04.10 Roofs
Open full view...Categories: Structural Systems
Introduction
The structural design of roof components follows the same general format outlined within section 04.05, Stick Systems, taking due account of the particular conditions outlined below.
Loading
Roofs tend to be inclined surfaces and this introduces the need to consider snow loading within the structural analysis as it may add substantially to the applied loads. The snow load criteria are set out within BS6399: Part 3 and depend upon site location, building size, roof slope and whether or not access is provided. These result in minimum snow loads of between 0.6kN/m2 – 1.5kN/m2 measured on plan, dependent upon the conditions applying. The snow load as with all other gravity loading will act vertically downwards.
Note that when dealing with wind loads on inclined roof surfaces that such load will be applied normal to the surface being considered. This will result in both horizontal and vertical components.
Combined load cases may benefit from due consideration of climate conditions. Snow loading will tend to accumulate on north facing slopes and drift in valleys and troughs, and be associated with northerly or easterly winds, whereas peak wind loads will normally be associated with the south westerly direction.
Design criteria
The limiting deflection criteria for roofs tend to be more relaxed as they do not tend to support any other primary components and are far enough away from public view such that induced curvature is not a visual concern. However, deflections still need to be identified particularly in Atrium situations. The high glazed component within atria roofs leads to very high solar gain, which can give rise to very high thermal movements or stresses. In such situations connection details and sealents need careful attention to detail to ensure continuity is maintained and leaks and/or breakage do not occur. Poor ventilation and heat flow characteristics can exacerbate the situation.
Safety criteria associated with glazed roof components will also require attention with due consideration given to safety aspects of the glazing, Section 08.08.