Truss bottom chord fly brace accumulation of axial load in purlins

[ENG-T]

I'm busy designing a fairly large steel canopy, open on 3 sides, 36m clear span.

I've used 1.8m deep trusses and need to consider my bottom chord bracing options for the high uplift loads for an open canopy.

Providing bottom chord runners with bottom chord brace bay (wind girder) would be one option. I'm looking at the alternative using fly-braces:

Considering the 2% brace design rule for axial compression with fly braces on both sides of the truss:

The question is whether I would need to design purlins for not only the uplift and fly brace "point loads" at each truss, but also the accumulation of axial load within the purlins up to the brace bay/wind girder? So in other words, the purlins at fly brace positions would have substantially higher load than other purlins as they would also act as struts. To what extent would the resisting of buckling of the bottom chord generate an axial force in the purlins and would this force add up at each truss until they are transferred to the brace bay? or is resisting the lateral buckling of the bottom chord a rotation of the truss and therefore this is countered by the rotation (bending) of the purlin and no net axial load will accumulate in the purlins?

I'm looking at pure uplift where the only lateral load is generated by bracing the bottom chord in compression.

 

[KootK]

or is resisting the lateral buckling of the bottom chord a rotation of the truss and therefore this is countered by the rotation (bending) of the purlin and no net axial load will accumulate in the purlins?

Mostly that in my opinion. The purlins are predominantly acting as "roll beam" torsional bracing as you've intimated.

Since your truss to purlin "moment connection" is really a three member truss itself, that does kick some axial load into the purlins locally. That said:

1) I see that axial load getting resolved locally, at each truss, rather than accumulating down the length of the building.

2) With fly bracing on both side of the truss, one might lay claim to only the compression brace being active at any point in time such that the purlins get loaded with the much friendlier "bending + tension" loading as opposed to beam-column loading. Of course, this is predicated on you comfort level with telling the compression brace not to participate and having it listen. Alternately, perhaps you want only the tension brace to be active so that you don't have to consider buckling in that member. Realistically, so long as your brace length is relatively short compared to the overall purlin span, putting just the ends of your purlins into compression probably won't affect their design appreciably.

 

[ENG-T]

Thank you, your response is in line with what I was considering, I'm glad to get confirmation of my thought process.