INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Reinforcement Detailing of Simple Beams

Reinforcement Detailing of Simple Beams

(OP)
Hello Everyone,

In the design I am working on now, I have secondary beams supported by main beams. The main beams are not designed to resist torsion. This means that the secondary beams should be simply supported on the main beams in order to ensure that moment does not transfer from the secondary beams to the main beams.

So, how can I design a simply supported connection? What is the difference in reinforcement detailing between a simple connection and a fixed connection?

I appreciate if you could provide me with example drawings (if possible).

Thanks in advance!!

RE: Reinforcement Detailing of Simple Beams

I think you need to design the primary beams for compatibility torsion if they will see it. It's likely that minimum stirrups would provide enough torsional strength

RE: Reinforcement Detailing of Simple Beams

X2 for Jayrod's comment. Additionally:

Quote (monir87)

So, how can I design a simply supported connection?

You don't. The strategy that I'm familiar with is this:

1) Design the secondary beams as simply supported.

2) Design the girders for compatibility torsion.

3) Provide the secondary beams with top steel = 1/3 bottom steel.

From a detailing perspective, these faux-simple span beams will look like any other concrete beam.

In my opinion, it's important to maintain some negative flexural capacity in monotonically cast beams, even when they are designed to be simply supported. The beams will attract negative moment no matter what you do and, without some negative flexural capacity, your shear capacity may be adversely affected.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Reinforcement Detailing of Simple Beams

(OP)
Thank you jayrod12 for your prompt response. I am also confused between the reinforcement detailing of simple connections and fixed connections. What would be the difference between them? I read something related to the influence of development length on controlling the connection behavior, but I am not sure how that works. Any thoughts about this issue?

RE: Reinforcement Detailing of Simple Beams

(OP)
Thank you KootK for your great explanation as always :)
It is very clear now!

RE: Reinforcement Detailing of Simple Beams

(OP)
Just a quick follow-up. In Etabs, should I release the moment at the ends of the secondary beams? Or is it better if I keep the joints as fixed?

Thank you!

RE: Reinforcement Detailing of Simple Beams

That depends on what you're designing. For the secondary beam bottom steel, moments released would be conservative, for secondary beam top steel fixed would be the conservative route. For primary beam design then fixed would be the conservative route.

In this case where you want this to act as a simply supported beam then you would want to model it with end moments released.

However as KootK pointed out, design your primary beams for the compatibility torsion, and then provide about 1/3 of the secondary beam bottom steel as top steel that extends into your primary beam. It is always beneficial to maintain a bit of negative flexural capacity.

RE: Reinforcement Detailing of Simple Beams

I vote for pinning the joints in ETABS. I think that strategy will provide you with the most useful output for design purposes.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Reinforcement Detailing of Simple Beams

(OP)
Thank you very much again jayrod12 and KootK. I will model the secondary beams assuming pinned ends in Etabs. This will give me the maximum bottom reinforcement required. Then, I will add top reinforcement = 1/3 of the bottom reinforcement near the supporting girders. Meanwhile, I will make sure that the girder is designed to be torsion compatible by checking if the stirrups are sufficient.

RE: Reinforcement Detailing of Simple Beams

For compatibility torsion in an edge beam, you don't really design, you just follow the rules for minimum continuous reinforcement top and bottom, and minimum stirrups. With secondary beams framing into the edge beam, the twisting is resisted by flexure of the secondary beams.

RE: Reinforcement Detailing of Simple Beams

Monir87, you ought to invest in the CRSI Detailing Manual or ACI Document SP-66. They have a lot of real life or close to real life examples of reinforcing detailing.
Like hokie66, I prescribe to "Field of Dreams" detailing theory. If you assume it, it will behave that way. Just make sure you reinforce according to your assumptions.

RE: Reinforcement Detailing of Simple Beams

"Field of Dreams" is ok as long as it provides a load path to supports. If your dreams have been twisted by inappropriate inputs into or outputs from the brain, then it may be useless!

In terms of crack control, it does not matter what you dream, cracks will happen where elastic stresses are too high, before redistribution to whatever field of dreams you have chosen. So if you want crack control (and this includes compatibility torsion) you need reinforcement where the elastic stresses say you need it.

RE: Reinforcement Detailing of Simple Beams

Quote (hokie66)

"the twisting is resisted by flexure of the secondary beams."

So in other words, if you design your secondary beam so that it complies with deflection limits as simply supported it will automatically create minimal torsion forces in your primary beam.

RE: Reinforcement Detailing of Simple Beams

Yes, the rotation will be minimal, thus the torsional shear stress will be low. You still need to provide continuous top and bottom bars in an edge beam, as well as code required closed stirrups.

RE: Reinforcement Detailing of Simple Beams

Hi
So if i understand well detailing doesnt have to do much since whether we assume pinned or fixed end the detail will always be the same for the connection between secondary and primary (top bars anchored in the primary beam with a 12db hook). At the end the structute will behave as you design it in condition to do all the neceasary checking. Please correct me.

RE: Reinforcement Detailing of Simple Beams

Quote (SteynvW)

if you design your secondary beam so that it complies with deflection limits as simply supported it will automatically create minimal torsion forces in your primary beam

This will be true most of the time but designers still need to keep their wits about them. Whether or not the girder is shielded from torsion stresses is a function of the relative stiffness between the flexural load path in the secondary beams and the torsional load path in the girder. A common trouble spot is where you get a secondary beam tying into a girder close to but not at a column. The torsional load path at such a location is very stiff and some serious girder torsional cracking can take place en route to redistribution. These issues can, of course, be ameliorated by making thoughtful layout choices to begin with.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Reinforcement Detailing of Simple Beams

One place I have heard of problems is where two beams frame into opposite sides of a supporting beam, but they're slightly offset (like two feet). The span between the beams is subject to torsion, no matter what you do. If this is happening, it needs to be laid out differently.

