Fly brace supporting Purlin 3

[Italo01]

Hello,

I participate in a Whatsapp group with some engineers of my region(Brazil) and one of them said that he uses fly brace to reduce the span of the purlin. He introduces the fly brace on the software and the purlin acts as a continuous purlin.

I said that consider this wrong because the bottom chord of the truss is not adequate to receive the horizontal load and that he may introduce torsion on the truss, but he doesn't believe it because the analysis show no problem when he introduces the fly brace and the purlin, who otherwise fails, pass.

I think that the loads are balanced and the horizontal loads are absorbed by the opposite fly brace and suggested, without success, that he analyses the model with live load on only one span.

What do you guys think of this?

I've seen that a lot of the engineers on the group agree with him and do the same.

 

[phamENG]

I think you're absolutely right. If continuity is introduced into the system in this way, alternating span loading (skip loading, as some call it), must be considered.

 

[human909]

It might actually pass the analysis alternating span loading. In which case his approach might actually be appropriate. Though his refusal to consider the issue you raised indicates that he doesn't fully understand the assumptions he is making, it also seems like he is overly reliant software without questioning the model.

I personally like designing efficient structures. But using fly braces as load carrying member to reduce the span of purlins is a bit much for me. Also I fail to see the benefit given that fly braces aren't cheap and generally aren't require on every purlin.

 

[hokie66]

Fly braces are for bracing the rafter bottom flange. Assuming at the same time they support the purlins is foolhardy, no matter what the fancy computer model shows.

 

[Tomfh]

Not a good idea.

 

[human909]

Sorry I like being a little contrarian. I agree it is a abnormal idea and certainly quite questionable whether the designer appropriately understands the structure and load path they are analysing. But that doesn't mean it isn't structurally functional.

Flip the whole scenario on it's head and ask the question DO YOU? consider the torsional effects of fly braces on the members they attach to in circumstance with alternating span loading? I know I don't, and it is normal amongst my peers to not explicitly model the fly brace load path. I don't model purlins or fly braces explicitly. I merely have a UDL on a rafter and LTB restraints at the locations of the fly braces.

I'll reiterate, that I question do sense in this approach. But likewise I'm posing the question that is outright dismissal warranted? In the real world reality the effective span of the purlins ARE very likely between the fly braces where they exist. That is you will get load on the fly brace from the purlin and hogging about the support.

 

[Tomfh]

Flip the whole scenario on it's head and ask the question DO YOU? consider the torsional effects of fly braces on the members they attach to in circumstance with alternating span loading?

The purlin(s) resolve it.

 

[human909]

The purlin(s) resolve it.

Sure the purlins resolve it, that is pretty clear. But they don't resolve it without some rotation of the truss/rafter, and therefore some twist & torsion.

Again I'm not suggesting that this is a problem. But simply dismissing an approach simply because "it is not what is normally done", isn't good engineering. A model that suitably models the purlins and the fly braces is more detailed and more accurate than one which doesn't explicitly model them.

 

[Tomfh]

Yes there will be some compatibility torsion. My instinct is that it’s not going to be an issue, but I’ve certainly never checked it, which I guess is your point.

 

[human909]

I too agree it is quite unlikely to be an issue so I don't see the sense in chasing that thread. Just that I recognise that it is there. Likewise while we are playing that game I am also pointing out that a reduced span for purlins with a fly brace could be quite reasonable. Though I would question the sense in taking that into account when normally a fly brace for every purlin isn't required.

 

[concepto.ed]

I agree that one could make it functional but no matter how we consider stability, load paths, torsional effects, etc, uncertainty will be higher than not consider the fly brace to act as a support. For me, it becomes too unpractically, it is best to use them for only one purpose, in most cases to brace de truss or beam girder.

When purlins are attempt to work "continuous" I prefer to overlap them some portion beyond their support.

 

[Denial]

I have been involved with only a very small number of roofing structures of the kind the OP is describing, and those were an embarrassing number of years ago.[ ] The main truss (or beam) would be analysed using linear-elastic frame analysis software, and the purlins would be analysed by simple hand-calculations.

