Concrete edge beams
Concrete edge beams
(OP)
During a luncheon at work, we had several interesting discussions that I thought I would share with you folks:
One of my colleagues said that a concrete edge beam does NOT need to be designed for torsion if you pin the edges and design the secondary beams and the slab for the moment. However, another colleague stated that the above is simply incorrect and that the edge beam must always be designed for torsion, but did not state why. A third colleague said that it all depends on the rebar detailing? As you can imagine, I was left more confused than when the conversation started. I'm curious as to which side everyone here would take in this debate.
The discussion then jumped on to steel. If we have a secondary beam spanning perpendicularly into the primary beam, with the primary beam supported by a column on each of its end, and the connection between the primary and secondary beam is a clip angle, would we design the primary beam for torsion? The point that was raised is that the double clip angle can experience significant rotation/deformation as long as it is not too thick, and thus the end of the secondary beam can be treated as a pin without any significant torsion on the primary beam. Does that make sense to anyone? Because I don't know if I buy it. What about the stiffness of the primary beam, does that play any role?
Ah what the heck, since I'm posting a thread, I might as well ask another question that's been bothering me: I have a reinforced concrete slab (150 mm thick or 6") with a whole bunch of openings. I am worried that my slab will not act as a rigid diaphragm because of all the openings. How can I calculate and prove that my slab can act as a rigid diaphragm? Also, I was told that my slab has to match the rigidity/strength of my shear wall for it to be effective. What is the rational behind that? I do not understand that statement and have never heard it before until recently. How about you guys?
Clansman
One of my colleagues said that a concrete edge beam does NOT need to be designed for torsion if you pin the edges and design the secondary beams and the slab for the moment. However, another colleague stated that the above is simply incorrect and that the edge beam must always be designed for torsion, but did not state why. A third colleague said that it all depends on the rebar detailing? As you can imagine, I was left more confused than when the conversation started. I'm curious as to which side everyone here would take in this debate.
The discussion then jumped on to steel. If we have a secondary beam spanning perpendicularly into the primary beam, with the primary beam supported by a column on each of its end, and the connection between the primary and secondary beam is a clip angle, would we design the primary beam for torsion? The point that was raised is that the double clip angle can experience significant rotation/deformation as long as it is not too thick, and thus the end of the secondary beam can be treated as a pin without any significant torsion on the primary beam. Does that make sense to anyone? Because I don't know if I buy it. What about the stiffness of the primary beam, does that play any role?
Ah what the heck, since I'm posting a thread, I might as well ask another question that's been bothering me: I have a reinforced concrete slab (150 mm thick or 6") with a whole bunch of openings. I am worried that my slab will not act as a rigid diaphragm because of all the openings. How can I calculate and prove that my slab can act as a rigid diaphragm? Also, I was told that my slab has to match the rigidity/strength of my shear wall for it to be effective. What is the rational behind that? I do not understand that statement and have never heard it before until recently. How about you guys?
Clansman






RE: Concrete edge beams
CONCRETE:
I use ACI 318 (you didn't state your own location/code) and within that code the ACI explains that if you have secondary beams framing into a primary edge beam, as long as you design the secondary beams for pinned ends, you can then design the primary beam for a minimum torsion loading instead of a full analysis which would assume fixed ends and calculated torsion.
They even have a couple of 3D sketch-views of two structures showing the difference between a structure which doesn't need the torsional resistance for stability and one that does need the torsional resistance for stability.
For a typical exterior bay of a building, where the interior joists or beams are designed with assumed pinned exterior ends, then the exterior beam's torsional resistance is not theoretically needed for the structural stability of the floor.
For a beam that has a single cantilevered slab hanging off the edge of it, the torsional strength of that beam is essential for the cantilevered slab to remain cantilevered...thus for that sort of case you must include the torsional aspects of the design.
For my own personal "standard" I almost always will close any type of edge beam stirrups with torsional stirrups instead of open "U" stirrups...just because.
STEEL
Similar condition except that normally your primary beam end connections are not designed to resist torsion - therefore you would get a slight natural twist in the primary beam that is normally neglected. There is also some minor flexural stiffness in a concrete floor slab that helps minimize this twist as well.
You mentioned "what about the stiffness of the primary beam, does that play any role". If you are talking about torsional stiffness, remember that WF shapes have very little torsional stiffness at all.
I'll leave the diaphragm questions for others.
RE: Concrete edge beams
RE: Concrete edge beams
RE: Concrete edge beams
Compatability torsion will have redundancy to some degree, and if torsion cracking occurs and a reduction in torsional stiffness, stress will be eleviated and alternate load paths used.
