MWFRS Dumb Question

[JStructsteel]

Im debating my rational on wind loads on a gable roof (Wood truss roof). I calculate the wind load, and resolve to a horizontal pressure. Do you apply that to the median roof height, or to the top of the ridge to get your horizontal load resultant?

More and more I do wind loads, it bugs me I dont think I know enough. Whats a really good reference for MWFRS wind loads for buildings?

 

[Enable]

What height you take depends on overall height of structure, pitch of roof, etc. In Canada, we use the NBCC and for a low-rise building and steep roof we would use a reference height of walls + half vertical distance of roof. See page 23 clause 3.3.2 of NBCC for Steel buildings (will be same for your wood building).

For simple structures that fall under the prescribed procedure in the NBCC the document I linked to lays it all out fairly well.

 

[JStructsteel]

Enable, I think I understand that, mean roof height as described in the code. But for the calculation of the shear load, i would apply the results to the vertical plane that goes to the ridge to calculate my base shear correct, or does the calculation for wind factor in the ridge height, and the load is only applied to the mean roof height?

 

[phamENG]

JStructsteel - you have to work out where the shear load is resisted horizontally. Since you have a sloped diaphragm I agree it can be a bit tricky. Placing it at the midheight between eave and ridge is probably a bit conservative for a trussed roof but sound - I would not put it at the ridge. If you take the load through the roof diaphragm and out to the gable end walls, you'll end up with triangular shear walls in the gable end. These will have a shear anchorage requirement along the bottom that get transferred into the top of the top floor shear wall and overturning reactions at the ends (or wherever you place them). If the entire wall is a shear wall, then it's easy. If it's only partial, then it can be a bit more complicated (see this article).

This response assumes you're looking at the analytical model, not elevations for load determinations per the code.

 

[Enable]

For me it would depend on which direction you are looking at. On the side perpendicular to the ridge I would place the load at the eave height because the wind pressure is calculated using a reference height to mid-height of the roof (see example on page 23 of American Wood Council Presentation 2016). On the gable end wall side, I would place the load along the rake itself (see page 24 of same document). You'll still want to decompose uplift pressures into their horizontal / vertical components for ridge connection design though (see page 16 of same document).

phamENG is correct that if you have a structure that is intended to transfer the load at a particular location and will attract that load (i.e. is relatively stiffer) then you'll probably want to place the load closer to that point wherever it may be. But for typical residential construction I do not think you'll have an issue with this.

 

[JStructsteel]

Thanks Enable, it talks about using the vertical projection of the roof which is the answer I am looking for.