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How do you design a T-beam?
3

How do you design a T-beam?

How do you design a T-beam?

(OP)
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!
 

RE: How do you design a T-beam?

There is a considerable amount of literature available about the design of T-Beams.  I am not sure what kind of help you are looking for.  

In positive moment regions, T-Beams provide a wide compression block with a high center of gravity.  This increases effective depth and reduces the amount of positive reinforcement required.

In negative moment regions, however, a T-Beam behaves pretty much the same as a rectangular section.

BA

RE: How do you design a T-beam?

(OP)
BA, thank your quickly reply.
I do as same as you said. But I want to know how arrange the top reinforcements (negative moment)
Option 1: All arrange in the top of rectangle
Option 2: parts arrange in the top of rectangle and the other parts in the two side of T flange?
I think the negative moment is resisted by the rectangle with two side of flange together, consequently I think the option 2 is right.
Do you think so?
 

RE: How do you design a T-beam?

Option 2 sounds good,
Don't want to be putting all the reo in the rectangle, because then you have trouble getting the vibrator down for the bottom steel.

Don't know your code that you are designing too, so don't know if there is any rules for effective width of the T beam, bar spacing ect. So how best to distribute these bars will have to be in accordance with your code in regards to the flange.


 

When in doubt, just take the next small step.
 

RE: How do you design a T-beam?

In addition, before I send you up the path, remember to keep enough longitudinal reo in the rectangle for the shear stirps as a minimum.

When in doubt, just take the next small step.
 

RE: How do you design a T-beam?

(OP)
Rowingengineer,
Thanks your reply.
I am designing according to ACI318, but I don't find how percent reinforcements shall be arranged in the two side flange?
I know the range of the flange from the ACI318 clause 8.10
 

RE: How do you design a T-beam?

You may even arrange it uniformly on the top width of the head, provided that the shearing force at the interface between web and flange protrusions stays in permissible range. It is easy to calculate, (assuming a negative moment bending situation) count the numbers of bars  in the protrusion to one side, multiply for area and standing stress in the bars; do that in 2 sections 1 ft apart, near support; the difference between the two is the shearing force in such foot of an arm of concrete head; for the stress, divide by the area in such ft of the interface, i.e. 1 ft x thickness at root of arm, and compare the standing shearing stress with the factored or nonfactored stress permitted for the case. If too much, you can't put that number of bars in the protrusion; or more exactly said, you must reinforce in shear the shear interface to meet the code provisions; a shear-friction calculation may be permissible for the case but since the section will be for the factored loads likely cracked in tension make sure whatever the code says about compressive struts or limitations on the allowable shear force due to extenuation of the compressive capacity of the concrete for the situation is met. It is better to assume a reasonably low permissible stress in shear at the interface, say, 1.5 MPa under factored forces, or what the code allows, and if not met, place more main rebar in the web projection, and less outside.

All what above mainly meaning that the amounts can be quite precisely calculated; you only need a section analyzer that gives you the status of the section, in your case being of interest the stresses in the steel. I did some worksheets on Mathcad that give the status of T-beam sections under positive and negative  bending action, and should still be available at mathsoft's site in the Collaboratory section for Mathcad 2000.

RE: How do you design a T-beam?

Two things to consider

1. For serviceability design, use the effective flange in the calculation for I, this will stiffen up the section for deflections.

2. For ultimate design, only use the solid rectangular section for the ultimate design, this will be critical for negative moment where you require steel in the top face, I would not distribute the tensile steel over the entire flange width of your Tee setction.  

RE: How do you design a T-beam?

Tension steel should ideally be well distributed across the full effective width. When you have a lot of bars in the flanges you should also ensure the slab has sufficient longitudinal shear capacity to develop those bars.

RE: How do you design a T-beam?

There is a clause in ACI that requires you to distribute some tension steel in the flange and not just on the rectangular portion.  

RE: How do you design a T-beam?

Take a look at clause 10.6.6 and R10.6.6 for crack control.

When in doubt, just take the next small step.
 

RE: How do you design a T-beam?

Yup, that's the one.

RE: How do you design a T-beam?

Just because i'm aussie i will also give the AS3600 referances, Cl 8.6.1 (B) and CL 8.1.8.2

When in doubt, just take the next small step.
 

RE: How do you design a T-beam?

I try to avoid using Tee beams for negative bending.  When you add a good amount of reinforcment into the flange(even just for crack control, or shrinkage or such) then you can end up "over-reinforcing" the beam based on the relative small width of the compression stem.  This then leads to potential non-ductile beam failures....  