All rectangular concrete cross-sections have torsional stiffness to some degree and will attract torsion, deep narrow beams more so the wide flat beams.
As for the steel, the secondary beams spanning onto the edge beam will be stiffen the edge beam if the connection does not allow rotation. However, the edge beam will not take torsion, it will rotate compatibily will the flexural rotations of the secondary beams because of it's lack of torsional stiffness.
As for the 150mm slab, I don't see why it won't work as a rigid diaphram.
What is a WF beam. I'm from Australia where our I-beams are designated as UB or Universal Beams.
RE: Concrete edge beams
Regarding the steel - I agree with the point that was raised. All simple shear connections are designed to accommodate the deformations required to behave as a pin (thus not transferring any torsion to your primary beam).
Regarding the slab - one way to tell is to span it between two walls on each end. Double the stiffness of one of the walls compared to the other one. Place a lateral load in the center of the slab and run it as a membrane mesh with the actual stiffness properties of the slab (including openings etc). If the wall with twice the stiffness picks up about twice the load, then its rigid. If they both pick up half the load, then its flexible. That being said, I would just assume its rigid and move on.
RE: Concrete edge beams
RE: Concrete edge beams
It seems to me that the torsion in a steel beam is created by eccentricity of load as opposed to an imposed rotation with concrete beams. I have always assumed that the steel torsion is resisted by a couple between the slab and the connection.
RE: Concrete edge beams
A WF beam (wide flange) is basically the same as a UB, but they have lots more sections to choose from.
RE: Concrete edge beams
I don't think the connection can be considered pinned and no torsion is transfered to the primary beam. If the connection is a welded side plate, the reaction is going to be applied to the beam with an eccentricity introducing torsion. The only way to avoid this is to design the connection so it doesn't allow rotation and the secondary beam is essentially continuous. No torsion will be transfered to the primary beam of a WF section because of the small J value.
RE: Concrete edge beams
RE: Concrete edge beams
I saw one case, where a designer modelled a grid beams layout in sap without releasing end restraints at primary beams and primary beams literally twists causing wall finishes above to be cracked alongwith the cracks above masonry partition walls on secondary beams.
RE: Concrete edge beams
That's the reason why some designers use a flexible end plate (one welded to the secondary beam's web only) bolted to the primary beam's web. This results in no eccentricity and minimal rotation.
RE: Concrete edge beams
Would the spandrel beam be designed for torsion with a torsional span = distance between secondary beams?
RE: Concrete edge beams
RE: Concrete edge beams
WillisV mentions to pin the ends, or I think the torsion at the ends of the spandrel beam could be released. However, this would increase the deflections considerably.
Do you upsize your beam for the deflections or are there any other ways? It seems like you can't have your cake and eat it too, meaning make it pinned to release torsion and then assume it to be fixed for deflection calculations.
Any ideas?
RE: Concrete edge beams
RE: Concrete edge beams
The redistributed torsion reinforcement is intended to keep cracks tight so that you maintain aggregate interlock when the permiter beam cracks and rotates. I have seen some nasty torsion cracks in perimeter beams. It comes with longer span secondary beams/joists. This is because the rotation of the end of the secondary member imposes an angular roation on the perimeter beam. The width of cracks is a function of the angular rotation.
RE: Concrete edge beams
RE: Concrete edge beams
The real nitty gritty is this:
1. Can we just neglect the torsion by pinning in our model (therefore zero torsion from model, so zero torsion entry in our design spreadsheet),
OR
2. Do we have to re-run our model with fixed ends (even though we will eventually detail secondary beam as simply supported)in order to see whether this torsion exceeds the 'threshold minimum' in ACI318-08, in which case we can design for the less of this design torsion or the 'maximum Tu in event of redistribution' - clause 11.5.2.2 in ACI?
I.e. do we still need to put in extra stirrups for the (minimised, redistributed) torsion force even if our pinned model shows zero torsion force in the beam?
Thanks!
RE: Concrete edge beams
Too quote the commentry ACI C8.6:
"Two conditions determine whether it is necessary to consider torsional stiffness in the analysis of a given structure:
(1) the relative magnitude of the torsional and flexural stiffnesses, and (2) whether torsion is required for equilibrium of the structure (equilibrium torsion) or is due to members twisting to maintain deformation compatibility (compatibility torsion). In the case of compatibility torsion, the torsional stiffness may be neglected. For cases involving equilibrium torsion, torsional stiffness should be considered."