RE: Reinforcement Detailing of Simple Beams

chekre,

No, the structure will perform in the way it wants to. We just try to predict and detail accordingly.

The situations described by KootK and JedClampett are essentially the same. As they said, layout choices can prevent this problem.

RE: Reinforcement Detailing of Simple Beams

Hookie66, i meant by detailing the connection details between the secondary and primary since as far as i know there is no hinge detail between elements built integrally.

RE: Reinforcement Detailing of Simple Beams

Cast in place concrete structures don't often have intentional hinges. We may model joints as hinges, but that is just to make analysis simpler.

RE: Reinforcement Detailing of Simple Beams

One more thing. Kootk stated that shear will be affected without negative flexural capacity. Can u please elaborate ? Personally when i assume pinned connection in the analysis i put some minimal reinforcement which will.not depend of the bottom that i have unless i am in seismic zone for the sake of being consistent with my assumptions.

RE: Reinforcement Detailing of Simple Beams

Minimal reinforcement i mean the top bars in the discontinous element assumed as pinned

RE: Reinforcement Detailing of Simple Beams

Yes, some top bars at discontinuous edges is standard practice.

RE: Reinforcement Detailing of Simple Beams

Quote (Chekre)

Kootk stated that shear will be affected without negative flexural capacity. Can u please elaborate ?

Sure. Imagine a fixed end beam that has developed a negative moment flexural-shear crack near the end and has no top steel because it was designed as a pin ended beam. What is "d" for the purpose of determining shear resistance? I would argue that it is the distance from the bottom steel to the bottom of the beam (~65mm). Not good obviously. There's a similar failure mode for precast planks that develop end moments as a result of accidental restraint.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Reinforcement Detailing of Simple Beams

Quote (KootK)

Sure. Imagine a fixed end beam that has developed a negative moment flexural-shear crack near the end and has no top steel because it was designed as a pin ended beam. What is "d" for the purpose of determining shear resistance? I would argue that it is the distance from the bottom steel to the bottom of the beam (~65mm). Not good obviously. There's a similar failure mode for precast planks that develop end moments as a result of accidental restraint.

I wonder how many people's mind will be blown by your statement.

It's extremely similar to shear friction bigsmile

RE: Reinforcement Detailing of Simple Beams

Quote (jayrod12)

I wonder how many people's mind will be blown by your statement.

It still blows my mind unfortunately. I've made my peace with it at supports as I know how to deal with it. It's at locations where "detailing" top bars stop short of elastic inflection points that things get murky for me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Reinforcement Detailing of Simple Beams

Thank you kootk.
Do u think your above statement is applicable also for end pinned beams having some minimal top bars - less than the 1/3 that u have proposed or it is just for beams having none at all of top bars ?

RE: Reinforcement Detailing of Simple Beams

I think that's precisely what he's saying. You can call it a pin ended beam all you want. But it will act as a fixed end beam until the flexural crack forms. If you have no top steel extending into the support then your crack with propagate down to the bottom of your beam essentially taking your Vc to zero.

RE: Reinforcement Detailing of Simple Beams

Koot,

As long as your bottom steel is fully anchored and is sufficient for the member to act as a simply supported member, I would think that it will be exactly that. A large flexure crack at the support where no -ve moment capacity has been supplied. Resulting in a simply supported member with tension in the bottom everywhere and compression in t=he top, so shear capacity is being supplied by the bottom reinforcement with an effective depth to the top.

With one very wide crack at the support. My worry is the effect of that big crack on the connection to the support, which is why I would never do it!

RE: Reinforcement Detailing of Simple Beams

3
The image below is taken from the FIB document on precast connections and illustrates some of my concerns. Tell me those sketched failure modes don't get your pulse racing a bit?

I'm fairly confident that most well detailed systems could undergo the redistribution required to survive flexurally without the top steel. However, when we make our redistribution argument as engineers, we rarely seem to consider the shear condition other than at the "field of dreams" end state. The truth of the matter is that shear needs to work at that end state and all intermediate points in time along the load history from zero to that end state. Including reversals if required.

As an interesting aside, most codes limit the amount of moment redistribution that designers can employ to reduce elastic moments. In Canada, it's around 15%. In many other code, the allowance is much more liberal and often based explicitly on strain and reinforcing ratio. So, if one assumes a pin (zero moment), and the elastic moment was anything other than zero, then technically your %redistribution was 100%. Not sure how we feel about that at faux pins.

Quote (chekre)

Do u think your above statement is applicable also for end pinned beams having some minimal top bars - less than the 1/3 that u have proposed or it is just for beams having none at all of top bars ?

Honestly, I don't know the answer. I'll share some thoughts instead.

1) I consider no top steel to be a definite show stopper per our previous discussion and the clip below.

2) If you show me a gigantic, continuous transfer beam with 4-15M as the nominal top steel, I'll veto that (this has happened to me on multiple occasions).

3) In practice, if I've got 1/3 top steel and and it extends at least 0.25 x clear span, I'll sleep pretty easy. This is what I generally do at faux pins.

4) In my heart of hearts, I question the validity of any shear design at a location not designed for the expected moment at that location limited to a fairly modest amount of redistribution. While we, as designers, often treat shear and moment capacity as independent, they usually are coupled phenomena. Remember all the dust up a while back when that university of Michigan study indicated that stud rail punching shear provisions were non-conservative? Turned out they were non-conservative when insufficient flexural reinforcing was provided.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Reinforcement Detailing of Simple Beams

Kootk,

Fig 3.10b was my concern in my last sentence!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close