Store stålrammer – behov for afstivning: Guidance for Steel Frame Bracing
The official guidance Store stålrammer – behov for afstivning is published by the Danish Transport, Construction and Housing Authority (Trafik-, Bygge- og Boligstyrelsen). It provides a technical framework for the bracing of large steel frames in hall-type structures such as industrial warehouses, sports arenas, stables, and supermarkets. The document serves as a regulatory supplement to the Danish Building Act (Byggeloven) and the Building Regulations 2018 (BR18), specifically referencing Eurocode 3 (DS/EN 1993-1-1) for steel structures and Eurocode 1 (DS/EN 1991-1-3) for snow loads.
The scope of this guidance encompasses both the planning phase for new construction and the structural assessment of existing buildings. It details the technical requirements for Tværafstivning (transverse bracing) and Kipningsfastholdelse (lateral-torsional buckling restraint). Regulatory content is organized into chapters covering formal legal requirements, documentation standards based on Konstruktionsklasse (construction classes KK1 and KK2), bracing principles, and inspection protocols for identifying typical failure points like missing kropplader (web plates) or incorrectly executed rammehjørner (frame corners).
This document outlines the professional responsibilities of building owners, designers, and contractors. It provides specific tolerance values and load-bearing principles intended to prevent structural collapse or significant deformation under wind and snow loads.
Legal Requirements and Static Documentation
According to Byggeloven, the building owner is responsible for ensuring the structure is safe and properly maintained. For new projects, the documentation requirements depend on the assigned Konstruktionsklasse (KK).
- KK1: Includes agricultural and operational buildings without livestock; requires basic structural documentation for the building permit.
- KK2 and higher: Requires a certified structural engineer to coordinate documentation as per BR18, § 529.
- Statisk dokumentation: Must include specific methods for frame bracing and calculation of loads transferred from gables to the foundation.
- Kontrolplan: A plan identifying joints and structural elements subject to independent inspection during the construction phase.
Principles for Transverse Bracing
Transverse bracing (Tværafstivning) must secure the building against longitudinal horizontal loads, primarily wind pressure on gables and friction along the roof surface.
- Vindgitter: A system of bracing rods and purlins typically placed in the second bay from the gables to transfer loads.
- Tryk-/trækstænger: Rods used to connect gable columns directly to the wind girder system.
- Tagåse and løsholter: Components that hold the frames in place against the bracing system.
- Gerberbeslag: Specific joint types (e.g., Type A) required to transfer normal forces between timber purlins.
Lateral-Torsional Buckling Restraint
Under vertical loads like snow, negative moments occur at frame corners, creating pressure in the inner flange. Kipningsfastholdelse is required to prevent local buckling.
- Kropplader: Web plates must be welded into the steel profile at points where buckling restraints are attached.
- Dobbeltåse: Reinforced or double purlins used to provide sufficient moment stiffness for the restraint.
- Sprængværk: An alternative method using steel rods between wind girders to stabilize the inner flange.
- Rammehjørner: Corners must have continuous flanges and internal web plates to ensure structural integrity.
Snow Loads and Tolerance Standards
Specific environmental and geometric conditions dictate the required safety margins for steel frames.
- Special Snow Loads: Required for buildings near taller structures or those with east-facing facades (NNE to SE) in open terrain.
- Planar Tolerance: The maximum allowed deviation from the frame's intended plane is 10 mm.
- Inclination Tolerance: Frame legs must not tilt more than h/500, while general column inclination is limited to h/300.
- DS/EN 1090-2: The standard governing execution tolerances for the fabrication and assembly of steel structures.
Frequently Asked Questions
Who is legally responsible for the safety of a steel frame building?
Under the Danish Building Act (Byggeloven), the building owner is always responsible for ensuring the building is safe, properly maintained, and compliant with current legislation.
What is the maximum allowed deviation for a steel frame from its vertical plane?
The maximum allowed deviation out of the frame's plane during fabrication and assembly is 10 mm.
When is a certified structural engineer required for steel frame projects?
A certified structural engineer is required for buildings in Construction Class 2 (KK2) or higher, which typically includes industrial buildings and livestock housing.
What happens if a building has east-facing facades in open terrain?
According to the national annex to DS/EN 1991-1-3, buildings with facades facing between NNE and SE in open terrain must account for up to 50% extra snow load due to snow drifting.
Why are web plates (kropplader) necessary in frame corners?
Web plates are necessary to prevent local buckling and ensure that forces are correctly transferred through the continuous flanges of the frame corner.
Can portal openings affect frame stability?
Yes, if a door opening is placed between two frames, the rails (løsholter) must be dimensioned to function as buckling restraints, or the frame must be designed to stand without them.
