MWFRS and C&C dome external pressure coefficient

[SuG89]

Hello,

Please refer to following figures in ASCE 7-10.
1. Pg- 265, Figure 27.4-2, MWFRS Domed roofs
2. Pg-344, Figure 30.4-7, C&C Domed roofs

Usually components and cladding are designed for higher wind loads than the main wind-force resisting system. Why is it that the negative external pressure coefficient is higher for a MWFRS dome compared to C&C dome?

Regards,
Su

 

[JAE]

One table is offering you Cp values.
The other is offering GCp values.

So if you include a 0.85 value for G in the MWFRS chart values this will reduce those loads a bit.
Looks like GCp for MWFRS then varies from 0.21 to 1.27 vs. the 0.9 for C&C overall.



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[SuG89]

Thanks for your response JAE.

Variable 'hd':
I have a case in which the domed roof is supported by vertical walls. The vertical walls are supported on top of the roof of a regular building. In my opinion, variable 'hd' in Figure 27.4-2 and 30.4-7 is the height of vertical walls using building roof top as the reference point. And the velocity pressure exposure coefficient is calculated using the height of dome top with reference to the ground level.

Do you think this interpretation of 'hd' is correct?

Regards,
Su

 

[JAE]

I think you have to tread a bit careful with that.
The dome values given were probably derived with the assumption of a ground-supported dome, either on walls or not.

Putting a dome on top of a higher building seems to create a different sort of wind pressure condition as you have significant amounts of wind (on the main building below) "squirting" upward and over the roof and dome areas vs. an on-grade condition.

Sometimes with strange geometries, you have to look at ASCE 7's wind values in several views to get a potential range of values and then use your engineering judgement from there.

Alternatively you would go the wind tunnel route but that may not be an option for you here.

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[SuG89]

Thanks again JAE!