Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations KootK on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

two way slab opening in column strip. 3

Status
Not open for further replies.
GBMRAO

GBMRAO

Structural
Sep 4, 2018
31
Hi Structural Engineers:
First of all thanks to all engineers who are helping hospitals/clinics/urgent care for analysis and upgrades to help covid patients accomodation.

I have a two-way flat slab supported by concrete columns and perimeter beams.
The contractor made openings in the column strip along the end bay. Initially, it was shown there by the MEP engineers but a later structural consultation revision was sent to make an opening at a different location in the middle of the slab. but accidentally they made an opening in the first location.

Does anyone know any way how to analyze and support and transfer the loads to the adjacent slab in case of opening in column strip? i could not find any documentation for openings in column strip and how to distribute in a manual calculation. see attached existing plan with opening.
My thought process is to model it in software, find moments around opening and provide respective steel below and extend to adjacent rebar based on their capacity. I may be wrong but need some expert suggestions.

ANy help is appreciated.
 
 https://files.engineering.com/getfile.aspx?folder=75f66a48-7ffc-4917-9da9-213641ea2ce6&file=Two_way_slab_opening-_error.pdf
Replies continue below

Recommended for you

Sort by date Sort by votes
Has it already been poured? That would change a lot of the thought process. If not, why not move them appropriately?
 
Upvote 0 Downvote
its an existing slab. They already cut it. But for other locations, they are following my suggestion to keep it in the middle of the slab bay in two directions.
 
Upvote 0 Downvote
Those are pretty substantial sized openings, I highly doubt you'd be able to load share over to the middle strips, and then back to the columns appropriately. I'd be putting steel beams picking up everything.
image_dhsf29.png
 
Upvote 0 Downvote
ok thanks that's what I was scared about. i may have to do a a drill and epoxy headed anchors to engage the slab to the beam. or and edge angle around teh opening and weld to steel beam like you suggested.
thanks for your time and suggestion
 
Upvote 0 Downvote
I don't think that would be necessary. I'd be having them grout pack between the slab and the new steel beams. The most critical connection I could see would be the steel beams to those edge beams.
 
Upvote 0 Downvote
The second pic is scarier than the first. It shows the rectangular opening rotated so that it takes a 42" chunk out of the column strip very close to the column. If these are field cut openings in an existing slab and if your second pic is close to scale, you will likely need to provide steel beams under the slab, one each side of the openings to prevent a punching shear failure.

BA
 
Upvote 0 Downvote
I was a bit slow, so I didn't see the suggestion by jayrod12. I would hope to avoid the beams running east-west by hanging the north-south beams through the slab.

BA
 
Upvote 0 Downvote
BA said:
I would hope to avoid the beams running east-west by hanging the north-south beams through the slab.
Click to expand...
I would too, but I have my doubts that those column strips there can now take the point loads from the beams. Potentially they can, but only the OP can figure that out once he figures out the loads accurately.
 
Upvote 0 Downvote
Demo the concrete and restore the slab with some strengthening, if necessary.
 
Upvote 0 Downvote
Yes, that is an expensive mistake to correct.
 
Upvote 0 Downvote
thanks all for your suggestions. I will do my finite element analysis verify the moments around opening and provide steel beams or Carbon fiber reinfrocement adjacent to opening.

Will keep you all posted. Thanks
 
Upvote 0 Downvote
BA,

The large opening is right on two orthogonal column strips, punching shear... too many complications.
 
Upvote 0 Downvote
Yes, complications for sure. I wonder if the contractor took care when cutting the holes not to run past the opening dimensions. There is already a stress riser at the 90[sup]o[/sup] corner but over cutting would make a bad condition worse.

BA
 
Upvote 0 Downvote
You are absolutely correct. I think they need to shore the entire work area, or even jack up a little to release the built-in stresses.
 
Upvote 0 Downvote
May be I was a bit late...

I will suggest repair of the rebars with welding and the steps for repairment,

i) Make accessible the existing rebars with chipping the concrete,

ii) Temporary support the column strip with props and apply jacking,

iii)Add new rebars with welding to the existing ones,

iv) Formwork for the holes,

v) Apply epoxy to the surfaces and pour new concrete,
vi) Remove props after conc. gain strength and test the slab with loading.

typical_rebar_weld_xcx49i.png
 
Upvote 0 Downvote
Good procedure. May use mechanical coupler too.
 
Upvote 0 Downvote
HTURKAK,

Sounds like an excellent idea.

The first sketch shows orthogonal bars being welded together, but that is something that would not be done in a normal installation. Normally, bars would be tied together with wire to keep them in place while pouring, so I am wondering if that type of weld is necessary.

The second sketch shows bars welded to restore their axial strength. The splice bar is eccentric to the existing. A mechanical coupler was suggested by retired13. Alternatively, two bars could be used, one on each side of the existing to avoid the eccentricity. It's a small area, so the amount of material is not too significant.

BA
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

johndeng
  • Locked
  • Question
two way slab middle strip reinforcement
Replies
16
Views
154
johndeng
johndeng
ITsSB
  • Question
Simply supported two way slab definition
Replies
2
Views
170
Tomfh
Tomfh
mrzift
  • Locked
  • Question
Shear reinforcement in two way slab 8
2 3
Replies
48
Views
549
hardbutmild
hardbutmild
Rsd789
  • Locked
  • Question
Stirrups for one-way shear in two-way slabs
Replies
5
Views
211
Rsd789
Rsd789
johndeng
  • Locked
  • Question
two way VS one way concrete slab reinforcement 1
Replies
8
Views
257
rapt
rapt

Part and Inventory Search

Sponsor

Back
Top Bottom