# Roof Live Load Reduction



## Will777 (Nov 3, 2007)

Can someone please check my attached roof live load reduction and application calc?

I was not sure at first if when designing a single member in a truss, if the reduction should be based on the entire truss tributary or on the individual web or chord member I am designing. But if you design each individual truss member for its own smaller tributary you end up over-designing the entire truss. (2 pages attached)

Will


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## Will777 (Nov 3, 2007)

Also, I am not sure if the live load reduction is supposed to be based on the tributary area along the length of the sloping beam or based on the horizontal projected tributary?


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## kevo_55 (Nov 6, 2007)

Will,

The tributary area will be per whatever you are designing. For example if it is a column, then it would be the tributary area of the column. If it is a rafter, then it would be the TA of that rafter.

Now, if you're looking at a sloped rafter then your length would be that slope length. There is another way to calculate it.

Take a look at this PDF.


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## Will777 (Nov 7, 2007)

Kevo-

The tributary I was wondering about is for the roof live load reduction calculation. In my calculation, I took the tributary of the rafter to be the length times the width. But for reducing roof live loads, I now believe it should be the tributary based on the horizontal projection of the rafter. Look at slide # 49 here:

http://docushare.subr.edu/sudocs/dscgi/ds....otes%5B1%5D.ppt

It says: "Determine the tributary width within the horizontal plane" when it is figuring the roof live load reduction.

Also, the IBC defines "Lr" as:

Lr = Reduced live load per square foot (m2) of horizontal

projection in pounds per square foot (kN/m2).



kevo_55 said:


> Will,
> The tributary area will be per whatever you are designing. For example if it is a column, then it would be the tributary area of the column. If it is a rafter, then it would be the TA of that rafter.
> 
> Now, if you're looking at a sloped rafter then your length would be that slope length. There is another way to calculate it.
> ...


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## kevo_55 (Nov 7, 2007)

Will,

You are right, the lengths are in the horizontal projection of the roof.

Then again, the LLr will also be in the horizontal projection as well.


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