Bottom longitudinal reinforcement at supports for simply supp. beams

[almait]

Hi to every one.
I'm an italian engineer.
I'm studying the ACI 318 Code and at the moment I'm interested to shear desgin. Reading the code I didn't find any provision about bottom longitudinal reinforcement to place at supports and capable to withstand a tensile force equal to shear value V.
I saw some shop drawings of precast beam (Reverse T and double tee) and there's no similar reinforcement ...
Can someone explain me why the ACI Code doesn't prescribe this reinforcement at supports? Is it a difference of theories used to study the shear failure? It seems that this kind of reinforcement is needed for deep beams and not for "normal beams".
Thanks to every one wants to help me to understand better.
Sorry for my english.
Best regards

 

[Lion06]

The shear in concrete beams is resisted by vertical stirrups, not by longitudinal reinforcement. The idea is that shear in concrete beams will cause diagonal tensile failure of the concrete and these vertical stirrups will cross those diagonal sections and provide the necessary reinforcement.

 

[apsix]

Many (most?) codes have a requirement for the anchorage of positive moment reinforcement at simple supports to ensure adequate shear capacity at the support. I believe that is what alemanda is asking about.
I don't use ACI318 and therefore can't comment definitively, but I would be suprised if it does not have support anchorage requirements.

 

[JAE]

ACI 318 does prescribe requirements for bottom bars extending into the supports. If you read through chapter 12, there is a traditional requiremnt that bottom bars extend at least 6 inches into the spports.

There are also requirements for bottom bars to extend further and either be hooked or lap spliced. These are in Chapter 7 under "Structural Integrity".

There is also another equation for bottom bars in simply supported beams. I am away from my desk - but I'll look it up on Monday. It has, I think, an M/V ratio in it but I'm not sure.

 

[hokie66]

alemanda,

I haven't used ACI318 in a number of years, but ever since ACI318-673, there have been some provisions for anchorage of positive moment reinforcement at simple supports. Am sure JAE will point you in the right direction.

The Australian Code AS3600 requires development of a tensile force of 1.5 design shear force at face of support, where the shear force is taken at distance d from support. There are a couple of other provisions, but you are correct that the extension of the bottom bars past the potential diagonal crack is required.

Prestressed beams are more complex, as the prestressing forces themselves change the shear stress and also increase the shear load capacity.

 

[UcfSE]

The requirement that JAE mentions using Mn/Vu is in chapter 12 under development of positive reinforcement.

One thing to remember about the ACI is that shear load within a distance of d from the support is assumed to transfer in compression into the support, not through shear.

The difference you note is probably due to different approaches. I would expect that if the ACI does not directly address what you are asking then it is probably covered elsewhere, perhaps in the positive reinforcement development mentioned above.

And don't worry about your English

 

[almait]

Hi to every one,
thanks for your replies and contributes.
I read teh chapter 7 and chapter 12 of ACI code, but I didn't found the provision about the minimum area of bottom reinforcement to place at the supports for a simply supported beam ... better, there's a minimum area but it is very different than that one prescribed by EC2. In fact EC2 prescribes that at the supports an area of bottom reinforcement equal to Vu/fy (Vu is the design value of shear and fyd is the design value of the steel tensile strength). Example: simply supported beam; span = 10.00m, depth of beam = 70cm; DL (including self weight) = 5000 daN/m, LL = 5000 daN/m. The design value of shear at support is Vu=(1.2*DL+1.6*LL)*10/2=140000/2=70000 daN. According to EC2 I have to place at least 70000/3826 = 18.29 cm2 of well anchored reinforcement. How many cm2 of reinforcemen should be place according to ACI 318 code?

About theories ... Yes, I agree with Ucfse ... maybe the shear strength is derived from different theoretical approaches. I've read about Morsch strut and tie model (EC2) and MCFT (modified compression field theory) ...

Regards
Alessandro Mandalà

 

[kslee1000]

In practice, the longitudinal main (positive and negative) reinforcement shall be cut-off at a distance (development length by code) beyond the point at which the bars are no longer required according to the moment curveture, and the number of the remaining bars shall be no less than two. These longitudinatal bars can be counted when designing the shear reinforcement (stirrups), however, the effect is usually small, and thus ignored.

 

[hokie66]

Almait,

Previous posts by StructuralEIT and kslee1000 seem to discount your concern. If they mean the anchorage of bottom bars at simple supports is unimportant, I beg to differ. Sorry I can't give you guidance in studying the ACI Code, but the provisions must be there somewhere, probably with the requirements for flexural reinforcement rather than the shear (diagonal tension) provisions.

