What is a stiffening rib?
A stiffening rib, also called a stiffening beam, is a reinforced concrete beam cast together with a strip footing in a shallow foundation. In Dutch it is called a verstijvingsbalk or verstijvingsrib. The strip spreads the load over the ground; the rib on top makes the whole much stiffer, so the foundation deflects less and loads are spread over a greater length. It is also referred to as a shallow foundation with rib. It is a specific application of a stiffened girder.
When is a stiffening rib needed?
- Non-uniform ground: if the bearing capacity of the soil beneath a building varies, a soft spot can lead to differential settlement and cracks. A stiff rib bridges such a weaker zone.
- Large wall openings: with a wide shopfront or garage door, the load of the facade is concentrated on the piers beside it. The rib spreads those point loads over the strip.
- Varying loads along the foundation, for example from a heavy chimney or a taller part of the building.
Build-up
The foundation consists of a wide, relatively thin strip footing with a narrower, taller rib on top. Strip and rib are cast in one go or joined with a proper construction joint, and reinforced together. Cross-section, height and reinforcement follow from the structural calculation based on the ground investigation.
Application
You come across the stiffening rib in new build and in large extensions on shallow foundations, especially on sandy soils where a piled foundation is not needed but the ground varies locally.
Tips and points of attention
- Have the ground investigated: a cone penetration test shows whether a shallow foundation is possible and whether there are weak layers.
- Have a structural engineer do the calculations: the dimensions of strip and rib are not a standard size.
- Plan openings: pipe penetrations through the rib must be made with sleeves before pouring, at positions approved by the structural engineer.
- Extension against an existing house: watch out for differential settlement between the new and old foundation; a movement joint is often needed.
Related terms
- Strip foundation
- Settlement
- Stiffened girder
- Reinforcement
- Cone penetration testing
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