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Placing of tension reinforcement in flanged cross-section above the mid support

Placing of tension reinforcement in flanged cross-section above the mid support

Placing of tension reinforcement in flanged cross-section above the mid support

(OP)
I dont agree with my code on how to place rebar in a flanged beams at intermediate supports.
The flange is in tension above the suport, so the story of effective width cannot apply....yet the code says that the top (tension) rebar can be spread across the effective width wich is the same width as the one at the mid span where the flange is in compression.
Quote:
At intermediate supports of continuous beams, the total area of tension reinforcement "As" of a flanged cross-section should be spread over the effective width of flange (see 5.3.2).

This is Eurocde.
Whats your code guidelines and your thoughts on what is said in the previous eurocode quote.

Thank you

RE: Placing of tension reinforcement in flanged cross-section above the mid support

Our NZ code only allows 10% of the reinforcement in the beam to be relied upon in the slab, unless you do a strut and tie to ensure you can utilise reinforcement placed in the slab, (for example this basically means you need the same reinforcement going orthogonal to the beam to support a strut and tie model with 45-degree struts). The code allows a slightly more favourable angle to be utilised, but you get the idea. This seems far more logical than simply stating all of the reinforcement within the effective width can be utilised. Relevant clauses below for your comparison:-

Code Clause (note 9.3.1.2 referred to gives the calculations for the effective widths in compression)


Commentary clause

https://engineervsheep.com

RE: Placing of tension reinforcement in flanged cross-section above the mid support

This figure is also referred to as well in commentary:-

https://engineervsheep.com

RE: Placing of tension reinforcement in flanged cross-section above the mid support

Quote (mar2805

I dont agree with my code on how to place rebar in a flanged beams at intermediate supports.
The flange is in tension above the suport, so the story of effective width cannot apply....yet the code says that the top (tension) rebar can be spread across the effective width wich is the same width as the one at the mid span where the flange is in compression.
Quote:
At intermediate supports of continuous beams, the total area of tension reinforcement "As" of a flanged cross-section should be spread over the effective width of flange (see 5.3.2).)


Dear mar2805

I have copy and pasted the full paragraph below;

(2) At intermediate supports of continuous beams, the total area of tension reinforcement As of a flanged cross-section should be spread over the effective width of flange (see 5.3.2). Part of it may be concentrated over the web width (See Figure 9.1).
(3) Any compression longitudinal reinforcement (diameter φ) which is included in the resistance calculation should be held by transverse reinforcement with spacing not greater than 15φ.



The relevant Figure 9.1 ,



You do not agree with the code on to place rebar in a flanged beams at intermediate supports .But you agree with the same at midspan .. ( at midspan, tension reinf. is below the NA and at assumed cracked zone , still effective )..

There is nothing wrong with the code .

But , if the question is for seismic zone, the same rebar may subject to compression. so, all the reinf. shall be confined and placed in the web . The code also clarifies this case at item (3).

Good LUCK..


RE: Placing of tension reinforcement in flanged cross-section above the mid support

You should read the section you are quoting!

5.3.2 of Eurocode gives different effective widths in support zones compared to span zones. It is all based on the distance between points of zero moment, Lo.

By default, in positive moment regions that is taken as .7L, in negative moment regions it is .3L.

So the effective flange width in support regions is less than half of that in span regions.

RE: Placing of tension reinforcement in flanged cross-section above the mid support

My local code seems to handle this less rigorously than others. The sketch shown below is what I've done myself in the past. In essence, it's a poor man's version of the strut and tie approach that Agent666 recommended. Even at that, I try not to get too carried away with the effective width and try to keep it within 25% of the beam span. That's usually pretty easy to do, at least in new construction.

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