Consider it as MWFRS or C&C? 2

[jiamin]

I am recently participating in a job to estimate a building roof wind damage (gable roof). The roof is caved in at windward and ballooned out in the leeward and the roof rafters and joists are buckled. The estimation report will directly affect the determination of which side insurance company will pay for the repairing. The calcs show that if the roof is considered as a Main Wind Force Resisting System and use ASCE7-05 Figure 6-6 to determine the wind pressure coefficient Cp, the conclusion will be that the roof design meets the code requirement. But if use ASCE7-05 Figure 6-11 to determine the wind pressure coefficient GCp (consider it as component and cladding), seems the conclusion will be the opposite way.

My question is for the roof rafter and joist buckling failure, should I consider it as Main Wind Force Resisting System to do design checking or should I consider it as component and cladding to do design checking?

Appreciate your oppinion, thanks.

 

[ChipB]

It's a good thing you two don't work together! ;-P

JAE:
I think it is a construction issue more than a design issue. I value your opinion. You have kept me straight in the past. Please re-read the failure description of the O.P. then read my response and let me know if you agree or disagree.

Thanks

 

[ishvaaag]

I have checked how Spain's CTE sees the issue and goes conservative by recommendation of taking the local coefficient for the general elements of the structure to which they are tributary; I quote

"Si la zona tributaria del elemento se desarrolla en dos o más zonas de las establecidas en las tablas, como es el caso de análisis de elementos estructurales generales, el uso de los coeficientes tabulados opera del lado de la seguridad, toda vez que no representan valores simultáneos
de la acción de viento"

If the tributary area of the element is developed in two or more areas of the established in the charts, like it is the case of analysis of general structural elements, the use of the tabulated coefficients operates on the side of the security, since the values don't represent simultaneous values of the action of wind.

so at leas at one hypothesis, irrespective of taking lower overall whole structure wind values, CTE by absence of other insight directs to take the worse at every place as the code defines it.

 

[JAE]

ChipB - I don't disagree with what you are saying in regards to the blocking. I'm not sure a windward cave-in and leeward blow-out would be the result of the lack of blocking though.

I suppose it depends on the direction of the wind with respect to the gable ends, right?

 

[ChipB]

Depends on direction of wind with respect to gable ends: Right

I assumed perpendicular to gable. Thus the failure across the roof. If it were blocked, I'd expect to see a more localized failure.

 

[vandede427]

When I have a girder truss with less than 700 sf trib area, I specifiy for the truss manufacturers to design girder trusses as C&C. But they rarely do on the first set of shop dwgs, then I mark them up, then they call angry b/c their software won't allow them to do it.