Pitched Roof Rafter Major Axis Unbraced Length

[Vickna]

Hi,

I have a 30m length building with a slope of 1:10. using I-section (built-up) can i take the major axis unbraced length of the rafter to 15.0m. (Considering them like two cards from a deck inclined to each other).

thanks.

 

[molibden]

You need a ridge beam and a roof bracing. Account for lateral forces from rafters in compression.

How would you calculate unbraced length differently than 15 m? If you have duo pitch roof then you absolutely need some kind of bracing of the ridge.

 

[KootK]

I have a 30m length building with a slope of 1:10. using I-section (built-up) can i take the major axis unbraced length of the rafter to 15.0m. (Considering them like two cards from a deck inclined to each other).

We're talking about a pre-engineered metal building system here, right? I'll assume so. It's not an easy question to answer. Here's what I think I know:

1) At such shallow slopes, one starts to wonder if snap through buckling at kL = 30 m becomes the issue dujour.

2) Traditionally, using the equivalent length method, designers have assumed kL = 15 m for slopes as low as 1/4" per foot. And there haven't seemed to be any problems to my knowledge.

3) The direct design method, where second order effects are accounted for explicitly with K = 1.0, would seem to be a more appropriate design method.

4) Research has indicated that the reason for kL = 15 m has less to do with the "two cards" business and more to do with the rotational end restraint provided at the rafter ends. And that restraint varies with column height which makes the whole thing quite difficult to nail down.

For me, it's:

1) the direct design method OR

2) use kL = 30 m in combination with rotational springs at the ends calculated with some consideration for the softening effect of axial loads in the columns OR

3) Look at snap through buckling at kL = 30 and, if that's far in excess of kL = 15 m capacity, go with kL = 15 m.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Vickna]

Hi KootK

Thank you for your input regarding this. yes i am adopting the PEB concept for Design,

is there any literature supporting this. i tried get my hands on some but unable to find anything solid on the web.

can the enclosed explanation be adopted?

the problem faced with this type of assumption is with consultants who do the conventional way they insist on having the KL knee to knee.

 

[KootK]

- AISC has a design guide on web-tapered members that is excellent and covers thus topic.

- I feel that your simplified analysis is much too simple for the case at hand. For one thing, the 2% rule assumes that the far end of the brace stays put. Here, it moves around.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Vickna]

Hi KootK

Thank You for the info. went through it and did the buckling analysis using STAAD for the Frame by applying horizontal forces (opposite to each other) at the eave so that the rafter would undergo buckling. got a K value of 1.01.