New Openings in Existing Insulated Precast Panels

[erictlee]

I am working on a project where the owner would like to turn an existing single story warehouse into a multi-story office building that has existing insulated precast panels around the exterior. Based on the GPR, the panel has a very similar profile to the attached image. Where the existing openings are or relatively heavy loads are attached, the panels are grouted from bottom of opening or connection to top of foundation. In hollow core panels, I have specified a very similar detail, but I have never retrofitted an insulated precast panel:

1. Is there any other way to remove the insulation for grout besides cutting the inside face shell, removing the insulation and filling everything with grout? - I believe independent framing systems could be a better option if there is not other way.

2. The second floor windows are not too concerning as there will be a rigid diaphragm below the opening which will redistribute the in plane and out of plane loads in the panel. However, the first floor windows will have point loads and roughly half of the lateral load a few feet above the opening. If the panel doesn't have sufficient capacity, is there another option of reinforcing the panel besides filling the cores with grout?

3. My initial concept is to have a channel or HSS member for the lintel and sill with a plate welded at each end parallel to the member and bolted into the nearest core outside of the window which would be filled with grout. Is there a more efficient lintel design that allows for the lateral in-plane moment to not cause cracking at the corners of the new openings?

 

[erictlee]

Forgot to attach file

 

[KootK]

Some questions.

- can you post a typical elevation sketch? Where and how big the new openings are will be an important aspect of this.

- is there any kind of in plane shear connection between existing panels?

- are these precast panels prestressed?

- is there any transverse reinforcing in the panel top and bottom flange? WWF maybe?

Is there any other way to remove the insulation for grout besides cutting the inside face shell, removing the insulation and filling everything with grout?

Not that I can think of.

If the panel doesn't have sufficient capacity, is there another option of reinforcing the panel besides filling the cores with grout?

For gravity loads, you might bolt vertical channels to the inside of the panels as columns braced by the panels for stability. You mostly just need something that you can attach your beams to. For lateral loads, with the cut panels behaving like moment frames, I struggle to think of much that you could do other than getting fancy with stuff like FRP reinforcement etc. If you had lots of panels, were cutting very narrow windows in the centers of the panels, and had shear connection between panels, something could probably be justified. If you could leave a panel or two of each long wall segment un-penetrated, that would help too.

 

[erictlee]

KootK,
At your convenience, please see the attached elevation and the following responses:

- can you post a typical elevation sketch? Where and how big the new openings are will be an important aspect of this
o All windows are 3'-6" x 4'-0", but will likely be slightly larger due to the steel in the opening (Lintel, sill, and verticals acting like jambs).

- is there any kind of in plane shear connection between existing panels?
o All panels have a shear connection between them in the second floor and first floor. Unfortunately, some of the windows will require the reinforcement tying the shear connectors together to be cut. Less of a concern for a single or double bank of panels with windows, but will need to be considered

- are these precast panels prestressed?
o We have not been able to do any destructive testing (some of the building is being taken down as well) for the structure yet as the closing on the property keeps getting pushed back due to the current circumstances. However, I strongly believe the panels are prestressed.

- is there any transverse reinforcing in the panel top and bottom flange? WWF maybe?
o The GPR tests indicated very little transverse reinforcement. Essentially only across the shear plane between connectors and in the corner of existing openings.

The structure is not small (240ft x 300ft), but has a pretty good aspect ratio with similar stiffness's so the in-plane forces are pretty reasonable (<350 lb/ft). However, the structure is 30 yrs old with no information (drawings, contracts, etc.) and poor soils leaving me with difficult assumptions to make.

 

[KootK]

- Thanks for extra info. Serious structural renovation work is not for the faint of heart. That said, careers have been made by structural engineers willing to think creatively and take some strategic risks in order to further the interests of their clients.

- For what it's worth, I do a little precast sandwich wall design. Modern systems though, nothing like this.

- My gut feel is that you're fine for both lateral and and the transmission of roof loads around the openings. For that, I wouldn't bother with a lintel as you've effectively got a very deep concrete beam under the roof system which, by virtue of stiffness, will tend to draw all of the load even if you do install a lintel. What I've done and advised in the past is that channels be installed over the openings as shown below. They're not lintels though. What they are is really ties for a deep beam arching mechanism for the unlikely event that the panels are unable to function as unenforced beams which will be their natural mode of load resistance until a crack forms. So they're tension members. If they ever get utilized, there will be some serious cracking, the villagers will frighten, and repairs will likely be necessary. But everybody stays safe.

- As for the jambs around the windows, I suspect that you'll be okay there too. You'll still have one full cell intact on either side of the openings and, as you intimated, you'll be cutting the vertical span of the wall panels roughly in half.

- Without knowing the details of your second floor framing scheme, I second your suspicion that an independent, internal framing system for that will make the most sense. It'll be pretty difficult to connect your beams to the wall panels for any serious loads without cutting and grouting the heck out of the panels.

 

[erictlee]

KootK,

Thank you for the response I greatly appreciate the input. I had a thought last night about providing a vertical element bolted to the precast panel (acting like a strut of sorts) below the second floor girders down to a spot where the wall is grouted whereby eliminating the need to grout any cells and limiting the extent of foundation expansion from a post with a baseplate. There are obviously a few additional items to consider, but on the surface would this be a viable solution or am I missing something?

 

[KootK]

There are obviously a few additional items to consider, but on the surface would this be a viable solution or am I missing something?

I'd see that approach working like this, assuming a channel as the vertical piece:

1) The axial load delivered by the channel still needs to be passed through a base plate and into a capable foundation element.

2) The channel would be bolted to the wall panels using a shallow depth drop in anchor as with hollow core, perhaps with vertically slotted holes. In my mind, the connection to the wall panels over the height of he reinforcing would be for the purpose of laterally bracing the channels only.

If you can actually transfer your axial load out of the vertical reinforcing and into the wall panels before the reinforcing touches down at the foundation, great. I could definitely see that being a challenge to calc out though.