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!!
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
RE: Reinforcement Detailing of Simple Beams
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
RE: Reinforcement Detailing of Simple Beams
It is very clear now!
RE: Reinforcement Detailing of Simple Beams
Thank you!
RE: Reinforcement Detailing of Simple Beams
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 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
RE: Reinforcement Detailing of Simple Beams
RE: Reinforcement Detailing of Simple Beams
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
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
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
RE: Reinforcement Detailing of Simple Beams
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
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
RE: Reinforcement Detailing of Simple Beams
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
RE: Reinforcement Detailing of Simple Beams
RE: Reinforcement Detailing of Simple Beams
RE: Reinforcement Detailing of Simple Beams
RE: Reinforcement Detailing of Simple Beams
RE: Reinforcement Detailing of Simple Beams
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
I wonder how many people's mind will be blown by your statement.
It's extremely similar to shear friction
RE: Reinforcement Detailing of Simple Beams
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
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
RE: Reinforcement Detailing of Simple Beams
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
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.
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
Fig 3.10b was my concern in my last sentence!