Thus I assume your edge beam is in compatibility torsion, thus you can neglect you torion stiffness, but there will be detailing requirements, you will need to have fully enlosed ties i would think. you also need to provided shear ligs as per Cl 11.5.5.3, (I don't know your code as well as i should and am unsure about tension steel effects, if any)If you were in Australia, there are some tension requirements but I am yet to see some inculde the tension reo, unless it is a signification compatibility situation.
is this the best design, well that is upto you.
When in doubt, just take the next small step.
RE: Concrete edge beams
Just to clarify the reason for this.
As stated above, equilibrium torsion means that the member requires torsional stability in order to stay stable. This needs to be specifically designed.
Compatability torsion is only a serviceability issue, if there was no torsion capacity then the section would crack and rotate alleviating this torsion. The minimal torsional reinforcement noted above is therefore provided to minimise the torsional cracking rather than for capacity.
While we tend to treat concrete as an elastic material in analysis, it is important to realise that it is not. Many of the rules that we use in design are extrapolated from ultimate inelastic failure criteria such as yield lines.
RE: Concrete edge beams
When in general do you think it is appro ate to increase the Longitudinal reinforcement amount due to compatibility torsion? Would you increase the Longitudinal reo for an edge for instance? If so, how about a interior beam with different spans each side of the beam?
When in doubt, just take the next small step.
RE: Concrete edge beams
RE: Concrete edge beams
Just to confirm what you are saying, there is a minimum requirement for the amount of reinforcing steel for compatibly torsion, but this reinforcement is not additional to the flexural steel?
When in doubt, just take the next small step.
RE: Concrete edge beams
RE: Concrete edge beams
RE: Concrete edge beams
Also the rigidity and strength needs to be strong enough to adequately transfer the load, not necessarily equal to that of the shear wall, unless you require 100% of the shear wall capacity(strength and/or stiffness).
RE: Concrete edge beams
RE: Concrete edge beams
good point, The slab restraint would give you a lot more shear and torsion strength. AS3600 is based on isolated beams, for shear and torsion.
I now have another question, how many people take into account the adjacent slab restraint in beam design on a regular basis?
if so how do you take it into account in your torsion cal's, rangan and hall recommend a slab strent factor of 4-6 the stength for the shear/torsion interation, I have heard of other intration restraint factors based on the depth of the spandral/torion beam?
When in doubt, just take the next small step.
RE: Concrete edge beams
I dont take it into account other than occasionally using a few bars for the bending top steel over supports. There is a difference between knowing you have this up your sleeve and using it in your calculations.
This is taken into account with the compatability torsion rules and is the reason why internal beams are generally not designed for torsion.
RE: Concrete edge beams
This should be the new engineers slogan.
A star for you csd72
When in doubt, just take the next small step.
RE: Concrete edge beams
I would like to bring up this question about interior beams and why they don't have to be designed for torsion. We have an engineer who argues that interior girders should be designed for torsion in cases where you have unequal span beams framing into them.
For example, if there is a 50' bay and a 30' bay, he suggests the girder will need to be designed for the torsion. The argument against it has been that slab restraint will prevent the beam for twisting. Can you shed some more information on that?
RE: Concrete edge beams
I have noticed that many PT design/construct firms do not include minimum torsional reinforcement (e.g. closed ties at whatever centres) in spandrel beams. I've spoken to a few of them about it and their response is generally "we do it this way".
Does anyone have any comments with regard to PT structures?
RE: Concrete edge beams
The intent of redistributed torsion design is to provide enough reinforcement to keep any cracks tight and preserve aggregate interlock. I have never seen an interior girder with torsion cracks. I think this is because concrete beams aren't very stiff in torsion so they rotate and release most of the torsion.
RE: Concrete edge beams
"One more reason why this torsion can often be 'neglected' - in order for a concrete beam to fail in torsion it needs to elongate but the adjacent slab reinforcement will help prevent it from doing so."
Aren't these statements contradicting each other?
RE: Concrete edge beams
Other than the fact that the prestress help a little for torsion/shear strength, PT and reinforced slabs should behave the same in torsion and shear, thus the discussion is no different, in theory. The reason they like to use open ties if for practicality, they need to be able to install there shear ties then beam pt, Closed ties would cause hassles.
When i design PT slabs I normally let them use open ties to the interior beams and closed to the edge beams. However, you should note that I would normally design my edge beam as reinforced anyway, with PT provided for deflection control only, Guess I'm a bit conservative in that way.
When in doubt, just take the next small step.
RE: Concrete edge beams
RE: Concrete edge beams
When in doubt, just take the next small step.