That being said section 10.6.6 of ACI gives some specific requirements for reinforcing the flange. though I'm not sure that really helps us deal with the issue above very much.

As far as spreading the reinforcement around the stem vs. just in the Tee. I will try to put as little reinforcent in the flange as possible.... Whatever is based on Temp/shrinkage or crack control. If more reinforcement is required then I think it would be preferable to put it in the tension side of the stem.  But, it would also probably be costlier as well.   

RE: How do you design a T-beam?

I agree with Josh.  There is nothing which says that a rectangular beam with attached slab has to be considered as a tee beam.  The top bars need to be within the stem width or close thereto.  If there are too many bars to fit into the stirrups, either design with two layers or move a couple outside the stirrups.  Distributing over the full width (as the codes suggest), in my opinion, moves the bars too far from where they are required.  Now as to crack control, the "flange" will crack due to differential restraint shrinkage, and those cracks need to be controlled for serviceability reasons by added shrinkage reinforcement.

RE: How do you design a T-beam?

Why are you providing stirrups in the flange area? Your maximum shear core is going to be through the rectangular web of your beam, this is the only location where stirrups would be required.

RE: How do you design a T-beam?

I agree with KBVT - limit the stirrups to the rectangular section.

Mike McCann
MMC Engineering

RE: How do you design a T-beam?

Option 1 has 2 layers of 6 bars for a total of 12 bars.  Option 2 has 5 + 6 + 5 = 16 bars and a greater effective depth, so the options are not equivalent.

In Option 2, you could use 3 + 6 + 3 = 12 bars which would be permissible if the force in the bars in the slab can be transferred to the web by shear in the slab.  Slab bars should be fairly close to the web of the beam, say within L/20 where L is the beam span.

I don't think you should ever place more than 50% of the top bars outside the beam web.  For an L beam, this would be 25%.
 

BA

RE: How do you design a T-beam?

(OP)
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 75% in the rectangle.
right?

RE: How do you design a T-beam?

(OP)
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  

RE: How do you design a T-beam?

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.

RE: How do you design a T-beam?

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.
 

RE: How do you design a T-beam?

Quote:

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?

RE: How do you design a T-beam?

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.

RE: How do you design a T-beam?

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.



 

RE: How do you design a T-beam?

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.
 

RE: How do you design a T-beam?

rowingengineer,

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

RE: How do you design a T-beam?

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

RE: How do you design a T-beam?

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.
 

RE: How do you design a T-beam?

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

RE: How do you design a T-beam?

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.
 

RE: How do you design a T-beam?

Here is the Australian code commentary.

RE: How do you design a T-beam?

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).

RE: How do you design a T-beam?

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

RE: How do you design a T-beam?

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.
 

RE: How do you design a T-beam?

They are referring to flexural cracking, not shrinkage cracking.

 

RE: How do you design a T-beam?

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.

RE: How do you design a T-beam?

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.

RE: How do you design a T-beam?

Hokie,
You wouldn't happen to have a reference on you observation, maybe a few photo's, I am very interested this as the thought has never occurred to me.
But I also have thoughts about varying restraint in the slab, thus without really thinking it out, could the cracking be variable restraint shrinkage cracking. Hence removing some of the restraint contained in the beam to in the slab could reduce the cracking found in the slab?
 

When in doubt, just take the next small step.
 

RE: How do you design a T-beam?

ishvaaag,
I hate to take over this thread with talk about programs, my Apologies itdog and everyone else but I think you have hit the nail on the head, Program developers these days don't care about how long it will take to learn their program. That being said if you don't spend the time to learn it (especially engineering programs) it will bite you later on, this happened to me when i first started to use FEA, designed it once with the FEA then redesigned it again using normal methods because the FEA was giving me such good answers didn't know wood armer was the difference at that stage. Normally the cost of the program is half what it takes to train someone to use it, and then some.
.

When in doubt, just take the next small step.
 

RE: How do you design a T-beam?

Mxy got me the first time as well.

RE: How do you design a T-beam?

asixth,
you seem to have spent most of you saturday afternoon in and around the forum. Lost your weekends too?

When in doubt, just take the next small step.
 

RE: How do you design a T-beam?

good to see i'm not the only person that got caught with Mxy.  

When in doubt, just take the next small step.
 

RE: How do you design a T-beam?

