Should outward opening windows be used in high rise buildings?

Outward opening windows have been recorded as falling from buildings, creating near misses, causing injury and even death of pedestrians below.

There are no rules specifically forbidding the use of outward opening windows, but their risks above that of alternative opening styles (i.e. sliders, open in etc.) require careful consideration in a risk assessment to ensure that no greater danger is posed by an outward opening window than that of the equally available alternatives, as by their very nature, outward openers always project an element beyond the line of the façade or building face in their open condition.

One reason outward openers may be identified as being advantageous over that of the alternative styles is the ventilation area offered, as for similarly sized windows, outward openings may provide more free open area for ventilation than their inward opening counterparts.

Following the introduction of Approved Document O (ADO), new overheating requirements are likely to result in the amount of open area required in most residential buildings increasing dramatically.  This could favour outward opening windows.

Increased use of these types of windows to prevent overheating could lead to situations where many more outward opening windows are left open accidentally, including during extreme weather conditions. If several windows are now available per room, for example, if all are open, will an occupant or user remember to always close all of them? To meet ventilation requirements, windows may have to be fitted at lower levels (with appropriate guarding). Such windows could be left open on a regular basis as background ventilation, especially if, by virtue of having guarding in place, they offer little security risk while leaving the building unoccupied for periods of time and are potentially more awkward to open and close than more traditionally located windows.

If outward opening windows are to be used, consideration of accidental conditions in which windows are left open during storms or high wind conditions need to be addressed in line with the usage of the building. It would appear there are two general approaches, design of restrictor/stay, hardware and/or other components to maintain an accidental or extreme loading condition in the case of being left open. Or design the window, frame, and hardware for the condition where the restrictor/stay or similar becomes released in an accidental situation, that prevents the falling of the casement or any other components that may cause injury.

BS EN 14351-1 gives some consideration to testing a restrictor/stay, hardware and other components in place through the requirement of the system to hold a load of 350 N over a period of 60 seconds. It is not clear how the magnitude of this load was chosen, but it does lie between the point load values for residential barriers (i.e. 250 N to 500 N for domestic categories) from BS 6180.

It does not, however, appear to approach the load one might expect on a window during high winds or storm conditions. Even the most common small factor opening windows used in the UK when applied with the CWCT minimum wind loading (800 Pa) would still result in a load on the casement that is greater than 350 N. Therefore, consideration must be given to ensuring that the window is closed during extreme weather events if following BS EN 14351-1. If this cannot be controlled adequately, consideration could be given to providing restrictors/stays or other components that can withstand the expected wind loading in the accidental conditions as described previously.

Following the approach from BS EN 14351-1 “…threshold strength shall be demonstrated by means of tests carried out as described in EN 14609 or EN 948 (reference methods), or by calculation.”, it would seem reasonable that this approach could be considered for an accidental wind load condition as well. Bearing in mind that the test is not only checking the restrictor, but also the hinges, frame and associated other hardware, in which case a static calculation assessment should follow suit.

As it is an ‘accidental’ condition, one could reasonably expect to adopt this as an Ultimate Limit State condition, so deformation of the components, but ability to continue carrying the load may not result in a ‘failure’.

Revision 1, 25 May 2023

Previous post Next post


Leave a comment