Truss Structures
Truss Bracing System for Long-Span Excavation Support
What Is a Truss Bracing System?
On excavations with long spans, bracing opposite faces with a dense grid of simple struts (conventional cross-lot bracing) means many members and intermediate posts, badly crowding the work area. A truss bracing system carries the same load with far fewer members, keeping the excavation floor open for soil removal and construction of the permanent structure.
How Does a Truss Bracing System Work?
A truss consists of two horizontal chords — top and bottom — connected by diagonal or vertical web members. The truss’s depth, the distance between the chords, lets it carry the lateral load from opposite excavation faces as a tension-compression couple in the chords, instead of needing a dense network of simple struts and intermediate posts. It’s the same principle that lets a bridge or roof truss span long distances with far less structural weight than a solid beam of the same span.
Connection to Soldier Piles and Column Bases
The truss connects at each end to the soldier piles of the retaining wall. Designing this connection — along with the bases of any columns supporting the truss — has to properly transfer the truss’s concentrated reaction force into the wall system. Getting this detail right is one of the most critical parts of designing a truss bracing system.
Why Choose a Truss Over Simple Cross-Lot Bracing?
• Long spans: once strut length passes the economical limit for a simple section and buckling becomes the governing design issue, a truss is the more efficient alternative.
• More working space: removing the many intermediate posts a simple cross-lot system would need leaves the excavation floor far more open for equipment and permanent-structure construction.
• Less structural weight: for the same span, a truss is usually lighter overall than the equivalent set of simple struts.
Fabrication and Installation
Truss members are typically shop-fabricated and assembled on site. Fabrication and installation proceed in step with the excavation: each section of the truss goes up once the corresponding excavation level is ready, so no face of the excavation is ever left unbraced. Design Considerations • Thermal effects: like any steel bracing structure, temperature changes affect the internal forces in the truss members and must be accounted for in design and monitoring.
• Overall stability: lateral and out-of-plane stability of the truss, especially at the soldier-pile connections, must be checked separately.
• Staged loading: the load on the truss changes through the excavation stages and must be checked separately at each stage.
Where Does Truss Bracing Apply?
This method is the best option for long-span excavations where simple cross-lot bracing would need too many intermediate posts or oversized struts — particularly when open access to the excavation floor for equipment or fast construction of the permanent structure matters.
Frequently Asked Questions
How is a truss bracing system different from simple cross-lot bracing?
Simple cross-lot bracing uses straight struts, which need many intermediate posts on long spans; a truss covers the same span with a triangulated arrangement of chords and web members, using fewer members and leaving more open space.
Why does the truss-to-soldier-pile connection matter so much?
All of the truss’s bracing load eventually has to pass through this connection into the retaining wall; a weak detail here can compromise the performance of the entire bracing system. At Ziggurat, we design the truss, the supporting column bases, and its connection to the soldier piles, and carry out fabrication and installation in step with excavation progress.