building addition - 5 story wood

[m1ke99]

I am looking at an expansion for an existing 5 story wood frame structure. (Residential)
The existing doesn't have capacity for additional gravity loads so the structure will need to be independent.

With the effects of settlement (footings) and shrinkage, I am concerned about the differential movement between new and existing.
What details are typically provided at the transition to deal with the movement?

 

[KootK]

Are there basements involved? Both systems are shallow foundation, right? Whenever possible, I'll tie the two foundations together vertically.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[CELinOttawa]

A gap. No sarcasm... Specifically a seismic / wind drift gap, or movement joints. It tends to be a problem for you and the Architect both. If you don't have the capacity and must not load the existing, keep your structure stiff and the gap 50% larger than you calculate you need. The unknowns in the existing will get you every time.

Possibly helpful: I had a gap with a steel cover plate and the Architect used a PVC siding product and gaskets to cover for a VERY budget-obsessed client. Ironically the larger the gap in your case the better. Build with some (maybe 2%) slope up to the new building and detail for / live with the eventual reversal of this slope.

 

[CELinOttawa]

Good point Kootk: Either tie foundations together and make your new one able to take the differential settlement out, or separste them as per the superstructure. Not at all difficult, unless you have to have deep basements or high water table issues...

 

[XR250]

Why not tie the foundations AND the framing together while still designing the addition to carry its own weight so to speak. Why leave a gap?

 

[CELinOttawa]

XR250: You can, but now your new building needs to do double duty. Effectively the reasons are complexity of modelling/behaviour, and greatly increased construction cost.

That is, however, how I have upgraded both URM and post-disaster structures in the past, so it depends on the case at hand.

 

[XR250]

XR250: You can, but now your new building needs to do double duty. Effectively the reasons are complexity of modelling/behaviour, and greatly increased construction cost.

The OP was worried about gravity loading. Are you concerned about deflection compatibility?

 

[CELinOttawa]

Both deflection compatibility and settlement. Over five storeys it doesn't take much differential settlement to get one structure effectively "leaning on" the other...

 

[XR250]

Over five storeys it doesn't take much differential settlement to get one structure effectively "leaning on" the other...

Is that really a problem? Wood construction seems to have the ability to accommodate these issues nicely. Probably beats having to detail and implement a movement joint which may cause trip hazards within the building.

 

[CELinOttawa]

You're likely right, and in isolation with good geotech design and construction it probably isn't an issue at all. The issues with keeping the two structures apart remain, regardless. I've never worked in a jurisdiction with low enough both wind and seismic to allow those kind of forgiving conditions, but maybe the OP does.