What is a strut frame?
A strut frame (also called a bracing frame, Dutch stempelraam) is a temporary structure of mainly horizontal struts that stops the walls of an excavation, usually steel or concrete sheet pile walls, from failing under the pressure of the soil and groundwater outside. The struts span across the excavation from one wall to the opposite one, so the pressures from both sides balance each other.
How it works
A strut frame has two main parts:
- Walings (walers): horizontal beams, often steel sections, fixed along the inside face of the sheet piling. They collect the load from the individual piles and pass it to the struts.
- Struts: compression members between the walings on opposite walls. At the corners, diagonal raking or corner struts carry the load.
Deep excavations may need several levels of struts. The usual sequence is: install the sheet piles, excavate partway, fit the strut frame, then dig on to formation level. Once the permanent slab and walls can take over, the frame is removed. Ground anchors tying the wall back into the soil outside are the main alternative when struts would get in the way.
Application
- Have it designed as temporary works. A geotechnical or structural engineer designs the frame based on a site investigation and groundwater levels.
- Plan the build around it. Struts obstruct cranes, formwork and reinforcement. Agree their positions with the builder in advance.
- Ensure full contact. Gaps between sheet piles and walings are packed, for example with grout bags or steel packers, so the load transfers evenly.
- Remove in the right order. Only take out struts once the permanent structure or an intermediate prop has taken the load.
- Protect against impact. A strut hit by a machine can lose capacity. Keep plant clear and have any strike checked immediately.
Common mistake: digging deeper than the design allows before the next strut level is installed. The wall deflects and the surrounding ground settles with it.
Related terms
- Excavation pit
- Sheet pile wall
- Dewatering
- Berlin wall construction
- Retaining wall
- Propping
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