Need a book? Engineering books recommendations...

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

RE: Canvas patio covers and the code????

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Sasha,
I remember this to be the case when I was with Hughes Helicopter in Culver
City. The type of structure I am refering to is a simple shed roof type
system drawn tight over lightweight aluminum trusses.
Unfortunately, 150 miles away from Los Angeles, the local building
departments have not historically regulated or enforced code requirments on
cavas structures. Only in the last year or two have I been receiving calls
regarding this type of structure - all in Palm Springs (none in the
adjoining communities)and almost all from Hotels. However, there are enough
residential canvas structures to keep three or four retailers in business
for the last twenty years.

I'm not making light of this, but since one city has just started to enforce
this, they have created a very hostile market segment.

As far as my project, I assumed the uplift wind load based upon the
following:
Ce=1.13 ; Cq=0.9 I=1.0 and Qs=16.9 psf based upon an 80 mph wind. The
structure appears to be only about 3.0 psf based upon light weight aluminum
parallel chord trusses (ladder trusses) and a 0.5 psf vinyl canvas.

Inasmuch as the canvas is tied to the trusses, I did not consider
deformation of the canvas due to the applied load. I did increase the uplift
force by a multiple of 1.5. The uplift worked out to be 2200 pounds per
column and the foundation (3x3x2x150 pcf) is 2700 pounds.
I don't have the authorization to get more involved with the analysis and
have not considered the connection of the canvas to the trusses.

Would you let me know if you think this is a reasonable conservative design.
The area of the canvas is 20'x20'. I assumed that the unsupported end had 2
columns, but the final installation will actually have three columns.
Therefore the uplift reduces and without changing the foundations, the
factor of safety in resistance to uplift becomes greater.

Unfortunately, none of the six engineers out here get much call for this.

Thanks for the suggestions and offer.
Dennis

-----Original Message-----
From: Sasha Itsekson [mailto:itsekson(--nospam--at)jps.net]
Sent: Wednesday, August 26, 1998 7:45 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Canvas patio covers and the code????


Dennis,

This is simply not true that the awning companies never obtain permit for
the installation of the canvas structures.  I have personally dealt with the
awning company in LA area that asked the firm I used to work for to provide
analysis and design for several types of moduled awnings.

There should not be any difference between design requirements of canvas
structure and any other prefabricated structure (steel buildings, skylights
etc.).  They all should provide shop drawings and calculations signed by a
registered California Engineer.

The wind design on the fabric structures could get pretty complicated.  It
is in part because of the odd shape and in part because of large
deformations under the load (change in shape and hence the wind factors).
There are number of books available on this subject.  There is also
commercial software available to generate wind and other loads and analyze
fabric structures.

Contact me or call Craig Huntington at 510-251-9200 if you need any help on
fabric structures.

Sasha Itsekson, PE

-----Original Message-----
From: Dennis S. Wish PE [mailto:wish(--nospam--at)cwia.com]
Sent: Wednesday, August 26, 1998 6:08 PM
To: seaint(--nospam--at)seaint.org
Subject: Canvas patio covers and the code????


For years the desert cities in my area did not inforce the design or
installation of lightweight canvas structures - such as those used for a
patio cover or carport. I was told that these were considered temporary
structures, however I fail to see how something intended to be left standing
is considered a temporary structure. The City of Palm Springs tagged one
such structure as being in violation of the building code and required the
owner to have an engineer investigate it. The clients neighbor reported the
structure to the code enforcment department who then sited it for
compliance.
The awning company who created the structure claims that they have never
obtained a permit for installation in the twenty years they have been in
business in the Palm Springs area.
Since I get a number of calls per year regarding Canvas installations, I
contacted the building offial to find out what they required. Their position
is that they are in an 80 mph wind region and want assurance that the
structure is safe. Inasmuch as it is a proprietary system using non-standard
sections I informed the building official that I had not reasonable way to
check lateral stability of the structure nor could I determine at what
pressure the canvas would come free of the structure. I was hoping that they
would have developed an acceptable minimum standard within their
jurisdiction for installation of Canvas structures. Instead, I feel that
they "pawned" it off on the engineer to make the recommendation.
I informed the owner that I could not predict the systems stability due to
lateral loading. I also felt that regardless of the light weight of the
structure, the wind was suffient so that the 2" square x 1/8" thick columns
would not be suffient to resist collapse. I informed him that at some point
the structure could collapse and damage his automobiles parked below.
I was, however, able to determine if the structures connection to the
existing unreinforced concrete slab was sufficient to resist uplift and also
able to check the anchorage for tensile capacity.

The numbers proved that the 400 square foot area had a net uplift force of
14 psf. The connection to the existing unreinforced concrete was found
inappropriate for the column reactions as was the size, number and depth of
embedment of the expansion anchors used.
Now the project has grown into cutting through the flatwork to install two
pad footings 3' square x 2 feet deep (there was almost 4500 pounds of uplift
per column)and more robust anchorage.

The owner - a retired contractor from the Bay area - is upset, not with me
but with the building department, his neighbor and the awning company. He
argues the same question that I'm sure we all hear day in and day out.
"Permits were never issued, I've never seen one of these fail in 20 years
etc."

Am I missing something or is there a specific code requirement (UBC) that
makes structures like this exempt from code requirements? If the numbers are
typical of other canvas installations, how do the local engineers address
the political side of the issue so that we are not accused of trying to
create a market or putting some merchant out of business?

Dennis Wish PE