How do I change the layout of this roof structure to reduce horizontal deflection due to wind

[7788_011]

Hello,

Below is the plan view of roof structure for a residential jobs, which is 6m x 9.4m x 4m high.
The yellow lines you are seeing is the c purlins @ 1200 cts while the blue ones are rafters.
Wall bracings are placed under rafters and structs
Now I have an horizontal deflection issue about the red ones which are struts (or wind beams if you call it this way) on the external walls because the external wall thickness is limited to 90mm, which means the width of the strut member is limited to 90mm only.

 

[7788_011]

I then come up with something like this - use the bridging of the purlins to form a truss to reduce the deflection
Is there any issue with this? Or do you have any other ideas?
(PS: service wind load is 0.8kPa so roughly 1.6kN/m applied on strut)

 

[SRL V1.0]

Given you have (vertical) wall bracing under the rafters, the member you've called a strut can easily be analysed as a simply supported over the 6.0m span.
This member would typically be called a "wind header".
To resist lateral wind loads (from the wall system - which presumably spans vertically to the "wind header") you could use a PFC (channel) member on its side. It's be best to have the wind header toes down and have a simile top of steel level as the rafters at the point they meet (the header at each end will be at different levels given the roof should be falling in one direction for drainage.
Then the roof bracing would only be required to resist wind load coming from the other orthogonal direction. In that case, put a strut horizontal on the page and at midspan of the rafters, where the bracing meets on each side. This strut (compression only member) and the bracing is under the purlins.
In short, wind header can span 6.0m by itself (no horizontal roof truss req). Where as a horizontal roof truss would required to resist the wind hitting the 9.4m long face.

 

[SRL V1.0]

Also, don't use bridging to form a truss. Bridging is just for purlin capacity design.

 

[7788_011]

Thanks for the quick response SRL V1.0.

Yes I called it struct because they are structs for the other orthogonal direction wind. In the middle I would just use double purlins. I am not worry about the wind coming in this directions.

It is the wind coming the direction as shown the plan that kinda concerns me. Like you said, I did analysed it as simply supported beam with weak axis subjected to lateral wind loads of 1.6kN/m. With 300 steel channel the deflection is 33mm already. It is the deflection the issue here for me.

As for bridging, I do agree with what you said. But what is the problem if I take them as structs in the wind direction shown so my lateral deflection will be reduced? The axial force in the bridging is very small anyway?

 

[jayrod12]

I would think you'd be able to get an HSS203x102 to work for bending. I assume you were looking at the channel in weak axis bending to get that magnitude of deflection. So go to a stronger weak axis member like a closed tube.

 

[7788_011]

I tried 200x100 rectangular hollow section to reduce the deflection to 20mm but am just wondering if changing the layout would be a better idea instead of changing the section or size just to pass defection check?

 

[jayrod12]

Without seeing the bigger picture, we can't comment on what would be best. However generally speaking flipping the orientation of the purlins would likely be less cost effective than just increasing a single member at each end.

An alternate would maybe be to block the last purlin space at say 1500 o/c and have that blocking attached to the roof decking. That way the last member really only spans between blocking instead of the entire way. I don't know for sure if that's an option for you, but it's a possible solution.

Where I practice, labour is generally a bigger driver on decisions. If we have to make one or two members heavier to reduce the labour, that is generally the most cost-effective option. Installing the blocking is time consuming. Installing a 200x100 HSS is almost negligible difference in installation labour from the typical purlin. Only cost increase is strictly material.