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Live Load for canvas covered structures
3

Live Load for canvas covered structures

Live Load for canvas covered structures

(OP)
My office is designing an arched truss shade structure. It is basically a quonsat hut with canvas covering instead of sheet metal. The frames are arches spaced at 8 feet. Purlins are minimal, designed to transfer longitudinal forces only.

These are not designed to be walked on during construction or maintenance. Torn or damaged canvas is repaired from inside, or entire panels (sections) are replaced when damaged.

My Question: Do I need to design for Live Load forces? It seems to me I should be able to neglect a force which is contrary to design.

Any additional thoughts / suggestions / concerns are appreciated.

-DD

RE: Live Load for canvas covered structures

A little late to respond but would you have riggers on the trusses during assembly?  Uplift wind loads will be applied to the frame at the fastenings, of course.

RE: Live Load for canvas covered structures

I'm not sure where in the world you are, but here in the UK roof imposed loads are intended to reflect snow loads.  If this is a temporary structure which will only be in place for the summer, it may be possible to omit the live load from design.

If this is a permanent structure however I would include roof imposed loads in the design.

RE: Live Load for canvas covered structures

What about wind loading?

RE: Live Load for canvas covered structures

I think he was asking about construction loads.  Not any other transient load, i.e: snow, wind, etc.  I would agree with your conclusion Dairy, but it would ultimately be up to the building official.

RE: Live Load for canvas covered structures

(OP)
These structures are intended for use in very hot climates, where snow load will not be a factor. Scottiesei is correct, the concern is for construction loads and worker loads, which we do not expect.

Yes, Wind Load controls design.

Related question: for an arched section like this, it seems to me that wind will not behave similar to wind hitting a cubical, right-angle building. I imagine there will be greater uplift near the top (apogee??) of the arch.

For example, wind pulling a sail deforms the sail to an arch (in cross section). But perhaps this is due more to design of the sail support system than to actual distribution of load? Any insight or knowledge is appreciated!

-DD

RE: Live Load for canvas covered structures

ASCE7 has specs for arched roofs.

RE: Live Load for canvas covered structures

DairyDesigner...check Section 4.9.1 of ASCE 7-02.  The R2 factor for an arched roof might allow you a hefty live load reduction.  

RE: Live Load for canvas covered structures

I've designed a few in this locale as well as in the Lindsay, Ontario area and I've used Part 4 of the NBCC with unbalanced snow loading as well as wind loading.  They are often all season structures.  For the unbalanced condition, I've found that only the arches that are 'trussed' work.  Single tube/pipe frames generally cannot accommodate unbalanced loading.  Foundations are nearly nil with screw augers for uplift.  The drape of the fabric between frames generally provides lateral restraint for the arches...

Dik

RE: Live Load for canvas covered structures

(OP)
Dik,

Please elaborate on "The drape of the fabric between frames generally provides lateral restraint for the arches".

We are planning to put some light members between the arched trusses of our design for lateral loads.

I have ASCE 7-05. 4.9.1 in 7-05 says that a reduced LLr is NOT allowable "unless approved by the authority having jurisdiction." Then, it is stated the min. LLr shall be 12 psf.

That load exceeds the UBC min of 10 psf for unoccupied ag., our usual design standard.
Thanks,
DD

RE: Live Load for canvas covered structures

(OP)
I am designing the second phase of a pipe-column (or "pole barn"), shade structure. All footings are cylindrical pier footings with #3 hoops holding (8) #5 bars vertically. the #5 bars are placed 3" from the outside of the 48" diameter pier footing.

The reviewer is requesting I "check concrete footings for minimum reinforcing per U.B.C. sections 1907.12.2.1 and 1910.5.1."

the x-section area is 1810 in^2. *.0018 = 3.26 in^2 required. my (8)#5 bars provide 2.45 in^2.

The 8.625 O.D. XS pipe has an area of 12.80 in^2 and this column extends to within 3" of the bottom of the footing.

How can I justify the existing reinforcement? This is a U-3, "occupied agricultural" building.

Can I include the area of the steel in the column as part of my "reinforcement"?



Any suggestions?

-DD

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