The ACI provisions suggest distributing the negative top steel in the EFFECTIVE flange width which, I think, satisfies hokie66's point that spreading them out too far seems wrong.

There isn't a whole lot of shear lag in the flanges so this doesn't really create a huge concern - ACI's effective flange width is limited by slab thickness, beam spacing and span.  

Most engineers in the US detail as much steel in the center stirrups as they can as steel workers prefer building beam cages of rebar all together and then dropping them whole into the forms.

JoshPlum, I'm not sure I agree with your statement that you should avoid T beams in negative bending.  Most all building beams ARE T beams in that they have top flanges - the floor slab - and you should consider it.  

The negative moment rebar is limited by the width of the compression flange (the stem width) so the top, tension flange condition doesn't create situations where the beam could get over reinforced presuming the engineer checks the limit.

 

RE: How do you design a T-beam?

(OP)
Tomfh,
Will you provide the Australian code, I am much obliged to you for your help.
I don't find the content of the reinforcement details in Australian concrete structure code 3600-2001

RE: How do you design a T-beam?

(OP)
Jae,
please tell me which code suggest  distributing the negative top steel in the effective flange. I dont find the provision in the ACI318-05.
Our New York office engineer tell me American engineers usually distribute part of the steel in the effective flange and the other in the rectangle. But in china the odetails of the collective drawing adopt my upper option 1.
I think the option 1 cause the steel crowed in the top of rectangle and donot construct.
In fact,based on the finite analysis the effective flange with the rectangle resist the negative moment at the end support. But I dont ensure the how much constribute are the two side (flange and rectangle)  ?
 

RE: How do you design a T-beam?

Itdog
EC2 (EN 1992-1-1)also has that requirement;

Cl.9.2.1.2(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).

RE: How do you design a T-beam?

asixth,
Some good info you have posted, Very interesting in regards to the effect width over a support 0.15 x(span1+span2). Never had given this much thought before, always adopted the full flange width as constant across the span. This would support the idea of keeping the reo close to the web.
 

When in doubt, just take the next small step.
 

RE: How do you design a T-beam?

Comment No. 35...
For placing heavy bars outside of stirrup there's a rule of thumb: bar diameter in the slab shall be less then 1/10 of slab thickness.

RE: How do you design a T-beam?

(OP)
yakpol,
your rule of thumb isnot written in the code and I think that isn't feasible. For example we usually use dia 25mm in designing 200mm thickness flat-slab.

RE: How do you design a T-beam?

Ltdog,

Using M25 bars in a 200 flat slab is not usually good practice.  The contractors may like it, as there are fewer bars to place, but the larger bars do not control cracking as well as a larger quantity of smaller bars.  I have rarely used anything but M16 and M20 bars in a 200 slab.  Sometimes M12, but mats of bars made of M12 are not stiff enough to support big-footed concreters.

RE: How do you design a T-beam?

(OP)
hokie66, I know but thanks for you remind anyway.
If you design a underground park structure that the top slab cover the 1.5m height soil and fire truck, you will choose a feasible dia. Besides you use the small and dense bars, the concrete isn't casted and ensure the construction quality.

Good practice need to adapt the structure and to ensure the structural safety, construction feasible.

 

RE: How do you design a T-beam?

Ltdog,
1/10 diameter recomendation comes from Friz Leonghard. For the sake of the good practice it is as good as any statement in the code. Bend it as you wish, it's legal.
P.S. For undeground structure 200mm slab in a contact with earth is not a good practice either.

Yakov

RE: How do you design a T-beam?

(OP)
yakpol,
please give me a good practice for underground structure.
let me learn from your exprience.
thanks!

Ltdog

RE: How do you design a T-beam?

Ltdog,
As I understand we are talking about T-beams system serving as a roof for underground parking garage. Assuming structurally 200mm slab is sufficient to span between the beams, roof still needs to be watertight and fire-resistant. Having waterproofing membrane and passive fire-protective layer you may get away with 200mm.
Without membrane crack control under permanent loads will govern slab design in transverse direction.
Check project fire requirements. In the case of a Hydrocarbon Fire (just could be in a garage) concrete spalling will be about 40mm. It means you need to provide larger cover to reinforcing (consequently reducing effective depth and capacity. Having large permanent load (1.5m earth) bending capacity needs to be checked for reduced due to temperature E, f'c and fy.
My experience comes from public transportation projects where waterproofing and fire-resistance requirements are very high. I have not seen less than 300 roof slabs undeground. May be building code requirements are more lax...

cheers

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