Experience has shown that steel windows are suitable for the majority of sites and exposure conditions in the UK, where an exposure rating of 1200 Pascals satisfies most locations. Extensive laboratory prototype and production audit testing give typical results tabulated below, classified in accordance with BS 6375-1. The extra weatherseals introduced in W40 to provide an external rainscreen, internal airseals and a pressure equalised cavity have significantly improved resistance to air and water penetration in severe exposure conditions, particularly for windows that open inwards.
European harmonised testing methods are specified in the window and door product standard BS EN 14351-1, and guidance on their application in the UK can be found in BS 6375-2 Performance of Windows and Doors – Operation and Strength.
Strength is measured by subjecting casements to twisting with three corners held tight whilst the fourth is moved by a calibrated force. The idea is to simulate attempts to release a jammed sash. W40 casements achieve the maximum class 4, surviving a 350 Newton force on one corner with negligible deflection, exceeding the UK requirement of class 3 (the ability to resist a force of 300 Newtons).
W40 steel windows have also been subjected to weights hung from the handle jamb, designed to simulate an accidental vertical load. Casements barely moved out of square and promptly returned to their original shape when the load was removed. The UK requirement is class 3, resisting a load of 600 Newtons, and W40 steel windows satisfied the highest class 4 (a load of 800 Newtons).
Throughout the test programme for weathertightness and strength, units were continuously checked to ensure they satisfied ease of operation requirements by measuring the forces required to open and close them. All met class 1 requirements, a torque of no more than 10 Newton metres to turn the handle and a maximum force of 100 Newtons to push them open or pull them closed.
The inherent strength of steel ensures maximum rigidity once the window frames have been installed and glazed. The most secure steel window is a regular, standard window fitted with key operated locking devices and divided into small panes of glass, with vertical and horizontal bars tenon-riveted or welded into the frame and strong intersecting joints. Even if the glass is broken, entry cannot be gained through this type of window. Specifying laminated glass makes it even more resistant to intrusion.
A high measure of security can be achieved with large pane steel opening windows by using multi-point locking devices. A supplementary ventilator, mounted in the top glazing rebate, will give permanent or controlled ventilation with locked window security.
Following successful tests for enhanced security to BS 7950, W40 products, both open in and open out, fitted with concealed multi-point locking bolts and friction stay hinges or brass butt hinges, satisfy the Secured by Design standard. Doors of cold-formed tubular steel have been tested in conformity with EN 1627 for burglar.
Safety in Use
To ensure that a window holds its glass in place and remains closed even under impact from a heavy body, it can be tested to BS EN 13049 where a heavy impactor, consisting of two lorry tyres enclosing a steel cylinder and weighing 50kg, is swung into the window. Five classes are defined for this test, ranging from dropping the impactor from 200mm (class 1) to raising the impactor to 950mm before letting it go (class 5). The full range of W40 steel windows has been submitted for this test, including fixed lights, hinged and pivoted casements, and they survived the maximum impact to satisfy class 5.
It is important to verify that safety restrictor devices are sufficiently robust to withstand abuse. Those fitted to steel windows by the manufacturer have been tested to ensure that they remain engaged when subjected to a horizontal load in excess of 350 Newtons (35 kg).