How do you design a T-beam? 3

[Ltdog]

How do you design a T-beam?
1, We adopt a rectangle section in the model and the moment of the inertia should be based on the effective flange.
2, The top reinforcements are arranged in the rectangle section or part of reinforcements in the rectangle and part of reinforcements in the T flange?

Please help me to share your experience,many thanks!

 

[Ltdog]

BA,sorry i dont draw clearly.
the number of the reinforcements is only signal and not real.
i think 20% reinforcements can be distributed in the flanges, and 80% in the rectangle.
right?

my up answer is wrong,sorry

 

[ishvaaag]

Stirrups in the flange MAY be needed if the concrete contribution to shear strength at the interface between protrusion flanges and web is not enough to meet the shear force therein present out of the placement of main rebar in such arm protrusions. Other way, some rebar in the arms can't be warranted effective in collaborating to take the negative moment action.

 

[rowingengineer]

Another point to think about for negative beam bending is column bars in option 1. You will have a large congestion around the column bars.

When in doubt, just take the next small step.

 

[rowingengineer]

attached is an extract from the IStructE, that maybe of some help.

When in doubt, just take the next small step.

 

[Tomfh]

Distributing over the full width (as the codes suggest), in my opinion, moves the bars too far from where they are required.

Why do you feel the codes are wrong on this matter?

 

[hokie66]

Tomfh,

The top bars in tension are opposing the bottom part of the web in compression. The most direct route for force transfer is best. For negative bending, there is no point in distributing the bars outside the stem unless there is congestion. A "tee" section in negative bending doesn't really know it has a flange. I don't define a rectangular beam with a slab cast monolithically over the top as a "tee beam". As long as the slab parallel to the beam is reinforced with the appropriate amount of crack control reinforcement for the exposure condition, I think that the code conditions are satisfied.

 

[asixth]

Itdog,

If you need two layers of reinforcement for negative moment over the supports you may want to look at increasing the depth of the section at this location.

rowingengineer,

Was that attachment from the IstructE detailing manual?

All,

What I was referring too earlier when I mentioned putting all the steel within the rectangular "web" was from the Australian concrete code commentary C8.8 referring to T-Beams and L-Beams:

"For flexural strength of a T-Beam or L-Beam, the concrete in the flange has no effect when the flange is in tension (negative moment regions) and has little effect when the flange is in compression (positive moment regions). On the other hand, the flange width has a significant influence on the flexural stiffness of the beam and hence the deflections".

Citing the references CEB/FIP Model Code 1978 and some other technical papers. I cannot find a similar reference in ACI318 though.

 

[rowingengineer]

asixth,
yes that was an extract from the IstructE detailing manual, I love detailing manuals.

I also don't interpret the code the same as you, I believe the commentary is referring to the concrete in the flange, not the steel in the concrete in the flange. I conclude this due to the two figures given in the commentary fig C8.1.8.1, clearly showing that the reo is preferably distributed into the flange.


When in doubt, just take the next small step.

 

[asixth]

rowingengineer,

what are you doing on this forum at this time on a Saturday afternoon?

 

[asixth]

You are probably right, I might dig out the references and see if I can read any further into it.

 

[rowingengineer]

Truth be told, I am still at work finding excuses to not do work and this one is the best.

Guess i am the GFC lucky/unlucky guy, was working in a firm of 6 engineers, when the GFC hit the boss let go (fired) 3 of them (two senior and one grad, 40 years of experience out the door). And now I am left with myself and an equivalent 3yr grad. And no senior drafting staff.

And now i don't get weekends, but i have a job.


When in doubt, just take the next small step.

 

[asixth]

Are you doing your own drafting and what program are you using?

 

[rowingengineer]

Don't do my own drafting but spend way to much time checking, they use revit (architural) for structural drawings, hence the details ahve to be redrawn each time. and make the same mistakes each time.

But back to topic, I will also do some research as i would be intrested in some test reulsts.

When in doubt, just take the next small step.

 

[Tomfh]

Here is the Australian code commentary.

 

[hokie66]

Tomfh,

I don't have any argument with that, and I think it is entirely consistent with my comments above. What I was arguing against is distribution of the flexural bars into a much wider "flange" which can sometimes be the case in accordance with 8.8.2. The section (b) just shows bars moved outside the web to facilitate concrete placement and consolidation. The cracks referred to as occurring in the flanges are due to differential shrinkage (thin sections shrink faster than thick).

 

[asixth]

I know about the trouble with using revit, we are in the process of developing procedures using revit as well.

 

[rowingengineer]

Have fun with the revit procedure; from an engineer’s point of view who doesn’t draft, it takes about twice as long to get the same result as AutoCAD. But this isn't true if the architectural are done in revit, Here things get easier. Main problems i have is with the detailing, The sections just aren’t as good.




When in doubt, just take the next small step.

 

[Tomfh]

They are referring to flexural cracking, not shrinkage cracking.

 

[ishvaaag]

I have just completed my 1st project with Revit Architecture. I didn't dare to make structural nor mechanical plans within it, with my skill would be far too slow. Revit is another fantastic tool ... hard to master (we have a number of these, by now). Even what is its maing advantage, inmediate change of everything turns a problem, some things resist to the change (again, more skills needed). And I think seems somewhat restraining for complex forms (I didn't use complex forms in this one). So I maybe turn my eyes to Autocad Architecture to see what it delivers (even if life is too short).

Those selling these things must be scratching their heads: they know for sure that everyone practicing will need a fantastic amount of time to master their tools: literally, we don't have the time. And the implementations look short of the seepdily ways we see, say, even in this forum: we need (and know!) to work far faster than that.

In what I have seem, to draw 3D complex shapes I would use Inventor for everyting complex of shape, structural and non structural. Maybe Autocad 2010 is closing the gap but not yet. And for everything else, drawings, Autocad. If I use other things is to learn and use specific tools.

 

[hokie66]

Tomfh,

They do call it flexural cracking, but they are wrong. The cracks in the flanges form first as shrinkage cracks. If the beam then cracks over the stem flexurally, the shrinkage cracks then further open flexurally. This is from many years of observing these cracks.