FAQ 002924 EN-US 04/15/2019

Markus Baumgärtel Modeling | Loading RFEM RSTAB

When generating wind loads, some external pressure coefficients have a value of 0. What is the reason for this?


For duopitch roofs with a roof iclination of > 5 °, the roof areas F, G, H, I and J have to be separately classified according to the windward and leeward side. For the wind direction of 0 ° (wind in longitudinal direction), positive as well as negative aerodynamic coefficients have to be taken into account for roof inclinations of up to 45 °.
For these cases, this results in a total of 4 possible wind combinations, depending on the building side (see Figure 1).
For the wind direction of 90 ° (wind on gable side), however, there are no positive external pressure coefficients for a roof inclination of > 15 °. For a building with a roof inclination of 45 ° you would receive 10 possible wind load cases (0 ° = 4 * 2, 90 ° = 1 * 2). 

LC w+:

Only positive (pressure) aerodynamic coefficients per roof area are used.

LC w-:

Only negative (suction) aerodynamic coefficients per roof area are used.

LC w-/+:

Negative (suction) aerodynamic coefficients for the windward side and positive (pressure) coefficients for the leeward side of the roof are used.

LC w+/-:

Positive (pressure) aerodynamic coefficients for the windward side and negative (suction) coefficients for the leeward side of the roof are used.

If there are, for example, only negative coefficients for a load position, then only negative loads are applied to the roof surface. Consequently, there is no pressure -> therefore these values are set to 0. A load case, which thus contains only values with the size 0, could also be deselected during the generation.

For example, this is always possible, as already described, for the LC w + with a wind direction of 90 ° (gable-sided wind) and a roof inclination of > 15 °. 


Wind Aerodynamic coefficient External pressure coefficient Combination wind Wind direction

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