Snow Load 7-02, 7-05, Cold Roof Cs Questions 1

[DaveVikingPE]

I have a cold roof with Ct = 1.1. Roof slope is 6:12 or 26.6 degrees. It's an unobstructed slippery surface.

Using Figure 7-2b, it looks like Cs = about 0.8.
Using equation C7.4 2(a), Cs = 1.0 - (slope - 10°)/60° = 1 - (26.6 - 10)/60 = 1 - 16.6/60 = 0.72.

Which is it: 0.8 or 0.72?

Are there some notes I'm missing? Are my eyes playing tricks on my concerning using the graphical solution rather than the equation?

Any assitance is most appreciated, etc.

 

[chichuck]

DaveViking,

I don't think you missed anything. I compared the graph to the equation in ASCE 7-0052. Note on the graph in Fig. 7-2, at the top end of the inclined portion of the dashed line. It's marked 10 degrees, but that's not where its located. I'd say they drew that line wrong in the graph. If you pencil in a line that goes from (0,70) to (1.0, 10) it looks like your case reads about 0.72 on the graph, just like the equation.

In ASCE 7-02, the graph looks like it is correct.


regards,

chichuck

 

[DaveVikingPE]

Holy Cow! You're right!!!

Alert to ASCE: REVISE THE FIGURE!!!

 

[JAE]

It looks right in my 7-02 - I get 0.74 on the graph.

 

[DaveVikingPE]

I should have checked with ASCE first...

http://www.seinstitute.org/publications/errata.cfm

 

[chichuck]

Their correction of Fig. 7.2 in ASCE 7-02 looks like it changes the graph in Fig. 7.2b from right to wrong. I don't see any correction to Fig.7.2b in ASCE 7-05 at that page.

regards,


chichuck

 

[DaveVikingPE]

You are correct! The erratta needs an erratta! It turns out (according to research outside of Eng-Tips) that a lot of people have copies of ASCE 7-02 and 7-05 with the correct figure and a lot of people have them with the incorrect figure. My advice to the folks in my office is to simply plot the equation an past it into the book. That, or draw a red line in the corrst location in the book.