Roof live load - would you design for it?

[Serhiy2]

Good Day,

I'm working on a project where OWSJ reinforcement is required to accommodate new rooftop unit. Building was originally built in 1978 in Canada. OWSJ erection drawings say that it was designed to 14 psf dead load and 36 psf snow (snow load matches current roof snow load). National Building Code Part 4 requires 20 psf minimum live load to be used for roof. Would you consider it? It makes significant difference in the extent of chords to be reinforced when compared to analysis without the LL. I'm thinking of considering it, since local authorities can be very particular and impossible to reason with. Thanks

 

[Ron]

Yes. Snow is not the only loading the roof will see over the dead load

 

[JAE]

I agree with Ron. I can’t believe that the Canadian codes require load combinations that include BOTH roof live and snow loads at the same time.

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[jayrod12]

I've never done it. I need to double check the code, which I don't have with me, but I'm fairly certain theres a clause indicating the 1 kPa is only when the snow loads don't exceed that number

 

[Serhiy2]

I got thrown off by the the comment in appendix a for table 4.1.5.3 saying: "Minimum Roof Live Load:
Articles 4.1.5.3. and 4.1.5.10. stipulate a minimum uniform roof live load of 1.0 kPa and a minimum
concentrated live load of 1.3 kN."

But then clause 4.1.5.5 has this: "Except as provided in Sentences (3) and (4), roofs shall be designed for either the uniform live loads specified in Table 4.1.5.3., the concentrated live loads listed in
Table 4.1.5.9., or the snow and rain loads prescribed in Subsection 4.1.6., whichever
produces the most critical effects in the members concerned."

In my case snow and rain combination produce most critical effects so I don't need to consider LL of 20 psf. Topic is closed. Thank you.

 

[msquared48]

If you have to apply the live with the snow, let the code writer personally apply his load on a 4:12 or higher pitched snowy and icy roof.

Never done it and never will. One or the other, not both.


Mike McCann, PE, SE (WA)

 

[dhengr]

Serhiy2:
I think that the code’s real intention is: that while there is a snow load on the roof they will probably not be holding dance parties on the roof deck area or doing significant maintenance operations on the roof, and the like, thus the 20lbs./sq.ft. (1 kPa) LL may not be applicable in addition to the snow load; but the “concentrated live load of 1.3 kN” (300lb.) a worker with tools, fixing a mechanical unit certainly might occur during a snow loading. Thus, Ron’s post seems quite appropriate. The roof system should be designed to support all the loads which might reasonably be applied to it.

 

[OldBldgGuy]

The code intent is to apply a minimum live load of 1 kPa. If the snow load or snow plus rain exceeds that, as it does almost everywhere in Canada, then snow governs. For those in Lotus Land, you still have to allow something even if your snow all comes in liquid form.

 

[MotorCity]

Make sure you are reading the load combinations in the code correctly regarding LIVE LOAD and ROOF LIVE LOAD.

LIVE LOAD is meant to be applied to floors. You could have live load on the floors of a multistory building and snow on the roof.

36 psf is roughly 2.5' of snow. You may have 1 or 2 people on the roof in 2.5' of snow for whatever reason, but you will never have 20 psf of people up there in 2.5' of snow.

 

[NorthCivil]

@OldBldgGuy,

Coastal BC actually has the highest snow loads in the country I believe - due to the fact its high density slush.

 

[pmtrevisan1]

for owsj the live load is applied to the bottom chord to account for walkways for maintenance. if it meets requirements for clear space ( lt and width).

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