Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: canopy footing

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Steve,
 
        This seems scary to you?
 
        As I understood the question it was related to the geometry of the concrete cube only so I made no comment on any other part.
 
        As I read the description he had a T shaped structure 16' by 24'.  That's two columns (each with its own cube foundation) in a row 16' apart with a 12' cantilever on each side of the T.  Considering live load on one side only with wind (say 30 plf, just to pick a number) and taking moments about the base of the cube we have
 
Live load = 12*8*20psf = 1920# (one side only)
 
Dead load = 12*8*10psf = 960#
 
Wind = 30*8 = 240#
 
Moment = 1920#*6' + 240#*(14'+5') = 16,080 ft.lb. per cube foundation.
 
Total weight of foundation = 5^3*150 = 18,750#
 
Total weight at base of foundation (for overturning case only)
 
Wt = 18,750 + 1920 + 2*960 = 22,590 #
 
Eccentricity, e, = M/W = 16,080/22590 = 0.712
 
Factor of safety against overturning = footing width/(2*e)
 
F.S. = 5/2/0.712 = 3.5  <----------------
 
        This analysis is a bit simple, perhaps excessively so; but the basic design looks O.K. to me.  Did I miss something?
 
Regards,
 
H. Daryl Richardson
   
----- Original Message -----
From: S. Gordin
Sent: Monday, February 13, 2006 2:02 PM
Subject: Re: canopy footing

Scott,
 
Plain concrete is not allowed for structural applications in California. Anyway, the specified reinforcement of the "cube" was inadequate (as many other things in this design were).  For example, the restraining slab (as designed) is not even in contact with the "cube."
 
My main problem with the design is that - according to CBC/UBC Section 1806.8 preamble - the formulas 6-1 and 6-2 are both for distinctively column-like (3:1 aspect ratio) foundations.  Apparently, this is how it was tested back in the 1930s-1940s.  I am not sure that these formulas were intended for such use at all.
 
When drawn to scale, the canopy on such footing looks scary.
 
Steve Gordin SE
Irvine CA
 
 
----- Original Message -----
Sent: Monday, February 13, 2006 11:17 AM
Subject: RE: canopy footing

I have seen lots of canopy designers use pole sign formulas when I was a plan reviewer.  Formula 16-2 is the constrained formula.  If they are using that one they need to have an adequate portland cement concrete slab around the foundation to resist the thrust.  Up here they frequently put a small PCC slab around the pumps to resist spilled gas but have asphalt pavement close by.  If you do have an adequate slab I would tell them they need to use formula 16-1.
 
 You are not allowed to use plain concrete to resist seismic forces for other than houses/duplexes.  Are they reinforcing the "cube"?


From: S. Gordin [mailto:mailbox(--nospam--at)sgeconsulting.com]
Sent: Monday, February 13, 2006 9:47 AM
To: Seaint@Seaint. Org
Subject: canopy footing

Good morning,
 
I am reviewing a "standard" design of a canopy - a steel-framed structure with two cantilevering columns 14' tall supporting a light "low-pitched V" roof measuring 16'x24' with 12' cantilever. 
 
The engineer used UBC formula 6-2 (Section 1806.8) to justify the adequacy of the footings for the columns - 5'x5'x5' "cubes."  According to the UBC Commentary p. 297, the formulas of UBC 1806.8 are historically applicable to "pole-" or "column-" type footings. 
 
To me, these "cube" footings do not even look right for the subject application.  Any comments on the situation will be highly appreciated.
 
TIA,
 
Steve Gordin SE
Irvine CA