Bottom longitudinal reinforcement at supports for simply supp. beams
Bottom longitudinal reinforcement at supports for simply supp. beams
(OP)
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
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






RE: Bottom longitudinal reinforcement at supports for simply supp. beams
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
I don't use ACI318 and therefore can't comment definitively, but I would be suprised if it does not have support anchorage requirements.
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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.
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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.
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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.
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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).
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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?
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
PCA hollow core book
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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.
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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.
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
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.
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
In calculations follow through the theories, the positive bars may not necessary to get into the support. However, since the design assumptions are flawed (Hook's law, homogeneous material), and the properties and behaviors are difficult to master even in the lab, not to mention in the real world. To cover these shortcomings, the ACI does have requirement for ectending the positivement as pointed out by JAE and others. I think this is holding true for all of the codes around the world, but just arranged and mentioned in different manners. As a final reminder, a few positive reinforcement shall be extended into the support, but not to make the interface too rigid thus losing ductility.
RE: Bottom longitudinal reinforcement at supports for simply supp. beams
If you're trying to visualize the effect of the bottom longitudinal steel in shear keep in mind that almost all shear theories, especially ACI, use some sort of truss analogy. The bottom long. steel is the bottom chord and is vital for the stability of the system after cracking. ACI's shear provisions, which are now long-in-the tooth, doesn't explicitly check the strength of this "bottom chord" though newer codes do. Under ACI the design of the bottom steel is generally covered off by minimum steel and embedment requirements.
If the bottom long. steel is understrength then the axial strain, say mid-height, can increase, reducing the shear strength of the concrete through reduced interlock, etc.
There is now interest in the US, including ACI, in looking at the long. steel since it's recognized that increasing the long. steel, or even just accounting for it, can reduce the transverse steel requirements. They work together. It's also more important whether there isn't any transverse steel (i.e. hollowcore).
My two cents (three cents Canadian and rising)