Secondary beam at lower level than primary beam. 2

[struggle66]

Hi everyone,

Good Day,

My secondary beam is lower than my primary beam please see the sketch below if I use shear links to transfer secondary beam reactions to the top of primary, is it ok? Any other things to take into consideration?

Thanks in advance for your replies.

 

[ooox]

Link

Page 42 and 43.

 

[KootK]

I think that's the crux of it. Other minor points:

1) Put top steel in supported beam.
2) Run supported beam bottom steel over girder bottom steel.
3) Maybe comparability torsion reinforcement in the girder.

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.

 

[canwesteng]

In my mind,the supports for the secondary beam are taken as the centroids of the primary beams, the reinforcing is detailed accordingly (so your bottom bars run into the the primary beam, I would run them to the end and hook them so they can tie them to your primary beam steel), and then your additional stirrups resist load as shown (designed in the D region as strut and tie). As long as you detail it as a simply supported beam running to mid point of the primary beams, I don't think you need to consider torsion, at least I don't.

 

[KootK]

With reference to the torsion issue and the sketches below, I see things unfolding like this:

1) For early stages of loading, there's torsion in the girder and some fixity in the end of the supported beam.

2) The amount of torsion will depend greatly on the supported beam's proximity to the columns supporting the girder. If the supported beam ties in at girder mid-span, there's probably considerable pre-cracking torsional flexibility available. If it ties in near a column, you'll develop large torsions in the girder.

3) With enough additional load, you'll morph away from the torsion condition and towards a pure tensile hanger situation similar to the OP's concept. At that point, the supported beam loses its fixity.

4) Getting form the torsion situation to the pure tensile hanger condition probably requires redistribution of two sorts:

4-1) Torsional cracking of the girder.
4-2) Opening up of a crack at the right face of the girder as shown below.

If this were my baby, and especially if it were in a corrosive environment, I would probably do the following:

1) See if I could deal with all of the hanger tension in just the stirrup legs adjacent to the right side of the girder. This ought to minimize crack width.

2) Put in some sleep easy minimum torsion reinforcing.

Much depends on the relative stiffness of the two beams. The greater the flexural stiffness of the supported beam relative to the torsional stiffness of the girder, the closer you'll be to a true hanger. And one nice thing about torsion is that, if you can tolerate a little torsional cracking, the torsional stiffness will diminish by something to the tune of 90% in an instant.

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.

 

[Ingenuity]

2) Run supported beam bottom steel over girder bottom steel.

Will be tough to do with the OP's primary beam BOTTOM at the same level as the secondary beams TOP.

struggle66, I am not sure of the spans and magnitudes of your loads in this, but in the past where the span/load configuration and repetitiveness justified it, I used some vertical PT bars and vertically 'stressed' the secondary beams to the primary beams.

 

[KootK]

Will be tough to do with the OP's primary beam BOTTOM at the same level as the secondary beams TOP.

My bad. Here, substitute carrier bar for girder bottom steel. Conceptually identical.

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]

Depending on dimensions, I would think you would need a cog/hook at the ends of the secondary beam bottom bars for development!

 

[KootK]

It's probably prudent to hook the bottom bars for good measure whether it's required by the numbers or not. What design situation leads to a comdition like this. Seems like an odd thing comstructability wise as well.

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.

 

[struggle66]

Thanks All

KootK,

I will provide minimum torsion bars in GIRDER. Maybe I will only use 40% or 50% yield strength of the bar and put more links close to the right face of the girder

Ingenuity,

Vertical PT is a good idea. Thanks

RAPT

Thanks I will make sure the force fullly develop.