Metal Purlins as compression members in roof bracing system

[MasterrrrBlasterrrr]

Hi

Anyone had experience (in large steel framed buildings) in utilizing the reserve compression (or tension) capacity of roof purlins (cold formed metal C's or Z's) as part of the roof bracing system for restraint against wind loads.

I have seen use of "battened purlins" - in lieu of typical hot rolled sections - as strut (or tie) members in the roof bracing. "Battened purlin" being a pair of purlins nominally 300mm apart, battened to each other so they are each stiff enough to avoid compression buckling about their weak axis.

In particular I would like to hear comment on the effect of the slotted holes typically used in such purlins - and how this may affect the serviceability performance of the structure.

Bolts are standard M12 or M16 (they are NOT tensioned beyond snug tight). Slotted holes are standard 22mm long.

 

[hokie66]

Some people do it, but I wouldn't. Not for large buildings as you stipulated...maybe for small ones. I prefer the use of circular hollow sections for the compression members, and for the ties, you could use angles, rods, etc. Remember to support these members for self weight sag.

 

[dtoli]

These kind of sections have little capacity in axial load (are slender) and so if you cope with large buildings then will be inadequate. I consider two major reasons for that:
• Your main frames will be quite apart and so buckling length,
• you certainly have to cope with large forces.
Also you should consider that transmitting forces from horizontal truss to main frames you will create eccentricities, reducing purlins resistance capacity.

For these reasons putting your figures on paper you will end close to what proposed by hokie66.

I would avoid using slotted holes in horizontal truss. You know that slotted holes have less bearing resistance than normal ones while on slot direction cannot transmit forces.


URL:

http://www.diolkos-eng.gr

 

[MasterrrrBlasterrrr]

Thanks for your feedback
I note the following:

I tend to agree that using purlins for this purpose is problematic because of the slotted holes, but only from a serviceability point of view. In regards to the ultimate capacity the structure cannot fail (if it is designed properly) as it only takes a small movement to have the bolts engage. This movement however occurs in each bay, and the cumulative effect on the serviceability performance of the structure seems very problematic to me.

Having said that though, I know of several large projects by very reputable and knowledgable engineering consultancies (in Australia) that have produced this type of design over many years (20+), and have been exposed to this type of design myself, but did not understand how it works from a serviceability point of view. I was wanting to hear feedback from others - which you provided - so thanks, again. If you have any further feedback please add that.

In regards to the ultimate capacity of the purlins in compression, the "battened purlins" only are designed to take the roof bracing forces, not the other purlins. The "battened purlins" consist of two purlins nominally 300mm apart bolted to each other to create a little verendeel truss (in plan view), and the buckling capacity of these "battened purlins" is then dictated by their major axis capacity.

In regards to the connection capacity to cleats at the main rafters, the ultimate capacity per bolt is about 30kN for an M12 bolt, and 4 bolts for each pair of purlins gives more than enough capacity.

In regards to eccentricities, yes this has to be catered for. The typical detail at the "battened purlin" connection to the rafter involves having a fly brace, so the top and bottom flanges are held (albeit by purlin members with slotted holes!). With rod bracing (typically 16mm rods, grade 250MPa), the tension force is not that large and the web of the roof beam can be justified through yield line design approach.

So, the only aspect I am uncertain about is the performance under service loads. I agree it is problematic. I have asked about this with some colleagues who were presenting these designs to clients and having them built. I was offered some advice but did not find it useful. It is the case of those with "special" knowledge not fully passing it on. I am talking about senior people with 40 years experience who are renowned experts in their fields. But, it doesn't mean they are necessarily correct! The opposite side of that coin though is that they have produced this deign for buildings with large gantry cranes and slewing jib cranes that regularly impart large horizontal forces, and to date there has not been a problem in their structures due to this type of detail.

Another point I would make is that purlin companies promote in their engineering brochures what the compression capacities are for purlins of certain lengths - obviously implying they are used to take these loads.

Finally - in modelling "large" industrial buildings in 3D - there are significant movements of "braced members" that occur regardless - through the extension of tension members for example. defining a building as "braced" can be a bit arbitrary in some cases. It might the case that the additional movements at "battened purlins" - which in theory is +/- 3mm for a 16mm bolt in a 22mm hole (assuming M16 bolts are used at "battened purlins"), may not be too difficult to accomodate within the allowable deflection limits? I have not tried to model for these movements however - and they are only +/- 3mm on average. Some may be +0/-6 or vice versa. I dont think it is something that can be designed for, except perhaps conservatively at +/-6mm?


Cheers

 

[csd72]

MasterrrBlasterrr,

I have used this method on industrial portal frame buildings in various parts of Australia though I would suggest some precaution:
1. it can cause stability issues during construction as the purlins are very slender when not braced by the roof sheeting.
2. Normally you will require double purlins at the bracing bay.
3. These purlins have virtually zero tolerance to fire and therefore there needs to be alternative load paths during a fire or disproportionate collapse event.
4. If you use the charts then they require stronger bolts e.t.c. than usual.

With regards to your concerns on servicebility, I would suspect that what is happening is that the loads are transferred via a combination of axial load shared between the purlins and membrane stresses in the roof structure. Bothe of which are very hard to prove.

 

[dtoli]

While it is very good to have feedback from projects, that doesn't mean that are well designed just because are still standing.

I found myself, in the past, being fooled this way and because I am very suspicious in things that don't follow logic I never had the unpleased feeling to see the truth. So to my view is that sound structures not only have to look good but to be proved so.

That doesn't mean that I take a position on the method you describe (I only have a text description) but this is the way I think.

You say that in ULS there is no problem but before the support is fully mobilized you must be certain that the structure is between limits (i.e. will not buckle any member before). So it may not be an SLS condition only.

If you still need to have this tolerance you may consider to use high strength bolts with pretension.


URL:

http://www.diolkos-eng.gr

 

[jgailla]

I'm not a structural engineer, but I put up metal buildings in the U.S. for a little while.
The purlins I am used to seeing are 16-gauge or 14-gauge z-sections. Because of their slenderness there is essentially no resistance to compressiion forces.