Problem for changing beam alignment 3

[dr. rifat]

Hello,

I am in confusion on changing beam alignment.Here are two beam alignment.Beam size and reinforcement are given also.But I want to know which is structurally safe or not and why.Please experts gives your best opinion.And the reinforcement and concrete section is sufficient?

Information: Say slab cantilever 4' from column face.f'c=3 ksi.

Image:

Sorry for my bad English.

Thank you!!

RIFAT

 

[jayrod12]

Actual rebar aside (since we aren't here to do your design for you) both are stable as best as I can see. However, I've been told be concrete formers that they'd prefer to have everything in line as shown in the first layout.

 

[KootK]

1) Both layouts are sufficiently strong and stable in my opinion.

2) As jayrod mentioned, layout #1 wins with respect to constructability. Simpler formwork and less congestion at the beam to column connection.

3) Layout #2 is probably somewhat safer. All other things being equal, which they're not, it's usually better to get your loads to your columns via as direct a route as possible. Additionally, layout #1 is going to create an awkward condition where beam bending delivered so near the girder support may result in some rather extensive girder torsion cracking. Some thoughtful detailing and proportioning can mitigate that though.



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.

 

[dr. rifat]

Additionally, layout #1 is going to create an awkward condition where beam bending delivered so near the girder support may result in some rather extensive girder torsion cracking. Some thoughtful detailing and proportioning can mitigate that though.

I agree with this.If I choose Layout #1 then load transfer might be disturbed.In my practical experience I see that when just release form work this type beam crack in middle portion.So I am scared about this,now KootK whats your suggestion about Layout #1?

RIFAT

 

[KootK]

So I am scared about this,now KootK whats your suggestion about Layout #1?

I can see two possible strategies:

1) Pound the end of the girder full of closed stirrups. That won't prevent the cracking but will preserve capacity and keep cracks from opening up too wide. You've probably got a fair bit of steel in there already for hanger reinforcing.

2) Make sure the supported beam is nice and stiff. The stiffer that beam is, the less torsion you'll see being transferred through the supporting 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.

 

[Agent666]

Remove the beam if it causes issues like discussed, avoid issues altogether and saves you some design time, I assume it's all tied together by the slab anyway. Loss of one beam won't change fundamental behaviour that much 8 would have thought.

Might need to cantilever out another beam to support edge slab spanning up the page from the two magenta columns similar to the cyan columns below.

 

[BAretired]

Another possibility is to use two cantilevers, each aligning with the beam on the opposite side of the column and each extending a short distance from the column. The slab should be able to span between the tips of the cantilevers.

BA

 

[dr. rifat]

Another possibility is to use two cantilevers, each aligning with the beam on the opposite side of the column and each extending a short distance from the column. The slab should be able to span between the tips of the cantilevers.

If we want to use cantilever,then it not might be economical,because of slab thickness.(I think so)

I can see two possible strategies:

1) Pound the end of the girder full of closed stirrups. That won't prevent the cracking but will preserve capacity and keep cracks from opening up too wide. You've probably got a fair bit of steel in there already for hanger reinforcing.

2) Make sure the supported beam is nice and stiff. The stiffer that beam is, the less torsion you'll see being transferred through the supporting girder.

Thank you for your informative suggestion.



RIFAT