Reinforcement Detailing of Simple Beams 5

[StructuralAddict]

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!!

 

[chekre]

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.

 

[chekre]

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

 

[hokie66]

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

 

[KootK]

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.

 

[jayrod12]

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

 

[KootK]

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.

 

[chekre]

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 ?

 

[jayrod12]

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.

 

[rapt]

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!

 

[KootK]

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.

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.

 

[rapt]

Kootk,

Fig 3.10b was my concern in my last sentence!