Differential Building Movements

[PostFrameSE]

I've got a quite long wood-framed project where there are two different foundations under the building. The bulk of the building length is setting on a drilled pier foundation with minimal expected settlement and the other is on a floating slab and is expected to move up to perhaps 2". This foundation transition occurs where my roof line and walls are continuous (no step-downs or anything.) I'm interested in knowing how others have considered similar situations and what type of expansion/slip-joint type of details have been employed. I am using wood trusses and corrugated metal roofing for the entire roof. My wall cladding DOES change materials at this juncture where the foundation types change and where I would expect to see movement between the two areas of building.

It seems to me that some sort of bunched up rubber membrane would be in order along the walls covered by a trim that is only attached to one building. For the roof...................I'm thinking that my wood trusses spaced 8' o.c. and purlins at 24" o.c. that I could "hinge" both ends of my purlins and allow the metal roofing to change slope over the course of 96" without any crinkling, oil-canning or leaking.

Any thoughts would be appreciated.

Thanks.

 

[JAE]

Expansion joints and allowance for the differential movement across those joints is of course in order.

However, you might still have the issue of floors being offset, which you cannot allow (trip hazards, etc.).

I did a project some years ago where it had drilled piers on one half and spread footings on the other.

What we did was alter the spread footing sizes as we approached the joint such that the soil pressures were dropped very low, sequentially increasing the sizes each bay.

The idea was that we would reduce the differential to essentially nothing as we approached the drilled pier structure.
That costs extra money to do of course.

The only other option would be to bridge to the joint - similar to an approach slab on a bridge (i.e. highway pavement transitioning to pile supported bridge over 25 to 50 feet).
For the spread-footing bay adjacent to the joint, use a suspended slab that spans from spread footings on one side to the piers on the other. This will not avoid the differential settlements but it will spread them out over a bay length so you don't have vertical dislocations all at the joint.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[PostFrameSE]

Thanks JAE! Interesting thought. I appreciate you responding.

 

[hokie66]

I support providing structure to bridge the first bay of the soil supported part onto the piled section. I would also provide movement joints in the roof structure, roofing, walls, wall cladding, finishes, etc. In essence, there needs to be a structural separation joint. If you don't provide it now, the structure will provide it later.

 

[asixth]

It's a tough problem to face. Has a geotech provided expected settlement for an advised working load. Even then the real behaviour of footings varies a bit, particularly when high and low levels are in the mix.

I would try a foundation system that mitigates the heave/settlement issue. But I'm guessing the cost to construct is preventing that.