Wind Uplift
Euroclad load span tables for our range of profiles, prepared by the SCI working to the new Eurocode standards, are due for publication early in 2008. Alongside these the publication of load span tables working to the new Eurocodes for our range of Eurobar, Eurobar Extra and Mast spacer systems will enable designers and engineers to have ultimate confidence in our systems as a whole. This is reinforced further by BBA Certification for Elite Systems which cross checks published performance data against a rigorous testing regime.
For a System covered by a BBA certificate, the whole system will be tested including spacers, liners and fixings etc.
Tests include both imposed and suction UDL (Uniform Distributed Load) tests at a number of different to establish:
(i) the loading under deflection criteria for both positive and negative loading.
ii) the failure load and method of failure when tested to destruction for both positive and negative loading.
Wind Load Overview Part 1: Calculation of Loads
Wind loads directly affect the wall and roof finishes of buildings placing both imposed (+ve) and suction (-ve) loads of differing magnitudes on different areas of the building envelope.
These loads need to be considered in the design of both the roofing and cladding and the supporting structure. For design purposes the wind loads predicted for buildings should currently be calculated according to BS6399: Part 2: 1997 Code of practice for wind loads.
The calculations should be carried out by a competent person such as a structural engineer and will help guide design decisions regarding both structural steelwork and any cladding which is fitted as the building envelope. The steelwork setout may dictate the cladding profile and gauge chosen or the choice of cladding may influence the steelwork set out.
Reference should also be made to BS EN 1991-1-4:2005 Eurocode 1. Actions on structures. General actions. Wind actions This recently published European Standard, a part of the BS EN 1991 Eurocode series, provides guidance on the determination of natural wind actions for the structural design of building. It is intended to predict characteristic wind actions on land-based structures, their components and appendages. The UK National Annex to this standard is expected to be published December 2007.
Loads as calculated will vary according to factors included in the calculation which are project specific. Location factors such as geographical location, whether the building is in open country or a town, proximity to the sea, wind exposure categories and terrain factors form the first stage of a calculation. Site specific and building geometry factors are then also built into the calculation such as height, single or multi storey, roof pitches and further modifying factors such as presence of dominant openings, proximity of other buildings and funnelling effects.
The calculation will ultimately produce a “zoning map” of the building envelope areas showing the imposed or suction loads applicable in those areas. On a typical roof the perimeter areas will have higher loadings than the middle of the roof with the most concentrated loads occupying zones at the eaves corners and ridge corners. The setting out of structural steelwork directly affects the performance of any roofing or cladding fixed to it as the performance under wind load of the roofing/cladding is usually determined by the span between structural supports to which it is attached.
On a typical wall the highest loadings will often appear at corners in the lower half of the elevation. Wall loadings are usually less onerous than the worst case perimeter roof loads. Wind suction loads are often the limiting factor for envelope products and particularly for roof sheets. Snow loads as an imposed load may also need to be considered for roofs and some elements such as areas adjacent to parapets may need careful consideration.
Part 2: Euroclad Profiles Performance
Profiled sheets for external cladding of both roofs and walls as produced by Euroclad are supported by load/span tables. These tables allow a building designer to check that the sheet is capableof taking the wind (and snow) loadings to which it will be subjected.The sheets must not become too stressed or deflect to an extent where the profiles primary functions of weathertightness, air tightness and durability are compromised.
Load Span Tables – Design Case Definitions
Moment - is the ultimate bending moment capacity of the profile under positive or negative loading either at the support or at mid-span.
Inertia - is the moment of inertia or second moment of area of the profile and determines the deflection characteristics at working load within the elastic range.
Reaction capacity - This is the capacity of the profile webs to support the concentrated reactions at the supports (either internal or ends).
Interaction - This is a check to see if the combined bending and web crippling stresses at the internal supports do not exceed the critical stress capacity of the webs (e.g. control of web buckling under concentrated stress).
Bending & Shear – This is the capacity of the profile to resist bending and shear forces
BS EN 1993 Eurocode 3. Design of Steel Structures.
This European Standard, a further part of the BS EN 1991 Eurocode series covers several key areas of concern regarding cladding design eg:
BS EN 1993-1-1:2005 covers General Rules and Rules for Buildings. UK National Annex documents are expected to be published in Dec 2007.
BS EN 1993-1-3:2006 Supplementary Rules for Cold Formed Members and Sheeting. The UK National Annex to this part of the standard is expected to be published in 2008.
The current recommended deflection parameters for profiled metal sheets are given in BS 5427-1: 1996.
For roofs: Dead load: L/500, Dead and imposed loads: L/200, Dead and wind loads: L/90
For wall cladding : Dead and wind loads: L/120.
Where L = the span distance for the sheet (in most cases the distance between supports which are dictated by steelwork positions.
Sheet deflection limits are calculated dependent on the material specification and geometry of the sheet and are cross checked against test data for validity. The sheets deflection limit is a Serviceability Limit State rather than an Ultimate Limit State and as such can be checked against unfactored loads. However, a further check for Ultimate Limit State is required which uses factors of safety from BS 5427. They are 1.6 for normal imposed loads, 1.4 for wind loads and 2 for attachment.
The attachment of the SF500 profile side lap, for example is BBA tested to Ultimate Failure. The failure is checked under Ultimate Limit State factored to 2.0 (attachment) against the sheet deflection limit under Serviceability Limit State.
So long as the deflection limit is not exceeded at the design load and the Ultimate Failure load is 1.4, 1.6 or 2 times greater than the design load depending on the load case and failure mode, then the BBA Certification requirements will be satisfied.
