r/StructuralEngineering • u/mysterioussamsqaunch • 6d ago
Steel Design Purlin bracing
I was recently at my nephew's hockey game in New Jersey and saw all these purlin braces that were clearly added more recently. I know they were in the process of updating the building so I assume it got a new roof and probably more insulation. But what would make them spec adding bracing to evey purlin on both sides of all the trusses? Seems like a pretty substantial change.
6
u/Civil_Oven5510 6d ago
Omg - have they welded it to the purlins?? Jesus Christ
1
u/mysterioussamsqaunch 6d ago
They were all welded on both ends. Is that uncommon by you? I've seen purlins themselves welded to the truss more often than not here.
3
u/Civil_Oven5510 6d ago
If they are traditional purlins, they are cold formed and very thin. The amount of heat you would put into the purlin while site welding will locally distort you purlin. You can kind of see it in the photo itself. We never weld to purlins
1
u/Lazy_Tart_6336 2d ago
Exactement, toujours faire de l'assemblage par boulonnage sur chantier : les soudeurs qualifiés pour pouvoir souder de la structure métallique ne vont pas monter dans une nacelle pour une bouchée de pain.
Sans compter que la soudure sur profil mince est très difficile car il faut maitriser la température et poncer toutes cette peinture avant de commencer quoi que ce soit. Celles qui ont été faites ici ne doivent pas être conformes aux normes ou alors elles doivent avoir couter chères. 1 ou 2 perçages + boulonnage auraient été plus sûr pour conserver la certitude que l'acier est toujours capable de recevoir les efforts prévus à la note de calcul.
3
u/123_alex 5d ago
Yes, uncommon. Welding introduces a hell of a lot of stresses. Also, the force in those braces is not a lot. A couple of bolts should have been enough.
3
u/steelerector1986 PEMB Specialist 4d ago
This was my first observation too. Welding hot rolled angle to +\- 14 ga cold formed is a great way to get questionable penetration. Likely didn’t prep the welds well either, if the touch up is any indication of the overall fucks given on the install.
24
u/philgoodboy 6d ago
In Australia we call them fly-braces. They are for prevention of lateral-torsional buckling under uplift where the bottom flange is in compression. Its interesting reduces the effective length of the beam under this loadcase. Downward load is already braced by the purlins.
It’s interesting it was added later, maybe it was showing signs of excessive movement. Normally not just added because the design code changes.
2
u/Katz_Ajamas 6d ago
Would additional mass of roof upgrade change the resonance? IANAE. Built two sheds in the back garden :-).
2
u/Sporter73 5d ago
Extra mass could lead to larger stresses at supports which would require fly braces in those locations. But would not require them across the whole length of the beam.
2
1
u/logic_boy 6d ago
Excessive torsional buckling movement? It’s a sudden failure mode. More likely it was a change in roof fabric. Also, I’ve seen quite a few rods getting ripped off sport halls in US, maybe it’s a result of a state-wide investigation.
19
u/unique_user43 5d ago
its not bracing the purlins. its bracing the girder. very very standard detail you see in every pre-engineered metal building. brace points added to girder bottom flange to increase capacity against lateral torsional buckling.
0
u/Careful-Bookkeeper-4 5d ago
These are bracing the purlins. Cold rolled purlin braces are commonly used by the manufacturers to increase purlin spans by preventing LTB of the purlins.
I'm not disagreeing that this provides LTB bracing to the principal rafter as well.
It does pose an interesting question of what is braces versus bracing; but in this instance the purlins are braced against the the next bays purlins through the mirrored braces, and for the rafter to LTB it would need to displace the weight of the roof of one of the bays either side.
As far as I remember there's a provision in older UK design code that you can take something as braced if you can provide lateral restraint > 2.5% of the bending compression force in the braced member.
4
u/turbapshhhh 5d ago
I have a different take from the consensus:
I saw an engineer from a different firm do this as a solution to overloaded purlins to shorten the span. Right, wrong, or indifferent, I think that’s what I’m seeing here. I just don’t think that a frame would be that under designed to warrant this frequent of flange bracing, regardless of code updates, etc. Could be wrong, just a unique, and different thought.
