Cutting openings in existing tilt wall panels

[conradlovejoy]

Architect wants to cut new doors and windows in existing panels. I've never dealt with this issue before and want to hear some sound engineering advice. In theory, the portion above the opening just becomes something of a really skinny deep beam spanning between columns that are the full height portions of the wall on either side of the new opening. I know they obviously aren't reinforced for this case, but its likely this theoretical beam is plenty strong enough to span a reasonable new opening length. I know that panels can easily be designed with large openings, but I want to know what kind of theory surrounds cutting into existing solid walls. I've seen other engineers talk about bolting channels around the perimeter of the opening on the inside to "reinforce" the opening, but I am not sure what they mean by reinforcing. I don't have much in the way of existing structural drawings so I am not sure how this building is handling lateral forces. I understand that concrete panels are pretty robust when it comes to in plane forces. I suppose out of plane bending would be more affected by removing a portion of the wall area/reinforcement.

I want to do more than just say "yeah cut some holes, its probably strong enough" and if reinforcement is a typical practice, what is it for and what kind of calcs are associated?

 

[JLNJ]

I disagree that "panels are pretty robust". I've seen a few cases recently where I couldn't figure out how the heck the panel worked without any openings. Panels can be prestressed or not and composite or not. Or partially composite. Or just designed wrong to start with. Casting tolerances and workmanship quality is all over the map. Lots of bowed, ugly panels out there. Big BIG caulk joints.

The best way is to get the original shop drawings and see what you are dealing with. There are guys who specialize in these designs and may be of some help - for a fee of course.

Or, you pay the piper and design a steel frame to accept all the forces.

 

[conradlovejoy]

so I have seen engineers describing a steel channel frame around the opening. Is it assumed that the weight of the concrete above the opening is then entirely supported by a horizontal channel across the top of the opening and that horizontal channel is supported by vertical channels on either side of the opening...the theory being that the load is "shared" into the horizontal channel that is mechanically connected to the panel and that load is transferred into the verticals that can "share" that load back into the full height panel sections on either side? In other words, a steel frame that is supporting the weight of the now "unsupported" concrete but is connected to the panel so the load is jumped back and forth from the panel to the frame and back to the panel. Also, is the load from the concrete above the horizontal "arched" sort of like the 45 degree theory for masonry loads?

Sorry if this is nonsense...

 

[msquared48]

Working on one of those right now and I have posted such in another thread.

Depending on the size of the opening and the proximity of the top of the cut to the top of the panel, I add vertical tube columns to either side of the cut and a cross tube above the door opening, all after-set bolted to the panels with welded angles. Members are usually HSS 4X6 if varying thicknesses. May have to use 6X6... Set back columns and beam a minimum of 1'-0" from the edges of the new opening for after-set bolt clearance.

The verticals and horizontal are needed to support the panel in wind, and may be also needed for vertical load.

Mike McCann, PE, SE (WA, HI)

 

[conradlovejoy]

Are the bolts installed into the panel and tubes with pre-drilled holes installed over the bolts? I guess that could be considered means and methods if a detail just shows a bolted connection to the panel.

What do you think about my question regarding the loads? I can see how the frame would help the panel resist out of plane wind loads coming from the storefront installed in the new opening. In terms of vertical load, does the designer ensure the frame is strong enough to support the weight of the concrete above the frame? And if that is the case, does the "load-sharing/jumping" idea I described above hold water? In other words, does the load jump from the panel into the horizontal, then to the verticals, then back into the panel and into the foundation? This is all assuming the concrete panel has no strength and the entire weight of the panel above the cut must now be resisted, which I feel is conservative, but doesn't bother me that much to assume in this case.

 

[msquared48]

The angles are welded to the HSS tubes and bolted to the panels with Titen screw anchors.

As for the load to the beam, the panel can support most of its weight, but I take a 45 degree line from each corner for the area of panel to support, and of the triangle intersects the roof structure, include that portion of the roof load too.

Mike McCann, PE, SE (WA, HI)

 

[KootK]

This recent thread on a similar subject may be of interest: Link

As I mentioned in that thread, I've been both the designer of these kinds of panels and the reinforcer as EOR. My thoughts:

1) Yeah, unless the depth of panel left over the opening is very shallow, it will behave as a pretty competent deep beam for vertical loads. In all likelihood, it requires no reinforcing. When folks do the channels, the most common intent is to, effectively, replace the localized bottom steel that you would have wanted in this location as a tie if the opening were there from the get go. It's mostly about flexural crack control since, as you've surmised, there will usually be other horizontal bars near by that can perform this function. And really, even the channel is kind of BS in this respect. There's always some slop in the connections and, by the time that flexural strain actually engages the channels, you've probably got visible flexural cracking anyhow.

2) Another, less common reason for channels (more likely HSS) is to deal with out of plane wind etc that a window delivers to the bottom edge of the remaining panel.

3) Keep in mind that it is not a given that the exterior wythe is shear connected to the interior wythe. You may create a situation where the exterior wythe needs to be a deep beam to support it's own self weight as well.

 

[conradlovejoy]

Thanks for you all your responses! This was helpful.

 

[msquared48]

No worries...

Mike McCann, PE, SE (WA, HI)