PEMB frame deformation due to foundation settlement 3

[bones206]

I'm evaluating a 40 year old PEMB that has settled over 16" from front to back. The building is 200' x 220', with 4-bay frames spaced at 44'. I am involved in a relatively high-profile retrofit project at this building, with new mezzanines and equipment foundations, but the settlement findings have paused the project while we await results of a geotech investigation.

Meanwhile, I had the building laser-scanned and noticed significant frame deformation in the scan model. Looking in plan view, the frame beams at the roof are deflecting out-of-plane in a sine wave shape (i.e., buckling mode shape). The columns are twisted at the top following the deflected shape of the beams.

Obviously, this is not a good situation. I think the fly braces are doing a lot of work to prevent a full on buckling failure and frame collapse. The fly braces are L2X2X1/4 angles field-welded at the frame bottom flanges and the bar joist roof purlin bottom chords. These connections do not inspire confidence at all, but are critical to the stability of the building.

I'm being asked by the client to assess if there is a safety risk and to propose mitigation and repair options. Preliminary geotech results indicate that piles would be required to arrest the ongoing settlement. I have conceptual pile designs in mind, but I am looking for opinions on the building frame aspect. My inclination is to declare the building basically "totaled". I don't see how you can undo the buckling shape in the frames, and I believe that deflected shape indicates the frame elements are already stressed beyond their design limits. Given there is a lot at stake with this project, the client wants to see some more robust analytical "proof" that the building is not salvageable.

I am content to be the bad guy in this case, since my gut instinct is telling me the building might not be able to sustain a design-level snow load. On the other hand, the building has probably stood this way for decades, and maybe I am overlooking some structural heroics that could allow the building to be safely occupied into the future.

So my questions are: 1) how can I "prove" that a building is beyond saving, besides relying on my reputation and judgement? and 2) any longshot ideas on how to save the building?

 

[bones206]

@driftLimiter - I have a copy of ASCE 41-13 and have started wading through it. There may be an avenue here using a Tier 1 checklist assessment to show that it does not meet the basic Life Safety performance objective for seismic.

@Brad805 - here are some images. It's kind of hard to make out, so I added red horizontal reference lines along each frame line and attached the full image as a separate file. All the frame lines in the building follow a similar deformed sine wave shape.

Local distortion at sidewall braces, buckled roof brace, slacked roof flat strap X-brace (although the flat strap probably always looked like that from day 1):

Slack end wall cable X-brace:

 

[Smoulder]

Calculate stress from curvature if scans are accurate enough. Compare to yield less allowance for locked in stress of manufacture. Throw away if not still elastic.

If still in the game, look at jacking back to level off new piled foundation, or releasing and replacing bracing in sequence. Depends on dead load whether safe and feasible.

 

[bones206]

Smoulder - I think the scan model is accurate enough to do a rough analysis based on lateral displacements and curvature. I suspect the frame members are still in the elastic range, as these frames likely see significantly more deflection during the shipping and erection process. Less sure about the column twist being elastic, but an analysis could probably reveal the rough magnitude of stresses due to torsion.

I've thought about jacking the building back to level, but it seems too complex and perilous. I feel like I would be more likely to inadvertently break something (like the fly brace connections) in the process and further destabilize the frames. Maybe I could add some new, more robust fly braces prior to jacking the frames up. But in the end, it still might be cheaper to rebuild on a new foundation than trying to salvage.

 

[dold]

bones, in your second pic above it looks like the column is already yeilded at the brace connection, no? Code checks go out the window post yield so I don't know how you'd approach this. Like someone said earlier (maybe you?) you could probably pretty easily replace the end-wall columns (non-frame columns). Frame columns...not so much?

 

[bones206]

Correct, the column at the brace connection has yielded locally. That is just a wind column, so not hard to repair or replace. The same deformation is found in other wind column to brace connections throughout the building.

 

[SandwichEngine]

These things can bend a lot in their weak axis without yielding. If you can get it straightened out and nothing has yield line, then it's remained elastic.

https://youtu.be/V7zOZbCQ94M?si=qPRy6IQNFzt9Jw8p

I'm guessing a PEMB erector would be a good resource for how to get the rafters back to being straight. Or to say that you can't for some other practical reason.