eave struts

[mark416]

Hello all;

I am currently designing a single story steel framed building for an industrial application (W sections for columns and roof beams, cold formed channels for girts, and channels for purlins (roof heights have limited me to small C4's for purlins instead of using the traditional cold formed channels (CFC) that would have required me to use for example 8" CFC). The building is braced therefore not a portal frame and it is a single sloped roof at 1:12 pitch.

My question now is how can I detail the eave and gable ends of the roof without using an eave strut (i.e. a deformed channel for the lack of a better description “cantilevered” off the column). Is there a way I can use traditional members? My wall girts are 8" CFC that are mounted to the outside face of the columns (again I did this to increase the amount of space inside the building).

Thanks,
M.

 

[hokie66]

At the ends, I assume you have a W section rafter. At the two sides, you will need struts which will form the chords of your roof bracing truss. These struts are commonly CHS or RHS sections, sometimes W sections. The other roof bracing truss members could be a combination of angles, rods, or some other section, all depending on the size of the building and the forces involved.

For girts, why wouldn't you use Zed sections rather than Cee's? They are more economical, because they can be lapped over the supports to reduce the section required.

Connecting cladding to hot rolled channels is to be avoided if possible. Would you be able to get adequate capacity with lapped 6" Zed purlins?

 

[mark416]

Unfortunately I am very limited to roof height while maintaining the ceiling height inside. I am stuck with the hot rolled sections. I know I’m going to get flack from the guys in the field about that, but like I said, I have no room for the roof to grow up nor down.

I guess I could use z’ss for the girts, but my question is how can I detail the eave and gable ends of the roof. See attached sketch (hope it works, I have never attached anything to this forum before.)

Thanks,
M.

 

[MikeE55]

I would consider using a bent plate in combination with another shape to achieve the eave strut. Also, shouldn't you flip your roof purlins 180 degrees so that the toes of the flange are pointed up the slope - this way, the vertical load is closer to the centroid and puts less torsion into your purlin. If this is a standing seam roof, have you considered how the purlin is braced at the compression flange by floating clips, and how it resists uplift when the bottom flange goes into compression?

 

[mark416]

Can you please clarify the last sentence regarding the floating clips?

 

[youngstructural]

This is a standard problem with portal frame buildings and has several solutions, though when you are as confined as this case they aren't all that cheap.

Personally I use a custom welded bracket and pass a channel member from knee to knee of the portal frames, basically perpendicular to your page and connected at every frame. This is similar to what MikeE55 has said, and I would very much encourage you to do this... It does not have to be a folded plate; My personal preference being a custom welded T-section where the base is a shaped "arm" reaching up and out from the face of the column (or rafter; Many people mount this to the top of the beam-column joint).

I'll post a detail if I haven't been clear, however that'll have to wait as I am literally between jobs. *smiles*

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[MikeE55]

Mark416, you asked about my floating clip comment. The metal roof would either be a diaphragm roof or a non-diaphragm roof. I'm not sure how you are specifying it, but if it is a non-diaphragm, these are typically fastened with clips which allow the metal roof to expand and contract. Designing a purlin system for such a roof is a little more challenging than designing for a diaphragm system, which is why I asked the question.

 

[hokie66]

Mark416,

Your question about the eaves detail indicates that this is the first time you have done this. The purlin member at the eave needs to be able to support the gutter and also provide vertical support for the girts if they are not propped from the floor. So you have to detail a bracket off the column or rafter to support this eaves member. In Australia, we often use a special fascia purlin as the gutter support.

The eaves member for support of the sheeting should not be used as the eaves strut of the bracing system, because it is in the wrong plane. Refer my first post for that.

Metal roofing should not be relied on for diaphragm action. The fasteners are either through the crown of the sheeting, or else as MikeE55 suggested, they are concealed and the roofing floats in clips. Either way, no diaphragm should be assumed. Use a horizonal truss system.

The purlins should be braced against uplift with bridging between the purlins.

I think you need to find some drawings which have been done by others and seek the advice of someone more experienced in your office. It is not complicated, but there are tricks for young players.