Concrete edge beams 5

[GalileoG]

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

 

[rowingengineer]

pa4912, Please don't post twice, I have repeated my earlier post, but have read the above and you will find that we are all fairly consitent.
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.

 

[csd72]

I agree with the comments above regarding not needing to specifically design for compatability torsion but to provide nominal torsional reinforcement (depending on code).

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.

 

[rowingengineer]

I have a question in regards to longitude reinforcement minimum requirements of AS3600, 8.3.6, which is required by 8.3.7. I don't beleive ACI has an requiremnts for extra longitudinal reinforcment for compatibility torsion, have i got this correct?

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.

 

[JAE]

ACI does have requirements for longitudinal reinforcing for torsion (A_l. For edge beams where you are assuming the torsional restraint in the beam is not required for equilibrium, and you are designing it just with minimum torsion, I don't believe you need to add the (A_l.

 

[rowingengineer]

JAE,
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.

 

[slickdeals]

No, I believe JAE is saying that the longitudinal steel requirements do not apply for compatibility torsion, only for equilibrium torsion.

 

[JAE]

Yes, what slickdeals said.

 

[demayeng]

just a note on the last question on the diaphragm slab - you could draw a plan view and just check that there is a strut and tie model flow path of some sort..
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).

 

[csd72]

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.

 

[rowingengineer]

csd72,
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.

 

[csd72]

rowingengineer,

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.

 

[rowingengineer]

"There is a difference between knowing you have this up your sleeve and using it in your calculations."

This should be the new engineers slogan.
A star for you csd72

When in doubt, just take the next small step.

 

[slickdeals]

"This is taken into account with the compatability torsion rules and is the reason why internal beams are generally not designed for torsion."

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?

 

[Tomfh]

As discussed, under AS3600 you can neglect compatibility torsion provided you include minimum torsion reinforcement.

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?

 

[ron9876]

There is a level below which you can ignore torsion. If you calculate the applied torsion by modeling the torsional stiffness of the girder and slabs you will find that the girder won't attract much torsion.

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.

 

[slickdeals]

"I think this is because concrete beams aren't very stiff in torsion so they rotate and release most of the torsion."

"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?

 

[rowingengineer]

Tomfh,
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.

 

[JAE]

Keep in mind that "closed ties" in beams can be a combination of an open tie (with 135 deg. hooks) and a cap tie (one 135 deg and one 90 deg hook) so that PT can be installed after the main beam cage is placed and then finally closed off with the cap tie.

 

[rowingengineer]

While I am happy to use this detail on edge beams, and do it very frequently, it technically it doesn’t comply with my local code As3600. I again justify this with the slab being present on one side helping with torsion, but at corner columns, i do reduce my centres leading up to the column .

When in doubt, just take the next small step.

 

[rapt]

Yes, edge beams/bands, I would normally ignore torsion in the design but always use minimum torsion ties, and yes, AS3600 does not like open ties with closers any more, especially in shallow bands. I have never added extra longitudinal reinforcement for compatibility torsion but AS3600 seems to suggest that you should. But, then in most PT beams you do not need any or much flexural reinforcement and nominal bars are supplied.

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.