N.A. Location, doubly reinforced beam

StructuralEIT

How does providing compression steel reduce the long-term deflections? Apologies for asking questions within someone else's post

RE: N.A. Location, doubly reinforced beam

It does two things.  First it provides a higher Ieff.  This is benficial for immediate deflections as well as the long term, since the long term is a multiple of the immediate deflections.  The real benefit, however, is that the presence of compression steel directly impacts the magnitude of what that long-term multiplier is.  If there is no compression steel, then the multiplier is 3 (for long term loads).  If you have 1% compression steel, the multiplier is 2.33 (for long term loads) - this essentially knocks down the long term deflections by 22%.  That's pretty significant in my book.

RE: N.A. Location, doubly reinforced beam

Additionally, I just want to point out that there is a difference (even if it's only a small one) between the NA at ultimate strength and the NA at service loads.

RE: N.A. Location, doubly reinforced beam

asixth
In the context of AS3600 it reduces kcs, the load multiplier for long term deflection.

RE: N.A. Location, doubly reinforced beam

asixth,

SEIT and apsix have given you code based answers.  Compression reinforcement in practice makes a concrete beam act more like a steel beam.  So it removes some of the creep deflection.  Steel beams don't creep.

RE: N.A. Location, doubly reinforced beam

(OP)
"Additionally, I just want to point out that there is a difference (even if it's only a small one) between the NA at ultimate strength and the NA at service loads"

I believe in ultimate strength it will shift more up?

RE: N.A. Location, doubly reinforced beam

dgkhan-

You are correct.  So bars that may not be compression bars for strength considerations could conceivably be compression bars for serviceability considerations.  I'm not sure I would want to cut the gnat's behind that close on a regular basis, but I thought I would mention it because I was faced with this about 1.5 years ago.

RE: N.A. Location, doubly reinforced beam

Hokie66... only if they're designed correctly <G> or at elevated temperatures... (I couldn't resist it).

Dik

RE: N.A. Location, doubly reinforced beam

That is a very comprehensive spreadsheet, may I ask who the author is, consultant or academic?

RE: N.A. Location, doubly reinforced beam

Consultant.  

You can access almost all of his authored spreadsheets by doing a google search on him.  He's got a bunch.  The websites might not have his latest revisions to the spreadsheets though.  This one in particular, is the latest and greatest of this program.  I don't know how often he updates "Alex's Corner" but we (the company I work for) get his revisions as soon as he sends them.  His spreadsheet index with most of the current revisions is attached.  We got a couple this week and I don't think he's updated the MS Word document yet.

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