Mixed Systems

[haynewp]

What experience does anyone have on mixing foundations around the perimeter of a building where the facade is supported on shallow foundations independent of deep foundations supporting the columns/structure? There would be movement joints between the footings and drilled shafts allowing for differential settlement (a few inches of compressible material as the footings pass over the shafts).

Similar to thread256-152238 but my question involves the perimeter with precast facade and in the 4 to 6 story range.

 

[Ron]

There's no way I'd support the facade on a foundation system separate from the main structure. Differential settlement will put an inordinate amount of stress on the connection between the facade and the structure. The facade, the waterproofing, the windows...all will suffer.

 

[haynewp]

What it the connections from the facade to the structure were slotted for vertical movement, with the entire facade set out from the structure as a separate vertical system carrying only its self weight?

 

[hokie66]

It would depend on the type facade and the type of building. For a lightweight metal facade around a parking deck, the bottom supported, guided connections approach could work.

For a precast facade, I would always support it on the structure, and at each floor.

 

[BigHarvey]

Never mix deep foudations and shallow ones in a single structure.

 

[a2mfk]

4-6 story building with precast concrete panels?

Don't do it, unless you want to give Ron some forensic work in a few years. I have seen problems on single story houses that mix the two systems together.

I don't have much PCC panel experience, but does this not need to be supported by brackets at each story like Hokie said? Trying to imagine how an independent gravity system would even be possible.

 

[haynewp]

It's bypassing the frame similar to how curtain wall bypasses a frame. I didn't want to go this direction but we got value engineered which took out the grade beams and additional deep foundation requirements.

Fundamentally, I don't see why it can't work if the weatherproofing details account for the facade movement through the entire blg height, which is expected to be about 3/4" without remediation. I am most concerned with the differential where there is a switch from deep to shallow bearing due to long spandrels (they are hanging the panels from the columns at a couple of areas due to long spans). Again the columns go to the deep bearing.

The panel connections are all vertically slotted. But the panel horizontal joints won't line up where it switches from deep to shallow bearing. The geotech has suggested preloading to take out most all of the shallow settlement.

 

[hokie66]

Preloading takes a lot of time. How will that make the building more economical? I don't think it makes sense.

 

[haynewp]

Panel self weight, geotech said 90% of the settlement should be out in the first week. The whole project has been a total PITA.