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!

 

[rowingengineer]

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.

 

[rowingengineer]

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.

 

[asixth]

Mxy got me the first time as well.

 

[rowingengineer]

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.

 

[rowingengineer]

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

When in doubt, just take the next small step.

 

[JAE]

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.

 

[Ltdog]

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

 

[Ltdog]

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) ?

 

[apsix]

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).

 

[asixth]

Had a look through Eurocode 2, it is suggesting that the effective flange be taken as twice the width of the web. I agree with this procedure when designing deep narrow beams however I would not use it for designing wide flat beams.

 

[apsix]

asixth
The rules for effective flange width are more complicated in the 2004 version of EC2, see attached.

BTW, ENV.... means the code is post-draft but has not yet been issued as final, compared to the complete version EN....

 

[rowingengineer]

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.

 

[yakpol]

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.

 

[Ltdog]

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.

 

[hokie66]

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.

 

[Ltdog]

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.

 

[yakpol]

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

 

[Ltdog]

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

Ltdog

 

[yakpol]

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