Cutting Openings in Precast Wall Panels 1

[asixth]

Hi guys,

I have a 2700 wide x 8400 high precast wall panel which needs a typical door opening cut. The precast wall is reinforced with WWF mesh and is not prestressed. What they are proposing to do is cut an opening in the panel to widen an existing doorway. The problems that I see are they will be demolishing a base fixing of the wall panel and there will be no trimmer bars around the new opening. I have searched existing threads to see what is recommended for similar examples to this. What I have gathered is that the opening may need a steel hollow section framed around it to redistribute the loads. I have attached an elevation of the typical panels under consideration and the base fixing detail. The wall panels are restrained at the top by a steel channel section.

My questions are:

How can I reinstate the base fixing detail which will be removed when the new opening is cut?

Generally what is the procedure for cutting openings in concrete wall panels and what are good practices that I should follow?

Is there any good technical information for cutting wall panels and tilt-up design? I have a publication from an Australian concrete institution titled 'Guide to Tilt-up Design and Construction' but find it to be lacking in detailing and does not give advice for cutting panels.

 

[rowingengineer]

asixth,
Shouldn’t you be at the show?

Got a few questions for you,

1. Is this a panel that is required as a shear panel?
2. What type of footing do you have?
3. Do you have an external slab? if so does it lock in the base of your panel?
4. Is the panel grouted along its length?
5. What type of slab do you have? I would assume reo, 150mm???
6. is this for an industrial building? and dose the roof frame into this panel.


When in doubt, just take the next small step.

 

[asixth]

I haven't been to the show since 2000. I'm more into the show day races at Eagle Farm. I'm just glad to have a day off.

1. Yes, the roof bracing frames into the panels so the panel will need to work in shear. I haven't considered this but I will now have a look at it.

2. It is a suspended slab, so a 50mm (2") recess has been provided for the panels to sit in for waterproofing.

3. Same as 2.

4. Only the dowels are grouted, two dowels per panel.

5. The wall panels are 150mm (6") thick. The slab that supports the panels is a 1200 wide x 450 deep (48" x 18") perimeter beam (i.e. wide flat beam/slab thickening).

6. It is a retail building but is built similar to an industrial building. The UB rafter (W-Beam) does not frame into this panel.

 

[rowingengineer]

My thoughts on the situation.
If you don't go to teh show, they will take away my public holiday, and I really enjoy my public holidays.

Q1- to reinstate the base connection I would be tempted to create a recess on the inside of the panel in the top of the wide beam if possible (trying to keep this in the cover so no harm to structure). In the bottom of this recess I would drill a hole in the beam say 110mm deep? Weld a dowel to the bottom of an angle (say 150x100x10? angle 80mm long, with an N16 dowel), epoxy anchor this in the hole. this means that the recess only need be about 30mm deep and very easy to grout over. This angle then gets fixed to the wall with a epoxy anchor. Hopefully there is some lining to cover the bolt head, if not you could consider site welding the bolt to the angle (would suggest a puddle weld if you’re going to do this). I would do all this out of stainless steel or at the minimum galv the lot.

Q2- drill large holes in the corners of the new opening. Cut from hole to edge or hole to hole. This will reduce follow-on cracks from the saw cut. Before cutting the opening i would suggest packing in-between this panel and the panel with the existing opening at about 2/3 height, use no deforming packers. I would also grout for about 300mm in front of the angle, and seal such that it is water proof.

Q3- not that I know of.

hope this makes sense, if not i will do a sketch.


When in doubt, just take the next small step.

 

[csd72]

asixth,

rowingengineer has made some good suggestions but I will add a few comments as I have done this exact thing a few times.

Calculate the actual moment at the top of the opening. As this is only about one quarter of the way up then you may find that the existing reinforcement is adequate for strength and therefore does not require stiffening steel members behind it. You will need to prepare the client for the possibility of some cracking though.

If you do need to stiffen it then channels is the way to go, two vertically and one horizontally. Then use standard panel clips and mechanical/chemical anchors to fix the clips to the panels.

If you do the H frame as stated above then the base fixity is not required for stability. If you dont then 3 points of fixity may still be adequate.

 

[asixth]

csd72,

Thanks for the H-frame idea. What kind of cracks can be expected. Will the cracks be propagating from the corner of the opening?

The concrete has achieve a higher compressive strength, 50MPa (7,250psi), so it should be more durable than normal strength concrete. I am unsure whether this will help with minimizing cracking.

 

[csd72]

It helps but as there are no edge bars or corner trimmers the opening will act as a crack inducer under thermal movement.

following rowwingengineers suggestion in the corner will help minimise this.

The cracks should be fairly minimal though but it is good CYA to make the client aware of it.

I wouldnt expect much cracking if you use the H frames to support the panels.