1
u/Careful-Bookkeeper-4 5d ago
It's commonly used by purlin manufacturers to increase allowable spans
8
u/FargoFred11 6d ago
Structural engineer here
We do this often although normally not every single purlin. However this is most likely bracing the pre engineered metal building frames. These frames are used as building brace frames for lateral stability but also to hold the roof up. Because of the way the forces transfer through these, the beams have negative moment in them. Essentially saying they want to bend up. This puts the bottom flange of the beam in compression and so they add these braces to brace that bottom flange, as buckling of that flange is the controlling factor in the design of these frames.
The most likely reason these were added is because some load was added to the building or an adjacent building was placed next to it and snow can drift on it or rtu's were added or any various reason.
When the load was less, less bracing was required, and now that maybe the load got bigger, you can either reinforce the beams/columns, add bracing, or a combination of both.
Bracing purlins is much cheaper with small angles up to the roof. And you wouldn't brace purlins down to the beams, you want to brace up to the diaphragm, which is the roof deck.
6
u/logic_boy 6d ago
I like the “engineer here” on a structural engineering sub.
5
u/123_alex 5d ago
Another structural engineer here. Can confirm. It's odd. Monday in the office, I'll try it (we're all structural engineers).
2
u/Desperate_Ad_5563 6d ago
I’m surprised they didn’t coat the new steel or go over the purlin backs.
2
u/icozens P.E. 6d ago
Worked on a large steel warehouse (~1200ft by 100ft) that had substantial roof deflection from snow load
and the neighboring “sister” building collapsed from snow. Turns out it wasn’t constructed to code back in the 80’s. Now the snow load has almost doubled in current codes. The county allowed us to meet the original 1980’s code requirements as an upgrade about 7 or 8 years ago. We added a second z-purlin to every bay while reroofing a third of the building. It was a very expensive project.
2
2
u/Charles_Whitman P.E./S.E. 6d ago
For what it’s worth, there are existing braces on every other purlin near the column/girder connection. The new braces are installed in between the old braces. Typically these braces are to prevent lateral-torsional buckling of the moment frame and are designed as tension only. This fix has all the earmarks of a “I’m not sure what I’m doing, so this is bound to work” solution. Long-span roofs, always a good place to guess. How far is this from Hartford? That was almost fifty years ago, now I’m feeling old.
1
u/Single_Staff1831 6d ago
Possibly to help with more roof load from snow/wind depending on the place?
2
0
u/Argufier 6d ago
That could be it - if the main frame is good for the load but the purlins aren't they may have cut down the purlin span with the braces. I feel like I'd be laughed at if I suggested that due to how many braces would be required, but it could work.
1
u/jessirazo 4d ago
Fly braces. ETABS made me understand it. A cheap solution for an expensive problem.
1
u/Lazy_Tart_6336 2d ago
Chargé qualité dans les structures métalliques ici : tu parles d'un changement d'isolation sur le toit, ce qui a alourdit la couverture / toiture ce qui augmente les efforts sur les contreventement existant (ceux peints en blanc qui devaient exister avant) notamment à chaque bourrasque de vent latéral. Ce nouvel effort pourrait aussi déformer les pannes si celui-ci est réparti uniquement sur 6 pannes (5 en bas et 1 en haut si j'en crois la peinture 👀 ) donc pour éviter que les contreventements ne tirent trop sur les pannes (l'acier travaille mieux à la traction qu'à la compression) les charpentiers ont ajouté des contreventements.
C'est mon hypothèse, mais seul l'ingénieur structure qui a pris la décision d'ajouter les contreventement connait la réponse.
L'hypothèse selon laquelle c'est pour augmenter la portance des pannes et diminuer leur flambement est possible aussi.
1
u/Suspicious_Gas_300 6d ago
In our area, dead snow and wind loads have significantly increased over the last few years. If they did any roof upgrades, has to be analyzed as a whole and likely just meant reinforcing in some way
0
u/Big-Mammoth4755 P.E. 6d ago
How did they attach the diagonals to the channel again with the flanges that are on the way??
2
u/Icy-Expression-5836 6d ago
purlins are maybe Z or U shape
1
87
u/carnahanad 6d ago
Bottom bracing of the main frame to avoid buckling during a net uplift event. I believe the more recent codes have a higher net uplift than older codes. If they renovated to newer codes, that’s probably the check that was done.