Custom Roof
The Custom Roof option in MecaWind Ultimate will not solve all of your custom building problems, but it will handle some specific challenges that designers may face. The Custom Roof option is available for gabled, monoslope, and flat roofs in the Ultimate version of MecaWind. To summarize, the Custom Roof option allows you to design a roof that is not symmetrical. For example, in the image above, there is a gabled roof in which the ridge does not fall on the geometric centerline of the building footprint. The ridge is offset, and the overhangs are different sizes on each of the four sides. The Custom Roof option will allow you to model this situation in MecaWind. The monoslope and flat roofs have a custom option that allows their overhangs to be unique on each of the four (4) sides as well.
Custom Gabled Roof Example
The Custom Roof option is best illustrated with an example. Below is a summary of the dimensions of the gabled roof we are going to analyze. As you can see, the ridge is offset 8 ft from the centerline of the building, and the slopes are different on each side of the ridge.
Custom Gabled Roof Example
Main Wind Force Resisting System (MWFRS)
The Main Wind Force Resisting System (MWFRS) analysis must follow Chapter 27 Part 1 (assuming ASCE 7-10 or later is being used), since it is the most general method needed for the Custom Roof option. We do it similarly to a standard building, except that we must now check the wind loads from four directions. We consider the wind load acting perpendicular to each face of the building.
Custom Roof Wind Directions
The MWFRS design follows ASCE 7-22 Figure 27.3-1, which provides the coefficients to be used for roofs and walls. The wind parallel to the ridge (which, in our example, is Wind +X and Wind -X) will be similar; however, the wind normal to the ridge for Wind +Y and Wind -Y could differ because the slopes are different on each side of the ridge. Fortunately, the MecaWind Ultimate version takes this into account and performs that work for you. Below is a comparison of the roof pressures in the +Y and -Y directions, showing that the slopes have been taken into account for each direction accordingly.
MWFRS Roof Pressure comparison +Y/-Y
Components & Cladding (C&C)
The components and cladding will be similar to what is done on a standard roof; however, since the slope may differ on each side of the ridge, it’s also possible that each side of the roof follows a different C&C figure. To handle this, we use the C&C Location. The user will indicate whether they are interested in the Roof +Y or Roof -Y in the C&C Location dropdown.
C&C Location Selection
When selecting a location, the C&C entries are unique to the selected location. For example, if you enter several entries for Roof -Y, those entries will not carry over when you switch to Roof +Y. If you want the same entries, you would have to replicate them for Roof +Y.
In this example, we are following ASCE 7-22, and the slopes on the two sides of the roof are 27.84° and 43.88°. Both of these slopes follow Figure 30.3-2D, and thus there is not a different figure for each side of the roof. Consequently, the C&C Graphic will show the entire roof and the zones for the entire roof. If the figure followed for each half were different, then only half of the roof would be shown for each location selected when the C&C Graphics button is clicked.
C&C Zone Graphic
Monoslope & Flat
In a monoslope or flat roof, the Custom Roof option gives the user the ability to specify a custom set of overhangs on each of the four sides of the building. Similar to the gabled roof example, the wind is then checked from each of the four (4) directions for MWFRS.
Custom Monoslope Roof with Custom Overhangs
To select a custom overhang, the user must also select the custom roof option for the roof.
Summary
The Custom Roof option doesn’t solve all the problems of custom situations that can occur in building design, but it does provide designers with another useful tool in their toolbelt to handle some custom scenarios. In this example, we provided a high-level view of the major changes that occur when the Custom Roof is selected by the user. Here, we are also providing a link to download the MecaWind Input File and MecaWind Output file for your evaluation. All the Custom Roof options are only available in the MecaWind Ultimate version. If you would like to read more about other features included in the Ultimate version, then click here: MecaWind Information. If you have any questions or comments, please email us at [email protected].