Roof Live Load Reduction

[Will777]

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

 

[Will777]

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?

 

[kevo_55]

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.

 

[Will777]

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).

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.

 

[kevo_55]

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.