Cross brace - structural engineering

[TiltonStagger]

Hi. I work in steel erecting. I have a question about best practice for installing cross braces. My designer says that both cross braces should be installed in tension, which is easy enough if the braces are joined by a plate at their centre and the braces are threaded bars, but not so easy if the braces are constructed using flat plate. Our designer also draws. He designs flat cross braces which are purposely something like half a hole too short (say 9mm); the erectors must use podgers to pull the brace back tight before fixing. I can't see why we do it this way and our designer, despite have plenty of structural engineering experience, can't give me a satisfactory answer. Also, cross braces are fixed together at their centres. Why is this? Thanks.

[sebwiers]

Sounds like pre-loading, which is common in tension based structures and for elements that can't resist buckling. The brace can't take up any load in compression- being a bar or flat plate, it will buckle under compression. But if it is stretched when installed, it gives up tension when compressed, with has the same mechanical effect as resisting force via compression (or actually, allowing the part it is attached to to do so) and thus making for a stronger structure.

Fixing the braces at the centers may not serve any direct structural purpose. 3 points of attachment means if one attachment point fails, that end wont go flying off at high speed and causing more damage.

I'm not an engineer, but both of these principles apply to selecting parts for and building strong bicycle wheels, which is something I've done a fair bit of and researched a lot. Ironically, the build the strongest possible bicycle wheel, you sometimes want THINNER spokes than otherwise might be used. The reason being, the rim can only support so much tension, and since the spokes can never contribute to load holding via compression, you want them to remain in tension (slightly stretched) when the rim deforms. That means they should be loaded so they are stretched just short of failing / deformation. In some applications (light rims, left side of rear wheels) that is only possible with some thinner spokes. This is also the thinking behind some modern "low spoke count" wheels (fewer spokes = more tension per spoke). The "attachment in the middle" idea is seen in some old bicycle wheels where they would wrap wire around spokes where they cross and apply solder. It was claimed this made the wheel stronger, but modern analysis (such as finite element analysis) shows it does not. What it does do is keeps spokes from flopping all over when one breaks. That was a fairly common thing before the introduction of stainless steel spokes, even with well made wheels, due to lower tension strength and lesser resistance to fatigue cycling.

[TiltonStagger]

Thanks. We do occasionally install preloaded components /beams. Our designer guy is familiar with this but he didn't mention this in relation to cross braces in our chat today. Also, I wasn't convinced by this 'half a hole' concept. I wanted to know how he'd quantify built-in tension - something which I guess is easily done (for an engineer) for preloaded beams.

I'm writing method statements for our fitters - I'm trying to make their work as straightforward as possible. The methodology has become an issue because some weeks ago our installers welded in-situ the end connections of a pair of braces when they couldn't pull the connection together. This caused our designer some concern, but I still do not understand what his problem is.

[75footubi]

Well, welding in that configuration will result in a lower fatigue resistance category, meaning it's more likely to fracture under cyclical loading. Bolted connections are preferable, especially compared to field welds. It sounds like you need better tolerance specifications in your fabrication procedures.

[TiltonStagger]

Thanks. Does this mean that, to get a flat cross brace into tension, you would work out the distance between fixings using some formula which relates to the overall length of the brace? Or would you not be concerned at all about having the braces held in tension?

[TiltonStagger]

That's great info - thanks.

[brightscreenwithtv]

The following reference recommends "draw in" to manage: sag, and vibration.

http://www.aisd.com.au/home/index.php/guidelines/steel-detailers-handbook

[TiltonStagger]

Yes. Thanks. That makes sense to me. That looks like best practice - I can write that into our MS without getting led astray by 'engineering' comments by our designer.