Eaves beam in LFRS in a Portal Frame Steel Building

[Deener]

Hey guys,
Working in a post disaster category steel portal frame building in Ontario, Canada. We're adding some misc metals to the building which was erected about 10 years ago. While on site and while looking through the drawings, I like to imagine load paths. The building contains portal frames for the LFRS in one direction (N-S) and solid rod cross bracing for the orthogonal direction (E-W). The cross bracing is in a single bay and it's the end bay. There's also cross bracing in one bay in the roof of the building which is adjacent to the vertical cross bracing. Check out the two pictures below.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1719430994/tips/IMG_5895_x7mc0f.heic[/url]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1719431009/tips/IMG_5896_nuzyqv.heic[/url]

Here's the part I'm trying to wrap my head around. Only the last two bays have an HSS eaves beam tying together the tops of the columns of the portal frames. I can see that these eaves beams are transferring the load from the roof bracing into the vertical bracing. I would think that the eaves beam in the first bay would also be in compression and transfer wind forces on the exterior wall into tension for the cross brace. What I don't understand is why there isn't an eaves beam in every single bay if only one bay is cross braced? I.e. what happens when wind blows on the East wall? Could the masonry walls of the building with the lower roof be used for the LFRS?

Thanks for following along with my ramblings. Any insight is greatly appreciated.

 

[EngDM]

Wind hitting the east wall will load the roof diaphram and make its way to the shear collectors at the cross bracing locations. Assuming the diapgram has sufficient shear capacity, and the connections to the HSS are designed for the full shear, then this shouldn't be an issue.

Yes, you can load each bay and design it that way, but if you can get a shear collector system to work, you don't necessarily need all of that extra framing, especially when the purlins run to the pre-eng portal frames and not to the N-S walls.

 

[Greenalleycat]

Agreed with EngDM, looks like the roof diaphragm is doing the job of getting the load there
Pretty common form of construction though here it would be more typical to line some roof bracing up with the wall bracing

In practice, that x-bracing looks pretty steep and small in diameter
I suspect if you consider axial elongation in those braces then it's doing a lot of fuck all, the wall sheathing is probably far stiffer...

 

[human909]

In practice, that x-bracing looks pretty steep and small in diameter
I suspect if you consider axial elongation in those braces then it's doing a lot of fuck all, the wall sheathing is probably far stiffer...

I suspect that is the case in many cases of rod cross bracing!

But to be devils advocate, if it is tensioned up appropriately then it can be very stiff as you have preloaded the rod, the columns and the struts. But whether it is like that in practice depends on the engineers and the contractors.

 

[hokie66]

A 'post disaster' building should have real bracing, not roofing and siding adding as diaphragms. Roofing and siding shed water, not prevent structural collapse.

Just my opinion, I suppose, but I'm sticking to it.

 

[Retrograde]

The purlins could transfer the load across to the braced bay as axial compression.

 

[Klitor]

This would be my choice.
"Large" wind loads on front and back would be taken by eaves struts and roof bracing in first and last bay.
"LTB" bracing would be done by purlins (and/or fly braces) not pictured on global model.
In this scenario cladding is not doing anything (we mostly use insulated panels that are not designed for diaphragm action)