Bracing Cold Formed Section Purlins 2

[KB4444]

This is more of a general question but if a building has frames that are x ft apart, meaning the purlin span is x ft, when is bracing required on the bottom flange of the purlins?

For this example I am meaning that sheeting is through fastened to the roof every 2ft, which is counting the top flange as laterally braced. With a load case including SL, DL and LL the top flange would always be in compression. The bottom flange would be unsupported between connection points but it is in tension. Does the bottom flange need to be braced only if wind uplift occurs/if uplift is larger then the expected dead load?

 

[EngDM]

From what I understand, you are correct in regards to the wind uplift statement, however I usually go conservative and lower the roof dead load, since they are usually conservative in the downward direction, otherwise you might conclude that the uplift doesn't overcome the dead load, when that may not be the case. Also depending on your code, I know the Canadian code specifies a larger wind uplift on individual components so you'd need to look into that for your jurisdiction.

Are your purlins 100% pin-pin connections? If not you'll have bottom flange compression under gravity loads as well.

 

[hokie66]

Most buildings with cold formed purlins use lapped zee shapes for continuity and economy. They are flimsy on their own, requiring bracing. Use the manufacturer’s load tables and bracing spacing requirements.

 

[Tomfh]

Purlins are normally braced with bridging, at maximum of 20x purlin depth.

If you omit bracing/bridging the purlins are very floppy, even if it technically works. Eg roofers will have difficulty laying sheets and will complain that the purlins are unstable.

 

[rowingengineer]

There is a program that can assist from

https://www.rsgsoftware.com/

if you don't have manufacturers tables.

 

[SandwichEngine]

The cold formed code, AISI, has formulas for using the through fastened panel to torsionally brace the bottom flange of purlins. However, I believe the limit of what's been tested is with the through fastened panels fastened at 1 ft on center.

In practice for these types of buildings where there isn't a large snow load, wind uplift will almost always control. Because of this, supplying some type of bridging will almost always result in a reduction of purlin weight. BUT purlin weight isn't where the equation ends. PEMB manufacturers who usually use cold formed purlins are looking at cost, not just weight, and different types of materials can have vastly different costs. For the type of bridging my company uses, it costs approximately 9 times what the purlins cost per lb and 11 times the purlin cost if it's galvanized per the last time I checked. These material costs are very volatile so the equations you'd use to calculate lowest costs are constantly changing. A good design will usually not include bridging on low slope through fastened roofs which will increase purlin cost but limit total cost overall.

Through fastened roofs are becoming less and less common in favor of standing seam roofs. Standing seam roofs with sliding or floating clips hypothetically have zero ability to brace the top flange of a purlin and therefore have zero ability to brace the bottom flange of a purlin. Some companies have done testing on their own proprietary standing seam roofs to be able to rely on the fact that they do provide some lateral flange resistance but I believe that you wouldn't find a standing seam roof designed by an engineer in the real world without top and bottom chord bridging.

There's also the construction perspective. The policy I developed for my company is that even for through fastened roofs, bridging is required on through fastened roofs when the roof slope is 3:12 or greater to keep the purlins from slightly rolling during erection before the roof is installed. This is based on consultation with our construction team. We also require bridging when the bay spacing is 30 ft or greater for the same reason.

 

[KB4444]

Thanks for all the responses.

If the purlins were to overlap as everyone recommends then technically it would not be a pin-pin connection (creates a moment connection). That would place the bottom flange in compression as EngDM mentioned.

For an option depending on ease of construction, can a through fastened liner (i.e. sheeting) be connected along the bottom flange, as a replacement instead of the bracing? This would result in sheeting on both the top and bottom flanges.

 

[human909]

Thanks for all the responses.

If the purlins were to overlap as everyone recommends then technically it would not be a pin-pin connection (creates a moment connection). That would place the bottom flange in compression as EngDM mentioned.

For an option depending on ease of construction, can a through fastened liner (i.e. sheeting) be connected along the bottom flange, as a replacement instead of the bracing? This would result in sheeting on both the top and bottom flanges.

It sounds like you are overcomplicating things.

For starters unless it for some reason isn't applicable you should be able to rely on manufacturer specifications, guidelines and load tables. At least here in Australia you can pretty much just read numbers off a chart once you have your loads. No real engineering involved apart from knowing the loads. Pretty much all the advice included in this thread can be found in the manufacturers reference material.

Even without having a continuous member due to overlap you still end up with the bottom flange in compression from uplift. So you generally need intermediate twist restraint except for very short sections. In addition as others have said even if you don't need it for the ultimate loads your sheeting installers would want some twist restraint.

Threaded rod can be also used as twist restraint. But again best to refer to local supplier guidelines.

 

[KB4444]

I didn't realize something as simple as the image provided would count as bracing for the bottom flange as well. That's very easy from a fabrication side too.

Thanks everyone!

 

[Tomfh]

I didn't realize something as simple as the image provided would count as bracing for the bottom flange as well.

It's not very effective compared to real bridging:

 

[hokie66]

Not as effective or simple to install. Those sag rods are time consuming.

 

[human909]

I agree with the above comments. My emphasis was meant to be consult the purlin manufacturers guidelines.


I would not expect threaded rod to perform as well as other suitable bridging, nor is it as easy to install. It is not an approved twist restrain by the suppliers in my locality but if memory serves me correctly it used to be. But designs change. In Ireland it still seems to be used.

 

[jimstructures]

There are also "bangies", which, at the easiest, are an angle with a tab as the continuation of one of the flanges. The tab is inserted in a pre-punched slot in the purlin web, high and/or low, and banged down with a hammer to form a hook, which serves as a low-cost rotation restraint.

Jim