ASCE 7 rooftop structures factor

[FSS]

In calculating wind forces on a piece of equipment going on a roof, I noticed that ASCE 7-05 has added section 6.5.15.1 which says to multiply my wind forces by a factor of 1.9 This has caught me by surprise. Any ideas why we are basically doubling our forces now? And if that is the case, why would we only do it for buildings under 60'? I looked in the commentary but there is nothing on this section.

 

[Ron]

FSS...here's what the ASCE 7-05 commentary says:

ASCE 7-05 requires the use of Fig. 6-21 for the determination of the wind force on small structures and equipment located on a rooftop. Because of the small size of the structures in comparison to the building, it is expected that the wind force will be higher than predicted by Eq. 6-25 due to higher correlation of pressures across
the structure surface, higher turbulence on the building roof, and accelerated wind speed on the roof. There is now a very limited amount of research to provide better guidance for the increased force [Ref. C6-99]. Based on this research, the force of Eq. 6-25 should be increased by a factor of 1.9 for units with area less than(0.1 Bh). Because the multiplier is expected to approach 1.0 as
A f approaches that of the building (Bh), a linear interpolation is included as a way to avoid a step function in load if the designer wants to treat other sizes. The research only treated one value of A f (0.04 Bh).

The research also showed high uplifts on the top of rooftop air conditioning units, although the net uplift on the units was not measured. The consensus of the committee is that uplift forces may be a significant fraction of the horizontal force. Hence uplift load should also be considered by the designer.

Ron

 

[FSS]

Thanks. I looked again and found the commentary text in my copy now that you so kindly typed it out. I guess looking for commentary on the actual section number was my downfall.

 

[Ron]

You erroneously assumed there was some sort of organization to it!!