Minimum Snow Load on Fabric Structures 2

[amecENG]

I am looking for some guidance on the minimum snow loads required on fabric structures. My question really relates to the Cs (roof slope factor) and I am more concerned about the National Building Code of Canada although I welcome any commentary from other code users.

I know both NBCC and ASCE have similar equations for Cs where it is a function of roof slope and slippery vs non-slippery surfaces. Essentially for a slippery surface with a roof slope of less than 60 degrees the Cs will be > 0 and therefore the structure must be designed for some value of snow load.

I am wondering if there is any reference or code commentary that suggest that a lower snow load value could be used for fabric roofs. I beleive ASCE has one but I can't find anything on NBCC. These companies have been building these structures for years in cold climates and I am wondering how they justify it to meet code?

Thanks in advance for your help.

 

[BAretired]

I am not aware of any code requirement which would allow the snow load on a fabric roof to be reduced. In view of some recent collapses of fabric roofs from snow load, I would not see any reason why snow load should be any different for fabric than for any other material.



BA

 

[amecENG]

Thanks BA,

I completely agree but how is that these structures are built here in Canada? Does someone at the municipal level approve of their use? Since municipalities can choose to adopt the National Building Code or not, does that give them the authority to allow this type of construction?

The explanation I was given from a Fabric strcuture manufacturer is apparently from the commentary for ASCE-7-02 where it says "Glass, plastic, and fabric roofs of continuously heated structures are seldom subjected to much snow load because of their heat loss causing snow to melt and sliding. For specialty roofs, knowledgeable manufacturers and designers should be consulted".

According to this manufacturer a snow load of 6psf is the maximum load any structure will have to resist. This is 10 times less than the normal roof snow load in my City.

 

[JedClampett]

I'd say that you design to the code, not the commentary. Power (heat) has been known to fail during bad storms. What happens then? Oh well?
6 psf is not an adequate wind load, much less snow.

 

[dmoench01]

amecENG - I currently work for a fabric company and we use the ASCE7 for all U.S. projects. We take no deductions because we have a fabric roof. We take a similar stand for what we do in Canada, no special deductions because of the fabric.

 

[MiketheEngineer]

Agree

 

[msquared48]

dmoench01:

How do you deal with the degredation of the fabric, and it's strength, with time due to UV radiation? Special coating, or recommended replacement time of the fabric?

Seems like this would be a real issue with snow loads of any magnitude.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dmoench01]

msquared48:
Because the fabric does degrade over time, all the more reason to not reduce the snow load any more than is acceptable per the ASCE 7. Really not much different than a metal buidling roof as they degrade in their slippery factor over time as well. Type of fabric, special coatings, location in country will all affect the lifetime of the fabric.

 

[TLHS]

amecENG, muncipalities in Canada don't decide whether they're going to adopt the NBC. Municipal code officers/inspectors can generally interpret the code but except for one specific exception where the province has delegated responsibility (Vancouver) building code is a provincial responsibility. Municipalities follow whatever their province adopts.

I wouldn't reduce anything. In fact, I'd be worried about localized sagging leading to some kind of collection of snow if the geometry and fabric tension allows it.

 

[amecENG]

Thanks everyone for your responses. I'm just confused how these structures are all over Alberta if they are not designed for NBCC snow loads.

I have attached a short paper which explains why this company believes 6psf is a maximum load these structures need to resist. If you have a few minutes please read through it and let me know what you think.

 

[wadavis]

amecENG
I remember seeing some conversation about the old Cover-All sheds being under-designed to the NBC around the time they went bankrupt. If you can find any reliable sources it may provide some insight to the big picture.

 

[MiketheEngineer]

You are the engineer - its your stamp and liability. The ONLY way I might reduce the load a bit is if it is a wide open mesh that will allow snow, rain, wind to go through it. Still then - I am hesitant becasue a good ice storm could "clog" it.

 

[JStephen]

The report states "It is likely that the maximum snow load that a Sprung Structure has to resist is under 6.0 psf no matter where the structure is located...". That's not the same as saying "We design these for 6 psf" or "These structures should be designed for only 6 psf."

 

[amecENG]

The problem is they have their own engineer that stamps the structure. I am only required to do the foundations.

 

[woodman88]

Sorry, but are you the EOR or merely a sub-engineer on the project?
As an EOR, I would be telling them what loads to use. If they wanted to lower those loads, I would require documentation of where it is allowed in the codes with the calculations and test for it.
As an sub-engineer, I would calculate the loads to the foundations. Compare my loads to theirs. Use the higher loads for my design. (Unless I get their calculations and documentation for a lower loading.)

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

Ok.

So the OP is the EOR for the foundation, and the other engineer is the EOR for the fabric structure. This is the same situation as for many PEMB's where there is no designated EOR for the project, only parts of it.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[woodman88]

EOR (I believe) is the Registered Design Professional in Responsible Charge and per the 2009 IBC there can be only one EOR on a project.
2009 IBC Section 107.3.4 “...The registered design professional in responsible charge shall be responsible for reviewing and coordinating submittal documents prepared by others, including phased and deferred submittal items, for compatibility with the design of the building.”
If you are not doing this you are a sub-engineer responsible only for what you are sealing.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[BAretired]

Since municipalities can choose to adopt the National Building Code or not, does that give them the authority to allow this type of construction?

In the Province of Alberta, municipalities do not have the option of choosing the building code. They are required to use the Alberta Building Code (ABC) which is identical to the National Building Code (NBC) insofar as structural design is concerned. Climatic and seismic information for various municipalities is given in Appendix C of the ABC.

The Engineer of Record does not have the option of varying the snow load unless he obtains a variance from the authority having jurisdiction.

BA

 

[amecENG]

Can you reduce the snow load based on scientific research? If so, do you need to get permission from the authority having jurisdiction to use the scientific research to circumvent the code? I would have thought so but....

 

[BAretired]

I think you need permission.


BA