Shed truss- camber or not?

[Pete O]

Hi all, haven't posted in a while but I have a question for the brains trust;
As I have too much money and not enough to do, I decided to drop a tree on my machinery shed a couple of months ago. I'm fabricating a new truss and am wondering whether I should build some camber into it. The photo shows the damaged truss and the structure generally:
   

The span is just under 9 meters, height of the truss is 500mm and material is 50x50x3mm SHS with 25x25x2 SHS webbing.

I've welded up the long rectangle and cut the material for the webbing but, having zero experience with this kind of truss, I'm wondering if it is necessary / advisable to pull some camber (arch) into the main frame before I weld in the bracing. As you can see, it supports the centre truss in the middle of the span without any posts in the shed. I'm thinking maybe 40-50mm of camber but would appreciate input if anyone has any experience / knowledge with this.

[EdK]

Wow! I sure hope there are some structural engineers on this forum that can help you.

Ed

[Pete O]

I don't think any structural engineers ever had any input into this shed Ed. Just thought maybe someone had fabbed these type of trusses before and would know if camber is usually added. I've put up a lot of sheds but never with this type of truss, I'm following the original closely with the replacement as it has stood up for 40+ years until my little mishap. In fact I wouldn't be surprised if the frame was second-hand when it came here.

[Ed ke6bnl]

(05-19-2022, 01:51 AM)EdK Wrote: Wow! I sure hope there are some structural engineers on this forum that can help you.

Ed

sometimes you don't need to be an engineer if you just over build it. Ed ke6bnl

[vtsteam]

Well the point of a truss is to reduce flex, otherwise an arch will do. So making a truss an arch seems a little over-sophisticated for the purpose, which, after all isn't an aircraft wing or even a building resisting a snow load, Down Under. But not much is going to hold up to continued tree dropping, so I'd cut that practice out as soon as possible!

I find that worked for me...... fingers crossed....

ps. I imagine upward forces from wind loading are as much of a concern where you are.....

pps. I'd just make it like the others, if arched they will still remain so, and if straight, likewise.

[JScott]

A truss made of tubing over a short span like that probably doesn't need camber but I think 25mm would be plenty. Unless you're expecting snow load!

[rleete]

I remember looking at the new trusses for our building expansion here at work. None of them had any bow that I could see, and these were for an industrial building where we hang overhead cranes from the ceiling. The trusses were taller (3 ft.?), but I was kinda surprised how thin the steel was.

[vtsteam]

In heavy wind loading an upward camber would actually weaken a truss.

The last truss also carries roughly half the roof load of an inner truss, unless you're hanging more from it.

If the other trusses have no camber, and the replacement one does, the sheet roofing will have a buckle, and the ends may not finish flush -- if standard lengths have been used.

[Pete O]

Thanks for the input fellas. It's an unusual construction, almost certainly cobbled together by a farmer from second-hand trusses. The photo is probably not very clear but the actual trusses that the roof purlins are bolted to are perpendicular to this one, one at the top of each side wall and one down the middle of the two bays, then timber purlins between those. The damaged truss spans the front of the opening and bears the weight of the centre truss, where you'd normally see a post. So this truss is a one-off in the construction and bears load only at it's centre point. Others have suggested replacing this truss with a post but it's a big advantage to not have a post in the middle of the opening.
I'll get back to welding the replacement truss in the next couple of days and will probably do so without putting any camber in it.

[vtsteam]

In that case, it's designed strong enough in practice. It's still standing after a tree fell on it.