RE: Concrete edge beams
Internal beams/bands, I would normally ignore torsion but supply nominal ties just to support the transverse tendons/reinforcement (500 ctrs with legs at 500 ctrs across spacing also) and with nominal top bars in the slab direction with some extra tie sets at maximum spacing for a couple of meters eiter side of the columns where shear and torsion are the worst.
Except on a roof slab. Then for the internal bands I would increase the ties in the inetrnal bands and make sure there is extra continuous bottom reinforcement for temperature differential stresses (normally not a problem with RC but very significant with PT). Normally you will need N16 @ 200 to 300 longitudinal in the bottom for this, fully lapped at the columns.
RE: Concrete edge beams
This has been an immensely useful thread even though I think my understanding on compatibility and equilibrium torsion just got better. But this thread actually got me thinking further. Say, for example, I have a cantilevered slab with a back span. Now, can I say that the equilibrium torsion due to cantilever is some what nullified by compatibility torsion on the other side and only check the beam for effective torsion (i.e. equilibrium torsion - compatibility torsion)? Pattern live load will cause more grief, but that is a different aspect which we should consider anyway.
In any case I would just design for equilibrium torsion and be glad that the beam may not in reality experience so much torsion. But it would be interesting to know your thoughts on this situation, which happends all the time.
RE: Concrete edge beams
RE: Concrete edge beams
RE: Concrete edge beams
Rapt,
Why the increase I tie spacing for thermal effects, I wouldn't have thought that thermal effects would have effect shear or torsion. Is this to just ensure that the torion/shear cracks don't open up?
Here is extract from page 37-39 of Concrete international July 2009, it was an article talking about wood and armer but then is started talking about how to handle torsion in FEA programs, have included for information/discussion.
"Design Using Element Nodal Forces
Slab design methods based on element nodal forces
have been widely implemented in FEA software. These
methods are attractive because results are relatively
accurate even for very coarse meshes, can be used for
slabs containing beams or drop panels, and are easily
extended to design prestressed or post-tensioned floors.
In this approach, the critical question facing engineers is
how to account for the total torsion T computed across
the design section, which is separate from the total
primary bending moment M, as illustrated in Fig. 4. (sorry couldn't past the figure)
Principally aligned design sections
The effect of twisting moments can be ignored when design sections are aligned with the principal bending directions at all locations, as the twist on the design section vanishes in this case. A rule of thumb that is sometimes used is that torsion and twist can be ignored if T is less than 10% of M.12 If the torsion is greater than this value, then neglecting the torsion and twist effects may lead to unconservative results. Many commercial FEA programs are capable of plotting vectors corresponding to the directions of principal bending; and after reviewing such results, design sections can be chosen to be orthogonal to these principal bending directions. When these directions do not match with desired reinforcement directions,the components of required reinforcement are determined first with respect to the principal bending directions and then transformed via a simple change of axes into the desired orientations.
Bending modified by torsion
One approach to incorporating T is to directly combine M with ±T, similar to the Wood and Armer approach. From
a mechanics standpoint, however,this concept differs from the Wood and Armer method in that the moments are resultants from the entire design section, not an infinitesimal plate element. In addition, this approach
can lead to highly uneconomical designs when T is significant compared with M.
"Beam" torsion
Another approach that has been implemented in commercial FEA
software is to account for T according to building code provisions for beams in torsion. This approach resists torsion in the slab by hoop stresses, and reinforcement is proportioned to satisfy this load path.
Torsion resolved into transverse shear
The total torsion T in the design section can be decomposed into linearly varying transverse shear per unit length with a maximum value equal to 6T/L2, where L is the width of the design section. By multiplying this maximum shear by the width of the design section, an equivalent (but quite conservative) resultant shear force due to torsion, which is equal to 6T/L, can be considered during the shear design of the section."
When in doubt, just take the next small step.
RE: Concrete edge beams
For thermal, it was an increase in the nuymber of ties, so a decrease in the spacing.
RE The FEA quates above,
1 FEA is only accurate for the elastic analysis of the slab. It is generally not "accurate" for the design of the slab, which is normally not elastic.
2 The Mxy moment in FEA analysis for slabs is not actually torsion. To define the moments on an element to match the actual stress state in the element, it is necessary to define 3 moments, Mx, My and Mxy. If you are designing about the principal axes, Mxy is zero. Otherwise there are 3 moments. The longitudinal moments Mx, My and Mxy which defines the twist in the element. As it is not practical to reinforce for the principal axes (as they are at a different rotation at every point in the slab), we reinforce in an orthogonal pattern so have to deal with the 3 moments.
