Large Timber Trusses Bracing Guide – Danish Building Regulations Overview
The document Store træspær – behov for afstivning is a regulatory guidance issued by the Trafik-, Bygge- og Boligstyrelsen (Danish Transport, Construction and Housing Authority). It provides specific technical instructions for the lateral bracing of large timber lattice trusses, which are commonly used in wide-span buildings such as supermarkets and agricultural stables. These trusses are typically manufactured from 45-60 mm thick planks, making the structural components slender and reliant on a secondary bracing system to maintain stability and prevent collapse under vertical and horizontal loads.
The guidance is situated within the legal framework of the Danish Building Act (Byggeloven) and the Bygningsreglementet 2018 (BR18). It specifically references technical standards including Eurocode 5 (EN 1995-1-1) for timber structures and Eurocode 1 (EN 1991-1-3) for snow loads. The document covers regulatory domains such as structural design responsibility, principles for transverse bracing, and specific requirements for complex truss types including Saksespær (scissor trusses), Valmspær (hip trusses), and Spær med løs top (trusses with separate tops).
This guidance is intended for property owners, building designers, and contractors involved in both the planning of new wide-span buildings and the inspection of existing roof structures. It outlines the formal distribution of responsibility between owners and structural designers while providing an 8-point checklist for identifying structural deficiencies in standing buildings.
Legal Requirements and Responsibility
Under the Danish Building Act (Byggeloven), the building owner is legally responsible for ensuring the structure remains safe and compliant with legislation. Municipalities can order owners to rectify unsafe conditions under Byggeloven § 17, stk. 1.
- Bygningsreglementet 2018 (BR18): Establishes the general requirements for structural safety and references Eurocodes for specific design values.
- Bygværksprojekterende: The lead structural designer must be appointed during the permit application process to coordinate documentation for the entire structure as per BR18 § 529.
- Control Report: Documentation of the executed construction is mandatory to ensure compliance between the design and the physical building.
Principles of Bracing and Stability
Timber truss systems must be braced transversely to handle combined loads from self-weight, wind, and snow. The bracing system serves two primary functions: preventing buckling in compression-sensitive parts and transferring longitudinal wind loads to the foundation.
- Gitterbjælker: Lattice beams placed between trusses to provide lateral stability.
- Stringere: Longitudinal members placed near the ridge (Kipstringer) and the eaves (Tagfodsstringer).
- Vindkryds: Diagonal wind bracing, often made of steel straps, to absorb horizontal forces.
- Lodrette forankringer: Vertical anchors that secure the bracing system to the building's foundation.
Vertical Load Management
Vertical loads create compression in the Spærhoveder (top chords) and specific internal Gitterstænger (lattice members). Since these are slender elements, they act as columns that require lateral support to prevent outward buckling.
- Spærhoveder: Must be held in place by battens that are themselves secured to bracing elements like lattice beams.
- Gitterstænger: Specific members marked with TVÆRAFSTIVNING (transverse bracing) require additional support using battens or boards.
- Snelast: High snow loads can cause temporary deformations; buildings near higher structures or in exposed areas require special calculations according to Eurocode 1.
Inspection of Existing Buildings
Existing buildings with truss spans wider than 14 meters (or 12 meters for scissor trusses) should be inspected regardless of age. The guidance provides an 8-point checklist for site inspections.
- Structural straightness: Check if top chords are bowing or if the roofline shows an S-shaped pattern.
- Bracing elements: Verify that lattice beams are installed at each gable and at least every 15 meters.
- Modifications: Ensure gitterstænger have not been cut to accommodate ventilation ducts.
- Gable stability: Confirm that gable columns are properly connected to the roof's bracing system.
Frequently Asked Questions
Which buildings require a mandatory inspection of timber trusses?
Buildings with timber trusses wider than 14 meters, or scissor trusses wider than 12 meters, should be inspected for bracing deficiencies.
What are the spacing requirements for lattice beams (gitterbjælker)?
Bracing lattice beams should be placed at each gable and at intervals of no more than 15 meters along the building length.
Who is responsible for the safety of a roof structure under Danish law?
The building owner is always responsible for building maintenance and safety according to the Danish Building Act.
How do you identify if a truss member requires extra bracing?
Internal lattice members that require transverse support are physically stamped with the text TVÆRAFSTIVNING.
What does an S-shaped pattern in the roof tiles indicate?
An S-shaped deformation in roof tiles or sheets often indicates that the trusses have buckled due to insufficient transverse bracing.
What is 'spær med løs top' and why is it significant?
These are trusses manufactured with a separate top section for transport; the horizontal collar beam (hanebånd) created at the joint requires specific lateral bracing.
