Design purlin with seismic forces by asce7:2010

[caiomarcon]

How can i design girts and purlin (roof member) with seismic forces by asce7:2010? Can i utilise item 13.3.1 "seismic demands on nonstrucutral components / seismic design force"? How can i classify "ap" (table 13.5-1 or 13.6-1) and "Ip" (section 13.1.3)?

 

[MotorCity]

Unless they are part of your lateral force resisting system, they are usually not designed for seismic forces. The exception would be if they support some type of equipment, unique feature, or other architectural component.

 

[caiomarcon]

Hi MotoroCity,

This members (girts and purlin) aren't part for the lateral resisting system. They are only for suport roof deck.

 

[MotorCity]

The roof deck has a mass and theoretically would contribute to the seismic force but it would be EXTREMELY small. Unless you had cladding with significant weight, like a precast wall, the seismic force would be negligible. In such a case, you would simply select the appropriate ap value from the table and design the member accordingly

 

[JAE]

It may be that the girts are designed for all the typical D+L, D+S, etc. load combinations but also combos with the Ev (vertical seismic) component).


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[caiomarcon]

JAE, in your opnion, we need design girts and purlin with seismic effects?

 

[jayrod12]

I think they're indicating you at least need to determine if the seismic load combination could govern when compared to the other load combinations seen by the purlin. Most often the seismic loads seen by purlins do not govern their design.

 

[JAE]

Yes - what jayrod12 says...

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[wannabeSE]

For the vertical force use ±0.2S_DSW_p per 13..3.1. Usually the diaphragm resists the horizontal component. If not, I would assume it is similar to an exterior wall in table 13.5-1. The importance factor, I_p, is typically 1.0 except risk category in IV buildings where it is 1.5. As the other have said, the load combinations with snow or full roof live load typically govern.

 

[caiomarcon]

wannabeSE, thanks a lot.

In your opinion, we can design by (asce7:2010 - 13.3.1):
Purlin: vertical force = ±0.2·SDS·Wp (usually the diaphragm resist the horizontal components)
Girts: horizontal force = Fp = 0.4·ap·SDS·Wp/(Rp/Ip) · (1+2·z/h)
?