UBC seismic design

[rogerheadford]

I know this code is now superceded, but i have been asked to review a design performed in 2004
The main building structure is a pitched portal frame having 10M to eaves and 12M to ridge
Inside this building (which is the main subject of this query) is a large steel platform 4M high.
The span is 20M and designed as a portal frame with 6M bay spacing and defined as a "non building structure".
There is secondary beams spanning 6M to support grating.
The main purpose of the platform is to support a 3 level conveyor system which carry parcels
Equation 32-2 in UBC 1997 defines Hx and Hr in the equation and since the design is for the platform,
then it seems that Hx = 4 and Hr = 10 (or 12)
However, since the 3 level conveyor is also 4M above the platform, Hr has been defined in the design as being 8M
I believe this to be incorrect and Hr should be either 10M or 12M (see attached x section of platform and conveyor support)
In addition, the design includes for the full dead load of the conveyor frames and also for 25% of the live load at each level for the load on the conveyors
This load has been determined in for the total horizontal seismic force and distributed in accordance with equation 30-15

I believe this to be a total over kill since
a.) the conveyor system and platform is not "storage or warehouse"
b.) the conveyor frames are non structural components

I would appreciate other Engineers opinions

Regards

Roger Headford

 

[oldrunner]

Looking at your sketch it appears that the conveyor support framing is not connected to your building. If that is the case, then just review/design the support system per the code. The steel framing doesn't know it is not a building. Looks like this section will sway somewhat.

 

[rogerheadford]

To Oldrunner,

Many thanks for the comments.
Correct, the main platform and the conveyor frames are not connected to the main building, so, it seems that the original Hr as being the 8M to the top of the conveyor frame is correct.
However, Clause 1632 is specifically for "non structural components and equipment supported by structures"
So, this lateral force distribution is to be used for "the design of the conveyor support frames" above the platform and not specifically for the support platform below.
While it could be assumed that the resulting reactions from the design of the conveyor frame should be applied to the platform, UBC is not very clear on this. Obviously, the safest route is to apply those reactions, however, this leads to a very complex analysis for the platform design, since the conveyor layout is like a tin of spaghetti turned upside down.
I tend to think that the only loading for the platform is the total Self Weight of the conveyor supports, platform and grating plus the live load on the frames.
I do not think this should be subject to 25% of the live load for seismic and I don,t think it necessary to apply the horizontal distribution factors at the different storey levels from the conveyors. (equations 30-15 and 32-1 or 2
For Seismic on the platform design, this should just be the simple seismic percentage evaluated and applied at the top of the platform level (4M)

Regards
Roger headford