Fly braces were fitted to only one side of the truss, and the forces they were required to take were hand-calculated from the compression force in the truss's bottom chord.[ ] This was done by allowing for an out-of-straightness construction tolerance in that chord.[ ] This bracing force (which could be either tension or compression depending upon the direction of the chord's assumed lateral bowing) was to be resisted by the purlin, so it was taken into account in the design of the purlin.

So the fly-brace did not SUPPORT the purlin, it LOADed it.

The OP's WhatsApp mates have access to much more powerful software than I did way back then.[ ] However if they want to take advantage of the fly-brace's supporting possibilities they must also take account of its loading possibilities.[ ] We are not told how elaborately they model the situation in their software.[ ] To do it properly their models would need to explicitly include appropriate geometric out-of-straightness in their truss chords, and be analysed using a solver that incorporates geometric non-linearity.

 

[human909]

So the fly-brace did not SUPPORT the purlin, it LOADed it.

Except that isn't an accurate model of the real structural system. Sure might run a code check, I'd do the same. But running a code check isn't the same as the actual load path.

The actual load path would almost certainly involve the purlin loading the fly brace.


Of course I get that we don't always compute every load path in our models. I've designed structures pretty much exactly as you just described. However I wouldn't go as far as saying that the fly-brace loads the purlin rather than the other way around. (For you typical 45 degree fly brace)

 

[hokie66]

If there is a force in the fly brace, it loads the rafter at one end and the purlin at the other. It's just an axial force member, which can work in either direction.

 

[jimstructures]

In a standard pre-engineered metal building the purlins do not load the flybraces. The flybraces load the purlins and are loaded by the roof/rafter beams, which they brace. The span of the purlins is not reduced by the connection of flybraces. Every purlin is not provided with flybraces, normally the are only located in area where the is compression in the rafters/roof beams lower flange is under compression,

Jim

 

[human909]

In a standard pre-engineered metal building the purlins do not load the flybraces. The flybraces load the purlins and are loaded by the roof/rafter beams, which they brace. The span of the purlins is not reduced by the connection of flybraces. Every purlin is not provided with flybraces, normally the are only located in area where the is compression in the rafters/roof beams lower flange is under compression,

You sure about that?

People need to differentiate between an engineering model and the actual structural behaviour. Because it is pretty clear to me that the normal load path of loads on the roof would result in the fly brace being loaded by the purlin, this would be especially evident when the fly braces occur in pairs. Either side of the rafer.

 

[hokie66]

human909,
jimstructures is correct, whether pre-engineered or conventionally engineered. What your referred to is just incidental increase in the capacity of the purlin with the fly brace. No need to know exactly how it works, if it does.

 

[human909]

human909, jimstructures is correct, whether pre-engineered or conventionally engineered.

I disagree. Though I feel like I'm being extremely pedantic in continuing this discussion regarding fly braces. I do believe the deep point that this discussion has raised is an important distinction to recognise.

Under any particularly loading scenario there is a set of member loads that for an equilibrium where by the applied loads (such as those of the roof) are resolved by equal and opposite loads on the foundation. The path that these loads are resolved are the load path.

For a typical twin flybrace and rafter design it is trivial to show that the fly brace are active in the load path of either uplift or gravitational loads on the purlins. Furthermore the rafter typically do not load the fly braces as described by jimstructures despite it being a necessary structural check to ensure there is appropriate bottom flange/chord restraint.

What your referred to is just incidental increase in the capacity of the purlin with the fly brace. No need to know exactly how it works, if it does.

It isn't incidental, it is part of the REAL load path. I also dispute the claim that we don't need to know how it works if it does. Aren't we engineers here? Knowing how and why something works is surely what we do.

 

[canwesteng]

Is this a single fly brace or a double fly brace? The double fly brace will load the rafter, but not really consequentially. More important would be the effect of creating continuous purlins which change the end reactions on the purlins and would be unconservative. The single fly brace won't load the purlins in any measurable manner, since any small deflection required near the end of the purlin putting force into the fly brace results in a negligible twist in the member, and due to the low torsional stiffness, a negligible warping stress. If you're chasing that you might as well try and figure out wind load on the web of the rafter when a gust enters the building.

 

[Tomfh]

Human909,

I think by incidental they’re saying you can safely ignore the fly brace supporting the rafter, not that the fly brace provides literally zero support.

Similar to removing a “non load bearing wall”, that was nonetheless carrying some load.