Mean roof Height? 1

[tricalcim]

I have a 2-story house with the top plate height of 20 feet.
The top of the roof ridege is 15 feet above that. So, a total height of 35 feet from the ground. When I select the Ce factor, would you suggest using the total 35 foot height or a mean value. This would make a differece of designing for 16.2 psf or 17.9psf. Is it defined in the UBC. Any thoughts are appreciated.

 

[AlohaBob]

I use the roof high point for the exposure height exclusively in the old UBC. But where a mean height is figured these days, the average of the eave height and the ridge is the mean.

 

[nrguades]

Assuming you are using Projected Area method of UBC 1997:
Ce values varies with the height. You will have corresponding uniform wind pressure for a height of 0 - 15 ft from the ground, then varying wind pressures above that height.

Assuming you are using Normal Force method of UBC 1997:
Ce values varies at the windward wall side only. For leeward
wall, windward roof, & leeward roof, Ce value is based on the roof mean height.

 

[AlohaBob]

I don't have 97 handy.

In my '94 UBC Ce was dependent on the height of the average level of adjoining ground. This didn't make much difference except for those buildings on a hillside. I took a height from midpoint of the side profile to the ridge. The application of Ce to P was the same for either the Normal Force Method or the Projected Area - The same factor multiplier in either method. This interpretation seems to have been carried forward from at least '88.

The 2000 IBC uses Mean Roof Height. I would consider this the average ground elevation to the average roof elevation.

So there's a change occurred here.

 

[nrguades]

Section 1619.2 Method 1 (Normal Force Method - UBC 1994)

Method 1 Shall be used for the design of gabled rigid frames and may be used for any structure. In Normal Force Method, the wind pressures shall be assumed to act simultaneously normal to all exterior surfaces. For pressures on roofs and leeward walls, Ce shall be evaluated at the mean roof height.

Section 1619.3 Method 2 (Projected Area Method - UBC 1994)

Method 2 may be used for any structure less than 200 feet in height except those using gabled rigid frames. This method may be used in stability determinations for any structure less than 200 feet. In the Projected Area Method, horizontal pressures shall be assumed to act upon the full vertical projected area of the structure, and the vertical pressures shall be assumed to act simultaneously upon the full horizontal projected area.

Mean Roof Height (IBC 2000/2003-Definition)

The average of the roof eave height and the height to the highest point on the roof surface, except that eave height shall be used for roof angle of less than or equal to 10 degrees.

 

[AlohaBob]

I take a shortcut so regularly I don't notice I do it any more. I use the highest Ce for the structure from the ground up. I think you can use the incremental Ce's from the ground up to the highest however. The calcs get more rigorous.

 

[tricalcim]

thanks - you guys are good. My analysis/design will be in accordance with the 97 UBC (2001 CBC) projected area method. My calcs indicate that the pressure at the top plate due to varying wind pressures (Ce) are the same as if I were to use a mean roof height based on the eave height and the max ridge height.