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Mean Roof Height
2

Mean Roof Height

Mean Roof Height

(OP)
Dear All,

I am having one question on the mean roof height calculation. I am using asce7-98.

In one of my project, the eave height (roof slab) is 58' 6". There are some curtain walls are starting from roof to the height of 8'. Moreover there are few pockets going upto 10'-14' at elevator/stair areas. Now my Q is either to use Fig 6-3 or Fig 6-4.

Please explain.

-Murali

RE: Mean Roof Height

2
Murali27,

The mean roof height of your structure is 58'-6". The walls extending above that height may be treated as parrapet walls and not roof structure. For figure 6-4 you would get the external pressure coefficient GCpf for low-rise buildings (<60 ft). For figure 6-3 you would get the external pressure coefficient Cp for all heights. If the roof slope is <10 degrees the mean roof height is measured at the eave, which means you may use either figure 6-3 or 6-4.

Hope this helps, if you need more info please elaborate.

RE: Mean Roof Height

(OP)
Hi ERV,
Thanks for your reply.

This is a shearwalled concrete bldg. Roof slab is a flat surface & few pockets are going up by 10'-14'.
Enclosed bldg. Size: 230' x 95'

Can i assume roof angle as 0 deg or the avg of roof slab height & top elevation of pockets ? Pl explain.

External pressure coefficients are (if roof angle = 0 deg)

Fig 6-3 -> windward wall = 0.8
           leeward wall = -0.3
Fig 6-4 -> wind ward wall = 0.4
           leeward wall = -0.29
       
Obviously the forces are less if we follow fig 6-4 when compared to Fig 6-4. Pl explain what to do in this case ?

-Murali

RE: Mean Roof Height

The MWFRS is from the eave height down. If the roof is flat or nearly so treat it as a flat roof. By all means use figure 6-4 for your calcs (of course the forces are less in 6-4, this is for buildings <60 ft).

The projection above the roof (10 - 14 FT) should be treated as a parapet (or separate structure), with separate wind analysis (for structures over 60 ft tall).

RE: Mean Roof Height

(OP)
Thanks ERV.

In this project i got 1.5 times more wind load in Fig 6-3.
Since the force is too high with the matter of 2' height difference.

This probability theory of computation does not give any smooth transition with respect to limiting values.

Thanks once again.

-Murali

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