3D Wind-Load Generator - Roof Overhangs (protruding roof)

Wind load generation on roof overhangs is considered in accordance with the EN 1991-1-4:2005 (E), figure 7.3.

Roof overhangs - how to model properly

All the roof overhangs must be modelled as separate entities, e.g. load panels or slabs. For these entities, "3D wind" property needs to be turned on. Afterwards, for the corresponding "wind data", the "roof overhangs" property needs to be turned on. As shown in the figure below, this needs to be done for all the structural parts where the roof should be treated as an overhang (#A). For all the other parts of the roof, the property "roof overhangs" should be turned off (#B).

The roof overhang is a special type of the roof, where no Cpi coefficient is considered (it has no interior, but rather two exterior surfaces - upper and bottom). In order to achieve correct automatically generated wind loads, the normal of load panel (or slab) needs to point upwards (be on the upper surface of the roof overhang - #D). The arrow of the "wind data" should have the same direction (#C).

Pressure load on roof overhang consist of two parts:

1) Pressure at top side found from roof pressure - based on the Cpe coefficient of the corresponding roof part, the same Cpe coefficient for this part of the load is considered. Cpi coefficient is always considered as 0 for the load applied on the roof overhang.

Note: in the case below, the Cpe of the corresponding roof part is -0.181, and Cpi = -0.2. Hence, there is suction on this part of the roof, because the difference between these coefficients is (Cpe - Cpi) = -0.181 +0.2 = +0.019. However, the -0.181 of the Cpe introduces pressure, what is the reason why there is pressure on the roof overhang based on the load applied on the roof part.

2) Pressure at underside found from wall pressure - based on the Cpe coefficient of the corresponding wall part beneath the overhang, the same Cpe coefficient for this part of the load is considered. Cpi coefficient is always considered as 0 for the load applied on the roof overhang.

Note: in the case below, the pressure at the underside of the roof overhang is graphically plotted by the same colour as the suction. This is due to the fact, the pressure from the underside is equivalent to the suction from the upper side. The roof overhang (load panel, plate) has only one mid-plane towards which the surface load is considered, hence the character of the load (pressure, suction) is always related to the upper surface (defined by the blue arrows).

When the blue arrows (normal of the plate, or load panel) or the arrow of the wind data of the roof overhang are oriented differently then described in this chapter, the loads generated on the roof overhang might not be in accordance with the provisions by the EN 1991-1-4:2005 (E), figure 7.3.