 

[JAE]

Here are the relevent sections from the ACI code (318-02) that address bottom bars extending into supports:

7.13 Requirements for Structural Integrity
Specifically sections 7.13.2.1 through 7.13.2.4

12.10.3 This section indicates that reinforcing should extend beyond the point where it is no longer required to resist flexure - by an amount of 12 x bar dia. or d.

12.11.1 Development of Positive Moment Reinforcement
Requires 1/3 of As bottom bars of simple spans to extend at least 6 inches into the support for beams (1/4 of As in continuous members).

12.11.2 When the beam is part of a lateral load resisting system then this section requires further anchorage due to potential positive bending at the support.

12.11.3 This is the section that has the m/v ratio. It requires a limitation on bar diameter to meet the equation (12-3).

 

[almait]

Thanks to each contribute.
Back to my example but considering the length of the beam equal to 5.00 m.
According to EC2 As at supports is equal to 9.14cm2
According to ACI 318-02 a flexural reinforcement at midspan is required and it's equal to about 27.85cm2. One third of this area is equal to 9.28cm2. Similar results. Ok.

Now my question is.
Are the provisions you quoted referred also to prestressed members? In particular the hollow core slabs have not longitudinal ordinary reinforcement and they have not shear reinforcement.
Does a bottom reinforcement have to be placed at supports by means of a further grout into holes made at the extremities of the slabs?
Thanks
Regards

 

[UcfSE]

Typically with hollow core slabs, additional bars are grouted either into the cores or in the key ways. Where a bar is to be placed into a grouted core, the top part of that core can be field-cut to ease placement of the bar. The bar may be cast into the support, or placed after the slab by welding to a bearing angle or being expoxy-grouted into the support. Cast bars are usually not a good idea since they interfere with the placement of the slabs.

 

[almait]

OK. Also in Italy and Europe it is a common practice to grout additional bars into the cores or in the key ways.
How can I size the bars to be grouted into the cores at the extremities of the slab. How many cm2 of reinforcement should be placed according to ACI code? Where Can I find some provisions for this kind of reinforcement?
Thanks
Regards

 

[kslee1000]

Hokie66:

Please read the post carefully and draw yourself a simple moment diagram, define the location in which bars are no longer required, then develop the bars from there and make sure there are at least two bars. Can you see the final configuration of the reinforment on your sketch?

 

[UcfSE]

The Portland Cement Associations (PCA) publishes a book with some good information about design using hollow core slabs.

PCA hollow core book

 

[NS4U]

Correct my if I'm wrong but is this what almait is getting at....

ACI accounts for a component Vc, which is the shear capacity of concrete. Using Mohr's circle and rotating this "pure shear" 2D-element by 45 degrees, that element is represented by tension on one side and a compression on the other.

Looking at the tension... Since it's pointing 45 degrees it has a vertical (shear) component and a horizontal (longitudinal) component. Hence why you need both stirrups AND longitudinal bars. This is covered more extensively in AASHTO as Modified Compression Field Theory. Where AASHTO does require that you determine the amount of stress in the horizontal direction and design longitudinal bars accordingly.

I have always assumed that the ACI value 2*sqrt(f'c)*b*d is just so conservative that longitudinal bars are not needed. Where as with the modified compression field theory you could get values of 3*sqrt(f'c)*b*d or 3.5*sqrt(f'c)...

I hope that makes some iota of sense, I really need a white board to explain what I am thinking. I have always wondered why I never did that longitudinal bar check ACI but I had to in AASHTO, maybe someone can offer me a better answer.

 

[kslee1000]

If I remember correctly, from strength of material, the maximum shear of a rectangular section is in the mid section, then approches zero towards the free surfaces (top & Bottom). At the level of bottom reinforcement, the tension theoretically is very small, however, it is good PRACTICE to extend bars into support to account for other complications.

 

[hokie66]

Can't leave this one alone. Inclined cracking is not likely to occur at service loads. It is due to overload. Therefore, the shear reinforcement, including the extended bottom bars, only work after cracking to provide safety and ensure ductility. This is why most codes, if not the ACI, are clear in requirements for extension of the bottom bars. Anyone have a friend on the ACI Committe who could shed some light on this issue? I noticed years ago when I first came to Australia that it was standard practice here to not only to extend bars into supports, but cog them (turn up). This was then new to me, as it apparently is to some of you in the good old USA.

 

[JAE]

If you check chapter 7 of the ACI code it requires some percentage of bottom bars to be either hooked or lapped with adjacent beam bottom bars.

In a moment frame, many times you have to do this anyway as you get positive moments occurring at the face of the supporting columns.

 

[Tomfh]

mikehughes,

That's how I look at it. The load struts down to the support and the longitudinal bars ensure the horizontal component is rectified. If you assume 45 degree strut then the tension force equals V.