3 So Mxy is NOT compatibility torsion. It is the degree of twist in an element required to define the actual stress state in the element. This twist occurs in just about every element to varying degrees, even a perfectly symetrical, regular slab. In such a slab, the Mxy moments could be as large as 15-20% of the Mx or My values at the peak Mx and My points. For irregular slabs they could be significantly higher.
4 It is unconservative to ignore these moment in design. Several FEA analysis design programs have been ignoring it for many years, and producing under strength designs as a result. Once I heard it quaintly defined as "the effects of 2way action leading to reduced moments". Rubbish, it is under-design by ignoring part of the design actions on the slab.
It is nice for them to suggest that if it is less that 10% you can ignore it, but that is like saying that for a Equivalent Frame analysis you only need to design for 90% of the calculated moments. Convenient for the design but underdesign no matter what. And codes do not allow you do reduce your design moments by 10% because it is easier to calculate!
5 Many codes do not mention Mxy specifically. When I pointed this out to the AS3600 committee, their response was that the code does say to include it as it says that all actions must be included and it is part of the design moments on an element calculated as a result of the calculation method used, FEA. The designer is supposed to understand this and implement it correctly in design. They agree that it is definitely NOT compatibility torsion, as I think you will find the writers of your Concrete International will attest. Otherwise they would not have modified their own program recently to include Wood Ahmer calculations for design.
6 Also, note the "error" in that article where all of the "design strips" are the full width of a column panel, no column/middle strip distribution. I hope no-one in Australia is designing like that. It is not acceptable to AS3600 for RC or PT plat slabs.
RE: Concrete edge beams
I do agree with your comments about Mxy and FEA programs, but i would have thought that there were also torsion moments in slabs. I do believe the last part of this article is talking about torsion, I also think that torsion in slabs can be treated as compatibly torsion, same as beams.
I have many problems with FEA programs As discussed in thread507-249338: Shear in slab desig need to be considered or not. but that being said i trust many programs.
When in doubt, just take the next small step.
RE: Concrete edge beams
A simple way to tell if it is compatability torsion is envision the beam with a torsional swivel at the ends.
If the beam is still stable after you put the swivels in then it is compatability torsion.
In the case of the torsion at the end of the cantilever, with a swivel at the support the cantilever will rotate and therefore this is not compatability torsion.
rapt and rowingengineer,
Some interesting comments. I will not comment on this as my experience with FEM is very limited.
RE: Concrete edge beams
with "but that being said i DON'T trust many programs".
csd72, your not missing out on much.
When in doubt, just take the next small step.
RE: Concrete edge beams
If you have an interior beam with 20' long joists framing into one side and 40 ' long joists framing into the other side, which will create an unbalanced moment over the girder. This unbalanced moment will have to be transferred via torsion into the columns.
I realize that this is compatibility torsion which will self-limit itself based on the torsional cracking.
My question is this:
If the (unbalanced negative moment-20% redistribution allowed)exceeds the threshold torsion, should torsional reinforcing be provided in the beam to pick up the difference?
RE: Concrete edge beams
When in doubt, just take the next small step.
RE: Concrete edge beams
This is exactly the problem I face. I.e. can you just pin this in your model as you assume it cracks and therefore provide nothing for torsion. OR, still design for the (reduced, since its compatability) torsion as surely it must be in there somewhere.
From reading the above, I'd guess that some engineers are neglecting whilst others are designing for it. With apparently no ill-effects. Has anyone designed for no torsion and subsequently seen lots of cracking?
RE: Concrete edge beams
I designed a crash barrier kerb for torsion and saw a lot of cracking nearing ultimate load. Got to do the test on the design so is was kind of cool to see what happens. This was equilibrium torsion, not compatibility which is you direct question, I am yet to see an edge beam crack up such that anyone could notice, if that helps.
The reason many engineers will design with zero torsional stiffness and only supply closed lig ties, is because once you surpass the uncracked torsional strength the stiffness of the beam reduces dramically (about 80-95%), thus it is very hard to get your model to equilibrium, without a few iterations. Using the closed lig arrangement will ensure the torsional cracks are keep to a reasonable size for service loads, for compatibility torsional type loadings.
When in doubt, just take the next small step.
RE: Concrete edge beams
I have a torsional question for those who are using the new ACI 318-08 version.
It states (R.10.3.3) that to use 'moment redistribution', the beam must be designed for a Minimum Net Tensile Strain of 0.0075. Does this mean if we want to use compatibility torsion, our edge beam must meet this requirement? Or is it refering to the secondary beam framing into it?
Or both?
RE: Concrete edge beams
Please post your question in a new thread, as I doubt that your question will get much notice here. and if you do I will answer your question there.
When in doubt, just take the next small step.