making new opening in existing wall

[n3jc]

Lets say we have an existing wall and existing concrete slab on it. We want to make new opening in existing wall.

We have two options:
1. to make opening all the way up to the slab
2. to not make opening all the way up to the slab

Because we have existing slab on the wall, we have positive moments - tension in the upper zone of the slab, so there is reinforcement there.
If we make an opening as shown in figure 1 (see attached picture) we get tension in the lower part of the beam and compression in the upper part of the beam/slab.
My question - is that way existing reinforcement in the slab OK? I think there is no need to additional reinforcement because tension is in the concrete beam?
Id connect new beam with existing slab by drilling holes in the slab + epoxy + rebar. OK?

What about figure 2? We dont change static conditions for the slab that way either, so existing reinforcement in the slab is ok too? Am I right?

 

[KootK]

Both options will work. Option one would probably require tempora y shoring if the slab.

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.

 

[n3jc]

tnx for answer, but what about existing reinfrocement in the slab? Is it still OK? I think static conditions in the slab havent been changed after making an opening?

 

[SteynvW]

How big is the opening? If it is just a double door opening you might
get away without the beam. If the beam is still required and you dowel your
stirrups in the slab, roughen the slab soffit and use your wet-to-dry epoxy
you will be A for OK in opinion.

 

[KootK]

Yes, I believe that the slab reinforcement will be fine.

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.

 

[StrEng007]

Using a flitch plate (or channel) can be a simple solution. Do your load take down, design a flitch plate to span the opening. Use a group of post installed anchors at the end of the flitch plate and design the balance as a UDL along the length. Perform cuts AFTER plate is fully installed. This solution, if permitted by the values you calculate, alleviates the need for shoring and/or casting any new concrete.

Also, make sure you're not cutting a hole in shear wall first.

 

[RFreund]

You know for me the hardest part about new openings is justifying the out of plane strength. Usually the openings are in a CMU wall that you know nothing about. There might be other similar openings or maybe this new opening is bigger by 6". I struggle with how to "calc out" a realistic wall capacity with the opening.
I've heard some say, well openings less than 4' or 5' or 6' will be "fine" and for the most part they're probably right as I've seen even larger openings with no consideration for out-of-plane forces. Besides running a full on FEA of the wall I'm not sure the best way to approach these situations. One somewhat reasonable suggestion is to assume some sort of "effective jamb width".
I've stuck with the following:
[ol 1]
[li]Try the effective jamb width method (going up to 3*t is more than reasonable)[/li]
[li]If it is 3' or 4' I don't get too worried depending on the width of the pier on each side of the opening.[/li]
[li]Try to put in some sort of kicker above the ceiling. JAE had a nice detail a while back on this with angles that run vertically up the wall to roof. The kickers stay above the ceiling and are fastened to the vertical angles part way down the wall.[/li]
[li]Some sort of external frame/reinforcement. Or sawcut and add bars.[/li]
[li]I'd like to try the FEA route but can't say I have.[/li]
[/ol]

EIT

http://www.HowToEngineer.com

 

[KootK]

I've struggled with the same thing and come to the conclusion that it's just not worth it to try to get fancy. The dominant Canadian masonry text has a fancy section that deals with openings out of plane in plate fashion (similar to FEM). It indicates that you can use really wide effective jambs (8'-ish). Trouble is, the method gives no consideration to the simultaneous axial loads. Once you get into beam column territory, simple starts to look real attractive.

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.

 

[Akeee]

Please check if the building is in a earthquake zone and if the rezisting lateral structure (masonary walls) is designed by seismic forces. If it is only gravitational loads you are fine but if not you have to check that pier you split in 2 for shear redistribution.