mechanical platform kickers

[nitnelion]

I'm designing a mechanical platform sitting on top of a roof. It's in seismic design category D. I was planning to use kickers to transfer the lateral load down to the roof diaphragm. Are these kickers considered an eccentrically braced frame and do they have to be designed and detailed as such? The workpoint at the bottom of the kickers is the centerline of a stub column and beam intersection. However, the top of the kickers lands in the middle of a condenser support beam. Thanks.

 

[haynewp]

Is it just one kicker that attaches to the mid-point of the condenser support beam? Can you upload a sketch?

 

[nitnelion]

Well, let me try to explain. The top of steel of the platform is about 4' to 5' above the top of steel of the roof. The platform is rectangular in plan (32' x 45') with six posts up from the roof. 4 of the posts are on the corners and 2 are centered on the 45' long side of the platform. These posts do not go down to the ground. There would be 2 to 3 "kickers" per column (1 or 2 in each direction) to get the lateral load of the condenser platform down to the roof diaphragm. I was planning to make the kicker slope around 2 horizontal to 1 vertical. Thanks.

 

[haynewp]

I think what you have is an access floor by ASCE 7-05 section 13.5.7 and the R to be used is per Table 13.5-1. That means you would not distribute the vertical seismic forces from the rest of the building into the platform, and you would not use the R value of an eccentrically braced frame or detail it as an EBF. I expect that someone else here is going to disagree with me on this.

 

[haynewp]

I don't know what you decided to do about this, but the more I thought about what R to apply to the mezzanine the less sure I was. I don't know at what point you would consider a small mechanical platform as similar to an access floor (component) and when the size becomes large enough to treat it like any other floor in the building. If the mezzanine is 1/10 the area of the roof plan, I don't see combining it into the vertical distribution with the rest of the building.

But, does that mean it should still use the same R value as the building and have specially detailed frames supporting it also? Or should it be similar to any other component or partition attached to a floor and have its weight figured into the rest of the entire floor weight, and then use a 'component' R value for the elevated floor itself? I am somewhat perplexed on this one.

 

[prsconsultant]

I disagree with haynep. The kickers need not be detailed as an EBF. I would use an R=3 and no special detailing is required. R value used for Mechanical platform would have no impact on the R value used for the building.

 

[haynewp]

where did I say to detail it as an EBF?